voltage of dimmer output not reading same on two different DVM's

I am using a 12 V 20 amp lamp dimmer to control some LED lighting. While my two DVM's show nearly equal input voltage to the dimmer, they both read separate voltages on the output side. One is reading 9 V and the other 4 V. I am trying to determine wattage output at various dimmer settings. Why the different in voltage readings on two different DVM's and how to remedy?

Reply to
Loading thread data ...


** Both are correct, but only one of them is "true RMS " ?

OR the controller use high frequency PWM and one meter cannot read it.

Next time, leave out all the facts just to make it harder.

.... Phil

Reply to
Phil Allison

Sorry, the meters are a GE2524 Digital Multimeter that reads the 9V and the other meter is a Etek 10709 that reads 4 V. The dimmer is one from Amazon (sorry, 30 amps not 20 as I said originally):

formatting link

Not sure about dimmer frequency, but I can hear a faint whine as it runs.

Reply to


** What range are you using ???

AC volts or DC volts?

PWM voltage waves can be read as an average value on any DC meter - but true power will require something much fancier.

Why not just measure the DC battery current and multiply by the voltage?

Then subtract a watt or so for losses in the controller.

.... Phil

Reply to
Phil Allison

DC on both meters on lowest range, 20 V.

If you mean input from the battery into the dimmer, yes, tried that. Both meters show the same 12 VDC @ 7 A when the dimmer is set at maximum. Will have to check and see what happens as dimmer is adjusted.

Would I only lose a watt or two through the dimmer? If so, I guess I could take all my readings from the input side and go with those.

Reply to

These dimmers MUST be pulse width modulated, othwerwise the heat production would be huge. You can't expect a DVM to give accurate reading of a pulsating source. Possibly some true-RMS meters may do a decent job of averaging the result, but I would not trust such a reading without verifying it with another instrument, like an oscilliscope.


Reply to
Jon Elson




s I

Ok, just as a follow up, I did keep both meters on the input side of the

dimmer. Voltage remained constant and nearly the same on both meters,

11.97 and 12.01 VDC, respectively, with dimmer off. At dimmer maximum setting, voltage remained nearly constant (drop of -0.3 V) but current changed from nearly nothing to the 7 A maximum with dimmer at max setting. I swapped out the meters to read current just to be sure and each showed the 7 A maximum. Then, I just adjusted the dimmer to different positions, made some tick marks with current values indicated.

I think I'll just go with this. As you mentioned, losses through the dimmer are probably slight and not an issue, at least in this application. Thanks for your suggestions.

Reply to


** Still can certainly show the average DC voltage.

The usual input RC network doe the job nicely.

** Only if the have response down to DC and up to the high harmonics of the PWM.

.... Phil

Reply to
Phil Allison

=_Part_0_193160550.1604795784421 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit

i'd guess because they are non-sinusoidal outputs and one of the meters is not true rms

----Android NewsGroup Reader---- 
 Click to see the full signature
Reply to

I'd say neither of them are true RMS.

Reply to
Andy Burns

Good point--measuring DC current average is the appropriate setting, and there's no DC-scale on any of my meters that does RMS measure.

Voltage, averaged or RMS averaged, is going to deliver at-or-near nonsense numbers for the purposes of evaluating an LED illuminator.

Reply to

AHA, the keystone part of the puzzle. This blows Phil's statement out the w ater which is rare, there is no TRMS in DC.

Now you are down to the meters being plain old wrong, after all one just be , right ?

First of all realize that 100% LED compatible dimmers, which could be used on multiple lights must be the raw PWM. ON OFF ON OFF. If you smooth it off the a DC voltage, LEDS in parallel might fire at slightly different voltag es and screw ya all up. So PWM is fine with LEDs as long as you never excee d the Imax (current rating) As such your meters on DC are actually measurin g AC in the wrong mode.

It will help to know how the digital voltmeter works. Now when a voltage is applied to a DVM it is of course divided by the range control and whatever . After that it goes to a comparator. The other input to the comparator is a sawtooth wave, generated quite accurately. When your input voltage and th e voltage from the sawtooth cross, the comparator changes states and puts o ut an output. The exact time that happens is used to stop or read a counter to know just how much the internal voltage has risen.

It is easy to see that each DVM can have its own timing, or speed. higher s peeds read faster and they are more expensive if they are accurate.

Now if you want a DC measurement, which could be useful, then you must feed the meter DC, not AC. As you can see frigs it all up. So, an extremely sim ple way would be to get like a 5K resistor and like a maybe what guys, 68uF ? Take the resistor from the output, connect it and the probe there, and t he cap and the other end of the cap to ground. yyou just made an R/C filter . This should make the meters read the same.

That is because they are measuring DC on the DC range.

I can't remember when the last time it was I had to make an R/C filter like that but I know I did a couple of times.

Reply to

snipped-for-privacy@gmail.com wrote: ====================

** FYI:

" true RMS " = DC equivalent value for the *heating* effect on a resistance.

Repeating waveforms may have a DC offset = average DC value.

The "true RMS" value of such wave is:

sqrt ( DC ^2 + ACrms ^2 )

..... Phil

Reply to
Phil Allison


Known. but a DC meter might not be set up for that.To be, it would have to have some sort of filter and they don't really do that because they don't want the load. I think I told the guy to get a resistor and a cap, damn su ch uncritical values really, cana be 10K and a 47uF, or a 470 and a 10uF. I t just doesn't matter if that meter has an input of 10 megs.

Of course TRMS might come in handy for an incandescent or some sort of heat ing element, but for LED it is out of the game.

I would think most LED dimmers say rated 400 watts (?) would be ready for m ultiple LEDs in parallel. This absolutely requires the max voltage delivere d with PWM or they will not come on or dim at the same rate.

So he was trying to measure PWM on a DC scale, might work on an old style m eter with a real meter, but not on this.

The first time TRMS did me any good was on TVs. Of all things, the filament winding of the power transformer opened up. I decided rather than change i t, run the filament off the flyback. So I did and I wound the wire around t he core and the meter read about 6.3 or so on TRMS and I connected it. ZAM it worked.

I found out that with that half sine, 70KHz wave repeating at 15.7Khz came to the right voltage at 22 volts peak to peak off the fly.

I had adventures in that. Shorted H-K ? We I got a winding for that. Panaso nics seemed to be really affected by the capacitance though I tried to keep it low. So I EQed the video output circuit for it, just a cap in the emit ter of one of the transistors. Short intermittent ? Fine, I just put like a 4.7 K in there to make it shorted all the time.

I did all kinds of shit for that company that nobody else would do.

Now this devout, religious person who is partner wants me to lie, to put in a good review on their local whatever they got. I would in exchange for a really glowing review of me but I am not looking for work. I even stopped a dvertising my business. I got enough now.

I doubt the guy in Florida has my number, I might leave it that way. After all this starting a business to piss off the goniffs etc in this business and put them out of business with truth and nice reasonable repairs, I am g oing to lie ?


Reply to

snipped-for-privacy@gmail.com wrote: ============================

** You are not paying proper attention.

DC voltage and true RMS voltage are equivalent. DC meters read the average, DC value of a wave.

..... Phil

Reply to
Phil Allison

No dispute on that, just why a DVM might not read it right.

Anyway, of the TRMS, I have noticed my average responding meter comes fairl y close.

The TRMS one is a Fluke 8050A which I keep on the top shelf, it has more t han enough accuracy and the LED display (which is in perfect shape HAHA) is not as easy to read as the 8000A I use as a daily driver. The 8000A has on e less digit and is fast, I like it. When I need accurate then well... But then sometimes I read DC voltages with a scope. Put it on DC in most things you get like maybe 25 volts tops ? Five volts per division. Most shit you just need to know if it is there. I don't care from 4.96 or 5.03.

"Hack", say it.

Reply to


** Cannot possibly help but do so.

Analog ones do so by virtue of meter needle inertia.

DVMs do so by virtue of a simple LP filter at the input.

..... Phil

Reply to
Phil Allison

That really should be qualified.

The way I understnd it is that the RMS voltage produces the same ammount of heat in a resistor as a DC voltage will.

However RMS voltage could be a high peak value that will damage solid state devices. The meter could show 5 volts, peaks could be 50 volts.

Again DC meters could read many different values depending on the meter and wave form.

Reply to
Ralph Mowery

Ralph Mowery wrote: ===================

** No need exists.
** What I wrote was accurate.

Strange how so many are baffled by such simple stuff.

...... Phil

Reply to
Phil Allison

let's say the following:

TRMS meters generally have AC and AC+DC modes, but usually your interested in the AC portion. The TRMS voltage of a waveform will have the same heating value of a DC voltage set to the TRMS value.

AC meters have a frequency response. They also might be limited by the crest factor of the waveform. There were thermal AC TRMS meters at one time.

Non TRMS meters are what you might call. Average reading, TRMS responding. What does that mean? Take line voltage of 120 V 60 Hz sine wave. It flips the voltages below zero (i.e. precision full-wave rectify and it averages. Throws it into a capacitance filter. The result is the average value. The meter then multiplies that average value by a constant so it reads the RMS value of the hypothetical sine wave.

The meter basically assumes it's being fed a sine wave, averages whatever it gets and multiplies by some k. if you feed it a 60 Hz sine wave, you will get out the RMS value of the sine wave.

We talk about average, but the average of a typical sine wave is zero. it's really the average of the absolute value.of the waveform.

Reply to
Ron D.

ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.