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

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ImA4Wheelr
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Excellent dchomak.  Thanks for the added info in Post 28.  Nice approach with the excess solder to fine tune.

This may be an old thread, but it is quite relevant.  I have read the OP many times trying to figure out how to make one of these.  I think it would be nice for future readers if you update the OP.  I'm asking as I know I will be one of the future readers.

dchomak
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Thanks, ImA4.
I did a poor job explaining just how to go about creating one of these in the OP.
It takes a LOT of work to write something that is EASY to read.
I can’t imagine how much effort HKJ puts into his write ups, over and above all the work in the actual testing and documenting. It should be greatly appreciated.
I know I often spend MORE time writing about a mod than I do building it!

I will ad an addendum to the OP, a cut a paste of the instructions on how to fine tune the shunt. And a link to my post 28 which I hope explains my thoughts more clearly.

ImA4Wheelr
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Just wanted to post a big thank you.  I finally tried my external shunt in a high power light yesterday.  It is so cool.  I would have had to purchase a new DMM and probably would not have achieved the same accuracy if it were not for you posting this thread dchomak.  Thanks again Smile

I should have said this back when I made the shunt, but forgot to make the post.  Not sure if the OP says so, but letting the shunt cool before testing (when calibrating) is very important.

ImA4Wheelr
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Need input from an electronics guru

I have several DC Volt/Ammeters that don't include shunts for measuring current.  This one on Ebay is similar, but the pcb (and maybe some components) appears lightly different.  It does appear to use the same shunt spec wise.  It calls for a 50A/75mV shunt.  Ammeters have labels that either specify 50A/75mV or 100A/75mV.

Since, I don't have a way to push an accurate 100 amps DC through a shunt, I would like to make the shunt by flowing 10 amps through a shunt and measure for a 7.5mV drop.  So, my question is:

If I wanted to make my own shunt (or buy one and test it for accuracy), do I need to flow 100 amps through it to measure the 75mV drop?  Or does the spec have some other meaning like 75mV drop per Amp of current?

HKJ
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ImA4Wheelr wrote:
If I wanted to make my own shunt (or buy one and test it for accuracy), do I need to flow 100 amps through it to measure the 75mV drop?  Or does the spec have some other meaning like 75mV drop per Amp of current?

The specifications means that the meter shows 100/50 or whatever its maximum value is, when the voltage is 75mV.

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

ImA4Wheelr
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^

Thank you HKJ!  Smile

 

EDIT: I seem to be running into a problem if I go DYI shunt.  To get a shunt that can handle 100 amps and have 75mV of drop, I will need a very long shunt.  To get a shunt in the length I would want, I would need to go all the way up to 22 gauge wire (5.47 inches long at 77F).  I haven't tried to figure out if 22 gauge wire can handle 100 amps, but my intuition says it can't.  Even if it can, it will rise in temp at high amps which will throw off the measurement.  Guess I need to buy some of these to get these meters working.  Am I missing something.

 

dchomak
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At $3.39 that current shunt is definitely the way to go.

6 Gauge copper wire will handle 100A and you would need just under a 2 foot length of it for it to drop 75mV while conducting 100A. (From ohms law, and the resistance tables of copper wire.)

BTW, that shunt will only need to dissipate 7.5W. P=IxE 100A x .075V = 7.5W

And I can’t wait to see this light that draws 100A you’re working on Big Smile

ImA4Wheelr
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^

Ha!  I'd like to see it too.  Maybe when emitters have 1Vf at 100 amps.

Thank you.  Went ahead and ordered some of those shunts.

 

EDIT: I still haven't used the shunts I ordered above, but have been using bare 10 gauge wire with a fan blowing air over them to keep the temperature from rising.

LightRider
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Hello friend. Could you explain what is meant by cutting notches in the solder to adjust resistance? Do you cut a v out of some of the exposed solder or do you just squeeze it? Thanks

Phlogiston
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Just squeeze it very gently, especially while you’re getting a feel for the process. This is the third part of a coarse – medium – fine tuning process, so it’s only intended to make tiny adjustments.

You’re basically looking to deform the metal ever so slightly – thereby constricting the current path by the tiniest fraction – rather than cutting into it as such.

LightRider
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Phlogiston wrote:
Just squeeze it very gently, especially while you’re getting a feel for the process. This is the third part of a coarse – medium – fine tuning process, so it’s only intended to make tiny adjustments.

You’re basically looking to deform the metal ever so slightly – thereby constricting the current path by the tiniest fraction – rather than cutting into it as such.

Wow. Ok. That is some fine tuning then. Thanks

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