Suggestion wanted! 3A SCR

[Martin G7MRV]

Hi,

I'm wanting to put a crowbar circuit on a 24V PSU as overvoltage protection. I need an SCR that will survive around 3A for long enough to blow the fuse! (equipment draws about 2A max)

Can anyone suggest a 'ball-park' device? Ideally one that costs peanuts of course ;-)

Cheers
Martin

[turretslug]

BT151? TO-220 package. Though bear in mind that taking out the fuse pronto may involve rather more than 3A.

[G6Tanuki]

In these situations to ensure reliable fuse-blowing you need a thyristor/SCR that can happily sink loads of current.

TYN825 from NatSemi ??

https://www.st.com/en/thyristors-scr...es/tyn825.html

[Restoration73]

All this seems so last century. I would use a relay with the break contact in series with
the output. Thus the thyristor load is only the relay coil. It is resettable and it is even
possible to have a push to test switch. Use a fast relay, e.g. loudspeaker protection type.

[G6Tanuki]

A relay takes nastily-long-numbers-of-Mechanical-Milliseconds to pull-in/drop-out and safeguard the load - those milliseconds could easily mean expensive overvoltage-death to your load.

A big/cheap SCR/thyristor clamps the output much, much faster.

I've always worked on the basis that the power-supply is more-valuable than the-load-it-supplies.

[Martin G7MRV]

Thanks for the suggestions. Ive no look-up tables for these, and online lists on the supplier websites are too hard to read on a phone screen!

[Ed_Dinning]

If you can find one of the BTY series from some old equipment.
These beasts will take huge surge currents for 20mS or so, but it is best to ensure a good DC gate signal.

Ed

[G6Tanuki]

Originally Posted by G6Tanuki

I've always worked on the basis that the power-supply is more-valuable than the-load-it-supplies.

I mis-spoke here: I meant to say that the power-supply is _less_ valuable than its load.

[Martin G7MRV]

Originally Posted by G6Tanuki

I mis-spoke here: I meant to say that the power-supply is _less_ valuable than its load.

I realised that was the case In my case the PSU is worth less than a 20th of the cost of the equipment

[Argus25]

Originally Posted by Martin G7MRV

Hi,

I'm wanting to put a crowbar circuit on a 24V PSU as overvoltage protection. I need an SCR that will survive around 3A for long enough to blow the fuse! (equipment draws about 2A max)

Can anyone suggest a 'ball-park' device? Ideally one that costs peanuts of course ;-)

Cheers
Martin

It depends on how classy and beautiful you want your SCR to look. I have a fondness for the 2N3228, so I bought a number of them a while back. They are in the wonderful T0-66 package. (I like metal rather than epoxy packages for power devices).

I think they are in the 3.2 to 5A range, I used one in a project here, but you will see if you scroll to the last page of the project, that it is dead easy to wire up a pair of transistors as an SCR if you want:

http://nebula.wsimg.com/c5d98e1b7320...&alloworigin=1

I know that UK folks don't much like buying parts from the USA with the extra costs, but I just had a look for you, and these parts are in Birmingham UK for a reasonable price , if the mods allow the link:

https://www.ebay.co.uk/itm/2N3228-50...4AAOSwk1JWgFjj


Hugo.

[McMurdo]

can you use a TVS instead?

[kalee20]

BT151 for me too.

Lots of thyristors have an I-squared-t rating for fusing. As long as the fuse that you intend using has a smaller let-through energy, you're fine.

Failure mode of thyristors is short-circuit anyway, so unless you have a six-inch nail as a fuse, the load should be pretty well protected.

[emeritus]

As the application is to blow a fuse, dissipation shouldn't be a problem. In the 1970's my boss at Plessey once mentioned he had had a requirement to provide protection for the front end of the radio receiver part of a R/T radio being designed for a military application to prevent damage in the event of the transmitter still being active when switching to receiving. An SCR capable of handling a high current for a brief time was called for, and he had used such a device in a previous employment in connection with SCR control of electric traction motors in railway locomotives. The device he had in mind used a chip mounted in a large stud package to dissipate the heat generated when passing the rated current continuously. That package was mechanically unsuitable, and as the device would only have to handle a brief pulse of current, he got the makers to mount a few chips in smaller transistor packages. They worked perfectly, so they were given a NATO stock number and identified as something like protection devices, but basically they were the high power SCR chips in a small package.

[jimmc101]

Can I be different and suggest a TRIAC.
The BT137-600E is an 8A sensitive gate device(10mA max) and can easily be driven from a TL431.
It's certainly cheap (10 for under £3 from UK suppliers).

Attached is a circuit I've been using to protect a 5v supply.
The inductor is included to limit to limit di/dt since the TRIAC is soft triggered.

At 24v the TL431 would be dissipating a maximum of 250mW just before the TRIAC fires, should be OK but a small heatsink would help if your ambient temperature is high.
Of course in normal operation the dissipation is a lot less.

Jim

[Diabolical Artificer]

I've used the C106D a lot in crowbars and other protection circuits, it has sensitive gate triggering. I used them to protect LM317AHV power supply's to protect valve heaters from over voltage. With the C106D bolted on the back of the HS that the 317 was on and using thin copper clad board, I could fit the crowbar into the same space as a TO220 package, IE the zeners and cap etc fir onto a daughter board that sits vertical on the main board, two copper pegs made out of 1.5mm lighting cable connect the two.

Andy..

Andy.

[Martin G7MRV]

Ive decided to go with the TYN825 - ample capacity and cheap. Just waiting for them to arrive.

I actually found a stud mount BT106 while searching my spares boxes, but its not practical to mount it in the case this PSU has to fit.

Martin