Design of PVA circuit

[Hughjohnson]

I have varios components such as diodes and capacitors but not the sizes recommended in the circuit on Google.
the recommendation is 1A diode 400v and 22 micro farad capacitor. can I us a 500 or larger capacitor.which I have ??
Any ideas or should i just go shopping . I know I am a cheapskate. like many boaters.
Hugh

 

[bruceb58]

eBay

electronic surplus store.

You are talking a couple $ of components here.

Nothing really magic about 22uF. You can probably even go higher. Will just take more time to decay if you don't change the bleed resistor.

 

[gm280]

The cap is used to store the voltage long enough so you can read their potential. Using a higher rated cap of that size could easily offer false readings. I would fork out the little bit of money to get the proper parts and then you will never guess if it is working correctly. I mean how expensive can those parts cost these days. JMHO

 

[Crosbyman]

http://www.electronics-tutorials.ws/accircuits/rms-voltage.html

dva adapters basically convert a AC volatge to DC by rectifying it in the diode and the capacitor flattens out the rectified peaks to be read by a DC meter

​AC voltages made up with peaks and valleys can be averaged out to a DC equivalent by fancy mathematics (see link)

The DC (DVA) equivalent of the stator AC is ..about .70% of the the AC voltage. ( actually % depends on the sine waves/second)

​as a test when you have your DVA working could you post what AC voltage you got off the Stator and the DVA value you ended up with on the DC side of your DVA adapter.

​if you end up with a DC reading about 70-75% of the AC input maybe DVA adapters can be done away with and we can all assume that the
" DVA equivalent "is... 70-75% of stator voltages

​Some AC meter give out the RMS value which should be pretty close to a DVA adapated meters

 

[bruceb58]

http://www.electronics-tutorials.ws/...s-voltage.html

dva adapters basically convert a AC volatge to DC by rectifying it in the diode and the capacitor flattens out the rectified peaks to be read by a DC meter

​AC voltages made up with peaks and valleys can be averaged out to a DC equivalent by fancy mathematics (see link)

The DC (DVA) equivalent of the stator AC is ..about .70% of the the AC voltage. ( actually % depends on the sine waves/second)

​as a test when you have your DVA working could you post what AC voltage you got off the Stator and the DVA value you ended up with on the DC side of your DVA adapter.

​if you end up with a DC reading about 70-75% of the AC input maybe DVA adapters can be done away with and we can all assume that the
" DVA equivalent "is... 70-75% of stator voltages

​Some AC meter give out the RMS value which should be pretty close to a DVA adapated meters

Totally incorrect info. A DVA allows a meter to catch the peak voltage. It is not a sine wave so an RMS voltage is useless. The DVA captures the peak voltage of the pulse. Most multi meters have very slow sampling so most meters will not be able to capture the pulse even if it had peak detection.

 

[Crosbyman]

good point crow again for supper:facepalm: ... I stand corrected:black_eyed: ....

​

 

[F_R]

I have varios components such as diodes and capacitors but not the sizes recommended in the circuit on Google.
the recommendation is 1A diode 400v and 22 micro farad capacitor. can I us a 500 or larger capacitor.which I have ??
Any ideas or should i just go shopping . I know I am a cheapskate. like many boaters.
Hugh

If I read this correctly, you are asking if you can use components with higher voltage readings. The answer is yes. In fact it would be preferable. 400V on the diode is just barely adequate--if that.

 

[Hughjohnson]

thank you all for your replies, sorry for the delay I was away. took your advice and bought new components ( incredibly cheap ). so I built a DVA with 2 amp 700 PIV diode 2200pf capacitor (1600 v )and 200 ohm series resistor. and I meg ohm discharge resistor across the capacitor. so I then tested this circuit using 120 volt AC from house supply. the result was surprising. DC voltage across Cap was 62 v . I expected to get a peak voltage of 120v times Sq.root of 2 ( 1.41 ) which is about 170 volts. so now if my DVA only measures 62 volts when we know the peak on the household sine wave is about 170 volts it is not much good to me.
please can anyone explain this . I thought I understood electrical circuits . I also built a DVA using different components with similar results..
Hope you can help .

 

[bruceb58]

A 2200 pF is not the right size....way too small. That is your problem.

 

[Hughjohnson]

thanks Bruceb I thought I had read 2.2 micro farad which is what I bought . what would you recommend.. My other circuit I made was 100 micro farad electrolytic and gave similar results of 62 v DC across cap.

 

[Crosbyman]

almost looks like that DVA is picking up an RMS equivalent on household AC peaks ... I doubt the folks at Hydro are pumping out peaks of 170 volts on a 120 ac line ... but you never know time to whip out the old heathkit scope from the garage

 

[F_R]

Your single diode is only a half wave rectifier. That is it is only collecting half of the AC sine wave and storing it in the capacitor. So half of 120 = 60V. Normal.

EDIT: Although in some cases it may be used on AC waves when testing ignition systems, the primary use is holding DC voltage pulses. It will give true readings for that.

 

[gm280]

Sounds like to me your PVA is working perfectly. Like F_R stated, one simple directional diode is doing exactly what it was designed to do. Allow the voltage peak to be stored in the cap while blocking the negative swing form draining that peak voltage so you can read it. JMHO

 

[F_R]

That's one way of saying it. Another way is to look at the pulses output. The voltage normally will be zero between pulses. The suddenly a, lets say, 300V comes along and nearly instantly vanishes. Then it is zero again till the next pulse (elapsed time varies according to RPM). Those pulses happen so rapidly that an ordinary DC voltmeter cannot read them. The DVA stores the pulse average peak value long enough for the meter to display it.

 

[gm280]

That's one way of saying it. Another way is to look at the pulses output. The voltage normally will be zero between pulses. The suddenly a, lets say, 300V comes along and nearly instantly vanishes. Then it is zero again till the next pulse (elapsed time varies according to RPM). Those pulses happen so rapidly that an ordinary DC voltmeter cannot read them. The DVA stores the pulse average peak value long enough for the meter to display it.

You are absolutely correct. But if there wasn't a directional diode (half wave rectifier if you wish), the negative swing would negate the positive peak stored in the cap instantly as well. And the faster the RPMs, the more accurate that peak will read. Because it doesn't have as much time to bleed off between positive peaks.

 

[Hughjohnson]

thankyou all again. I do not have an osciloscope but I got out my old textbook.
a sine wave has a peak voltage of 1.4 times the RMS value. my household supply is 120 volts which is the RMS value . its peak is therefore 169 volts.(RMS X 1.4) in both directions. or conversely RMS is 0.707 times peak.
If you use a single diode then yes you get half wave rectification and will get about 53 volts .(45 % of RMS )) full wave rectification should give you 106v (90% of RMS )
Now the whole point of the PVA is to collect the peak voltage and store it in the Capacitor so that we can measure it with a DC voltmeter.
so the PVA should be showing 169 volts with my 120 volt AC supply and instead I am only getting about 60 volts. which is pretty close to half wave rectified voltage. I wish I had an oscilloscope now.
I am beginning to think that the PVA might just give Half wave voltage and the that the actual peak is far higher.
do any of you have a PVA that you can test across your house AC supply to see if you get the same results I am getting.
the mystery remains ??

 

[F_R]

You wrote "so the PVA should be showing 169 volts with my 120 volt AC supply and instead I am only getting about 60 volts. which is pretty close to half wave rectified voltage. " That is only partly correct. Your DVA should be showing the peak of the half wave, not the whole 169 volts of the full sine wave (the second part of your statement. Only the positive peaks make it through the rectifier (or negative peaks, depending on which way you connect it). You are measuring with respect to neutral (the center of the sine wave).

 

[F_R]

Aw heck, now I'm so confused I gotta get my textbook out. Later----

 

[bruceb58]

It is not putting out sine waves! The capacitor is holding the peak voltage...not the average voltage either. RMS calculations are not to be used at all here.

 

[bruceb58]

I doubt the folks at Hydro are pumping out peaks of 170 volts on a 120 ac line ...

Uhhhhhh yes! They are 170 volt peaks. 340V peak to peak