I have a 55 A, 100 V, SCR, TO-218AC in a car battery charger that is obsolete , the closest replacement i can find is a 55 A, 200V, SCR, TO-218AC. How critical is the volts? are there tolerances rules as with capacitors and resisters in +/- where the circuit will still do as design? how high can that voltage rating be raise and it still work? is it like caps where you can use a higher voltage cap but not a lower one? thanks in advance

 

how high can that voltage rating be raise and it still work? is it like caps where you can use a higher voltage cap but not a lower one?

Exactly. Your replacement is rated to handle the same maximum current as the original and will withstand twice as much voltage, so it should be good.

 

so you are saying the voltage can be doubled and will be ok ? for future reference is this a standard for replacement? would higher then double cause problems? Am I correct in assuming a lower voltage would be a no no? I tinker with a lot of old odd equipment and your info has help a lot ..thanks

 

A component's maximum voltage rating (like 200 volts for your replacement SCR) is exactly that: a maximum rating. It's the maximum voltage the part can take without damage. Same goes for the maximum current rating: both the original and replacement SCRs are good up to 55 amps. Any more, and you risk damage.

That 200 volt rating doesn't tell you the part "needs" 200 volts; it tells you the part can't handle any more than 200 volts.

And yes, a lower voltage (or lower current, for that matter) rating would be a no-no.

 

I know about the voltage and current in other components, this the first time I have run into dealing with a scr. thank you so much for your time. apparently an old dog can learn knew tricks

 

You're welcome. Good luck with your project.

 

But what if the voltage is 201V? Will SCR survive

 

What voltage is the existing SCR operating in, if on the secondary side, you may even be able to go higher, the original may have been over-spec'd.
Max.

 

the original is 100v, so if the voltage is 201v it will be letting all the smoke installed at the factory escape

 

The point I was making is the 100v rating of the original could be overkill if this is presently being used in a low voltage circuit.
You mentioned battery charger.
SCR's are very seldom seen on the supply/primary side.
Max.

 

Two things to watch out for. This is more for that "other equipment" that the current question.

1. When changing a semiconductor to one with a higher rating (usually voltage or current), note that other device parameters will change. For example, the SCR's forward voltage when it is conducting probably will be a bit higher with the higher voltage device. Almost certainly nothing to worry about, but something to check. Also, the gate turn on voltage might be different. A common situation with power MOSFET's is that higher voltage devices tend to have higher ON resistances (other parameters being equal), leading to higher operating temperatures, higher voltage electrolytic capacitors have higher ESRs and absorption factors, etc.

Again, this probably will not be an issue in this case. But in general, try to get the datasheets for the old and new devices, make a table with the old and new main parameters (voltages, currents, resistances) and evaluate the differences in the context of the application.

ak

 

Max you are right it is on the secondary side of a car battery charger that also will jump start a car. I have been just changing boards at work so long that I am having to fight through the cob webs to get back to basics. this is just a play thing doing a favor for an old friend, actually more for personal satisfaction to prove i still got it. In my real world, I don't have the time to do any real electronics trouble shooting

 

You just don't thinking too deep, and you could thinking this way:
55 A, 100 V, SCR, TO-218AC -- The original rating values, you could treat it as the stuff you want to buy.
55 A, 200V, SCR, TO-218AC -- The new rating values, you could treat it as the money in your pocket.
Do you know what to do now?

 

Max you are right it is on the secondary side of a car battery charger that also will jump start a car.

I think I would be interested in upping the current rating than worry about a higher voltage rating.
Cranking amps can reach 100amps.
Max.

 

not worried about upping anything, the parts/unit are obsolete, no parts or info available, upping the voltage on the scr is due to cost prohibitive NOS parts and nothing i found to match so far other then the higher voltage rating with the original other specs that match closest. Fortunately it is a small circuit with few parts and if it doesn't work it won't be the end of the world. It has been so long since i have designed a circuit, this is more of a diversion then anything. The original question was more out of curiosity then anything else. I am impressed and very thankful to find out if I do run into a problem, that the folks at this forum are willing and ready to lend a hand

 

so you are saying the voltage can be doubled and will be ok ? for future reference is this a standard for replacement? would higher then double cause problems? Am I correct in assuming a lower voltage would be a no no? I tinker with a lot of old odd equipment and your info has help a lot ..thanks

As long as it can handle the same (or more) current, a higher voltage rating is just more safety margin than you had before.

Those aren't the only ratings you have to watch out for - higher voltage SCRs sometimes have less sensitive gates.

 

ian, I had just read about that yesterday, this article said past a point, in that story they went from 200v to 800v and it total changed things.
I am just going up 100v because of availability. should be fun seeing what it will do.

 

I am guessing that with that SCR circuit in the charger it's one of those old style 'smart chargers' that never properly charges a battery worth crap or give you it's full cranking amps either.

I've pulled dozens of those circuit boards out or hard wired around them over the years in cheap battery charges I picked up at auctions, garage sales, scrap yard iron piles and garbage dumps for next to nothing because they hum but won't charge a battery and gave them away or stripped them down to build power supplies out of.

If it was mine I would just get rid of the circuit all together and have a good reliable simple charger unit.

 

I agree. A, "dead" battery charger contains a big transformer that is just begging for a proper regulating circuit. Meanwhile, you get the metal box it came in, a switch or two, and a crude amp or volt meter.

The next level of boon is a UPS. Most of them get tossed for a bad battery. Add a battery and you just got a device worth most of $100.