Low current triac or .. ?

[walker1]

I am constructing a totempole power output switching between + and - 40 V with 50% duty cycle, and I would very much like to short-circuit protect it.
I.e. both upper and lower transistor goes off in case the output current exceeds a certain level. And stays off till I press a button (or something).

It is fairly easy to construct a one-sided fold-back limiter, and I guess it is possible to double/invert it for two-sided use, but are there other ways ?
A low current triac, for instance, that can bypass the output drive and lead my less than 1 mA input current (resistor limited) to the output ?

I have so far not found an easily available (in Denmark !) triac with a holding current lower than 6 mA, and I haven't yet found out how to construct a triac myself.

 

[nbucska]

What is the load? Simplest would be to switch e.g.
between + & - 45V and drop with a R to 40V .
If it takes 1 mA at 40V it has 40kohm input R2. Add a series 4.99Kohm 1W -- it will limit a short to 9 mA.


<nbucska@pcperipherals DOT com> subj: eng-tips

 

[walker1]

Well. the load in normal operation is up to +/- 1.8 A,
but yes, I have a resistor in the input as sugested. 33K or so.

In the meantime I have thought about placing a conventional current limiter (resistor with a transistors BE across) on each of the push-pull branches (common 0R47 resistor for both branches if I can get away with it ,
and then use the x-tra voltage drop over the input resistor to trigger a shut-down.

I have opto-couplers and relays available and plenty of other circuitry at +5V, so ...

It will not be an instant shut-off, but with the output current limited, a few 100 ms for a relay to trip shouldn't create too much heat in the output stage.

In all my system requires 4 of these power outputs, so using integrated parts at digital level would make the power end simpler.

 

[Skogsgurra]

Hi Walker,

Your relay idea is not elegant, but very practical. Especially if you combine it with current limiting. It could also be used for other safety purposes (knowing nothing about the application). And a nice surprise is that most relays pull in and drop out in 10 - 20 ms and not hundreds of ms. So you are right, not much heat build up there. The latching function can be had using a self-holding contact. And it works "hvergang".