Re: programmable circuit breaker

On 05/03/2025 21:29, john larkin wrote:

On Wed, 5 Mar 2025 18:05:55 -0000 (UTC), piglet
<erichpwagner@hotmail.com> wrote:
 

Liz Tuddenham <liz@poppyrecords.invalid.invalid> wrote:

john larkin <jl@650pot.com> wrote:
>
[...]

Maybe I shouldn't diode clamp the relay coils. I'll have to test the
dropout time.

>
Put a resistor in series with the clamp diode, the value can be chosen
to keep the inductive spike just below the maximum allowable voltage and
that will give the fastest turn-off.  By using higher voltage switching
transistors you can have a higher voltage spike and a faster turn-off -
but there is the risk that the spike might be capacitively coupled to
somewhere vulnerable to damage.
>
>

>
Or even just a resistor and no diode! The extra few percent current
consumption is often not a problem and a resistor is very reliable, more
durable than a diode.

 I tried a BSS123 logic-level mosfet driving my 12-volt 33 mA relay
coil. It avalanches at about 120 volts and runs cool at 20 Hz.
 I'l let it run for a zillion cycles, but it's noisy.
 https://www.dropbox.com/scl/fo/i9xxdnw7bveuegm373sky/ALKsKJsIlT1fWhCehkHaa8w?rlkey=5ttkfyzl52prxvyd7xztguzh4&dl=0

When I tried a similar experiment I found that there was no
significant improvement in relay opening time once the flyback
reached 5 times the operating voltage.
Maybe a 50V mosfet that avalanches at about 60V would achieve the
same benefit but with less noise.
There are mosfets intended for automotive fuel injector drivers
that are rated for repeated avalanche operation.
John