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DIY DC millivoltmeter

djQUAN

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Joined
May 27, 2013
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1,154
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There have been times that I needed to measure very low voltages (millivolts and sometimes down to microvolts) and my multimeter is a typical autoranging type with 1mV resolution which isn't good enough.

When I'm measuring low voltage drops across high current shunt resistors or contact points or adjusting bias currents in amplifiers, I needed a few digits below millivolts for accuracy.

Here's what I did in an afternoon to fill that need (without having to buy another multimeter) and using parts already at hand.

The parts: the familiar digital panel meter, switches, project box, dc-dc boards and connectors
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cut the rectangular hole using my mill. I also used the mill to make accurate pilot holes for the switches and banana jacks.
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Drilled the holes for the switches and jacks
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Here's the meter wired up. There is a diode protection circuit to prevent blowing the meter input when a high voltage is accidentally applied. The grey pot at the switch is the cal adj for measuring the internal battery voltage to check for battery life. A step up converter board wedged on the side powers the 5V panel meter from a 4.2V lipoly cell.
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I used a li-poly pack from a broken ipod touch. This li-poly cell is only about 2mm thick and has a protection PCB already. A 4pin female header is used as a charging port.
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Adding some labels and here's the finished meter along with the dedicated ammeter that I made a while back.
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I calibrated the meter by wiring it in parallel with my calibrated multimeter to a resistor voltage divider which is then connected to an adjustable bench supply. I adjusted the supply so the multimeter reads 199mV and calibrated the DIY meter to have the same reading.

The meter range is 0-199.99mV with 10uV step. It isn't lab grade but would get me by for those odd jobs that I need to measure small voltages. :) (if anyone is curious, the ammeter has two ranges, 0-20mA and 0-10A but it can measure up to 20A in the high range with short duty cycles. It also uses a li-ion cellphone cell and same charging port on the side)
 

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Joined
Jan 14, 2011
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A man who can, successfully, make his own tools, is a man who knows what he's doing. Good job!
 
Joined
Dec 11, 2011
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4,364
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Oy, you don't know how jealous you made me with that mill. You have any idea how many hours I've spent drilling corner holes and working my way across with a hack saw blade, and filing out the corners to make it "square"?

Excellent build though! I typically just use a differential probe on my scope for small voltages, but I typically don't need better than +-1mV accuracy.
 

djQUAN

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Joined
May 27, 2013
Messages
1,154
Points
63
That's the cheapest mill I found a while back. search for Proxxon micro mill. I found it very useful all around the shop. I was doing the drill-and-file method also back then and took a lot of time and resulted to not so perfect rectangular holes. :p

I needed microvolt accuracy since I need to measure very low resistances by using the CC mode on the bench supply and measuring it with this meter. Worked pretty well for the purpose.
 




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