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Trouble activating a mosfet with a 555 PWM circuit

Braeden Hamson

Feb 18, 2016
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I'm using a standard PWM circuit like so:
https://cdn.sparkfun.com/assets/7/d/0/2/4/52433239757b7fb7798b4567.png
The timer circuit is powered to ~4.25 v with an LM317. The load which at this point is some LEDs is being powered by an IRLZ44N mosfet. Both circuits work, just not together. When put a 3v LED on the output of the 555 IC and connect it to ground it blinks away happily and I can control the duty cycle no problem. When I connect the gate of the mosfet to Vcc the lights turn on as expected and off when I connect the gate to ground. With I connect the gate to the output nothing happens.

Mid post edit, I ran the timer circuit on 5.2 v and everything worked fine. But what I really don't get is that I touched the Vcc with my hand and then touched the gate wire and everything turned on, then I touched ground and it went off. I can use my body to turn the mosfet on, but not a timer IC which can turn on an LED running at a low voltage. The data sheet for the mosfet says the gate threshold voltage is min 1 v and max 2 v so I'm pretty confused.

Save you a google: http://www.mouser.com/catalog/specsheets/international rectifier_irlz44n.pdf

Post post edit: It doesn't work all of a sudden I have power and a good output the transistor cycles with VCC and ground it works. When I connect the gate to Vcc then to the output of the 555 the lights turn off. Mosfets are weird...
 
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Harald Kapp

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Mosfets aren't that weird.
You do have a common ground for the 12 V circuit (Mosfet, LEDs) and the 4.25 V circuit (555)?
You haven't mixed up source and drain of the Mosfet?


By the way: you may want to reduce the value of R3 to a much smaller value, e.g. 100 Ω. Even if a Mosfet requires no static gate current to operate, a comparatively large current pulse will be needed to charge the gate-source capacitance which in this case is typically 1.7 nF. With R3 = 10 k the time constant of the resulting RC term is 17 µs -.> the Mosfet is turning on and off rather slowly.
 

Braeden Hamson

Feb 18, 2016
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I got to the bottom of it. I was attempting to do high side switching ( I think it's called ) the transistor was on the positive side of the load trying to switch the positive voltage. I switched it to the ground side and it worked fine. I'd like to know why that matters.

And I don't understand why the small amount of power through my body was enough to trigger but the voltage from the 555 wasn't.
 

Harald Kapp

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I was attempting to do high side switching ( I think it's called ) the transistor was on the positive side of the load trying to switch the positive voltage
It's kind of difficult to check a circuit when the diagram doesn't show the actual setup :(.

And I don't understand why the small amount of power through my body was enough to trigger but the voltage from the 555 wasn't.
You don't need much power. When you touched Vcc (I assume that's the 12 V) your body charged the gate to 12 V which is sufficient to turn on the Mosfet even in your high-side switching application - albeit not fully as the source will be at Vsource = Vgate - Vthreshold = 10 V .. 11 V depending on tolerances.
When you operated the 555 circuit from 5.2 V, the gate voltage to the Mosfet was also near 5.2 V which in turn provided approx. 3.2 V ... 4.2 V at the source, just enough to power your 3 V LED.

Also: your circuit lacks the required current limiting for the LEDs. You need to add a resistor between the drain of the Mosfet and the LED string to limit the current in the on state of the Mosfet.

Mosfets are weird...
In that case it's not the Mosfet that's weird, it's your circuit and the fact that the schematic diagram doesn't match the real circuit. Show us a diagram of the real circuit if you have further questions, please.
 
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