How to read 10A, 15A, 20A and beyond with a cheap meter

42 posts / 0 new

Pages

dchomak
Offline
Last seen: 4 years 7 months ago
Joined: 03/17/2012 - 20:10
Posts: 4122
Location: Connecticut
How to read 10A, 15A, 20A and beyond with a cheap meter

How to read 10A, 15A, 20A and beyond with a cheap meter

______________________________________________________

08/15/2015

I added an addendum to the bottom of this post with detailed instructions on how to build the shunt resistor. And here is a link to my post #28 in this thread that I hope explains things more clearly

______________________________________________________
.
.

Up until recently, I have never had to measure very high levels of current. Any measuring I have done were for single emitter lights that had current draws of no more than 5A. For that, I had used a DMM set to the 10A scale with better than stock leads. Although that was a OK for those purposes, I came to realize that it was NOT OK for the much higher current levels of multi emitter lights where the emitters are wired in parallel. What I tried next was soldering 14 gauge wire directly to the banana jacks inside of the meter like this:

While that was better, it still wasn’t good enough.

The problem with measuring high current levels up until now, is that ALL of the current going to the LEDs has to pass down the test leads into the meter and back out before it reaches the LEDs. All of the resistance in this path can significantly alter (lower) the amount of current that will flow through the emitters. This is especially true for multi emitter lights in parallel. A very significant amount of current will be required. A lot of us here have dealt with the issues of preparing a good electrical path for that current. What I will attempt to do is provide a good method to measure that current without significantly increasing the resistance in that path. Actually there will be LESS resistance than other very expensive meters.

What I am going to use here is a couple of cheap Harbor Freight meters.

These meters, and many others, measure current by passing the current through a shunt resistor, and measuring the voltage drop across that resistance. The actual voltage drop is displayed as the measurement of the current. In the case of the HF meter, on the 10A scale, this shunt resistor is 0.01 ohms. This .01 ohm resistor is actually quite low for a cheap meter.

If 700 mA passes through that shunt, then the voltage drop will be according to Ohms Law, I x R, or .7A x .01 Ohm = .007V or 7mV The reading that is displayed is actually a measurement of that voltage drop. Of course when attempting to measure high currents, it is a good idea to use high quality leads to reduce any additional resistance to the path. An even better idea is to solder heavy gauge wire directly to the banana jacks to also eliminate any contact resistance. Hence the use of cheap HF meters. You wouldn’t want to do this with your Fluke 87V.

The heavier 14 gauge leads are soldered directly to the inside of the banana jack on the top hole and the bottom hole. I was careful NOT to disturb any of the soldering in the vicinity of the shunt resistor as it has been calibrated. Any additional solder on the shunt wire itself would change its resistance and throw it off. BTW, I consider the design of this meter to be quite good in this area. Notice that the jacks are soldered directly to the circuit board all around the perimeter of the jack and that there is a very short path to the shunt resistor. This would enable this meter to measure 10A without any problems so long as 14 gauge wire is soldered directly to the jack as I have. In fact, I am tempted to say that it could possible handle more. For how long I don’t know, but 20A passing through a .01 ohm resistor will generate only 4 watts of heat. It might be possible, I don’t have a 20A source to test it out on.
Here is that meter measuring 15A into a 6 – XM-L2 SRK with those leads. Even though the scale on the dial says 10A, the meter will read up to 20A

Even with this improvement, there is still a problem for us in reading current levels of our flashlights. There is still too much resistance. As many of us know, resistance kills performance concerning these lights. Additionally, LEDs are non linear, meaning that small increases in operating voltage will increase the current disproportionately. In other words, small increases in voltage lead to large increases in current. We want as little resistance as possible in the measuring path. So what I am saying is that if there is resistance in the path measuring the current, we will perhaps not be measuring the true amount of current flowing to the LEDs. It is quite possible that the light put together is drawing more current than what we are able to measure because there is more resistance in the measuring path than there is in the actual light. After I did the 14 gauge mod, I realized that the 2 leads wire actually had twice the resistance of the shunt resistor!

In an effort to reduce the resistance of our measuring equipment, I had to first verify that what I said earlier about measuring the voltage drop across the shunt resistor is true. So what I did was solder a couple of cheap HF alligator clips across the inside of the banana jacks. I did this so I could use a second meter to measure the actual voltage drop and compare it to what the meter was telling me what the current draw was. Here is a pic of that connection.

Once again, I stayed away from the shunt resistor so as not to affect it.
As they appear from the front of the meter.

The test circuit.
What we have here is a wimpy, vintage Knight brand power supply connected to a 2.2 ohm power resistor. The meter on the left is set to the 10A scale and is measuring the current flowing from the PS, through the meter and through the resistor. The meter on the right is connected to the 2 alligator leads that I soldered across the shunt resistor of the meter on the left. It is set to the 200mV scale and is reading 7.3mV as that is the voltage drop across the shunt. Just as expected.

Looking at that picture, I found it hard to believe that there was twice as much resistance in those 2 14 gauge leads as there was internally in the meter. I looked it up and 14 gauge copper wire has a resistance of 0.002525 ohms per foot.
When measuring current, it’s a shame that the shunt resistor is in the meter. Wouldn’t it be a great idea to have the shunt resistor at the point of measurement and measure the voltage drop there. No need for the current to enter the meter at all. That way we can eliminate the resistance of the leads altogether. While we are at it, lets reduce the shunt resistor from 0.01 ohm to 0.001 ohm. So that is what I did. From the resistance chart, it works out that 4.75 inches of 14 gauge is about .001 ohm. So I cut off a 5 inch length of wire and soldered leads a little greater that 4.75 inches apart. I calibrated it with another HF meter on the 10A scale. At first it read a little high as it was a little too long, but I reduced the resistance slightly by allowing solder to wick into the wire until is was exact.
Here is the shunt resistor soldered to a jig to measure current of a SRK.

The final result, ALL of the current is flowing through the short white wire. The meter is measuring the voltage drop across that wire, a space of about 4.75 inches. There is only 0.001 ohms in that path.

08/15/2015

To fine tune or calibrate the shunt resistor, we need to make adjustments to the resistance.

While a known amount of current is flowing through the shunt wire, and the leads are hooked up to a volt meter we will use the meter reading to make the following adjustments to the shunt wire.

1 Coarse adjustment: Cut the wire a little long and solder the leads a little farther apart than .001 ohm. So we start off a little high. The reason to be a little high is because the next adjustment is perfect for lowering it a little.

2. Medium adjustment: Let solder wick into the stranded wire to reduce resistance a little. Add solder until is a little too low. The reason to be a little low at this point is because the next adjustment is perfect for raising it just a tiny bit.

3. Fine adjustment: Use a pair of diagonal cutters and gently squeeze a notch in the wire where the solder is. With a little patience, just the right number of notches can be created to zero in on an exact reading.

Done.

Edited by: dchomak on 08/15/2015 - 11:55
allan d
Offline
Last seen: 7 months 2 weeks ago
Joined: 09/01/2012 - 00:15
Posts: 345
Location: SW Florida

OK, now you have sent me back to the bench.

I recently made a 0.01 ohm shunt with 18 AWG wire, now I can see that I need to make a 0.001 ohm model.

Allan

djozz
Offline
Last seen: 1 week 1 day ago
Joined: 09/07/2012 - 17:04
Posts: 18319
Location: Amsterdam

Very useful tutorial, I will use your technique sometime when I need to measure higher than 10A currents.

Thanks for sharing!

wight
Offline
Last seen: 2 years 10 months ago
Joined: 11/27/2013 - 16:40
Posts: 4969
Location: Virginia, USA

Thanks for posting this. I’ve been turning over the idea of a better shunt in my head since earlier today when I saw your discussion with RMM in the other thread. Looks like you’ve got the problem under control!

For calibration I do not think another DMM is really necessary. A consistent, known load (such as 7135’s) should allow someone to tune their shunt.

Still fine, still on a break. One day I’ll catch up with you folks! previous wight catchup
list of my drivers & variants (A17DD, FET+1 stuff, WIP stuff, etc)

blfdemigod
Offline
Last seen: 5 years 3 weeks ago
Joined: 10/03/2013 - 17:55
Posts: 520
Location: San Antonio, TX

WOW, great post. Now I know!

Also, are those really cheap HFT DMM's? Or Free DMM's? look what I found this month in my Autoweek magazine! Plan to go tomorrow and pick them up!

Disclosure:

RMM
Offline
Last seen: 2 years 11 months ago
Joined: 07/23/2013 - 13:47
Posts: 4006
Location: USA

HF DMM, "the choice of flashaholics." TM

Mountain Electronics : batteries, Noctigon, and much more! What's new?

blfdemigod
Offline
Last seen: 5 years 3 weeks ago
Joined: 10/03/2013 - 17:55
Posts: 520
Location: San Antonio, TX

^^ I hope no one tries to print that!

Disclosure:

wight
Offline
Last seen: 2 years 10 months ago
Joined: 11/27/2013 - 16:40
Posts: 4969
Location: Virginia, USA

HF might Their biggest product endorsement is “Chosen by Truckin’ Magazine,” which they use for several items.

Still fine, still on a break. One day I’ll catch up with you folks! previous wight catchup
list of my drivers & variants (A17DD, FET+1 stuff, WIP stuff, etc)

FlashPilot
Offline
Last seen: 12 hours 5 min ago
Joined: 05/10/2010 - 16:07
Posts: 4939
Location: USA

Just a warning about these free harbor freight meters. I have three of them and they all read quite differently of one another. I changed out the batteries with fresh new tested energizers and they still read incorrectly. Since I have a few calibrated fluke DMM’s, I didnt bother to look any further and threw the HF meters in the parts box. There might be a calibration pot inside to dial these in. Just dont rely on your readings until you have checked them against a known calibrated DMM and adjusted if necessary. Then recheck for accuracy every month or so. I read that a lot of cheap meters are overly sensitive to the state of the battery voltage that powers them and can also make them read incorrectly. So as the cell ages, your calibration gets thrown out the window, even after a recent calibration. :Sp Maybe thats why these are free and have been for several years.

I might dig mine out and practice my throwing sling techniques. Just hold it by the leads and give it a good throw into a concrete wall

My new collection of 50+ Chinese knives

dchomak
Offline
Last seen: 4 years 7 months ago
Joined: 03/17/2012 - 20:10
Posts: 4122
Location: Connecticut

FlashPilot,at risk of being called a shill for HF, I must say that it seems HF is upping their game lately. It used to be that I could just not touch their stuff. Totally unusable, but lately their stuff has improved. Take for example this meter. It used to be the shunt resistor was longer and not mounted directly to the jacks. When passing current it had to travel through the circuit board, Now they are right next to the jacks. Here is an older #90839 and the newer #90899

Also, I was shocked when I recently was forced to buy some more alligator clips. In the past they were terrible, totally unusable as the wire was hair thin. Compare the ones I just picked up last week with the ones from a few years ago. They package claims 18 gauge.
The copper step flashing I used was cut with the free HF scissors (shears). Their regular price is \$1.99, on sale almost always for .99. Or you could go to Dollar Tree and get the same exact pair, in white, for \$1

As far as these meters go, unless there is something grossly wrong with one, they pretty mcuh agree with each other.

Woops, not that one, this one.

and this

Anyway, I am not pitching HF meters in this post but an idea. That idea is central to BLF, If you don’t own it, or can’t buy it, you can make it. And have fun doing it!

Muto
Offline
Last seen: 37 min 14 sec ago
Joined: 09/04/2012 - 16:42
Posts: 2823
Location: Southeast, PA

Really all a meter has to do is provide you with a basis, that is, a reference.
If it is not spot on the exact value, it does not matter, what matters is that you use the same meter and record the difference before and after mods.
Most cheap meters will work quite well once you have upgraded the leads, preferably soldered them as dchomak has outlined.

As they say, “it ain’t rocket science”

Some of you guys get sooo bent on a battery charger charging .01 volt over or under like it’s the end of the world.
Then HKJ will chime in and say, BFD, it’s within acceptable spec.
This just in, the battery isn’t going to explode because your I4 charged it to 4.22 volts.

Likewise, your flashlight is going to look the same at 4.185 volts as it does at 4.20 volts, and really that battery will be down under 4.15 volts before you can type this post if your light has any kind of decent draw, so big deal.

I like specs, I just will not be lured into being, or acting like a Spec Head.

Keith

The difference between Hoarding and Collecting is the illusion of Organization
.
.“I will get one of flashlight from patrol car”

“History doesn’t repeat itself, but it sometimes rhymes,” Mark Twain

After the Apocalypse there will be only 2 things left alive, Cockroaches and Keith Richards

FlashPilot
Offline
Last seen: 12 hours 5 min ago
Joined: 05/10/2010 - 16:07
Posts: 4939
Location: USA

Great post dchomak… you silly HF DMM mega-shill! :D. Holly smokes! Thats a lot of meters you’ve got there. :bigsmile: That pic says it all and Im impressed with the close voltage spread. Mine were off by more than .5V, as I recall. I’ll have to locate my 3 copies and see if they resemble yours. I think I got them last summer so they are less than a year old. Is there an adjustment pot anywhere on the board? Maybe hidden on the front side underneath? Hopefully one for current and another for voltage.

I got a free pair of black scissors like yours from HF instead of the free DMM last time… and I like it! Maybe I’ll keep going back with my free scissors coupon and become the HF Scissors Mega-Shill. How’d that be? :bigsmile:

My new collection of 50+ Chinese knives

ImA4Wheelr
Offline
Last seen: 1 year 4 months ago
Joined: 02/03/2013 - 14:51
Posts: 7935
Location: SC

Careful folks, dchomak is just trying to lull us into a false state of complacency as he silently builds his secret army of DMM's that will be used to attain complete and total World DOMINATION!

ImA4Wheelr
Offline
Last seen: 1 year 4 months ago
Joined: 02/03/2013 - 14:51
Posts: 7935
Location: SC

Sorry, I keep surprising myself as I continue to reach new levels of corniness.

Nice thread dcho.  Very interesting and informative.

dchomak
Offline
Last seen: 4 years 7 months ago
Joined: 03/17/2012 - 20:10
Posts: 4122
Location: Connecticut
ImA4Wheelr wrote:

Careful folks, dchomak is just trying to lull us into a false state of complacency as he silently builds his secret army of DMM’s that will be used to attain complete and total World DOMINATION!

That’s right, if I can get ahold of enough meters…….., well resistance will futile.

Actually there’s nothing to worry about, I’ve moved on to the free tape measures. Look for a post in the near future about my “collection” (Tongue in cheek, of course)

FlashPilot, it’s appropriate that you mentioned the Sling. A sling illustrates just what I was putting forth.

Ever hear the story of David and Goliath. Goliath was a warrior, a mercenary that fought for the enemy. He was equipped with the most modern weapons of war. He was so greatly feared that none of David’s countrymen would meet him in battle. The only one that was willing to take him on, one on one, was David. He was just a little sheep herder, not even experienced in the art of war. Nor did he have any sophisticated weapons, just a homemade sling and 5 smooth stones. And you know how that went, he got the job done, he slew Goliath.

I have a saying, “The true measure of a man is what it takes to stop him”
If it doesn’t take much to stop someone, then he’s not much of a man.
So, don’t have the tool? Don’t let that stop you!

The OP was my way of showing, members here that sometimes you may not need fancy equipment to get the job done. After all, some of us don’t have all the “toys”. Maybe they are young, or just starting out, or don’t have the money. 2 individuals that come to mind that do very well without the fancy tools is Old Lumens and ComfyChair.

And I wasn’t down playing having good tools either, that “hack” works with a Fluke also.

texaspyro
Offline
Last seen: 3 years 7 months ago
Joined: 04/29/2011 - 12:43
Posts: 4593
allan d wrote:
I recently made a 0.01 ohm shunt with 18 AWG wire, now I can see that I need to make a 0.001 ohm model.

Lotsa luck doing that… at 0.001 ohms (even 0.01 ohms) even a tiny amount of change in wiring/component leads/soldering can be more than the resistance value you want to make. I have meters that can read well below 0.000001 ohms and getting anywhere near 0.001 ohms takes a lot of careful work with them.

dchomak
Offline
Last seen: 4 years 7 months ago
Joined: 03/17/2012 - 20:10
Posts: 4122
Location: Connecticut
texaspyro wrote:
allan d wrote:
I recently made a 0.01 ohm shunt with 18 AWG wire, now I can see that I need to make a 0.001 ohm model.

Lotsa luck doing that… at 0.001 ohms (even 0.01 ohms) even a tiny amount of change in wiring/component leads/soldering can be more than the resistance value you want to make. I have meters that can read well below 0.000001 ohms and getting anywhere near 0.001 ohms takes a lot of careful work with them.

That’s exactly right TP. I was able to do it by cutting a little long, making my connections, and allowing small bits of solder to wick into the wire. It wasn’t easy, but I got ‘er done.

FlashPilot
Offline
Last seen: 12 hours 5 min ago
Joined: 05/10/2010 - 16:07
Posts: 4939
Location: USA

dchomak, I didnt mean to detract from your thread. I think it was an interesting post that demonstrated some good problem solving skills… and free is always better. Especially if its useful. :bigsmile:

The only reason I saved my HF DMM’s was because I might have a use for them as parts in future projects if they couldn’t easily be calibrated correctly. I picked them up thinking that a 0.1-0.2V variation would be good enough to give to others to check their cells from time to time. 0.5V or more just didnt cut it. But you know, there’s always some humor to be found while criticizing a cheap product thats completely useless. BUT, Ive been more than happy with my free HF scissors! I think there was a similar thread on BLF posted a few years back where someone used a pair of DMM’s in tandem to read higher amps but I cant find it now. Im lucky that I have access to a calibrated clamp meter if I need it.

Ive always enjoyed the saying, “dont bring a knife to a gunfight.”

My new collection of 50+ Chinese knives

Rufusbduck
Offline
Last seen: 9 months 2 hours ago
Joined: 04/04/2012 - 15:34
Posts: 10401
Location: Golden state

Interesting thread. I’ve always looked at meters as “black boxes” not understanding how the results were obtained. Learned a bit today, thanks. Mostly I just use mine to verify that all the 7135’s are functional or that the driver is in the ballpark but even for that I’ve had to replace the stock leads with shorter, thicker ones. This mod should get a link in one of the DIY sticky’s.

Three Tanna leaves to give him life, nine to give him movement. But what if he eats the whole bag?

Scott

ImA4Wheelr
Offline
Last seen: 1 year 4 months ago
Joined: 02/03/2013 - 14:51
Posts: 7935
Location: SC

I can't believe I still having followed the advice given in the OP and made one of these external shunts yet.  I generally use very short leads made each time using the emitter lead itself (use extra length for testing and then shorten for final assembly).  Probably not too bad a set up, but this is much better.

I hope to build one this weekend.  Thank you again for this thread.  Good stuff.

ImA4Wheelr
Offline
Last seen: 1 year 4 months ago
Joined: 02/03/2013 - 14:51
Posts: 7935
Location: SC

I really want to make myself one of these external shunts, but I'm having trouble picturing how calibrate it.  In the OP, you wrote:

From the resistance chart, it works out that 4.75 inches of 14 gauge is about .001 ohm. So I cut off a 5 inch length of wire and soldered leads a little greater that 4.75 inches apart. I calibrated it with another HF meter on the 10A scale. At first it read a little high as it was a little too long, but I reduced the resistance slightly by allowing solder to wick into the wire until is was exact.

Could you elaborate on how you did the Bold texted part above?

HKJ
Offline
Last seen: 2 months 1 week ago
Joined: 05/24/2011 - 12:23
Posts: 7455
Location: Copenhagen, Denmark

ImA4Wheelr wrote:
I really want to make myself one of these external shunts, but I'm having trouble picturing how calibrate it.

The easy way to calibrate is if you have a power supply with a good current limit.

Then you can measure the current, move the meter to the shunt and measure the voltage.

With tow good DMM's it is easier, because one can be used for the current and one for the voltage over the shunt.

A DMM with a very good low voltage readout is preferred for this!

My website with reviews of many chargers and batteries (More than 1000): https://lygte-info.dk/

ImA4Wheelr
Offline
Last seen: 1 year 4 months ago
Joined: 02/03/2013 - 14:51
Posts: 7935
Location: SC

Thank you HKJ.  I think I'm following you.  I will try that out this weekend.

My power supply is low end, but I have hooked up DMM's directly to it and have had matching readings on current (within a few milliamps).  I don't have a good low voltage DMM, but the rest of what you say should get me most of the way there.  Basically, I think you're telling me I should be able to get separate readings from 3 different units  (Power Supply Meters current and voltage, DMM current, DMM voltage) to get some comfort with the external shunt.

HKJ
Offline
Last seen: 2 months 1 week ago
Joined: 05/24/2011 - 12:23
Posts: 7455
Location: Copenhagen, Denmark

Yes, when you know the current in the shunt you can use the DMM to verify the reading.

With a 10mOhm shunt you need a meter that is fairly good at low voltages..

I do most of my chargers test with a 10mOhm shunt and a 6½ digit bench DMM, but because I do not maintain calibration factors for the shunts (or calibrate them) I have a few percent tolerance on the current measurement. If I had done some calibration I could get below 0.5%.

My website with reviews of many chargers and batteries (More than 1000): https://lygte-info.dk/

dchomak
Offline
Last seen: 4 years 7 months ago
Joined: 03/17/2012 - 20:10
Posts: 4122
Location: Connecticut
ImA4Wheelr wrote:

I really want to make myself one of these external shunts, but I’m having trouble picturing how calibrate it.  In the OP, you wrote:

From the resistance chart, it works out that 4.75 inches of 14 gauge is about .001 ohm. So I cut off a 5 inch length of wire and soldered leads a little greater that 4.75 inches apart. I calibrated it with another HF meter on the 10A scale. At first it read a little high as it was a little too long, but I reduced the resistance slightly by allowing solder to wick into the wire until is was exact.

Could you elaborate on how you did the Bold texted part above?

BTW, I did not make a .001 ohm resistor, what I did was take a piece of wire and solder connections to it, that after fine tuning, those 2 connections became exactly (well not exactly) .001 ohms apart. There is a subtle difference between the 2, but that difference makes this possible.

I am assuming you know how to make the shunt, what you are asking is how to calibrate it, right?

What I did is put the shunt in series with a HF meter while passing current through both. Then fine tuned the resistance of the shunt so that the voltage read across it corresponded to the current given by the HF meter on the 10A scale. Of course it would only be as accurate as the HF meter. If you have a known current source, there is an easier and better way.

HKJ wrote:

The easy way to calibrate is if you have a power supply with a good current limit.

Then you can measure the current, move the meter to the shunt and measure the voltage.

With tow good DMM’s it is easier, because one can be used for the current and one for the voltage over the shunt.

A DMM with a very good low voltage readout is preferred for this!

The key to this whole idea is that in constructing the shunt, the first step is soldering test leads to the shunt, before any calibration. This is what makes all this possible with cheap meters. This eliminates contact connection and exact location issues with the shunt resistor and BTW this is a 1 milli ohm resistor, for the lowest possible voltage drop while measuring current.

Once the initial connections are soldered in place and a dedicated meter is chosen (HF meters are cheap or free) The rest of it is fine tuning the resistance between the connection points.

I don’t see why a very good low voltage readout meter is necessary as during the tuning (calibration) process the meter – shunt resistor combo will be calibrated together. If the meter was off in the first place it wouldn’t even matter. Calibrate the combo for the known current flowing through the shunt. Now if a cheap meter is sensitive to a weak battery, that would be another matter.

If the shunt was a separate piece of equipment, then that would be a different matter and every piece of equipment would have to be of high quality.

HKJ
Offline
Last seen: 2 months 1 week ago
Joined: 05/24/2011 - 12:23
Posts: 7455
Location: Copenhagen, Denmark

dchomak wrote:
I don't see why a very good low voltage readout meter is necessary as during the tuning (calibration) process the meter - shunt resistor combo will be calibrated together.

Because I prefer a resolution in mA, not in Ampere. At 1mOhm 1mV is 1A, i.e. I would want a resolution of 10uV or better (10uV means 10mA in resolution) for most of my measurement.

If I am going to measure above 10A I would probably use a current clamp. If the current was supplied by my (now dead) power supply I could get the value directly from it and not worry about clamp or small value resistor.

My website with reviews of many chargers and batteries (More than 1000): https://lygte-info.dk/

ImA4Wheelr
Offline
Last seen: 1 year 4 months ago
Joined: 02/03/2013 - 14:51
Posts: 7935
Location: SC

Thank you both for your advice.  This is on my list of things to try to get done today.

I hope you all don't mind another question.  I would like to make one of these shunts for testing current flowing to the emitter.  So I will need some extra length to string it from the emitter base to the driver.  So the shunt will have the leads for reading voltage somewhere along the shunt wire well away from the ends.  When I calibrate, I will be sending x amps through the whole length via the bench top power supply.  Do I need to compensate for the extra length of shunt wire outside the voltage meter lead attachment points?  I think the answer is no, I don't need to compensate for that.

Another related question.  As I use this external shunt, I will periodically need to trim off some of the ends as they will deteriorate from repeated soldering.  Will the trimmings require recalibration?  I think the answer is no.

Actually, even bigger related question.  Then I am calibrating the shunt, I will have like 3/8" or so of each end clamped in the bench top power supply jacks.  But when I use the shunt, the ends of the shunt wire will be soldered to a driver and emitter base with no clamps.  Do need to alter how I attach the shunt wire to the bench top power supply to make it more similar to how it will be used for measuring current to the emitter?  Again, I think the answer is no.

Really, all the 3 questions above are the same question.  I think the important part is area between and including where the voltage measurement leads attach to the shunt wire.

Now a somewhat less related question.  Why are we not concerned about voltage drop along the leads that go to the DMM that will be measuring the voltage drop along the shunt?

HKJ
Offline
Last seen: 2 months 1 week ago
Joined: 05/24/2011 - 12:23
Posts: 7455
Location: Copenhagen, Denmark

ImA4Wheelr wrote:
Really, all the 3 questions above are the same question.  I think the important part is area between and including where the voltage measurement leads attach to the shunt wire.

You are correct, you do not have to worry about the part outside the DMM probes (Except it adds some voltage drop, but this has nothing with the current measurement to do).

Depending on the material the shunt is made of, the temperature may also affect the measurements.

ImA4Wheelr wrote:
Now a somewhat less related question.  Why are we not concerned about voltage drop along the leads that go to the DMM that will be measuring the voltage drop along the shunt?

We are, but generally the voltage drop will be negligible when the current is 1uA or lower. Remember a DMM usual has 10Mohm input impedance on voltage ranges.

My website with reviews of many chargers and batteries (More than 1000): https://lygte-info.dk/

dchomak
Offline
Last seen: 4 years 7 months ago
Joined: 03/17/2012 - 20:10
Posts: 4122
Location: Connecticut

ImA4, you asked me what I meant when I wrote what is in bold in my quote.

dchomak wrote:
From the resistance chart, it works out that 4.75 inches of 14 gauge is about .001 ohm. So I cut off a 5 inch length of wire and soldered leads a little greater that 4.75 inches apart. I calibrated it with another HF meter on the 10A scale. At first it read a little high as it was a little too long, but I reduced the resistance slightly by allowing solder to wick into the wire until is was exact.

You have brought up an old thread and I just now realize that I never fully explained all that was intended when I wrote that.
On the surface it means that I used the only thing I had during the calibration process, a HF meter on the 10A scale. The “10A” scale part is important, but I never explained just what I meant by that.

The purpose for making the shunt resistor is not to use it in measuring 1 to 3 amps. A stock meter with 14 gauge leads is OK for that. This shunt resistor is best used when reading current levels of say, 10A and above. As we all talked about, even though cheaper meters say they have a 10A scale for reading current, most will actually read and display up to 20A. When you calibrate this shunt resistor, you want to calibrate it using a current close to the current levels you will be using it for. In my case it was at some point between 10 and 20A. It was the current flowing through my 6-XM-L2 SRK.

Remember now, the primary reason for constructing the shunt resistor wasn’t necessarily to read currents of 10 to 20A, but was done to lower the resistance in the current measuring path. Measuring 15A with a stock meter and 14 gauge leads has a big impact on the current flowing through LED’s. Pass 15A through that .01 ohm shunt resistor* in a stock meter* and you drop 0.15V across it. The LED’s then only “see” 4.05V instead of 4.20V. I first realized there was a problem with stock meters and their .01 ohm shunt when I first charged up a set of Samsung 28A’s to 4.35V. That extra 0.15V gave me 3A more draw! That means in the case of my 6-XM-L light, when I took a current reading with a stock meter and 14 gauge leads, that current reading would be about 3A less than it would actually be when the light was put together!
Use an expensive and highly accurate meter with it’s 0.01 ohm internal shunt to measure the current, and it too will read 3A short of the actual current when the light is put together.
The reason why that 0.15V drop across the internal shunt has such a big impact is because an LED is a non-linear device and small changes in voltage have a disproportional effect on current draw.

That is the real reason for using this external shunt. It just so happens that larger currents can be measured with cheap meter also.

To fine tune or calibrate the shunt resistor, we need to make adjustments to the resistance.

While a known amount of current is flowing through the shunt wire, and the leads are hooked up to a volt meter we will use the meter reading to make the following adjustments to the shunt wire.

1 Coarse adjustment: Cut the wire a little long and solder the leads a little farther apart than .001 ohm. So we start off a little high. The reason to be a little high is because the next adjustment is perfect for lowering it a little.

2. Medium adjustment: Let solder wick into the stranded wire to reduce resistance a little. Add solder until is a little too low. The reason to be a little low at this point is because the next adjustment is perfect for raising it just a tiny bit.

3. Fine adjustment: Use a pair of diagonal cutters and gently squeeze a notch in the wire where the solder is. With a little patience, just the right number of notches can be created to zero in on an exact reading.

Done.

ImA4Wheelr
Offline
Last seen: 1 year 4 months ago
Joined: 02/03/2013 - 14:51
Posts: 7935
Location: SC

Very good.  Thank you again HKJ.

EDIT:  This cross posted when dchomak posted.  I will read that post and reply in a few minutes.

dchomak
Offline
Last seen: 4 years 7 months ago
Joined: 03/17/2012 - 20:10
Posts: 4122
Location: Connecticut

ImA4,
WOW, I just realized than in the 30 minutes it took me to write the above, you asked more questions.. HKJ I see, already answered them. What service BLF provides!