Miny SCR burn out/ replacement

I came across this schematic of a driver for a car spark coil, having built the driver it worked fine for about 3 short tests then one of the SCRs blew out. Clearly the amp draw is more than the capacity of it.

The type of coil I am using draws 5 amps with an inrush of 2x +. Can any one recommend an SCR that will fit the bill to replace the 100-8 and fit the application.





The application is peaceful………..I am building a plasma cutter and this is the igniter for the arc.

The author of that circuit is a resident of this forum, and may be along soon.

Did the scr's get hot, or just fail, if the latter then maybe the reverse (despite the diodes which take time to turn on) or even forward voltage was too high and caused breakdown, that being the case you might get away with putting Mov's across the scr's to limit the max voltage.
Also did you try 1uF for the coupling caps as per the suggestion?
This circuit may well be meant for older higher resistance primary coils, newer constant energy coils tend to be much lower, some less than an ohm.
I put together a circuit for Hf start not for a plasma for a tig welder, its uses a 555 and a Mosfet, current limit is provided by limiting on time.

The SCRs failed by sending shards across the board.
I like this circuit design due to the fact that it uses 120 volts and I do not need an additional transformer or power supply to feed the coil.
The coil I am using is an older style high resistance type and the 1uf coupling cap is in place.
Thanks for your input and Happy New Year.

Ok then.
Are the 1uF's class X or suitable for mains operation, might be worth checking with a meter for shorts.
Hope you enjoyed yuletide.

They

multis6 said:

I came across this schematic of a driver for a car spark coil, having built the driver it worked fine for about 3 short tests then one of the SCRs blew out. Clearly the amp draw is more than the capacity of it.

The type of coil I am using draws 5 amps with an inrush of 2x +. Can any one recommend an SCR that will fit the bill to replace the 100-8 and fit the application.

The application is peaceful………..I am building a plasma cutter and this is the igniter for the arc.

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Not sure what your doing to have it Burn out one of the SCR's, But I have been using this circuit and selling kits for it for Many Years and No Failures.
The 2.2uF caps can be lowered to 1uF. These Cap Should be Good Quality Polyester Caps with a 250 Volt rating and Prevent High Current to the Coil.
Mostly I just use the MCR 100-8 SCR, Rated at 800mA.

Want to Talk, The USA is a FREE PHONE CALL For Me.

multis6 said:

The SCRs failed by sending shards across the board.

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Shards? Can you Explain what you mean by this?
It Exploded?

Yes Shards.....one of the SCRs exploded.
This is good news that you have built this circuit and it has worked well. I guess the thing to do is to rebuild it and expect a different result. It could have been a bad component.
Thank you so much for the input. and Happy New Year !

multis6 said:

Yes Shards.....one of the SCRs exploded.
This is good news that you have built this circuit and it has worked well. I guess the thing to do is to rebuild it and expect a different result. It could have been a bad component.
Thank you so much for the input. and Happy New Year !

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I Designed This Circuit and it is on My Website.
http://chemelec.com/Projects/110-Car-Coil/110-Car-Coil.htm

Even if the Coils Primary was Shorted, It should NOT Damage either SCR.

Looks like chem's designed the circuit with the 2.2uf's to limit the current, even with a shorted coil the current will be about 40ma or 10 watts, half that with the 1uf's.
You need class x caps, I suspect they are not and have shorted, hence my suggestion to check them in #4, just see if you get a resistance with a multimeter, they should measure open resistance.
There are other circuits to do this, however theres nothing wrong with this one.

I'll check the caps and replace them with the class x when the parts come in.
Thanks for your interest & input.

dr pepper said:

Looks like chem's designed the circuit with the 2.2uf's to limit the current, even with a shorted coil the current will be about 40ma or 10 watts, half that with the 1uf's.
You need class x caps, I suspect they are not and have shorted, hence my suggestion to check them in #4, just see if you get a resistance with a multimeter, they should measure open resistance.
There are other circuits to do this, however theres nothing wrong with this one.

Click to expand...

Unless BOTH Capacitors are Shorted, I doubt it was Bad Capacitor causing his SCR to fail,
In Fact, Only One of these capacitors are Needed for the Current Limiting to the coil.

Would be Really BAD if he used Electrolytic Capacitors.

I Use BOTH for Greater Isolation.
Especially needed to Prevent a HIGH Current to a Coil that may have an Electrical Path to Ground.
Or a Person that is Grounded, Touching the Coil or it Wires.

A PICTURE of His Circuit Would be GOOD TO SEE!