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From: piglet <erichpwagner@hotmail.com>
Newsgroups: sci.electronics.design
Subject: Re: DC relay latching voltage
Date: Sat, 24 Aug 2024 22:10:49 -0000 (UTC)
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john larkin <jlarkin_highland_tech> wrote:
> On Sat, 24 Aug 2024 11:11:19 +0100, liz@poppyrecords.invalid.invalid
> (Liz Tuddenham) wrote:
> 
>> Pimpom <Pimpom@invalid.invalid> wrote:
>> [...] 
>>> Normal power supply decay will not release the relay quickly enough. So
>>> I've added a section that turns the 12V relay off when the decaying PS
>>> drops below 7.5V. Works fine in simulation.
>>> 
>>> However, due to interaction with other sections, the gate voltage of the
>>> transistor driving the relay rises again briefly to about 2.5V before
>>> the shutdown process is complete. But by this time, the 12V supply has
>>> dropped to less than 3V.
>> 
>> Add a few diode drops in the supply to the Base of the transistor and a
>> pull-down resistor from Base to Earth.  You could even use a 7.5v Zener
>> so the transistor switched directly off the PS and no other active
>> components were needed.
> 
> A 2-resistor voltage divider can reduce mosfet gate swing.
> 
> Or an R-C in his case of a transient gate drive spike.
> 
> Or both.
> 
> 

If the C is gate to drain (or base to collector) then the miller effect
slows rate of flux collapse enough to remove need for any additional
components across the coil.


-- 
piglet