Why DVA meter?

[jtexas]

Another thread I asked why we test powerpacks by measuring "peak AC", when a capacitor discharge is always DC; this is the answer I got, which totally makes sense to me (a non-engineer):

The capaciator is charged to roughly 300 volts DC. Which simply means it has stored a bunch of electrons. When the SCR is fired it causes the capacitor to discharge through the coil's primary winding as a short pulse of current. The rest of the time, the coil's primary effectively has no current. It takes a peak reading AC meter to read the voltage. Otherwise, the voltage level would be averaged for a reading which would be very low. The term AC can be somewhat misleading in that AC is short for alternating current. Which implies current flowing back and forth in opposing directions. The current from the power pack only flows in one direction. But, if you reference the measurement of that current as being the average level, the current does go in opposing directions past that point - thus AC.

Which begs the question: why would I need anything but a capacitor across my DC voltmeter leads to perform this test?

If the answer is, "no reason at all", the next question is, what kind of capacitor (capacitance, voltage, etc.) would work?

And the next question after that is, would that work to test the trigger output as well?

 

[F_R]

Re: Why DVA meter?

As another non-engineer, but one who has studied electronics, I suppose we are both examples of the old expression of knowing just enough about electricity to be dangerous.

Seriously, just giving the idea a bit of quick thought, I would say that the capacitor would charge up to some value when it receives the pulse, but it would just as quickly discharge back through the coil as soon as the pulse ends. I suppose a diode could be included in the scheme to prevent the discharge, then you could measure the voltage across the capacitor.

For some reason the law that says that the current leads the voltage across a capacitor by 90 degrees comes to mind. My brain is too tired to figure it all out.

Just my 2c worth.

 

[iwombat]

Re: Why DVA meter?

Just to correct the voltage current misnomer.

Usually voltage leads the current unless the circuit has a negative Reactance.

The diode-capacitor technique should work though as long as your capacitor is big enough to hold a charge for awhile, otherwise you're back in the same boat of measuring a brief pulse. I've had my ME hat on all week, when I get a chance I'll put on the EE hat and spend some time into figuring out the right size. Gut instinct has me saying something in the 100mf range should work though.

 

[iwombat]

Re: Why DVA meter?

I'm correcting myself now.

Capacitors have negative reactance, so in that circuit voltage will lag current. I really hate working with AC circuits - imaginary numbers and phase angles and all that.


Editing again: Of course, that doesn't apply here since we're in pulsed DC and not true AC here. If you ever did a lab with a capacitor, a square wave generator, and an ocilliscope to measure the decay rate of the cap, that's exactly what we're talking about here. That simplifies it quite a bit. I'll need to look up the decay rate of capacitors.

FWIW I still think 100mf would do the trick. You may need a load resistor at the tail end. 100k ohm is probably something that will work.

 

[F_R]

Re: Why DVA meter?

Seems to me that the capacitor should be small in order to charge up to peak voltage in such a small time. The larger the capacitor, the longer it takes to charge. Right???

 

[iwombat]

Re: Why DVA meter?

I think here you're more concerned about discharge rates. If it takes a few pulses to charge up, who cares. You want it to hold a charge. Think tiny battery.

 

[F_R]

Re: Why DVA meter?

"few pulses to charge up...that makes sense.

 

[iwombat]

Re: Why DVA meter?

See, if I thought about it long enough I knew it'd start making sense.

I've been dealing in rotational systems and damped oscillators and lions and tigers and bears for the last week. The mental switch-over can sometimes be drastic.

 

[F_R]

Re: Why DVA meter?

Warning!! If you touch the leads of a 100 mfd capacitor charged up to 300 volts it will knock you right out of your bvd's

 

[iwombat]

Re: Why DVA meter?

The key is you want a big enough load between the C and the R to not effectively dead-short your system to ground. Although, that would surely be a very predictable test.

 

[jtexas]

Re: Why DVA meter?

which of these looks mo betta to you guys?

 

[iwombat]

Re: Why DVA meter?

Scenario #2

 

[iwombat]

Re: Why DVA meter?

Okay, what was I thinking?

The second circuit - parallel is the correct one. With the pulsed wave, the pulse will go through the resistor, at the end of the pulse the capacitor will then begin to discharge. This all assumes there's enough impedance in the powerpack to not drain the capacitor. If not, you'll need to put a diode on the powerpack lead.

There, I feel better now.

 

[jtexas]

Re: Why DVA meter?

A diode between the cap and the powerpack...you're dealing with a rank amateur here...between pulses, would it have to withstand a huge reverse bias?

have an old stereo, might be able to scrounge a suitable capacitor......

thanks!

 

[iwombat]

Re: Why DVA meter?

Honestly, I'll bet the powerpack is already taking care of that for you. But, the diode would go on the common lead before it gets split between the resistor and the cap. Putting another diode in there won't hurt anything though.

It's basically a half-wave rectifier with a reservoir capacitor.

http://en.wikipedia.org/wiki/Reservoir_capacitor

 

[iwombat]

Re: Why DVA meter?

So, I double checked that with out EE here and got the nod. What that'll give you on a multimeter is the average voltage between the peek and the decayed voltage from the buffering capacitor. If you've got a slow enough decay, it should be close. It's probably exactly what the DVA adaptors are doing anyway.

We did come up with a nice active circuit to measure "true peak" if anyone wants to muck about with some transistors.

 

[Rock Hall]

Re: Why DVA meter?

I just read this thread. I think I'd like to try this circuit. Scenario #1 is correct..right? The resistor and cap in parallel with the meter.
I got a little confused at that part of the thread.

Thanks
Mark

 

[iwombat]

Re: Why DVA meter?

Yes scenario #1, aka scenario "A" - elements in parallel.


JTexas has ordinal issues.

 

[Molaker]

Re: Why DVA meter?

Sorry, jtexas, I've been away.

It's basically a half-wave rectifier with a reservoir capacitor.

http://en.wikipedia.org/wiki/Reservoir_capacitor

40 years in electronics and this is the first time I've heard the term "Reservoir Capacitor". But, what's in a name? The diagram in the above link should serve as a reasonable DVA adaptor. A common 750 ma - 1 amp power supply rectifier with 750 -1000 piv should do the trick. We're not talking high current or voltage here. The meter should be connected across the capacitor. The capacitor should be large enough to prevent the meter from impacting the reading. With most digital meters today, that will probably not be an issure as most are fairly high impedence, but I'd suggest a .1 mfd or larger because the frequency & pulse width is fairly low. The main thing would be to have a sufficient working voltage as a charge of up to 300 volts could occur. A 10 meg resistor across (parallel) the capacitor would serve as a load if the coil is not connected to a power pack or if the power pack is open and to discharge the capacitor when you complete your reading (you don't want 300 volts sitting there just waiting for your fingers to discharge it).

All that said, the accuracy of such an adaptor might be in question when you compare your readings with specs. So, it might be a good idea to, at least, measure a known good system just to see if you're in the ballpark.

 

[iwombat]

Re: Why DVA meter?

Thanks, Molaker for the additional info. I don't do this kind of stuff often enough any more to stay adequately sharp.