Rectifier reverse voltage quesiton. - Page 1
Forum section: electronics / projects

author: Deactivated-1
author_rank: Frequent Contributor
timestamp: August 22, 2022, 12:05:53 pm
content: [ Attachment Invalid Or Does Not Exist ]is this ok? the diode is 400V max reverse voltage rated, the B+ is around 500V.but the bias supply is trough a dropping resistor, the bias supply is around -40V so would it be ok? since voltage across the diode would be limited by the reverse leakage current of the diode, which would go up a lot when the 400V rating is reached...

author: Vovk_Z
author_rank: Super Contributor
timestamp: August 22, 2022, 12:33:51 pm
content: The diode doesn't possibly burn with a 47k current limiting resistor in series, but I don't understand why not to use any 1000V rated diode like 1N4007? 1N4007 are popular and cheap as a dirt.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 22, 2022, 01:14:29 pm
content: Also it probably breaks down at 900V or so; not great if line transients come along, but unlikely to fail.Putting in a 1kV part is easy insurance, certainly.Tim

author: mag_therm
author_rank: Frequent Contributor
timestamp: August 22, 2022, 03:15:30 pm
content: Is there a lower tap for the bias supply? - to reduce the present unwanted dissipation.At present there is ~ 525/50000 = 10 mA of DC pulses in the highest voltage ( most turns) winding from the half wave rectifier.This can cause yoke flux. If goss core, it might increase pri magnetizing current a bit ( hum & buzz) , maybe it does not matter.I would recommend a full wave rectifier to minimize that.In an old amp here, they deliberately used a low voltage winding/half wave rectifier for the push pull stage grid bias,then another winding on the transformer injects via adjustment pot, a level of full wave 60 Hz to the grid biasso user can null the mains transformer hum.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 22, 2022, 09:31:00 pm
content: Quote from: ELS122 on August 22, 2022, 05:09:49 pmNo, that's what I mean. Or do you mean you measured it as 400.0V? Because that's a very particular value, though a remote possibility. What I mean is, I've measured half a dozen makes of "1N4004" for example, that all break down near or above 1kV. Granted, I don't have a clue what a D226B is.Tim

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 23, 2022, 06:44:45 am
content: Oh, WTF? Pffbt, ancient junk, who cares, throw it out or replace it for all that matters... it's a couple cents, not even worth asking the question. Let alone my typing out an answer!The thing about the 1Nxxx, etc. system (JEDEC), is it is not based on type parts. It's a spec, and anything that meets that spec can be registered and sold as such.So, I've measured 1N4001s as low as 363V and high as 1600V. Evidently, the Vf and Ir specs are met, so who cares about Vr -- it's a limit, not a bounded characteristic.It's not simply "relabels": I have some 1N4007s that measure lower than 1N4004s, and vice versa. Which one is the "correct" part? That's the thing: they all are!These include well known western brands (ON Semi, Fairchild, Diodes Inc.), as well as eastern brands (MCC, Good-Ark, others I couldn't identify), and across time (a few salvaged probably as old as the 80s, up to recent production).Probably, if you found parts back when the specs were created (60s), you'd find things much closer to ratings. You might also find little or no robustness to avalanche (which is how I manage to measure this at all -- which also means they all passed, up to at least the reverse current I tested with!). Vf was probably worse back then, having poorer controlled doping, or die thickness, stuff like that; modern types are all going to be epitaxial I'm guessing, better controlled parameters, thus able to target much higher breakdown voltages while still meeting Vf. And breakdown causes spooky nuisance failures (e.g. due to surges), while an incrementally higher Vf (but still within ratings -- within expectations), so it's a much better business proposition to make as high a Vr as possible for a given type, and maybe put some marketing faff on there about "high reliability/robustness".Tim

author: Kleinstein
author_rank: Super Contributor
timestamp: August 23, 2022, 07:36:02 am
content: The maximum reverse voltage the diode would see is the capacitor voltage (supposed to be ~ 40 V due to the divider) plus the peak voltage of the transformer.So a 400 V diode could get away with only occasoinal break through on mains transients. Without the divider to reduce the capacitor voltage the diodes may suffer regular.Even with Soviet parts the break down voltage is usually a limit and it is normal to find diode that can withstand a significant higher votlage. They are just not tested for the higher voltage and one can not complain if they don't. Chances are 1N4001-4005 are often the same nominal dies, just tested different, maybe selected on waver / batch level. So a relatively poor batch would not be used for the higher grades, as there could be more rejects.The 1N4007 is usually slightly different from the 1N4001-4005. At least the old time parts were PIN types to get the high voltage rating. This has the additional effect of a reduced capacitance and usually rather slow reverse recovery. With modern parts from newer manufacturers there may also be simper PN diodes and still meet the specs.

author: Vovk_Z
author_rank: Super Contributor
timestamp: August 23, 2022, 10:11:49 am
content: QuoteD226B is aД226В- ancient soviet silicon diode. It is quite common in old soviet devices. So, I guess TS is somewere from Russia or Russia occupated state. It is quite common practice in russian forums to make devices from a garbage.

author: Vovk_Z
author_rank: Super Contributor
timestamp: August 23, 2022, 10:29:08 am
content: Quote from: ELS122 on August 22, 2022, 05:07:26 pmTbat is kind of typical for some types of diodes to withstand much higher currents. And it is VERY typical for all soviet semiconductors (except military grade #7/9) to have a real reverce breackdown voltage somewhere between 40%...140% of a rated value. So you always don't know a real breakdown voltage until you measured it. A military grade semiconductors (which fulfilled all requirements) where about 1% of a mass production.

author: Keri Szafir
author_rank: Contributor
timestamp: August 23, 2022, 06:24:54 pm
content: Measure the (sine wave RMS) center to end voltage on the transformer and multiply it by 1.41, that will be the peak voltage. Add 10% or even 20% for grid fluctuations. Using a diode with reverse voltage lower than the value you got would certainly fry the diode, and since diodes are cheap, just get something rated for at least twice the peak voltage. 1N4007 will be fine.By the way, I'd suggest a full wave negative voltage rectifier (use two diodes instead of one) for less ripple and added safety if the diode fails. Bias PSU must be reliable.Use higher resistances in the voltage divider: 100k in line with the diode, 22k || 6k8-33k trimpot. This will reduce the current draw twice, and you don't need to draw any significant current from the bias PSU.MLT are my favorite BTW. Good stuff, and there's plenty of them here in Poland. They're surprisingly hard to fry. I did some experiments with putting three or four times the nominal power and while they're clearly burnt, they're still going.

author: bdunham7
author_rank: Super Contributor
timestamp: August 23, 2022, 06:42:18 pm
content: Quote from: ELS122 on August 22, 2022, 10:11:10 pmNo, standard rectifiers are not predictable like zeners. The rating is just the guaranteed minimum. I have repaired a number of instruments that use a 1N4007 as reverse protection in an instrument rated to withstand exactly 1000V. I 'select' replacements from 1N4007 stock by applying 1600V--if they don't avalanche or leak, I deem them worthy. I have not yet had any 1N4007s fail this test! You could similarly select from your old stock by applying 600V--if they don't pop they should have adequate margin for your application.

author: floobydust
author_rank: Super Contributor
timestamp: August 23, 2022, 07:07:05 pm
content: OP these parts don't last forever and the bias rail failing costs a lot more than the few pennies for a diode. I have no idea of the quality of the Д226В, how they age.You can splurge and put two in series. This is kind of how EE's think.I find 1N4007 is too slow with HV and prefer parts that are fast recovery.I'm using RC snubbers on HV secondaries like 10nF+1kohm, you can use a scope to confirm no resonance. Although two of a 10nF gave no problems.

author: floobydust
author_rank: Super Contributor
timestamp: August 24, 2022, 04:56:57 am
content: Send some to Noopy for analysisThing is, diodes actually have changed over the past 50 years. The old ones from the 1970's have excellent hermetic packaging glass, metal - but they are slow switching and not really rated much for current. 300mA in a 11x9mm metal package? 1kHz max speed? Have to see pics of the die to know if the part really is make of oak.

author: Vovk_Z
author_rank: Super Contributor
timestamp: August 25, 2022, 08:21:30 pm
content: Quote from: ELS122 on August 23, 2022, 04:35:39 pmNobody forces you to buy garbage from China.Old soviet passive components are the only components that may find usage now (but not semiconductors). Some old soviet film caps still can be ok, but it is much easier and more convenient for me to buy modern ones. WIMA/KEMET/SR Passives/Vishay/EPCOS-TDK/Panasonic/JB Capacitors etc are much closer (accessible) to me now than К71/К72/К73-17 and so on.MLT, and OMLT (MF) are the only soviet passives I can sometimes use now (because there is some stock at my laboratory). And precision ones с2-29в, they are well aged now, and they are the older - the better.

author: eugene
author_rank: Frequent Contributor
timestamp: August 26, 2022, 05:47:48 pm
content: Quote from: ELS122 on August 23, 2022, 04:35:39 pmSo put two of them in series. Maybe put a 100k resistor across each one to encourage voltage sharing. Carbon comp, of course.

author: Wolfram
author_rank: Frequent Contributor
timestamp: August 26, 2022, 06:41:12 pm
content: The diode will see the sum of the negative bias and the positive peak. If the rectified voltage is 500 V, the peak voltage before the rectifier could easily be a few tens of volts higher, and the mains voltage also has some tolerance so the total voltage seen by the rectifier is in the 550 - 600 V range. As mentioned earlier, it might handle this fine or it might leak excessively above 400 V depending on your luck.Any leakage current will pull the bias voltage closer to the postive mains peak on the transformer secondary, and this can be a significant fraction of the forward current since the forward current is limited by a 47 k series resistor. Leakage often goes dramatically up with temperature, so you could end up with a bias voltage that diminishes with increasing temperature. This can then lead to a dramatic increase in idle current in your output tubes, with subsequent loss of the output tubes, the power transformer and/or the output transformer depending on your luck and fusing.It probably won't be a problem, but there's always a chance that it will. It's not a gamble I would take, given how catastrophic the damage can be and how cheap 1000 V diodes are.