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Old 18th Oct 2020, 6:30 pm   #1
murphyv310
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Default Discussion on rectifier and capacitor replacing metal rectifier

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"Also add a capacitor in parallel across the 1N4007 of 1nf and rated at least 250v ac to remove switching noise from the Diode."

I think 250V ac will not be enough as the maximum voltage will be higher than this as its the dc ht plus the maximum negative peak of the ac waveform. maybe 450V ac or 1000V DC would be better
Hi.
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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Old 20th Oct 2020, 7:54 am   #2
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Default Re: Metal Rectifier in Pam TR 26

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Quote:
Originally Posted by smiler411 View Post
"Also add a capacitor in parallel across the 1N4007 of 1nf and rated at least 250v ac to remove switching noise from the Diode."

I think 250V ac will not be enough as the maximum voltage will be higher than this as its the dc ht plus the maximum negative peak of the ac waveform. maybe 450V ac or 1000V DC would be better
Hi.
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?
the reservoir capacitor connected to the cathode of the rectifier will be charged to the ht voltage dc, when the anode gets a negative half cycle from the mains the voltage between the cathode and anode will be the dc charge voltage on the reservoir capacitor plus the negative voltage on the anode the total will peak well above the 350v peak of the 250v ac waveform for which the 250v ac capacitor would be rated The negative peak alone would be the 350V peak with the reservoir capacitor voltage on top. This does not concern the dc component of the current through the diode just the peak voltage difference
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Old 21st Oct 2020, 10:03 am   #3
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Default 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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Last edited by murphyv310; 21st Oct 2020 at 10:22 am.
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Old 21st Oct 2020, 2:00 pm   #4
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Default 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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Old 21st Oct 2020, 2:30 pm   #5
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Default 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.
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Old 21st Oct 2020, 2:42 pm   #6
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Default Re: Metal Rectifier in Pam TR 26

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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.
Hi Lawrence.
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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Old 21st Oct 2020, 3:18 pm   #7
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Default Re: Metal Rectifier in Pam TR 26

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Originally Posted by murphyv310 View Post
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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.
Hi Lawrence.
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.
But that's also in effect what's being implied, I always thought for half wave (or full wave bi-phase) that the PIV across the diode was VAC peak and if a reservoir capacitor is fitted then the PIV becomes 2*VAC peak under no load conditions?

Lawrence.
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Old 21st Oct 2020, 4:03 pm   #8
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Default Re: Metal Rectifier in Pam TR 26

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Quote:
Originally Posted by murphyv310 View Post
Quote:
Originally Posted by ms660 View Post
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.
Hi Lawrence.
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.
But that's also in effect what's being implied, I always thought for half wave (or full wave bi-phase) that the PIV across the diode was VAC peak and if a reservoir capacitor is fitted then the PIV becomes 2*VAC peak under no load conditions?

Lawrence.
Hi Lawrence..
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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Old 21st Oct 2020, 4:52 pm   #9
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Default 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
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Old 21st Oct 2020, 5:02 pm   #10
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Default 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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Old 21st Oct 2020, 5:20 pm   #11
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Default 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.
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Old 21st Oct 2020, 5:43 pm   #12
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Default 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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Old 21st Oct 2020, 6:55 pm   #13
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Default 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
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Old 21st Oct 2020, 7:07 pm   #14
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Default Re: Metal Rectifier in Pam TR 26

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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.
It would have been better if you had also done a quick explanation regarding the PIV across the diode.

Lawrence.
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Old 21st Oct 2020, 7:53 pm   #15
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Default Re: Metal Rectifier in Pam TR 26

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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
Hi.
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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Old 21st Oct 2020, 8:00 pm   #16
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Default 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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Old 21st Oct 2020, 9:31 pm   #17
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Default 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.
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Old 21st Oct 2020, 10:47 pm   #18
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Default 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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Old 22nd Oct 2020, 7:56 am   #19
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Default 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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Old 22nd Oct 2020, 8:00 am   #20
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Default 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
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