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Vintage Radio (domestic) Domestic vintage radio (wireless) receivers only. |
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18th Oct 2020, 6:30 pm | #1 | |
Dekatron
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Discussion on rectifier and capacitor replacing metal rectifier
Quote:
Never had an issue with 250v AC rated capacitors in this section. Perhaps I'm wrong but would not the DC content be insignificant in this application?
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20th Oct 2020, 7:54 am | #2 | ||
Tetrode
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Re: Metal Rectifier in Pam TR 26
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21st Oct 2020, 10:03 am | #3 |
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Re: Metal Rectifier in Pam TR 26
Yes I understand what you said but remember an AC rated cap has a much higher DC rating, most 250v AC caps are rated at 750v DC. I've been using 250v AC caps across diodes since the 70's with no known issues.
When the DC is sitting on top of the half wave AC it's still well below the cap's DC rating. In the forward direction the voltage across the Diode is around 0.6v so cap only sees the negative half cycles, that's added to the resultant HT less of course limiter resistors. The resultant is well under 450v dependent on the circuit and again under the caps DC rating, used to be 750v but more commonly 630v dc these days.
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21st Oct 2020, 2:00 pm | #4 |
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Re: Metal Rectifier in Pam TR 26
Hi.
I've done a little video of what to expect across a silicon half wave rectifier. I hope this is of help to anyone. https://youtu.be/WpTLBoPKz8s The waveform over Diode wasn't quite what I expected TBH.
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21st Oct 2020, 2:30 pm | #5 |
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Re: Metal Rectifier in Pam TR 26
So what you're saying is that there's no way that the PIV rating of the diode needs to be at least 2*VAC peak?
Lawrence. |
21st Oct 2020, 2:42 pm | #6 | |
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Re: Metal Rectifier in Pam TR 26
Quote:
I didn't say that, I'm just discussing the peak voltage across the diode that the snubber cap should be rated at. You can see on the video that is at 10% approx of mains voltages. Like I say I've always used a 250v ac cap as a snubber with no failures.
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21st Oct 2020, 3:18 pm | #7 | ||
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Re: Metal Rectifier in Pam TR 26
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Lawrence. |
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21st Oct 2020, 4:03 pm | #8 | |||
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Re: Metal Rectifier in Pam TR 26
Quote:
Yes the theory would seem that way but in practice it doesn't seem to add up unless I'm missing something?
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21st Oct 2020, 4:52 pm | #9 |
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Re: Metal Rectifier in Pam TR 26
You need to add the maximum voltage across the reservoir capacitor to the peak voltage of the negative half of the AC supply, because at that point in time that's what the voltage will be across the diode.
Lawrence. Last edited by ms660; 21st Oct 2020 at 5:02 pm. Reason: word change |
21st Oct 2020, 5:02 pm | #10 |
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Re: Metal Rectifier in Pam TR 26
Agreed.
With what I stated in the video. This came to 40v approx in the test circuit which is 10% of mains. This equates to 400v DC. The AC is of course sitting on DC component from the reservoir cap. So in my book well below the 630vdc the 250vac is dc rated at. Like I've said if we were over running a 250vac cap in this position I would have had a failure by now. All my AC/DC Radios and TVs have been done this way as have countless others over the years.
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21st Oct 2020, 5:20 pm | #11 |
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Re: Metal Rectifier in Pam TR 26
I wasn't questioning as to whether or not the capacitors you use are rated enough, was just stating what the PIV would be on which we are in agreement.
Lawrence. |
21st Oct 2020, 5:43 pm | #12 |
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Re: Metal Rectifier in Pam TR 26
Indeed Lawrence.
One of those questions that is easy to misinterpret. I'd be inclined to say that anyone who is unhappy using a 250v ac cap just go to a 400v ac cap. No harm in doing the video, sometimes easier to show what happens in a practical circuit.
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21st Oct 2020, 6:55 pm | #13 |
Tetrode
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Re: Metal Rectifier in Pam TR 26
The DC component of current through the diode has no effect on the peak inverse voltage this is a separate point. The reservoir capacitor will be charged to HT, which could be up to the peak of the ac waveform or 350V. When you get a negative peak on the mains the diode will see up to 700V across it so the capacitor must be rated for at least this level. This is why you use 1n4007 diodes and not 1n4001 to 1n4005 in this position
the peak inverse voltage across the diode is up to 700V and is not dependent upon the dc component of the current through it, only the ht voltage on the reservoir capacitor and the peak mains voltage. the 250V ac mains capacitors may be rated at 700V DC but there is then no margin and the part will be stressed |
21st Oct 2020, 7:07 pm | #14 | |
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Re: Metal Rectifier in Pam TR 26
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Lawrence. |
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21st Oct 2020, 7:53 pm | #15 | |
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Re: Metal Rectifier in Pam TR 26
Quote:
We are not discussing what PIV a diode can withstand, we are discussing the peak voltage the snubber cap has across its junction.
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21st Oct 2020, 8:00 pm | #16 |
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Re: Metal Rectifier in Pam TR 26
The PIV is the maximum reverse voltage a Diode can withstand laid down by the diode spec.
In all honesty I don't see why we are discussing the diode. See this short article. https://en.wikipedia.org/wiki/Peak_inverse_voltage The thread is now going to confuse the OP and perhaps it should be split.
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21st Oct 2020, 9:31 pm | #17 |
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Re: Metal Rectifier in Pam TR 26
At approx. 3 minutes into the video you say that you add the peak to peak voltage to the DC voltage, could you explain why?
Because as I understand it as your circuit stands in the video the maximum voltage across the diode is the sum of the maximum voltage across the reservoir capacitor and the peak voltage of the AC supply to the diode. Lawrence. Last edited by ms660; 21st Oct 2020 at 9:46 pm. |
21st Oct 2020, 10:47 pm | #18 |
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Re: Discussion on rectifier and capacitor replacing metal rectifier
Hi Lawrence.
I'd have thought it was obvious! The scope is connected between anode and cathode. The lowest part of the AC waveform is sitting on the DC level which was around 20 odd volts then you add that to the peak of the AC waveform which was bit under 30 v. So the maximum peak voltage was say for arguments sake 50v. The drawing is one I did before doing this on 230v
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22nd Oct 2020, 7:56 am | #19 |
Tetrode
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Re: Discussion on rectifier and capacitor replacing metal rectifier
When the diode is reversed biased there is no current flow so no voltage drop across the resistors so the whole voltage of the capacitor is applied across the diode. the dc voltage between gnd and the diode cathode will be the capacitor to gnd voltage, there is no 20v dc, the voltage across the diode is constantly changing because the anode voltage to gnd changes with the ac mains waveform and at its peak is up to 700V. The piv required for the diode was mentioned because the same piv applies to the capacitor and you would use a diode with 1000v piv and the same should apply to the capacitor, so any capacitor here I would recommend would be a 1000V dc capacitot
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22nd Oct 2020, 8:00 am | #20 |
Nonode
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Re: Discussion on rectifier and capacitor replacing metal rectifier
Having read this thread and watched the video I am confused!
My understanding of the theory of how a half wave rectifier works makes me agree with Lawrence. Only one thing for it. I'm off to the workshop to replicate Trevors experiment. Peter |