Soft Latching Power Switch Help #262

[will900]

Hi all, I posted this question as a response to an earlier post but not sure if that buries it so thought I'd post as a new topic, I hope that's not bad forum etiquette.

I'm trying to make the soft latching circuit from project #262, I've attached an image.

I've had to replace the IRF9110 with a (hopefully equivalent) MOSFET, I've used an IRF9160 (datasheet attached) as from googleing this is the replacement for the IRF9110.

What I can't understand about the circuit is why it would turn on when power is supplied first. I would have thought that as the gate is tied high at first it would be off then when the switch is pressed and the gate of the left transistor is brought high and it makes a connection to ground it would switch the MOSFET on and latch?

The problem I am having is that when I first apply power to the circuit the output is off, when I press the button it switches on and latches, then that's it, it stays latched on even when the button is pressed. I have to make a direct connection to ground, not through the transistor to turn it off.

The only difference between my circuit and the one in the video is that I'm using the IRF9160, my power source is 4x 1.5V AAA batteries, I used two initially but this did nothing.

Any help would be very much appreciated.

[Zero999]

Someone did respond to your other post and I can't see the image you say you've attached, perhaps you forgot?
https://www.eevblog.com/forum/beginners/understanding-values-chosen-for-eevblog-262-world's-soft-latching-switch/msg1013588/#msg1013588

If it's assembled correctly, ut should be off when power is first applied. Could it be that there's a capacitor on the circuit connected to it which is remaining charged, cause it to turn on after the power is disconnected and reconnected?

[fubar.gr]

Check your MOSFET. They can be easily damaged by ESD. Try swapping it with a new one.


I made a similar circuit the other day on a breadboard.

I noticed something weird, when I pressed one of the pushbuttons, one of the LEDs would dimly turn on for the duration the button was pressed.

That should be impossible. The button and the LED had nothing to do with each other. Yet, pressing the button did indeed generate a voltage at the LED's anode. I spent the next hour reviewing my schematic and the actual circuit, to no avail. The LED shouldn't come on.

What tipped me off was the fact that the problem got worse over time and the LED would now light up brighter.

The culprit was a p-type mosfet with a leaky gate. The gate was on the same node as the LED anode, so it was backfeeding the LED.

I swapped the MOSFET with a new one and then everything was working as it should.