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1967 Kohler 6.5 kw with L600 engine, replacing selenium rectifier

Kenny Crenshaw

Registered
I've read nearly everything I can find on the internet I about replacing the selenium rectifier in my generator. I've seen everything from "Nothing wrong with selenium rectifiers, they run forever", to "Replace that evil rectifier before it fries your transformer and kills a whole village". And "They sell brand new selenium rectifiers every day. There must be a reason why."

Not to resurrect this deceased equine simply to flog it again, but I would like to go one more time around the track.

My rectifier seems to be working just fine. Do I really need to replace it? If it fails, is it going to damage other things in my generator? Some say yes, some say no.

If I do, do I just mail order a 24 amp, 400 volt bridge rectifier in a screw-down aluminum case? Do I screw it to a piece of aluminum with a "motherboard" type heat transfer tape? Do I check the input and output voltage to see how much voltage drop there is through the selenium rectifier? Do I need to put a resistor in line to duplicate the voltage drop through the selenium? Do I need to load the generator to test the output voltage, then use a resistor to match the original voltage? Will the silicon bridge regulate the voltage similar to the selenium?

I am led to understand that Kohler used to sell a kit with the bridge rectifier and the proper resistor to replace the selenium, but it seems No Longer Available.

There seems to be much contention on this topic. Thanks in advance.
 

Vanman

Subscriber
Last Subscription Date
07/10/2019
Well, the first rule of living in the Misinformation Age is that there is far and away more Misinformation than actual, useful information.

I have only read here on the Stak that selenium rectifiers fail merely with age. Read that even a brand new, unused old one can be bad right out of the box. I have no personal experience.

But it is widely known that silicon rectifiers are very good, reliable, robust, and extraordinarily long lasting, so long as their voltage rating is not ever exceeded.

And I can tell you by looking at the circuit that, if the rectifier fails open while the set is under enough load, the transformer WILL be ruined. There's no ambiguity here. Somewhere right here on the Stak there is a thread where this happened to someone. He had it rewound at great expense as I recall.

That transformer has a ratio of something like 50:1.

Assuming yours is single phase, swapping in the silicon unit is very simple. The auxiliary field circuit is current based, not voltage based, so the lower resistance does not technically matter. It cannot result in an increase in current flow in the auxiliary field.

The lower voltage drop will result in a lower voltage across the primary of the transformer, which will result in a very slightly lower voltage drop across the secondary (which is in series with the load). So theoretically the output voltage would rise (ever so slightly!) as load is increased. This would be by only a small fraction of a volt. Not noticeable, and only detectable under ideal conditions with perfect instrumentation.

Because the current flow is only a few amperes, the heat dissipated by the rectifier will be low. I would think that securing it with a bolt to the sheet metal controller enclosure would suffice. This is what I did with mine. Have not really put it through a proper test, but I had it running my 4 ton A/C unit, delivering right about 4 kva, and as I recall it didn't even feel warm.

Keith
 

Power

Registered
I have had selenium rectifiers fail on equipment, particularly old TV sets. What I have seen is plate(s) goes bad, and then you see arcing at others, the whole stack shorts out and selenium fumes are released. The smell will not be forgotten. It is worse than rotten eggs, and people say the fumes are poisonous. Usually there is collateral damage- power transformer or other components.

Selenium Rectifier VS Silicon Diode - The Wasteland Tinkerer

Search domain tcpmeta.wordpress.com/2018/01/11/selenium-rectifier-vs-silicon-diode/https://tcpmeta.wordpress.com/2018/01/11/selenium-rectifier-vs-silicon-diode/
Here is a reason why you should convert to Silicone Diodes instead of leaving in a Selenium Rectifier. For one Selenium can just suddenly short out and put toxic fumes in the air, not to mention fry the power transformer of your device that you're probably restoring. In other words can be a fire/safety issue. Picking the proper Diode is easy.
 

Zephyr7

Registered
Get a 1000 PIV rectifier. You don’t really “need” it, but the higher reverse voltage rating costs almost nothing more, and gains a bit of robustness when handling any transients.

You don’t need any thermal grease or tape. The heat generated is relatively low, and the contact area is fairly large. You don’t need any insulation, either, since the rectifiers case is already electrically isolated from all four terminals. Just bolt the rectifier to a convenient spot on the metal chassis and you’re done.

Selenium rectifiers most certainly DO fail from age. There is a good reason why no one really uses them anymore! Yeah, you can still get them, but not big ones as far as I know, and no one recommends using them in new equipment. Replace your selenium rectifier with a silicon unit and rest a little easier knowing you don’t have a time bomb in your otherwise reliable genset.

Bill
 

Fred M.

Registered
I am led to understand that Kohler used to sell a kit with the bridge rectifier and the proper resistor to replace the selenium, but it seems No Longer Available.
Kenny-

I'm just a worry wart. What is this rectifier used for? If it is for charging a battery, then changing to silicon could shorten battery life. You need the resistor to limit charging current. I wonder if Kohler would provide specifications for the resistor used in their kit?

I agree with going for the higher rectifier voltage rating.

Fred
 

Power

Registered
Kenny-

I'm just a worry wart. What is this rectifier used for? If it is for charging a battery, then changing to silicon could shorten battery life. You need the resistor to limit charging current. I wonder if Kohler would provide specifications for the resistor used in their kit?

I agree with going for the higher rectifier voltage rating.

Fred
Unlikely in this particular application--Read Vanman's post 2
 

LWB250

Registered
The kit sold to replace the selenium rectifier consisted of a 25A full wave bridge rectifier, the type of which you see as a black "cube" with four 1/4" push on terminals.

It was supplied with a "puck" style GE MOV that was to be wired across the bridge to absorb spikes/transients that occur when the generator has a load removed or significantly reduced across it.

In all the years that I used these kits I never saw one with a resistor in it.

Replace the selenium rectifiers. Consider this an insurance policy, as you don't want to have to replace the transformer, if you even can.

Dan
 

Zephyr7

Registered
If that transformer lets out its smoke, I don’t think replacements can be found — your only option is to have it rewound which is expensive, and there aren’t many people willing to do it. “Expensive” is going to be at least a few hundred dollars.

Replace the rectifier for a few bucks and think of it as cheap insurance. Better safe than sorry!

Bill
 
Last edited:

mdj21

Registered
On mine it says GE-MOV V250 PA40B.

I didn't have a lot of luck finding a spare one, but since mine is still working, maybe I didn't look as hard as you probably will.

Good luck.
 

LWB250

Registered
The GE varistor that was previously used (I believe) clamped at around 500VAC and was good for 6500 joules.

That's a guess based on what I recall - you can easily find a radial device at Mouser or other major electronics supply in this range for a buck or two.

Dan
 

Power

Registered
The thing that concerns me about MOV's is that they are consumable. Each spike wears it. I have seen them fail shorted and fail open. If it fails shorted, it would take out the transformer. I have been using bridges with 1000 PIV, like Zephyr said in post 4. No MOV.
 

Vanman

Subscriber
Last Subscription Date
07/10/2019
In this circuit, if the MOV or rectifier fail shorted, nothing would happen to the transformer. You would merely lose generator compounding, and voltage regulation would suffer as a result, as the auxiliary field current would be shunted by the short.

The danger to the transformer is an open circuit. It is around 50:1 turns ratio, with the secondary in series with the load. If the primary is open circuited with a load on the set, the voltage across the primary would cause it to flash over and burn. Have to find the thread about that. I'm sure there are photos of the carnage.

The MOV would only have a chance of getting hit with a spike if the generator output was short circuited. Even then, I'd be surprised if it peaked at more than a couple hundred volts. Since the auxiliary field is wound along side of the main field, and the main field is not open circuited, transformer action between the two would likely keep a spike caused by a sudden increase in current to a low value. Would be interesting to put a scope on it.

Sudden removal of load places zero stress on any component. The field collapses, forward biasing the rectifier, it's energy dissipated in it's resistance. If the transformer secondary were to generate a spike from the sudden loss of current flow, it too would forward bias the rectifier, it's little bit of energy would also be dissipated in the auxiliary field winding.

Keith
 

Zephyr7

Registered
On mine it says GE-MOV V250 PA40B.
That’s a 250V AC rated MOV (330V DC, 384V clamping), 40 joules. The datasheet I found doesn’t give the lead spacing, but that’s probably not very critical here.

A suitable replacement MOV with equivalent ratings is littlefuse #V250LA10P available for 59 cents each here:

https://www.digikey.com/product-detail/en/littelfuse-inc/V250LA10P/F3013-ND/1009395

Better hurry if you need one, they only have 3,054 of them in stock :D

Bill

The GE varistor that was previously used (I believe) clamped at around 500VAC and was good for 6500 joules.
Probably 6500 amps, not 6500 joules. 6500 joule rated MOVs are BIG BEEFY BLOCKS. 6500 amps is a common rating for more regular size (leaded disc type) MOVs. The amp rating is only good for a millisecond or two, so it’s not as impressive as it first sounds.

Bill
 

LWB250

Registered
Probably 6500 amps, not 6500 joules. 6500 joule rated MOVs are BIG BEEFY BLOCKS. 6500 amps is a common rating for more regular size (leaded disc type) MOVs. The amp rating is only good for a millisecond or two, so it’s not as impressive as it first sounds.

Bill
Correct. My mistake. That would be a "hockey puck" or larger.

Dan
 

Zephyr7

Registered
Thanks all. I just ordered a bridge rectifier and an MOV from Digi-Key. Just think, a week ago I never heard of an MOV!
Digikey also carries my favorite crimp terminals: AMP PIDG and Solistrand. Both are soooo much better than cheesy hardware store crimp terminals. I highly recommend both of those good types to everyone!

Bill
 
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