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weird DMM voltage readings

kellogs

Jan 7, 2014
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Hello,

I am trying to debug some PCB - the LED below won't light up

Took ut my trusty DMM and measured
V_AC = 3.31 V
V_BC = 1.33 V
V_AB = 0 V <--- ?? should it not be around 2 V ?

Thanks!
odd_dmm_readings-png.44388
 

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davenn

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Sep 5, 2009
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Hello,

I am trying to debug some PCB - the LED below won't light up

Took ut my trusty DMM and measured
V_AC = 3.31 V
V_BC = 1.33 V
V_AB = 0 V <--- ?? should it not be around 2 V ?

Thanks!


You haven't told us what the MCU is .... presuming your LED is connected to one of it's outputs ?
does that output switch to ground (0V) when it is activated ?
if it doesn't, then that is your problem

In other words, does that MCU pin source or sink current when it is operated?

If it is a current source, then your circuit will not work
 

kellogs

Jan 7, 2014
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The MCU is an ATmega88 set to sink current, nothing out of the ordinary about it that I can think of. The pin is PB6 and I have also tried sinking into PC1 (another pin) - did not take readings on that one but the LED remained off.
Anothert thing I have attmepted was to connect 2 xAA batteries for 3.2 V right at A and B and the LED lit up.

But anyway, about those readings that I see on the voltmeter...
 

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
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V_AC = 3.31 V
V_BC = 1.33 V
V_AB = 0 V
odd_dmm_readings-png.44388

The key measurement is the voltage across the resistor. That will tell you if any current is flowing. It is possible you will find that value is also zero (although I suspect you won't).

You might then ask why 0 + 0 = 3.31, and my answer is that you may have a white led, or some other LED with a Vf greater than your supply voltage. Thus, effectively zero current flows and the loading by your multimeter measures the voltage drop across each component as zero.

Another option (and the one I'd lead towards) may be that you have killed your LED and it has now failed short circuit. if this is the case you will see the voltage across the resistor as the same as the voltage across A and C. It would also explain why the voltage across B and C is so much higher than zero when the output is presumably pulled low. For this to be the case, I would expect the value of R1 to be under 100 ohms -- possibly well under that value. What value is R1?

Do you have a diode check function on your multimeter? It should show the forward voltage drop (if it's low enough for your multimeter to measure) and light up dimly during this process. If it shows a near zero value, the LED is shorted.

Another option is that you have a wiring error on your PCB or a programming error on your MCU (for example, having it set as an output instead of an output, but I'll discount those for the moment.
 

Harald Kapp

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it has now failed short circuit.
A shot circuit doesn't explain the readings.
Nor does an open circuit would.

V_AC = 3.31 V is to be expected as this measurement gives the supply voltage.
V_BC = 1.31 V is a bit suspicious. A logic high or a logic low on the MCU's output should lead to V_BC= 0V or V_BC = 3.3 V. A voltage of 1.3 V indicates either a heavily overloaded output or a non-DC voltage e.g. from a pwm signal at that pin?
In any case standard circuit theory predicts V_AB = V_AC-V_BC and therefore a voltage around 2 V as expected by @kellogs , not 0 V as measured. FRFegardless of the components between points A and B.

Could it be the multimeter creates a current path that short circuits part of the circuit when measuring? Make sure you use a battery powered meter and there are no other connections of the meter other than the 2 measurement wires. Are you sure you measure between the right points as indicated by your diagram? You may think so but the pcb layout may be different (wrong) such that your actual measurements are between other points than indicated?
 

kellogs

Jan 7, 2014
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R1 is 270 ohm, and as I have said, taken out of its PCB - together with L1 and a 3.2 V battery it does light up. No shorts there, no wrong measure points, no bad PCB traces. It might be a wrong hex file for the MCU model, I don't know.

But that's just the context, the real question is why this DMM is giving the readings that it does. As far as I know, the DMM measures voltages by inserting a big gigaohm-ish resistence between the two probes and multiplying that by the current it senses. If this is correct, then I am with @Harald Kapp. The thing should indicate a voltage of 1.98V drop between A and B (given the other AC and BC measurements are correct).
I have just taken a reading on R1 alone, it shows 0.00 V as well
 
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