Question about PIV rating of diodes...

mr mojo · 23rd December 2005, 07:23 PM

Howdy folks,

Well I've run into my first snag with my DIY amp-the bias network.

Basically I'm tapping the HV secondary for bias. Set up:

HV secondary in series with .05uf 600v Sprague Orange Drop cap. Next a diode (1000 PIV, 4 amp) in series with the cap, going to a trimpot. The junction of the cap and diode has a 27k 1 watt resistor going to ground.

Last week the .05 caps shorted and took out the 27k resistors in the process. Replaced the caps and resistors, but still having very erratic behavior with the bias.

Someone told me the Orange Drops were the issue, so I bought some PIO caps from Angela, but then I got to thinking:

Shouldn't the diode's PIV rating be 2.8 times the voltage of the secondary? I read elswhere a good rule of thumb is to make sure the PIV is 3 times the secondary voltage.

I'm thinking maybe the problem has been the diodes all along, so I'm putting in some 1,500v PIV diodes tonight and seeing if that fixes the problem.

Anybody got any insight into this diode situation?

Thanks for any and all help,
mr mojo

poobah · 23rd December 2005, 07:26 PM

Schematic? Voltage?

mr mojo · 23rd December 2005, 07:37 PM

Hey poobah,

Having some server issues at work so I'm having trouble posting images, but if you check the post at the top of this link there's a working link to the schematic:

first DIY amp start up-yowza HELP!

800VCT, so I've got 400V on each half of the secondary.

Best,
mr mojo

poobah · 23rd December 2005, 08:17 PM

Hmmm,

You're talking about C25 & C26???

All I can think is that the voltage is just too much for the caps. 400 volts RMS means 566 Volts... if you are running right on high line voltage (125) instead of 117 you could wind up with 604 Volts. That should be right on the hairy edge but what if a spike came along? (they did both fail at the time did they not?)

You know usually there is good safety margin but these caps are seeing alot of AC current which heats 'em up, and eats the safety margin...

I wouldn't spend any money on boutique caps... the orange drops with a 1000 Volt rating would be better... the diodes in the circuit shoud have at least 800-1000 Volt. the diodes. You can put 2 diodes in series to jack up their PIV rating.

Why did 600 Volt papers work and not these... the papers weren't dealing with the nasty recovery of a jelly bean silicon diode maybe. I think all this is just TOO close to the limits.

Tom Bavis · 23rd December 2005, 10:49 PM

I would use at least a 1200V diode here, and at least a 1000V cap - cheap insurance. 1K or so in series with the .05 would reduce the surge current on transients, but it's probably voltage, not forward current that killed it.

poobah · 23rd December 2005, 11:04 PM

Hi Mojo,

Got to thinking... you said things have been erratic since.

You know if the caps failed and took out the resistors; it's quite likely the pots might have taken a beating as well. Just wondering about "erratic".

EC8010 · 24th December 2005, 12:24 AM

Quote:

Originally posted by mr mojo
800VCT, so I've got 400V on each half of the secondary.

Not so, I'm afraid. Assume that the transformer charges the reservoir capacitor to its peak voltage of 400V x root 2 = 565V. Now the polarity at the end of the transformer/diode changes and it's at -565V. Your diode now has 1130V across it.

mr mojo · 24th December 2005, 04:04 PM

poobah,

I really appreciate the help! Like I've said before, I'm not an engineer and the more I learn the more I realize how much I don't know!

I'm replacing the diodes with 1500PIV 1 amps as well as 630v PIO caps. The caps are temporary until I can order some 1600v Orange Drops when Antique re-opens.

Tom,

I think you and I are on the same page, your suggestion is just about exactly what I'm in the process of doing. Guess this is just part of the learning curve for me and part of the testing and evaluation of what's basically a prototype amp-even if the circuit was designed by the engineers as Westinghouse, it's not as if this is a widely built circuit with a lot of documentation from actual use!

EC8010,

Spot on-that's exactly what I'm doing. When I started researching diode ratings I came across the 1.6 FV and the 2.8 PIV rating and suddenly I beleived I had my answer-but like I said above I'm still scaling the learning curve so it's nice to have someone more knowledeable backing up my reasoning.

Thanks so much for you help folks-I really do appreciate it so much. This forum is the BEST!! I'll update this thread next week after my repairs. I'm also replacing the HV filter caps with 450v Spragues in series with voltage dividers so I'll post some pics of that whole affair as well.

Hope you all have a merry Christmas and a happy and safe new year!

Best,
mr mojo

poobah · 24th December 2005, 04:10 PM

Hey Mojo

You are most welcome. I are an engineer (a bad one), "and the more I learn the more I realize how much I don't know".

Hope you solve your problem!

Happy Holidays dude!

amperex · 24th December 2005, 07:12 PM

From years ago, problems went away when that 3x the PIV rating was used. Before Cary Audio existed, Dennis had some RF amps that had issues due to that rule.

23rd December 2005, 07:23 PM	#1
mr mojo diyAudio Member Join Date: Jun 2005 Location: Iowa	Question about PIV rating of diodes... Howdy folks, Well I've run into my first snag with my DIY amp-the bias network. Basically I'm tapping the HV secondary for bias. Set up: HV secondary in series with .05uf 600v Sprague Orange Drop cap. Next a diode (1000 PIV, 4 amp) in series with the cap, going to a trimpot. The junction of the cap and diode has a 27k 1 watt resistor going to ground. Last week the .05 caps shorted and took out the 27k resistors in the process. Replaced the caps and resistors, but still having very erratic behavior with the bias. Someone told me the Orange Drops were the issue, so I bought some PIO caps from Angela, but then I got to thinking: Shouldn't the diode's PIV rating be 2.8 times the voltage of the secondary? I read elswhere a good rule of thumb is to make sure the PIV is 3 times the secondary voltage. I'm thinking maybe the problem has been the diodes all along, so I'm putting in some 1,500v PIV diodes tonight and seeing if that fixes the problem. Anybody got any insight into this diode situation? Thanks for any and all help, mr mojo

Thread Tools	Search this Thread
Show Printable Version Email this Page	Search this Thread: Advanced Search

Similar Threads
Thread	Thread Starter	Forum	Replies	Last Post
811a rating question	nhuwar	Tubes / Valves	14	2nd March 2009 04:12 PM
Blackgate N voltage rating question	KT	Parts	1	28th July 2005 12:05 AM
A question about diodes...	mr mojo	Tubes / Valves	4	26th July 2005 09:16 PM
power supply diodes voltage rating?	Dominique	Parts	6	27th April 2005 02:45 PM
Current rating of diodes within a bridge?	tiroth	Parts	39	5th December 2003 03:26 PM

New To Site?	Need Help?
Register to Participate Search Privacy Statement Contact Us	Frequently Asked Questions Did you forget your password? Mark Forums Read

23rd December 2005, 07:26 PM	#2
poobah diyAudio Member Join Date: Nov 2005	Schematic? Voltage?

23rd December 2005, 07:37 PM	#3
mr mojo diyAudio Member Join Date: Jun 2005 Location: Iowa	Hey poobah, Having some server issues at work so I'm having trouble posting images, but if you check the post at the top of this link there's a working link to the schematic: first DIY amp start up-yowza HELP! 800VCT, so I've got 400V on each half of the secondary. Best, mr mojo

23rd December 2005, 08:17 PM	#4
poobah diyAudio Member Join Date: Nov 2005	Hmmm, You're talking about C25 & C26??? All I can think is that the voltage is just too much for the caps. 400 volts RMS means 566 Volts... if you are running right on high line voltage (125) instead of 117 you could wind up with 604 Volts. That should be right on the hairy edge but what if a spike came along? (they did both fail at the time did they not?) You know usually there is good safety margin but these caps are seeing alot of AC current which heats 'em up, and eats the safety margin... I wouldn't spend any money on boutique caps... the orange drops with a 1000 Volt rating would be better... the diodes in the circuit shoud have at least 800-1000 Volt. the diodes. You can put 2 diodes in series to jack up their PIV rating. Why did 600 Volt papers work and not these... the papers weren't dealing with the nasty recovery of a jelly bean silicon diode maybe. I think all this is just TOO close to the limits.

23rd December 2005, 10:49 PM	#5
Tom Bavis diyAudio Member Join Date: Jun 2002 Location: Macedon NY	I would use at least a 1200V diode here, and at least a 1000V cap - cheap insurance. 1K or so in series with the .05 would reduce the surge current on transients, but it's probably voltage, not forward current that killed it.

23rd December 2005, 11:04 PM	#6
poobah diyAudio Member Join Date: Nov 2005	Hi Mojo, Got to thinking... you said things have been erratic since. You know if the caps failed and took out the resistors; it's quite likely the pots might have taken a beating as well. Just wondering about "erratic".

24th December 2005, 04:04 PM	#8
mr mojo diyAudio Member Join Date: Jun 2005 Location: Iowa	poobah, I really appreciate the help! Like I've said before, I'm not an engineer and the more I learn the more I realize how much I don't know! I'm replacing the diodes with 1500PIV 1 amps as well as 630v PIO caps. The caps are temporary until I can order some 1600v Orange Drops when Antique re-opens. Tom, I think you and I are on the same page, your suggestion is just about exactly what I'm in the process of doing. Guess this is just part of the learning curve for me and part of the testing and evaluation of what's basically a prototype amp-even if the circuit was designed by the engineers as Westinghouse, it's not as if this is a widely built circuit with a lot of documentation from actual use! EC8010, Spot on-that's exactly what I'm doing. When I started researching diode ratings I came across the 1.6 FV and the 2.8 PIV rating and suddenly I beleived I had my answer-but like I said above I'm still scaling the learning curve so it's nice to have someone more knowledeable backing up my reasoning. Thanks so much for you help folks-I really do appreciate it so much. This forum is the BEST!! I'll update this thread next week after my repairs. I'm also replacing the HV filter caps with 450v Spragues in series with voltage dividers so I'll post some pics of that whole affair as well. Hope you all have a merry Christmas and a happy and safe new year! Best, mr mojo

24th December 2005, 04:10 PM	#9
poobah diyAudio Member Join Date: Nov 2005	Hey Mojo You are most welcome. I are an engineer (a bad one), "and the more I learn the more I realize how much I don't know". Hope you solve your problem! Happy Holidays dude!

24th December 2005, 07:12 PM	#10
amperex diyAudio Member Join Date: Feb 2005 Location: MI	From years ago, problems went away when that 3x the PIV rating was used. Before Cary Audio existed, Dennis had some RF amps that had issues due to that rule.

Currently Active Users Viewing This Thread: 1 (0 members and 1 guests)