How To Replace Selenium Rectifiers • Choose The Right Diode & Dropping Resistor

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Flux Condenser Vintage Audio Electronics

3 жыл бұрын

Selenium rectifiers should be replaced. They can catch fire and emit toxic fumes. You’ll need a diode, but what kind? You’ll need a dropping resistor, but what value and wattage? In this video I show how to use your meter to find your circuit’s current. Use Ohm’s law to calculate dropping resistance. An easy calculation to determine resistor wattage. And simple steps to build a selenium rectifier replacement circuit.
IMPORTANT! Selenium rectifiers are marked differently than what we've come to expect with diodes. With modern silicon diodes, the cathode (striped end) is negative and the anode (non-striped end) is positive. Seleniums, however, have their cathodes marked as positive. Please consider this when replacing selenium rectifiers and consult the schematic for your device for proper diode orientation.
To learn more about the fascinating RCA Sound Cartridge Recorder shown in this video, please visit this link: • The Huge Cassette!
#electronicsrepair #radiorepair #diode #rectifier #rectification #stereorepair #electronics

Пікірлер: 56

@FluxCondenser 3 жыл бұрын

IMPORTANT! Selenium rectifiers almost always have their polarity marked incorrectly. With modern diodes, the cathode (striped end) is negative and the anode (non-striped end) is positive. Seleniums, however, have their cathodes marked as positive and anodes marked as negative. Please consider this when replacing selenium rectifiers and consult the schematic for your device for proper diode orientation.

@45Victrola 3 жыл бұрын

There was no mistake. At the time the (+) correctly indicated the DC voltage polarity and yes it was the cathode end. The same is done today on a modern silicon bridge rectifier block, The common cathodes are marked (+) and the common anodes are marked (-) .

@FluxCondenser 3 жыл бұрын

@@45Victrola Whether we call it a mistake or not, selenium rectifiers and the modern diodes that replace them are marked in a completely opposite way. A full wave bridge rectifier, while composed of four diodes, does not behave as a single diode and should not be used for comparison to a single diode.

@45Victrola 3 жыл бұрын

@@FluxCondenser You missed my point. I don't disagree, they are marked differently. Care and caution should be used when replacing a selenium diode with a silicon diode, it can easily confuse the novice. Selenium diodes were in use long before silicon diodes. The way they were marked was the first standard. Later silicon diodes adopted a ring around the cathode end of the device and it's logical because it resembles the cathode of the electronic symbol for a diode. The only point I take issue with is that they were marked incorrectly. It implies that early engineers were incompetent. One of my peeves around rectifier diodes is the way everyone seems to refer to the resistor immediately following the diode as a voltage dropping resistor. It is actually a current surge resistor designed to protect the diode at powerup. The voltage drop is a secondary effect.

@FluxCondenser 3 жыл бұрын

@@45Victrola I didn’t miss your point as the first sentence in my reply to you began, “Whether we call it a mistake or not…” And I agree it’s a good point. Mistake is the wrong word. As you say, early engineers were far from incompetent. We’ve largely settled on using conventional current flow as the standard these days (positive to negative), but actual electron flow is the opposite (negative to positive). Seleniums were polarity marked for electron flow, and that simply doesn’t fit into the conventional current flow world.

@ix-Xafra 2 жыл бұрын

That might explain why some Fender amps have the polarity symbols the wrong way round on several of their schematics. It usually occurs in bias circuit drawings as that is the only part of his designs, Leo Fender used seleniums.

@bingojamas4786 3 жыл бұрын

Wow. I had never noticed that before on the selenium rectifiers, so I checked the ones I had removed. Yup, you're right. A big plus sign on the "output" side. I am used to working with the silicon ones, so I just referred to the schematic and oriented the band correctly. Appreciate the heads up!

@FluxCondenser 3 жыл бұрын

My pleasure, Bingo. Glad you found that useful.

@nickhill9445 Жыл бұрын

A couple of (hopefully helpful) ideas. If replacing a high voltage (multi-stack) selenium rectifier, and the output voltage is critical, and you have no alternative transformer taps to bring the voltage back down: Add the same number of silicon diodes in series as the selenium rectifier had elements - this way you closely match Se voltage drop. Be sure to arrange the diodes so that the heat they generate can be lost. If you want to replace a selenium panel mount half-bridge rectifier, use half of a silicon panel mount full bridge rectifier (use 3 of the 4 pins)

@levi2bear 3 жыл бұрын

Very good explanation you area gifted instructor

@FluxCondenser 3 жыл бұрын

Thank you! Very much appreciated.

@tonysfun 3 жыл бұрын

Love your videos and the way you explain everything clearly! Thank you.

@FluxCondenser 3 жыл бұрын

Thanks, Tony!

@justsumguy2u 3 жыл бұрын

Gotta have that dropping resistor. I've seen B+ be as much as 30v higher in the set without one

@Geoduck. 3 жыл бұрын

What a great useful topic thanks Chris!

@FluxCondenser 3 жыл бұрын

Glad you found it useful!

@christopherleubner6633 4 ай бұрын

Yup those Se rectifiers smell horrible when they fail. Replaced 2 very large ones in a forklift battery charger. They failed so bad that one meted to slag and tripped the 60A breaker on the charger. Replaced them with welder doodes and changed the tap on the big transformer to a 277 tap instead of the 208v one to get the correct float voltage. ❤

@francescogreco4113 3 жыл бұрын

Great video as always

@FluxCondenser 3 жыл бұрын

Thanks, Francesco. Appreciate the support.

@johnstone7697 3 жыл бұрын

You mis-identify the rectifier as an IN (eye-en)4007, when it is actually a 1N4007. Most diode nomenclature begins with a 1, meaning 1 junction. A 1N4007 is pretty much a universal replacement for any selenium diode, as it has far higher current and peak inverse voltage ratings. It will just coast in the circuit. Also, there are many cases where changing the series resistor is unnecessary (e.g. in line powered table radios). The small voltage increase will simply not matter. The main reason a resistor is added is because of the current surge when the electrolytic is being charged. The 100 ohm resistor (this value almost always works) helps absorb the current surge to keep from blowing a fuse at power up. But many circuits with selenium rectifiers already have a series resistor in place.

@FluxCondenser 3 жыл бұрын

Yes, of course thanks for pointing that out. It’s a bad habit of mine calling them I’s instead of 1’s. The 4007 is a good general purpose diode which I used for the main amp, but I used a more appropriate lower-current diode for the secondary solid-state amp. In general I disagree that a resistor is not required, especially in the case of vintage radios. They were never designed to operate on today’s higher household voltages, so the dropping resistor is not only best practice to get the voltage to where the selenium would be, but to also further drop voltage to where a 110 or 115 volt-rated radio running at 120V+ really should be.

@midmodaudio6576 3 жыл бұрын

Awesome !

@wellusee 3 жыл бұрын

Very good. I Like the music, its very like the music on "FranLab" with Blanch. Did you have to study the circuit a lot before making the video or are you just brilliant?

@FluxCondenser 3 жыл бұрын

Thanks for the great comment, that gave me a chuckle! Sadly, I’m only of average intelligence and this stuff just isn’t as complicated as it seems to the uninitiated. After you’ve been doing this for a while, it all starts making sense. Diagnosing problems requires more circuit analysis than this procedure which is fairly routine. You’ve got a component that needs to be replaced (the selenium rectifier) and you’ve got one choice: use a diode. The diode will allow more current to pass than a selenium, so you also have to add a resistor so the replacement components match the originals. From there, it’s a simple matter of knowing which way to install the diode, how to figure out the correct diode and resistor, and knowing how to install the parts. FranLab is great!

@obedjimenez2624 2 жыл бұрын

I nevwer knew that selenium rectifier could catch fire and become toxic. Thanks.

@joeygonzo 3 жыл бұрын

great video

@FluxCondenser 3 жыл бұрын

Thanks, Joe. Much appreciated.

@jonsnell4751 2 жыл бұрын

If you notice, there are vanes (or the heatsinks) for each selenium diode. Each selenium diode has a voltage drop of 1 volt, a silicon diode is approximately 0.6volts. So as a selenium diode segment is good for a back EMF of 20 volts, HT diodes have many segments, each one dropping 1 volt. Hence the need for a resistor to drop the extra voltage. A 12 volt selenium diode will drop 1 volt and silicon 0.6 volts. a 300 volt selenium diode is made up of 15 segments, so 15 volt drop. A silicon 1N400* has a 0.6 volt drop.

@FluxCondenser 2 жыл бұрын

Thanks for the added info, Jon.

@andydelle4509 3 жыл бұрын

One minor, probably irrelevant point in this example. When you measured the amplifier current, it was at 14 volts. The amplifier may pull more current at a higher voltage. To be more accurate, you could set the Variac so the amplifier is getting 12.7v and then measure the current. But as we have seen in this example the difference is minuscule but could be significant in other applications.

@FluxCondenser 3 жыл бұрын

Yes, good point. The current could have been measured before removing the selenium rectifiers to get a more accurate measurement. But, as you, say, it’s a tiny amount of difference and the proof is in the final voltage. Further adjustments to the resistor value could then be made.

@FyrFyter Жыл бұрын

So I am trying to replace a Federal 1005A selenium rectifier. I replaced them with 1N4007 diodes. All the circuitry on the appliance is 120 vAC. What kind of dropping resistor do I need? (I'm not very knowledgeable on the topic so please forgive me)

@glenwhatley4125 2 жыл бұрын

Microwave rectifier for voltage drop. I haven't tried this yet but it occurred to me that it would be more efficient to install one or two microwave rectifier diodes in series with the silicone replacement bridge to handle the appropriate voltage drop. Those diodes are multiple diodes in series to handle the HV they're rectifying. They also handle a fair amount of current. You can forward bias the diode and check the drop by inserting a 9V battery in series with your voltmeter. They have about 3 volts drop. So, just insert the appropriate number of microwave rect diodes in series with your bridge rect and voila! Instant voltage drop with minimum heat produced. Any ideas on why this might be a problem are welcome. NOTE:You'll have to buy the entire microwave oven at a thrift store as the appliance industry has decided these diodes are precious and want like $50+ each for these

@FluxCondenser 2 жыл бұрын

Any diode will drop voltage and yes, you can use a bunch of them together to attempt to get the voltage you need. There are advantages to do this over using a resistor (less heat), but it can take a lot of them as the drop per diode is less than a volt.

@glenwhatley4125 2 жыл бұрын

@@FluxCondenser Agreed. the advantages here are that you have a bunch of diodes that are rated for HV as well as relatively high current in one relatively small package that can be easily added. And, as you mentioned with minimal heat generated as well as the diodes won't make the supply spongy as a resistor in series would. The voltage drop across the HV diode will remain solid regardless current draw.

@FluxCondenser 2 жыл бұрын

@@glenwhatley4125 All good points, Glen.

@LaLaLand.Germany Ай бұрын

At 3:14 we see You use a pen- what´s that for? Kind Regards and Thanks, Mate.

@davidnguyen5028 Жыл бұрын

I wish you had slowed down the wiring at the end.

@FluxCondenser Жыл бұрын

Have you tried watching it at a slower speed? KZfaq gives several options for slowing down and speeding up videos.

@patprop74 2 жыл бұрын

I have a Selenium in my Triplett 3480 tube tester, it is off-gassing, because it is a multi-voltage output dc, do I test with the lowest voltage or highest or perhaps midway?

@FluxCondenser Жыл бұрын

The selenium should be replaced. When you say test with the voltage, which test are you trying to accomplish?

@roymercer6967 2 жыл бұрын

Thank you for the your content. Shouldn't your current measurement have been taken with your DMM set to measure D.C.?

@FluxCondenser 2 жыл бұрын

Thanks. The current coming from the transformer is AC so if you’re measuring before installing the diode, the AC setting should be used. If you measure current after installing the resistor, the DC setting should be used. In the video, note that I was testing current without the diodes inline.

@roymercer6967 2 жыл бұрын

@@FluxCondenser Thank you sir. It's hard to tell what's going to what from the video. You must be measuring from the primary side of the transformer. Thanks so much for taking the time to respond.

@FluxCondenser 2 жыл бұрын

You’re welcome. But no, I’m measuring from the secondary of the transformer. Transformers don’t create DC, only rectifiers do that. The primary and secondary of a transformer are AC.

@roymercer6967 2 жыл бұрын

@@FluxCondenser After looking back at the video It makes more sense. I see that it is the AC input. That is a strange AC power cord connector.

@colincampbell7027 Жыл бұрын

I think it's called a ONE N not an EYE N.diode.

@iamhe999 Жыл бұрын

The theory of calculating a series resistance is ok somewhat.... it assumes the current draw on the dc side of ther power supply is somewhat constant throughout the use of the device.. Sometimes however something on the load side of the dc supply requires more current.. that series resistance may result in the DC voltage being too low at that moment in operation.... If you are replacing what seems to be a perfectly operating selinium rectifier monitor the DC supply's output voltage through out all the device's operation, solenoids and motors may switch on and off during normal operation... then after making the change keep an eye on the DC output voltage through all the device's operation.. verify the DC voltage is not dropping down too far when solenoids and motors turn on..

@FluxCondenser Жыл бұрын

The silicon will be able to supply as much or more current as the selenium and all measurements should be done closely keeping the voltages to spec with a variable output AC power supply.

@iamhe999 Жыл бұрын

The more current the load draws on top of the idle currant the larger the voltage drop across the resistor.the lower the voltage drops to the load.. its called the IR drop.. no doubt the rectifiers will have no problem supplying the current but the ir drop increases with the load current demand.. the above technique for calculating the resistors would be ok so long as motors and solenoids are not being activated.. so do you calculate the resistance on the minimum current or at the maximum current.. may not be a problem or could be a problem.

@FluxCondenser Жыл бұрын

Been doing it this way for years with many devices. Never a problem.

@darthbubba866 3 жыл бұрын

Yeah, you don't wanna be in the room when a selenium rectifier shorts and dies. The fumes are toxic, but the smell stays around what seems like forever. Unless a customer threatens my life, I automatically replace seleniums with silicon diodes (and the proper dropping resistor) and return the selenium in a plastic bag.

@FluxCondenser 3 жыл бұрын

Thanks for sharing your experience with seleniums, Darth. After I posted this video on some Facebook repair groups, I had one particularly rude gentleman argue that I’m being overly cautious, implying there’s nothing to be concerned about. I’ve never seen one go myself, but I’ve heard enough experiences from people like you that I don’t take a chance and replace them ASAP.

@darthbubba866 3 жыл бұрын

@@FluxCondenser Well, denial works until it doesn't. There's no down side to making this change, other than the electronic device no longer being in "original" condition. If that's what the owner wants and is not going to power up the device except to demo that it works then I'll relent. Almost all of my customers appreciate that I make the device safer for them. Stay safe and healthy!

@FluxCondenser 3 жыл бұрын

@@darthbubba866 Couldn’t agree more. Thanks for the voice of reason.