Strange? Phenomena - 1N diodes and Mastech digital meter.

[slimjim]

Hello,

In the process of troubleshooting my synthesizer, I measured the output of it's simple +/-15v power supply. For some reason I had the meter set to read A/C, and it read around 32v on the positive 15v output! Why? I checked points back to behind the LM317 voltage regulator, the positive side of the full wave rectifier read 64v ac!! Why? The negative side of the diode bridge read 0v ac as it should. I began to frantically try to understand what was going on. Diodes are not supposed to pass A/C right?

Anyhow, I simplified the whole thing and just tested one diode by placing it in series with one end of the transformer secondary, connecting the centre tap to the black lead of the meter - the red lead to the striped end of the diode. Reading 20v ac and around 9v dc Why? reversing the diode gave the correct reading of 0v ac and -9v dc. Why? So I flipped the red and black leads and tested both directions of the diode again. Results reversed.

I would not expect so much ac voltage (ripple?) and comparatively little dc.

Can anyone explain to me why I am reading so much a/c voltage and relatively little dc, AFTER the diode, and only when the diode is oriented for positive rectification?

-Jim

 

[tvtech]

Using the incorrect earth...it's possible. Measure across the INDIVIDUAL smoothing cap(s) that the Diode(s) supply on DC. Black lead on Negative and Red lead on Positive of the Caps. On DC Volts range. Please.

Just a thought...

 

[slimjim]

Accross the smoothing caps with correct meter lead polarity I read -28.8v dc on the negative side, and on the positive side 28.8v dc. I'm cool with that, just not the ac that my meter tells me is there.

 

[tvtech]

Accross the smoothing caps with correct meter lead polarity I read -28.8v dc on the negative side, and on the positive side 28.8v dc. I'm cool with that, just not the ac that my meter tells me is there.

You probably have no AC ripple slimjim.

Use an Oscilloscope to be sure...outta here. Someone else here will surely help further.

Too tired to think now, been troubleshooting sets the whole day. Cheers.

 

[slimjim]

Thank you, I think so too - though unfortunately I have no access to an oscilloscope. I think it's something going on with my meter, or just sheer idiocy on my part. Still several transistors fried for no apparant reason so I'm left stumped for now. Thanks for your time

-J

 

[The Electrician]

Some questions.

1. What is the model number of your Mastech meter? Is it an autoranging meter? Is it a "true RMS" meter?

2. What does the meter measure as the full secondary voltage of your transformer, when the meter is set to measure AC? Is this value what you expect?

3. What does the meter say if you set it to measure AC and then measure the voltage of a battery, with both possible connections of the red and black leads? Try a 9V battery if possible.

4. If you manually change the meter ranges, do you get the same readings from question 3 for different ranges?

 

[slimjim]

Some questions.

1. What is the model number of your Mastech meter? Is it an autoranging meter? Is it a "true RMS" meter?

Mastech MAS830L non-autoranging. I doubt it is "true" anything...

2. What does the meter measure as the full secondary voltage of your transformer, when the meter is set to measure AC? Is this value what you expect?

42.8v ac measured from end to end of the secondary, which is about what I would expect from this 2a 18-0-18v xfo unloaded. Perhaps it's a touch high but not enough to make me really suspicious.

3. What does the meter say if you set it to measure AC and then measure the voltage of a battery, with both possible connections of the red and black leads? Try a 9V battery if possible.

Now this was very interesting! 18.2v measured with the meter leads on the correct poles of the 9v battery, 0v if reversed.

4. If you manually change the meter ranges, do you get the same readings from question 3 for different ranges?

There are only 2 ranges for ac, 600v and 200v but the answer is yes.

I had satisfied myself that this was an effect of the meter somehow, by wiring in a couple of leds in parrallell and ass-about accross the output (yes with appropriate i-limiting resistors), only the correctly oriented led lit, so I am satisfied there cannot be any ac.

But this meter thing really threw me for a loop for a while!

What do you think could be the reason? is this common with cheaper dmm's?

-J

 

[The Electrician]

Non true RMS meters usually just rectify the input and measure the average value, then multiply by 1.1107 to get the sine wave RMS voltage.

Your meter apparently only half-wave rectifies the input voltage, so it responds to a battery only in one direction.

Try putting a 1 µF non-electrolytic capacitor in series with the meter and then making your AC measurement; this should get rid of the DC component of voltage you're measuring. You will have to wait for a while, maybe a minute or so, for the capacitor to charge up to the DC voltage component.

However, if the lowest range for AC volts is 200 volts, you're probably not going to get a good reading for AC ripple voltages that may be less than one volt.

 

[slimjim]

Thanks that does make sense, but still, how does it explain 18.2v on the AC volts scale from a 9v battery?

9v -
0.7v (diode drop)
=8.3v

Surely that is the maximum dc component my cheap meter could see on the AC scale?

so if the meter sees 8.3 volts after it's internal diode then multiplies by 1.1107

we would expect

9.219v or thereabouts.

I read 18.2v on the AC scale from a 9v battery.

I am still confused as to why this would happen, although I guess it doesn't really matter, just a curiosity.

 

[The Electrician]

I was wondering that myself, and your finding that it measured a battery at 2 times voltage with the leads in one direction but zero with the leads in the other direction gave me a clue.

I think they are using a half wave rectifier so when you're actually measureing AC, the meter only uses one half of the AC waveform. That's why it has to multiply by 2 to get the final result.

Maybe they are using a simple opamp precision rectifier which is only half wave. The opamp could get rid of the diode drop.

It's just a guess....

 

[slimjim]

Thanks, I'm convinced now - time to buy a better meter.

-J

 

[slimjim]

Thanks, I'm convinced - time to buy a better meter.
-J