I am playing around with various configurations of a mosfet load switch and have stumbled across a problem i dont quite understand.
Ive attached the circuit below. Basically its a very simple P mosfet load switch. I dont understand why the mosfet does not turn on at the value of the zener. As soon as the zener conducts it should pull the gate low with respect to the source pin and the mosfet should switch hard on. However in reality it actually switches on at around 11 to 12 volts.
I am using an led as a simple load so its hard to see when it actually does switch on because there is no snap action, only a gradually increasing brightness until its fully on.
the mosfet will have some conduction at any gate voltage, with the small current requirement of the led you will easily pass enough current to switch it on. the value that the datasheet reports is the gate voltage required to fully switch the mosfet on to it's minimum resistance
It will switch at the MOSFET's threashold voltage plus the zener voltage. The turn on will be slow and the voltage drop across the MOSFET will decrease as the current rises, untill it's fully on, then the voltage across it will increase with the current.
Ive attached the circuit below. Basically its a very simple P mosfet load switch. I dont understand why the mosfet does not turn on at the value of the zener. As soon as the zener conducts it should pull the gate low with respect to the source pin and the mosfet should switch hard on. However in reality it actually switches on at around 11 to 12 volts.
It sounds like you expect the zener voltage to somehow drop once the zener voltage is reached. The doesn't happen. The zener does not pull the gate low. It conducts with a constant voltage drop across it (it looks much like a 9V voltage source). Thus the mosfet will start to slowly turn off as the voltage is increased above 9V plus the mosfet threshold voltage.
If you want the mosfet to fully turn on a 9V then you will need to add a comparator circuit to drive the mosfet.
thanks for the responses. I made the post because i didnt understand what i was seeing on the bench.
Ive actually posted a similar thread about a month or two ago and Mike answered with a very similar circuit. I dont see the difference between his and mine only the zener he has chosen is one of theose adjustable ones. His has hysterisis but i dont really see how that makes it any different to the circuit i posted.
Ive actually posted a similar thread about a month or two ago and Mike answered with a very similar circuit. I dont see the difference between his and mine only the zener he has chosen is one of theose adjustable ones. His has hysterisis but i dont really see how that makes it any different to the circuit i posted.
I'm sure it depends on the MOSFET and the load current.
That's also a pretty slow turn on, if the voltage rise/fall is very slow 100mV/min then that MOSFET is going to burn up when it passes through the saturation region. Adding hysteresis will make it so it's not possible for the output switch to be half on and half off which would be lead to meltdown.
thanks for the responses. I made the post because i didnt understand what i was seeing on the bench.
Ive actually posted a similar thread about a month or two ago and Mike answered with a very similar circuit. I dont see the difference between his and mine only the zener he has chosen is one of theose adjustable ones. His has hysterisis but i dont really see how that makes it any different to the circuit i posted.
You missed the critical point: There is much more to an LM431 than a Zener. The LM431 is used as an open-loop comparator with a voltage reference (2.500V Band Gap) connected to one of its inputs. As the voltage at the tap of the voltage divider (REF input to the LM431) swings from a mV above 2.5V to a mV below 2.5V, the LM431's Cathode suddenly switches from low to high, switching off the PFET with only a couple of mV of input voltage change..
Look at the equivalent circuit of what is inside an LM431:
Are you talking about R4 and R5 on my previous reply to this thread?
R4 limits the base current and isn't needed with a MOSFET because its gate is an open circuit, R5 is a pull-up to ensure the transistor turns off and serves the same function with a MOSFET.
Andy used a symbol for a NFET, but labeled it PFET. In my circuit, it is a PFET with the source connected to the + input. The LM431 based circuit has no intentionally added hystersis, but the opamp has an open-loop gain of over a 1000, so the circuit switches from On to Off with a very small change in battery voltage.