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Using SCRs

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Firefox

Mon Aug 04 2008, 12:02AM

Registered Member #1389 Joined: Thu Mar 13 2008, 12:50AM
Location: Pittsburgh, PA
Posts: 346

I have been browsing SCRs and looking at what I need to build some wire exploders, induction launchers, and the like, but I am unable to understand how you guys pick your SCRs. It seems like more than just the current rating, which to me makes sense as the most important item when you are talking about putting energy into a near short circuit. Someone said that with 60 or 120J, a certain (dont remember which) SCR could handle it, but in a short circuit, it would have slightly exceeded the pulse current rating. So is pulse current rating the most important aspect of the SCR, or is there more to it than that?

Myke

Mon Aug 04 2008, 12:09AM

Registered Member #540 Joined: Mon Feb 19 2007, 07:49PM
Location: MIT
Posts: 969

There is peak reverse voltage rating to take into consideration because there will be some high frequency ringing.

Hon1nbo

Mon Aug 04 2008, 01:00AM

Registered Member #902 Joined: Sun Jul 15 2007, 08:17PM
Location: Pacific Northwest USA
Posts: 1042

also consider the voltage spike coming from the gate of the SCR to the trigger circuit (had SERIOUS problems with this)... Then there is the switching time/voltage (how long until on, and how low E until off).

Shaun

Mon Aug 04 2008, 01:58AM

Registered Member #690 Joined: Tue May 08 2007, 03:47AM
Location: New Jersey, USA
Posts: 616

Well the thing is, the most common use of SCRs by amateurs (and 4hv members) by far is in coilguns. Coilguns will have peak currents that are FAR lower than the applications you discuss (induction launchers/wire exploders) of similar energy. The pulse length will be much shorter in duration, but things still are looking bad for SCRs under those conditions.

Not saying they absolutely can't be used, its just that a 300J coilgun will require a much smaller SCR than a 300J ring launcher. An SCR also has a rather limited dI/dt (relative to a spark gap), which is bad for induction launchers where you need the current to be able to ramp up as fast as possible. Again, SCRs can be used, but once you get higher than a few hundred joules one is almost forced to switch to a spark gap.

To answer your original question, I think the most important rating is Itsm, the non-repetitive peak surge current. Keep in mind, this rating is given for a duration of 8.3ms (half a 60Hz sine pulse). 8.3 ms is quite a while, even for coilguns. So its fairly reasonable that this rating can be overshot by quite a bit before your SCR detonates. Just how far over you want to try and go is up to you...scope+current probe helps here.

rp181

Tue Aug 05 2008, 12:40AM

Registered Member #1062 Joined: Tue Oct 16 2007, 02:01AM
Location:
Posts: 1529

I talked to some engineers at a power semiconductor company, heres what they said:
All rating usually have about 10% saftey margin, also, pulse rating is for 10ms, for coil gun applications, pulses are less then 1ms. The surge rating exponentially goes up as time goes down, it is safe as long as you keep the pulse short enough for the power, and wait a couple of seconds between shots.

...

Tue Aug 05 2008, 01:58AM

Registered Member #56 Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445

I would look at the I^2t 'fuse' rating, since that is what will dictate wether or not you will melt the die. Also very important is the di/dt rating, since you are talking about HUGE di/it for things like ring launcher.

Firefox

Tue Aug 05 2008, 04:07AM

Registered Member #1389 Joined: Thu Mar 13 2008, 12:50AM
Location: Pittsburgh, PA
Posts: 346

Ok, say I was discharging a 450V 2200uF pulse cap into a dead short with an SCR. What determines the rise time and A/us? I know the Io is determined by the voltage of the cap and resistance of the load. Is it a combination of the resistance and inductance of the load, or just the inductance?

Myke

Tue Aug 05 2008, 04:15AM

Registered Member #540 Joined: Mon Feb 19 2007, 07:49PM
Location: MIT
Posts: 969

Resistance and inductance. The resistance is pretty low but it's still there. Same with inductance. Don't forget the ESR and the inductance of the cap too. Isn't there a calculator out there that calculates the max pulse current when given the resistance, capacitance, and inductance?

Firefox

Tue Aug 05 2008, 06:55AM

Registered Member #1389 Joined: Thu Mar 13 2008, 12:50AM
Location: Pittsburgh, PA
Posts: 346

I'm not scared of a little paper/pencil and calculator work, so a formula (and perhaps an article/explination on this topic) would be fine too.

rp181

Tue Aug 05 2008, 04:52PM

Registered Member #1062 Joined: Tue Oct 16 2007, 02:01AM
Location:
Posts: 1529

what is di/dt?

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