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 Post subject: Dependable diode brand
PostPosted: Feb Sun 21, 2021 11:15 pm 
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Joined: Jun Fri 19, 2020 11:29 pm
Posts: 35
I've had some really weird voltage drops with bridge and discrete diode bridge circuits. On a 220v 200ma secondary I'll get the expected 280ish unloaded vac but when connected to (admitted generic no name 1n4007) bridge or center tap 2 diode rectifier, vdc is down by as much as 50vdc. When c1 is connected the vdc will go no higher than Ts of 220. This example is over various transformers and diodes from different vendors for different projects. All before even connecting b+ to actual circuit. I never encountered this using radio shack diodes off the street(now closed down). End of story question: does anyone have a tip on reliable low resistance 1n4007s online?


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 Post subject: Re: Dependable diode brand
PostPosted: Feb Sun 21, 2021 11:28 pm 
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Joined: May Wed 18, 2011 2:40 am
Posts: 5850
Location: Littleton, MA
Buy them from Mouser or Digi-Key. If you prepay, Digi-Key will give you free shipping.

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 Post subject: Re: Dependable diode brand
PostPosted: Feb Sun 21, 2021 11:51 pm 
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Joined: Jun Sat 09, 2007 8:14 am
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Location: Melbourne, Florida
Deleted.

RRM


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 Post subject: Re: Dependable diode brand
PostPosted: Feb Mon 22, 2021 4:06 am 
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Joined: Jan Fri 27, 2017 8:41 pm
Posts: 380
Location: Springwater, NY
Please post a schematic of your power supply circuit. "vdc is down by as much as 50vdc" raises eyebrows.


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 Post subject: Re: Dependable diode brand
PostPosted: Feb Mon 22, 2021 5:23 am 
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Joined: Oct Sun 15, 2006 12:57 pm
Posts: 7973
Location: Liberty, Missouri
Agree with NW2K, in that your voltages seem very strange.

GP Diodes are about as "jelly bean" type of part you can get. Last I bought were about 5 years ago, I got a thousand "4007"s from some HK ebay vendor for a penny apiece. Did basic QC tests and they passed.

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 Post subject: Re: Dependable diode brand
PostPosted: Feb Mon 22, 2021 5:02 pm 
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Joined: Jun Fri 19, 2009 6:34 pm
Posts: 11155
Location: Long Island NY
You need to do some testing here to see where the problem is coming from. For starters, you say the transformer has a 220-volt secondary. If its waveform is reasonably sinusoidal the capacitor should charge up to 220 x 1.41 = 310 volts with no load across it. The instant any load is applied, that voltage is going to come down. Could be a leaky capacitor or a diode with high reverse leakage. Or the capacitor could be open or high ESR, in which case it will never develop the full voltage it should. What testing has been done to confirm that the diodes are bad? You can't get 50 volts of forward drop on an ordinary low to medium voltage silicon rectifier that tests perfectly okay otherwise.

Silicon diodes, whether packaged individually or into bridges, are made by the billions per year. To minimize waste, the individual diode dies are tested before they are cut from the larger wafer they were fabricated on, and they are normally tested again after packaging. From end to end the whole process calls for strict clean room conditions and highly purified materials. So it's practically unheard-of to get a new diode from any of the major manufacturers that is defective right out of the bag. But there are some two-bit operators which buy finished wafers from the larger foundaries and do their own slicing and packaging. These are a different story; they do not all maintain strict clean room conditions nor can they afford high standards of QC. There is also a secondary market that buys unmarked, unbranded floor sweepings and rejected parts from the larger companies which they resell. Then there are intentionally mislabeled parts and outright counterfeits. If you got a 1N4001 masquerading as a 1N4007 it would not have the PIV to operate at 310 volts and if the power was limited by the transformer impedance, it would most likely go into reverse breakdown similar to a Zener. The best way to avoid problems like this is to buy name brand components from reputable distributors like the ones mentioned above.

PS: It's also healthy to question your test equipment from time to time. Many handheld DMMs will give erratic voltage readings when their batteries get weak, even before the "Check Battery" indicator comes on. Just something to consider.

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