Question about a MOSFET driver

[EasyGoing1]

I'm curious about something that I can't seem to remember ... but in a circuit like this, when the source signal feeding R1 is a PWM signal, what does R2 do besides divide the voltage? Does it have some effect on the MOSFET when the PWM signal goes to 0? In other words, if the source voltage that's feeding R1 goes to 0 and stays there for some time ... does R2 do anything to make sure that the MOSFET stays off?

[stj]

Re: Question about a MOSFET driver

r2 exists to shut the fet off if the microcontroller crashes or cant pull the line low or it has a low-power sleep mode
r1 is a current limit to protect the microcontroller if the fet shorts - you dont actually need r1

[EasyGoing1]

Re: Question about a MOSFET driver

r2 exists to shut the fet off if the microcontroller crashes or cant pull the line low or it has a low-power sleep mode
r1 is a current limit to protect the microcontroller if the fet shorts - you dont actually need r1

So what you're saying in a situation like this where PWM might be used to control the current through the MOSFETs source and drain is that R2 would be more sensical to ensure that the MOSFET turns off in certain situations and that R1 is generally not required?

In a situation where the gate will be receiving either +5 or 0, what are (generally speaking) good values to use for R2? Talking about common Arduino type of circuits with a MOSFET typical like an IR540.

[stj]

Re: Question about a MOSFET driver

if your trying to partially turn on a fet, you wont do it like that.
you either need an analog output from the microcontroller or an integrating cap to convert the pwm into a continuous voltage.
how exactly is a potential "voltage divider" going to turn a 5v pulse into something lower???
it's not.
you need a cap to smooth the pulses out and if the mcu output is rail-2-rail then you probably also want a diode to prevent the mcu discharging the cap every pulse.

[stj]

Re: Question about a MOSFET driver

btw,
what your doing is a bad idea,
most fets are not designed to run in the linear region and will happily cook or explode on you.
you need to select fets designed for linear work and make sure they have apropriate heatsinks for the load.

i have seen so many fets explode or short because people tried to repair sine-wave invertors or expensive audio amplifiers that i'v stopped keeping track of it.

[EasyGoing1]

Re: Question about a MOSFET driver

btw,
what your doing is a bad idea,
most fets are not designed to run in the linear region and will happily cook or explode on you.
you need to select fets designed for linear work and make sure they have apropriate heatsinks for the load.

i have seen so many fets explode or short because people tried to repair sine-wave invertors or expensive audio amplifiers that i'v stopped keeping track of it.

This application will be providing source current to a small heating element. And when I say small, I mean sized for a lizard ... literally lol.

I'd like to use the MOSFET to maintain a specific temp (there will be a temp sensor on another pin on the MC) without using a relay to just turn the element on and off.

[PeteS in CA]

Re: Question about a MOSFET driver

R2 will limit the effect of the Miller capacitance when the MOSFET is turning off. If you are using a driver with active turn off, putting a diode across R1 with the anode at the MOSFET Gate will also help with turn-off and the Miller capacitance.

[eccerr0r]

Re: Question about a MOSFET driver

Chances are he's not running fast enough to need to worry about Miller effect, but the problem he is most likely having is that he's not driving the MOSFET properly. If he's using an IRF540 with a TTL level output, this sure isn't going to work very well.

[stj]

Re: Question about a MOSFET driver

best thing is to use a logic-level fet and pwm the heater element.
keeping in mind the heater is probably a coil so it's probably going to radiate some RF at the pwm carrier frequency.

[redwire]

Re: Question about a MOSFET driver

OP you can think of 50% heater output by turning the mosfet on and off every second. So one second on, one second off. That actually works OK, it's an example of low frequency PWM.
Instead, having a mosfet 1/2 way on works too but the mosfet is wasting the extra as heat, so it is not ideal. Most mosfets are designed only to switch and not be 1/2 way on roasting. Then there is the huge heatsink as well.

If you are needing the heater to have some safety, an extra "master" mosfet is typically used. This one shuts off the heater- it overrides the Arduino and other mosfet if things malfunction. Lizard lives matter.

Your pc board, the connector and only one via aren't enough for say 20A. I use around 1A/via max. The trace looks thick too, they are around 7-10 thou but it depends on the heater wattage and size etc.

[stj]

Re: Question about a MOSFET driver

i saw a teardown of a pc mouse with a built in hand warmer, it was just a bunch of resistors across the 5v.
i think 100ohms - i'll try to find the link

[redwire]

Re: Question about a MOSFET driver

OP, about how many watts of heat is needed?

[eccerr0r]

Re: Question about a MOSFET driver

both these threads probably should be merged...this and https://www.badcaps.net/forum/showthread.php?t=114874 ... same topic pretty much.

Mainly because these must go hand in hand.

I suspect this probably needs to be on the order of at least 100-200W to even be somewhat useful and possibly even 500W depending on the cage size.

[stj]

Re: Question about a MOSFET driver

no, he wants to create a heated surface not heat the whole vivarium.

[EasyGoing1]

Re: Question about a MOSFET driver

This thread was moved to this thread. I thought I posted in here somewhere that I was starting a new thread about this but I don't see that post. My apologies about that.