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## Solid States Devices => solid state devices => Topic started by: gast on August 26, 2005, 11:06:59 AM

Title: How to get right values from an non-50Hz signal using a DMM?
Post by: gast on August 26, 2005, 11:06:59 AM
Hi all,

does anybody have an idea how to measure comparable values (voltage/ampers) of an sqarepulse-signal using only a digital multimeter?
I dont want to get absolute values (V/A), but I want to get linear values independent from the signals frequency.

I guess a multimeter which is build for 50 Hz measurement shows always wrong values on signals other than 50 Hz.
But I guess also that the "error-factor" changes with the frequency (= not linear).

My idea is to rectify the signal with 4 lowdrop-diodes and a big capacitor to get an DC-voltage.
Ampere measurements I would do by a voltage measurement over a small load (10 ohm or so).

Any better ideas (except "buy an oszilloscope ;-) )?

Gast
Title: Re: How to get right values from an non-50Hz signal using a DMM?
Post by: Warpspeed on September 17, 2005, 11:39:12 AM
One low cost but fairly accurate way to measure true ac power (watts) not reactive power (v/a) is to illuminate a suitably sized light bulb in a light proof box with your high frequency signal.

What you then do is measure the relative light output somehow. It doesn't need to be anything fancy, a photocell or light dependant resistor connected to your multimeter. So you get some sort of relative light reading.

You then connect the identical light bulb up to a variable voltage dc power supply, and adjust the brightness to an identical setting.  By accurately measuring dc volts, and dc milliamps you can then work out the dc power required to get the same exact light level.

This will work up to extremely high frequencies, and with funny non symmetrical waveforms too. The more careful you are, the more accurate it will be.

Even with an oscilloscope, measuring ac power is extremely difficult because the voltage and current waveforms can be well out of phase.
Title: Re: How to get right values from an non-50Hz signal using a DMM?
Post by: hartiberlin on September 17, 2005, 05:15:21 PM
Yes, Warpspeed , this is right ! Many thanks for bringing up the comparising method
via light bulbs.As the human eye is also very sensitive you can place 2 bulbs this way
side by side and compare the DC powered bulb to the AC powered bulb and thus
compare the wattage output relative easily.

Thanks for bringing this very usefull and cheap measurement method up.

Regards, Stefan.
Title: Re: How to get right values from an non-50Hz signal using a DMM?
Post by: bolt on October 16, 2008, 04:58:23 AM
And dont use compact power savers either they can light up from 1 watt to 50 watts and still not change much on brightness. Another way is find a value resistor on the input that just gets hot then take that out and put the same value in the output. If the output one goes up in smoke you have OU :)