IRF7319 Mosfet latching switch

Thread Starter

dumpystig

Joined Apr 10, 2013
23
I have breadboarded the circuit below but can't get it to work as intended. As soon as power is connected the load (an LED just for test purposes) illuminates and stays lit. Operation of the momentary switch doesn't have any affect. I have transposed the Mosfet pin numbers from the datasheet to the diagram in red, but think I may not have done it correctly.
Can anyone help me out with this?
 

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Dodgydave

Joined Jun 22, 2012
9,064
Remove the capacitor, at switch on it will be discharged and will conduct to put the P-channel mosfet on (pins 3-5), also connect the switch in place of the capacitor.
 

Thread Starter

dumpystig

Joined Apr 10, 2013
23
Thanks for the quick reply. Doing as you said doesn't have the desired result - the LED is still illuminated when power is applied, although the switch does turn it off until released and then it's lit again.
 

MikeML

Joined Oct 2, 2009
5,444
The load cannot be a LED in-series with a resistor. The load must be have an effective resistance of 1000 Ohms or less.
 

Alec_t

Joined Sep 17, 2013
11,105
The circuit is sensitive to the turn-on threshold voltages of the FETs.
In your original circuit try increasing the 100k resistor value (e.g. to ~200k)
 

Dodgydave

Joined Jun 22, 2012
9,064
Thanks for the quick reply. Doing as you said doesn't have the desired result - the LED is still illuminated when power is applied, although the switch does turn it off until released and then it's lit again.
Did you move the switch as i said earlier????
 

Thread Starter

dumpystig

Joined Apr 10, 2013
23
Yes, I removed the cap and put the switch in its place.

Have I transferred the pinouts from the datasheet to the circuit diagram correctly?
 

MikeML

Joined Oct 2, 2009
5,444
Is that with reference to the original diagram or to the ammended suggestion by Dodgydave?
The original circuit.

I simulated it, and posted the results of the simulations on ETO under the title "Momentry Power Switch Troubles"
If loaded with just the LED, the forward drop of the LED screws up the circuit. The impedance of the load must be low enough. The circuit was intended to switch a good fraction of an Amp...
 
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crutschow

Joined Mar 14, 2008
24,701
Remove the capacitor, at switch on it will be discharged and will conduct to put the P-channel mosfet on (pins 3-5), also connect the switch in place of the capacitor.
The capacitor is needed for the alternate action latching operation. The switch is momentary.
 

Thread Starter

dumpystig

Joined Apr 10, 2013
23
Thanks guys, adding a 1K resistor across the load has got the original circuit almost functional; but still when initial power is applied the circuit is on instead of off, although subsequent switch presses work as intended.

What about changing the cap value? Incidentally, the cap I'm using is a ceramic that I bought specifically for this build and I don't have a stock of any other values - would an electrolytic (polarised) suffice, providing I have the correct value?
 

RamaD

Joined Dec 4, 2009
328
While switching on, the capacitance across the DS of the P MOSFET, makes the N MOSFET to conduct, turning on the led!
Maybe, a 10kpF cap on the GS of N MOSFET should limit the gate voltage from turning on, assuming 320pF of Coss.
 

crutschow

Joined Mar 14, 2008
24,701
My simulation shows about 10nF across R1 causes the circuit to start in the OFF state. Much larger or smaller value and the circuit doesn't work properly. You may have to experiment for the optimum value.

A polarized type will work fine for the bottom capacitor.
 

Thread Starter

dumpystig

Joined Apr 10, 2013
23
I've sorted the output side of the circuit by using a relay to switch the load. The relay coil is 160R so seems to be of low enough impedance for the circuit to operate almost as intended. But there's still the problem of the circuit being on when power is applied instead of off.
 

Thread Starter

dumpystig

Joined Apr 10, 2013
23
While switching on, the capacitance across the DS of the P MOSFET, makes the N MOSFET to conduct, turning on the led!
Maybe, a 10kpF cap on the GS of N MOSFET should limit the gate voltage from turning on, assuming 320pF of Coss.
Cheers, that did it. As I only have a few values scavenged from old boards I had to try what I had. I couldn't find a 103 (0.01uF) with long enough leads but it turned out that it works with a 104 (0.1uF).

So thanks to you guys for helping me out on this, you have provided some very expert knowledge and I'm grateful.

I've also attached an updated diagram.
 

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