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I was measuring a 'steer' signal for switching on a relay. The oscilloscope, gave me back it peaks at around 2.64V. The width of the signal is 132ms.

I then hooked up my multimeter, to measure how much amp was drawn. However the reading varies (between 4 - 14 mA). Which made me wonder.

My question: is there a minimum amount of time a multimeter needs to 'register' a correct load? In other words, is 132ms long enough to get a decent reading?

If not, what would be a correct way of measuring?

If it matters, it might be depending on the 'quality' of the multimeter? I used a UNI-T (UT61).

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2 Answers 2

up vote 3 down vote accepted

A typical update rate for a multimeter is 3 to 10 times a second, so 132 milliseconds is likely not enough time.

Since you have an oscilloscope, you can put a low value resistor in series with the load and measure the voltage across the resistor with the oscilloscope.

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Im sorry. Im not sure if i understand. I'm trying to figure out the max ampere thats been drawn. But because of the 132 ms signal, as you explained, it probably not reliable. The max voltage i already have. –  Rogier Mar 15 at 22:57
    
Wherever you connected your ammeter, replace it with a resistor. Say you expect 100mA. You might use a 1 ohm resistor. Then measure the voltage across that resistor with the oscilloscope (careful about ground issues). Each mV you measure represents 1mA of current. –  Spehro Pefhany Mar 15 at 23:04
    
Aha, ok, super thank! –  Rogier Mar 15 at 23:13
    
Agreed, a scope -- especially a memory scope with the trigger set approrpiately -- would be the first thing I'd reach for. –  keshlam Mar 16 at 1:40

Update rates are not really a big issue - the circuitry in a multimeter (generalism alert) will probably be a low-pass averaging filter before it gets digitized and this will prevent you measuring peak currents as you thought you could.

The standard off-the-shelf multimeter reads average current not peak. Use your O-scope for this type of measurement.

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Most multimeters I've looked at are dual-slope integration (integration period maybe 83.3msec out of an overall measurement cycle of 333 msec) preceded by a ~1Hz low pass filter. So you're getting something pretty messy from a short pulse. –  Spehro Pefhany Mar 15 at 23:12
    
@Spehro I can't deny that trying to read something that is short duration on a standard meter is gonna be like a minor (well maybe not train-wreck but) derailment!! –  Andy aka Mar 16 at 0:26

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