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Old 21st Sep 2019, 1:31 pm   #1
Diabolical Artificer
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Default Calculating rectifier diode PIV.

Is there a rule of thumb or calculation for ensuring the correct spec, IE PIV for a rectifier diode or bridge?

TFL, Andy.
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Old 21st Sep 2019, 1:52 pm   #2
Paul JD
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Default Re: Calculating rectifier diode PIV.

1.4 x Vrms according to this http://www.valvewizard.co.uk/bridge.html
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Old 21st Sep 2019, 1:55 pm   #3
ms660
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Default Re: Calculating rectifier diode PIV.

Vpeak in x 2 plus a margin.

Lawrence.
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Old 21st Sep 2019, 1:55 pm   #4
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Default Re: Calculating rectifier diode PIV.

That's what I've worked on in the past Paul, but having read a few threads and wotnot on PSU design I was under the impression this is incorrect, due to back EMF of the tfmr and suchlike.

Written before Lawrence's reply... So the peak Vin x 2 + a bit on the safe side. This brings us to what's marked on tfmr's, Vpeak or Vrms. i guess measurement of the AC waveform is the way to go.

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Old 21st Sep 2019, 2:03 pm   #5
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Default Re: Calculating rectifier diode PIV.

Transformers with their voltages indicated or stated will indicate/state the RMS voltage, all AC voltages are RMS unless otherwise stated....

Regarding PIV, what I said was for bi-phase full wave.

Lawrence.

Last edited by ms660; 21st Sep 2019 at 2:29 pm. Reason: clarification
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Old 21st Sep 2019, 2:19 pm   #6
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Default Re: Calculating rectifier diode PIV.

Ok, down to specifics, I have a tfmr on the bench, sec is 330v 0v 330v. AC in 245v RMS on the 0v 240v pri taps, OP on one half of sec is 500v peak ( scope set to 20v/div = 5 divs, 10x probe, (5x 20) x 10). Lastly are tfmr specs usually given for loaded or unloaded? This tfmr says 330 etc @ 60mA, so I guess loaded, will load and check V out. Checked, with a 5500r load on centre tap and 330v, IE one half of total winding, I get 350v RMS, with load on the 330v 330v taps I get 337v RMS, about right.

So a diode with a PIV of 1200v would be about right, correct? I've been using ones rated at 800v PIV without issue, therefore why is 2x peak + a bit correct? Is it because the tfmr inductance back EMF?

Lastly is this calc for the loaded rating or unloaded?

Andy.
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Old 21st Sep 2019, 2:27 pm   #7
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Default Re: Calculating rectifier diode PIV.

If it's a centre-tapped transformer with two diodes - let's say 100-0-100V RMS - feeding a capacitor-input filter then think like this:

The capacitor will charge with DC to the peak voltage - which is 1.414x the RMS - so it'll be sitting at 141V

Then on the next half-phase the end of the winding will go to a peak-negative of 1.414x the RMS - in other words -141V.

So the diode is seeing 282V worst-case voltage difference across its terminals.

To leave plenty of "wiggle-room" to handle transients etc. I would prefer to specify a diode rated at 4x the transformer RMS voltage, though some people will specify a lower rating and then fit transient-suppressors etc.

Bridge-rectifiers - since there are essentially two diodes in series at any one time - are more forgiving.
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Old 21st Sep 2019, 2:31 pm   #8
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Default Re: Calculating rectifier diode PIV.

Thanks G6, 4x tfmr RMS rating, in this case 330 x 4 = 1320v, so not far off Lawrences peak x 2 + tad. "Is it because the tfmr inductance back EMF? " Transients, gotcha.

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Old 21st Sep 2019, 3:10 pm   #9
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Default Re: Calculating rectifier diode PIV.

Andy, scroll down to pages 8 and 9 to see the PIV the diodes are subjected to for the different rectification regimes:

https://www.ietlabs.com/pdf/GR_Appno...r%20Design.pdf

Lawrence.

Last edited by ms660; 21st Sep 2019 at 3:15 pm.
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Old 21st Sep 2019, 3:38 pm   #10
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Default Re: Calculating rectifier diode PIV.

Two 800V diodes in series should give adequate protection. I agree that a single 800V diode is pushing things a bit for 330-0-330.
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Old 21st Sep 2019, 4:05 pm   #11
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Default Re: Calculating rectifier diode PIV.

Thanks Paul, that's what I've done.

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