# SCR Selection

Discussion in 'Electronic Design' started by panfilero, Dec 17, 2012.

1. ### panfileroGuest

I'm trying to design an SCR based overvoltage crowbar circuit for a 100V to 24V DC/DC converter...I'm having some trouble finding the right part, here's my specs

My max load current is 1A. I have a 3A fast acting fuse at my input to the DCDC. I want an SCR that can handle 4A long enough for my fuse to pop, I want this to happen if my output goes to 30V...

so, I went off to go look for such a part... seems straight forward enough... I found this

but the part says 4Arms, 2.6Aavg.... I'm just working in DC so I imagine RMS is the way I want to go here....

I also don't get the spec on SCRs that's called Vt which is the voltage across the anode to cathode when it's on... and it's usually 1.5V or so... if you have 30V across it how is it's votlage 1.5V or am I reading this wrong?

my last question is, how much voltage is it safe to put at the gate to turn these things on... I've seen 0.8V on datasheets as turn on voltage... is it ok to give them 1,2, or 3 volts at their gate?

much thanks!

2. ### Robert BaerGuest

The most critical rating is the "Circuit Fusing (t = 8.3 msec)"
i^2*T" rating.
You are discharging a capacitor (power source + load) which will
produce a large current; if that is less than the current rating, you
could still have a destroyed part when it is triggered.

As far as gate voltage, 0.7V turns on a transistor or SCR; DO NOT use
a low-Z voltage source (!); 5V with 1K series to gate (ie: 5mA) will do
for that.

3. ### P E SchoenGuest

"panfilero" wrote in message
You should really go for an SCR with a much higher current rating, such as:
http://www.littelfuse.com/data/en/Data_Sheets/Littelfuse_Thyristor_Sxx40x..pdf

These are about \$2.50 each and handle 40A RMS and over 600A for 20 mSec for
capacitor discharge duty. These devices have an I^2t rating of 1122, so you
just need to be sure that the fuse has a lower value. The 3A fast acting
fuse will probably open in less than 10 mSec at 30A or more, which
corresponds to an I^2t of about 10.

If cost is a major factor, you can get this 25A device for less than \$1:
http://www.newark.com/jsp/search/productdetail.jsp?ICID=ILC-1C130&sku=45J0826

If I read you correctly, your fuse is at the input of your 100V-24V DC-DC
converter, but you want to crowbar the 24V output at 30V peak. There is no
guarantee of what current will be drawn at the input in case of output
overvoltage failure, unless it is caused by a series component failing short
and imposing the 100V input on the output. So the fuse should be on the 24
VDC output, and the SCR crowbar from output to GND. There will probably be
enough capacitance on the output to provide a surge current sufficient to
blow the fuse and protect the 24 VDC load.

The fuse at the input of the DC-DC should be chosen to protect against
overcurrent and short circuits, and for a 24V 1A output, nominal input
current will be about 250-300mA. So your 3A fuse should instead be about 1A.
You can also add a crowbar to the input to protect against overvoltage.

I do not agree with the idea that one should rely on a crowbar SCR failing
shorted to blow the fuse.

Paul

4. ### Guest

It's a DC circuit, he needs to confirm the fuse is rated to interrupt that large of a current at that large of a VDC voltage.

5. ### Fred AbseGuest

That's the way I've always done it.

If the PSU has foldback, it's necessary to pick an SCR with a holding
current an order less than the foldback current.

6. ### josephkkGuest

Yes. Perfectly valid point. This SCR is too weak for the task as
described.

?-)

7. ### Fred AbseGuest

In my world, the SCR has to survive. The only failure should be a fuse at
the PSU *output*.

I've made rigs to test multiple trips, preceded with, and followed by,
parameter measurements on the SCR.

The only time I would countenance relying on an input fuse is in the case
of a series buck regulator, and then I'd do my best to dissuade the client.

8. ### Fred AbseGuest

A bit too sensitive, too. Igt in the microamps range.

MCR12DCM (It(av)7.8A, I^2t 41A^2.sec, 40mA max holding, DPAK) would seem
a more likely candidate. Max Igt is a bit high at 20 mA, but a simple
zener trigger circuit is still feasible.

The OP stated a 1 amp load. A Littelfuse 370 series (1 amp rated, 0.2
A^2.sec) PCB fuse *at the output* should do nicely.