PWM circuit for HHO generator

[toolate]

Hi

I'm wanting to make a simple PWM circuit for a Hydrogen generator which will be for the car.

I am thinking of making cpemmas PWM circuit http://www.cpemma.co.uk/pwm_erg.html and use a IRF1405N MOSFET http://www.datasheetcatalog.org/datasheet/irf/irf1405.pdf, would I need to change the base resistor value?  Are there any other recommended adaptions to do to give it years of operation in the car? like maybe smoothing caps???  volatage reg or something???

I assume I'll need a good heat sink if running at or near 20A but I think it will be more like 1 - 10 A  How would I find the power dissipation when going at 20A?

your thoughts and feedback are very appreciated! 

[justDIY]

I'm not an expert with switching, but i'll relay the few things I've learned - hopefully others will point out in flaws in my understanding.

the key to switching efficiently with a mosfet is driving the gate "hard" and "fast".  A mosfet can conduct huge currents with very little loss, but only if it's kept fully on.  A long transition between off and on at a considerable current can lead to catastrophic heating of the mosfet and subsequent destruction.

Driving a mosfet "hard" implies using a high gate voltage.  The datasheet should break it down for you what the ideal gate voltage is for a given load current - typically the higher the current, the higher the gate voltage should be.

Driving a mosfet "fast" implies the logic controlling the gate drive voltage should go from low to high to low again as fast as possible - this is typically a "totem poll" or "push-pull" driver, where the gate voltage is actively forced on and off, instead of just pulsing it on and letting a resistor pull it down to off.

You should google mosfet drivers or mosfet gate drivers - there are some nice chips out there that take a logic level (3 to 6vdc) control signal and efficiently switch a large mosfet with it.  The fancy ones even employ charge-pump drivers to kick up the gate voltage so you can use a beefy and inexpensive N channel fet to switch the high side of your load.

As for generating the PWM pulse-train, you could start with a simple 555 based circuit, or a 556 (dual 555)  if you need more flexibility in the duty cycle, or you could look into a microcontroller, which would let you change frequency and duty cycle easily with either buttons or changing a few lines of code.

The faster you want to switch at, the more losses you'll incur in the switch and the more heatsink your fet will need.  A gate driver can help a lot with this, especially trying to switch 20 amps.

[toolate]

Thanks for the reply!

Ok I'll start with a 555 circuit.  though will I be able to get good duty cycle control?  I'm getting into Picaxe programing but I'm still at the beginning with writing code, so I think it would be over my head! also I think the PWMOUT can only go as low as 4Khz and I want to switch it some where between .1 - 1Khz.  Would I still need a driver for this low speed?

Basically I want a circuit that I can get full control 1-20A of the current going to my HHO cell.

I also bumped into this good mosfet info http://homepages.which.net/~paul.hills/SpeedControl/Mosfets.html

Thanks a lot!

[justDIY]

There are several free 555 circuit simulators out there - download a few and you can try out different things before you build anything.

The picaxe is an extremely limited development platform, the pwmout command is probably software based, and has to work within the constraints of the on board interpreter.  programming a PIC or AVR based microcontroller directly would free you from those constraints.  Consider the audrino platform - it's very popular hardware right now and has a powerful, free, c-like development enviroment and compiler available.

[toolate]

Thanks, I'll do some testing and see how I go!