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

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


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.



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.


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.

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

Thanks for sharing!

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! :slight_smile:

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.

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!

HF DMM, "the choice of flashaholics." TM

^^ I hope no one tries to print that!

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

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 :smiley:

FlashPilot,at risk of being called a shill for HF, :slight_smile: 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!

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.

JMHO, your amperage may vary:)

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:

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!

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

Nice thread dcho. Very interesting and informative.

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

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. :wink:

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.

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. :smiley: 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.”

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.

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.