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21st Sep 2019, 1:31 pm | #1 |
Dekatron
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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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21st Sep 2019, 1:52 pm | #2 |
Pentode
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Re: Calculating rectifier diode PIV.
1.4 x Vrms according to this http://www.valvewizard.co.uk/bridge.html
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21st Sep 2019, 1:55 pm | #3 |
Dekatron
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Re: Calculating rectifier diode PIV.
Vpeak in x 2 plus a margin.
Lawrence. |
21st Sep 2019, 1:55 pm | #4 |
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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. Andy.
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21st Sep 2019, 2:03 pm | #5 |
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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 |
21st Sep 2019, 2:19 pm | #6 |
Dekatron
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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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Curiosity hasn't killed this cat...so far. Last edited by Diabolical Artificer; 21st Sep 2019 at 2:27 pm. |
21st Sep 2019, 2:27 pm | #7 |
Dekatron
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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. |
21st Sep 2019, 2:31 pm | #8 |
Dekatron
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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.
Andy.
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21st Sep 2019, 3:10 pm | #9 |
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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. |
21st Sep 2019, 3:38 pm | #10 |
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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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21st Sep 2019, 4:05 pm | #11 |
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Re: Calculating rectifier diode PIV.
Thanks Paul, that's what I've done.
Andy.
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