MOSFET switch used as ON-OFF Delay timer

[lineup]

Today's result of my creativity & design in Lineup Audio Lab.


The mosfet I am using here, TO-92, should not switch more than 200 mA.
Usage for this circuit is for example in an amplifier
when you need a shorter time to charge things.
Then AUTO Turn OFF after some time delay.

As circuit is now it would be good for time delays from 5 to maybe 100 seconds.
Of course you can use some TO-220 or more powerful MOSFET.
To switch much higher currents.

The 10V Zener diode limits the voltage across timing cap C1 to ~10 Volt.
This circuit version should be good for use even with like to 20 Volt Supply.
But minimum is of course 12 Volt.


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[unclejed613]

interesting.... i had a problem with a certain model of video monitor, that used a vertical output chip that was pushed to the edge of it's current limits. i was repairing about 20 of these monitors a week, and some of them began being rework repairs (multiple repairs for the same failure, sometimes within a week). i found that the failure was caused by the fact that the chip was spiking current when the monitor was turned on, and this current spiking was well beyond the limits of the chip. i devised a turn-on delay using a mosfet and an RC charging circuit. the MOSFET would act like a variable resistor, with very little series resistance (0.1 ohm) after the cap charged up. this would, over the period of about 5 seconds ramp up the power supply rail to the chip, so the chip could get into it's operating mode without committing suicide. since it took about 7-10 seconds for the CRT filament to warm up once the monitor was turned on, the visual effects of the ramp-up were invisible to the user (if it had been visible, it would be reminiscent of an old vacuum tube TV set warming up where the picture "grows").

the circuit was a MOSFET, cap, resistor and a diode (to discharge the cap when the power was shut off), but still very similar to the one you have here, but then again, i had no need of a shutoff timer. when simple works well, they call it elegant.....

[lineup]

Originally posted by unclejed613
interesting....
----
the circuit was a MOSFET, cap, resistor and a diode (to discharge the cap when the power was shut off),
but still very similar to the one you have here,
but then again,
i had no need of a shutoff timer.
when simple works well, they call it elegant.....

Because of that MOSFET gate consumes almost no current,
MOSFETs are very well suitable for simple timing circuits.
... is a bit different when using bipolar transistors

Another use of my circuit is short ON period for a light bulb.
Like in Cars.
When you open the car door, a relay is triggered and will keep a lamp operating for a while for you to get orientated/settled in your car.

There are several other uses.
At Power ON of your amplifier is one.

But if you add a simple button/press switch = TRIGGER Button
to recharge my circuit
it will operate for this any time you have chosen.

I refer to the Timing Electrolytic Capacitor = C1

Nice thing with Simulation of this in Spice, is that you get a very good picture of what is happening:
= Transient Analysis, which you see I have used here.
This is not anything critical with use of Spice Models for this.
It is very basic and very accurate


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[lineup]

Using one IRF9610, for example
would switch more than 1 Ampere.

Would probably be a slight difference to
C1 = time in seconds
... if changing to MOSFET with a bit different VGS(on)

[Mooly]

I would use a CMOS Shmidt trigger like the CD4093 etc to drive the FET. Will switch it cleanly on/off. A lot of circuits don't like the supply being "ramped" up. Better for the FET as well. Still zero current consumption as well. You can use smaller values of "C" too