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Delay off Circuit 12V - 3A? Mosfet or Relay? Answered

Hi all,
I want to make a delay off circuit 12V - 3A. When the switch is ON, the circuit will operate during 1 to 3 minutes, and when the switch is OFF, the circuit should work during 0.5 second before it turns off.
I made a delay off circuit using NE555 and a mechanical relay and it works. However, I need to make a smaller, cheaper and more efficient circuit.
I need your help and your advice to gain knowledge and improve my circuit. Thank you.



9 months ago

The trick is to use the 12V to power the 555 and self disconnect after time out.

Ckt to follow if I have time soon.

Here is a circuit...

Start the 3 amp load with S1 which applies voltage to the 8A $1.19 mosfet gate through Q1 that latches the ISO LED..

When you push S2, U1 starts a .5sec timing pulse that trips U2 after it goes low..

U2 turns ON Q2 which turns off Q3 by shorting the mosfet gate voltage to near zero.

That raises Q3 drain voltage high to about 12 VDC

which turns off Q3 and both 555s and Q1...


You may have to adjust R1 & R2 values..

R2 is necessary to discharge a tiny leakage coming from Q1 NPN emitter because the gate-source of Q3 is a small capacitor which would charge and turn-on the Q3 Mosfet from a small current leak...

That circuit appears considerably more complicated for what it needs to do. IRF840 may not be the ideal mosfet to use, as this is a high voltage FET with considerable on-state resistance. My goto fet is the PSMN027, which is 100Vds, 48mOhms on resistance, and only costs a buck.

Yea, you save 19 cents and I agree a lower voltage like the PSMN027 is warranted, but I had the 840 in hand just in case someone (like you) might challenge me..

Of course a tiny micro could do this simpler but then the OP would not have mentioned the 555... Familiarity with the 555 is very broad even though a single 556 can replace both of those ubiquitous 555s.

Now then, this circuit does totally disconnect itself from power draw which is a real PLUS in the design... Where a micro would need a lower voltage supply and That rates a Minus..

Any way, this CKT is original from 2HRs of my own work..

Maybe someone (like you) will do a better design after considering these design constraints..

Chalange accepted! :)

After thinking about it for 5 minutes, I figured my approch would be to use 2 transistors, a P channel FET (IRFP9530) and a NPN (2N2222). The FET is the pass element on the HIGH side, with a 100k pullup. The collector of a NPN reansistor connects here, with the emitter connected to an LED (or zener diode) which then goes to ground. The output is switched on when Vb exceeds the LED or zener voltage plus 0.6V. The base goes through 1mehaohm series resistor and connects to an parelell RC network to ground

When you want the output ON, simply charge up the capacitor by connecting it to VCC. Leave it connected to power the load. Then when switched off, the capacitor discharges and eventually discharges below the cutoff voltage.

This circuit, too, draws essentally no current when off. In fact, I was using a 470uF capacitor, which had 1uA of leakage. Once removed the current went to zero. If an LED is used, it additionally serves as a useful indicator.


Very well done, cheaper and vastly superior...

I will go now...


Thank's Icing, I'll take that as a compliment. The design was largely inspired by Dave's simple latching circuit.

It has it's problems, I think I need to add a bit of positive feedback on the BJT to get some hysteresis for a sharper, better-defined turnoff. Adding a 1Mohm resistor between the base and output however causes the circuit to latch. Also P channel FETs aren't the best in terms of the ON resistance, and the timing is dependent on the input voltage. Replacing the BJT with a comparator and the LED with a voltage divider I think would resolve most of these outstanding issues.

It was a well deserved compliment.. And I forgot to mention kudos for using a single switch...

And here is an inverted version of your circuit using your favored N FET and a Darligton PNP and Yes, hysteresis would be desirable...


dry hard to get a cheaper option than the 555 you might be able tofi d a cheaper relay or use an FET see on the right>>>>>>>>>>>>>