# Why isn't Ohm's law working for this simple circuit?

I have a 70W 12V-500V DC power converter, and something in these pictures is not adding up.

I run the 500V output through a high-voltage 332kΩ resistor.

Now, using the left multimeter I monitor the current through the circuit, and it runs up around 320mA. Using the right multimeter I first check the voltage on the power supply leads: 511V. (So right away we know something's off -- the supply isn't even getting warm after a few minutes of testing.)

Then I use the right multimeter to check the voltage across the resistor: 12V!

I verified the resistor's value with both meters. If all of this is true then Ohm's law suggests that either the voltage across the resistor should be .320mA * 332kΩ = 106kV, or else the current through the circuit should be 511V / 332kΩ = 1.5mA.

(Of course the right multimeter itself is providing a path for current, but its resistance should be very high. Indeed: when I remove the right multimeter from the circuit the current increases only 2-3mA.)

My best guess is that the output of the converter is not very smooth DC, or has some characteristic that is causing these multimeters to produce erroneous values. If so, what characteristic might that be, and how can I adjust for it?

BTW, here's a close-up of the DC converter. Maybe the design will be familiar to someone.

And in case the wiring isn't clear from the photos, here's how it was connected

simulate this circuit – Schematic created using CircuitLab

And here's some more information. First, to remove all doubt, here's the resistor being measured after all this, so it does not appear to have fried.

And here is the whole shebang. If the ammeter on the DC supply can be trusted, and this power converter doesn't have the ability to produce power out of thin air, then the true circuit current is under 3.6mA. (This is consistent with the fact that I have observed no heat buildup on any component whatsoever.) In which case the question is: Why is the Extech reading ~320mA? If I switch the Extech to µA scale it reads around 3180 (still not right). The other multimeter reads 0A at all scales, which is consistent with the voltage drop seen across the resistor (which implies 36µA true current).

Epilogue: I opened the Extech multimeter to find its 250mA fuse blown. Replacing that made it behave normally. Evidently it just has a very confusing failure mode!

• did you measure the resistor's resistance? – hassan789 Mar 25 '16 at 20:08
• If you're putting 500V across that resistor, you're burning roughly 3/4W, which is well over it's 1/2W rating. Something to keep in mind. – Shamtam Mar 25 '16 at 20:52

## 3 Answers

Based on your voltage measurement, it looks like your resistor is about 38 ohms, not 332,000 ohms. The current limit on your ExTech multimeter is 200 mA according to the datasheet. The meter is showing about 60% over that limit. My guess is that you either partially blew a fuse or triggered some internal current limiting circuit, so most of the voltage is being dropped across the current meter. Alternately, if the resistor actually was 336k, it might have burned out and failed short after dissipating 0.75W. You'd probably have noticed a smell if that were the case, though.

• Good call on the ExTech. I rechecked the resistor: It's still metering 332,000 ohms. And the circuit is almost certainly not carrying that many amps, because the DC supply I've connected to the converter's input has an ammeter on it that, after the power-on spike, is stable at only 100-150mA. When I put the current through the 10A on either meter they both read zero to two decimals, which I guess means current is under 5mA? – feetwet Mar 25 '16 at 21:49
• If the resistor's really 332k and not shorted out somehow, then yeah, you shouldn't see more than ~1.5mA. I'd trust the voltage measurement more than the current on a cheap meter. Have you tried connecting the resistor directly to the power supply output (no current meter) and measuring the voltage across it? – Adam Haun Mar 25 '16 at 22:05
• Yes: Voltage across the 332k resistor is 12V. So 36 microamps?! One of the power sources (see update to question) must have put itself into a current-limiting condition. Probably the 12V source because it's really a low-voltage device test source and probably doesn't want to fry them.... – feetwet Mar 25 '16 at 22:17

Can't see much on your photo. But one thing is certain: Ohm's law is tough, but just. You haven't connected something right.

• Ohm is not just a good idea; it's the law. – user65586 Mar 25 '16 at 19:59
• Right. Like a constitution. – Gregory Kornblum Mar 25 '16 at 20:01
• and just like a law, you need to read the fine print. – helloworld922 Mar 25 '16 at 20:04
• And yet resistance is futile... – user_1818839 Mar 25 '16 at 20:07
• That's the current situation, yes. – user65586 Mar 25 '16 at 21:42

At a guess, your 332k resistor is about a 37 ohm resistor. The power supply has gone into current limit. Your nominal current (500 volts, 70 watts) is about 140 mA, so your real current is about twice this, which seems credible. Start by running your supply with no load and verifying that it will put out 500 volts no load. Then measure the resistance of your resistor. If both of those tests are OK, in the words of Gregory Kornblum,

You haven't connected something right

and looking at your tangle of wires I'm not surprised.

• It looks as though feetwet did that and it measures 511V (makes sense with a very light load) – scld Mar 25 '16 at 20:48
• Yeah, just rechecked resistor and no-load voltage. The thing that I doubt is the current reading, not only because everything should be heating up if it's true, but also because the 12V supply to the converter has an ammeter that reads under 150mA. I guess I should update the question to ask, "Given that real current must be under 3.6mA, why would the Extech show 320ma?" – feetwet Mar 25 '16 at 21:54
• Try checking against something like a 10k resistor and your 12 volt supply. Better yet, just swap your meters – WhatRoughBeast Mar 25 '16 at 22:11
• Yeah, I'm pretty sure now the true current is 36microAmps. The other meter gets 0. I'm going to try a different 12V supply into the converter and some heavier resistors to keep circuit protection from confusing things. – feetwet Mar 25 '16 at 22:20
• BTW, regarding the "tangle of wires:" any pointers or examples of a more organized workbench would be appreciated! (E.g., especially since I'm dealing with high voltage I'm assuming lining things up on a solderless breadboard is out of the question.) – feetwet Mar 26 '16 at 3:11