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Thread: Single chip solution pwm

  1. #1
    alpha-dog Guest

    Default Single chip solution pwm

    This is a TL494 pwm I've built for the P-Channel mosfets. It will work with the N-channel mosfets also but you need to reverse pins 15 and 16 inputs.


    This pwm does everything that the ZFF does and more. Although I only show here current limiting, duty cycle adjust, and PRF ( frequency ) adjust, there's also Dead time and a control that allows for two outputs with 48% max duty cycle at 180 degrees out of phase.
    Last edited by alpha-dog; 04-29-2009 at 06:10 AM.

  2. #2
    drumacc Guest

    Default bigger

    This post is 6 months old, I know, but do you have a larger version of your sketch? Can't make this one out...

  3. #3
    alpha-dog Guest


    This should be larger


  4. Default ?

    Sorry still cant see the values of the resistors and such. Looks great from what i can make out tho. Any chance you can make one slightly bigger again?

  5. #5
    alpha-dog Guest


    It's really big on my machine. I've had this problem before on this site.
    If you want to build one of these, I get my components from digi-key.

    digi-key part number

    1 $.52
    100k trimmer pot
    1 $ .86
    10k trimmer pot
    2 $1.64
    5.1k resistors
    5 $.32
    1k resistors
    5 $.32
    220 ohm resistor
    5 $.32
    .1uf cap
    .01uf cap

    tl494 pin-out
    pin 1 connect center conductor 10k trimmer and then one side of pot to grnd and the other side to Vref ( pin 14 )
    pin 2 connect to 5.1k resistor to pin 3 and connect another 5.1k resistor from pin 2 to grnd
    pin 3 connect 5.1k resistor from pin 3 to pin 2
    pin 4 connect to grnd
    pin 5 connect to .1uf cap ( Ct cap used in Osc. ) and connect other side of cap to grnd
    pin 6 connect a 1k resistor: connect 1k resistor to 100k trimmer pot connect center conductor of pot to one end conductor then end conductor to grnd
    pin 7 connect to grnd
    pin 8 Vc output connect to pin 11
    pin 9 Ve output connect to grnd
    pin 10 Ve output connect to grnd
    pin 11 connect to 220 ohm resistor and other end of resistor to pin 12: connect to .1uf cap and other end of cap to grnd: connect to 1k resistor
    and other of resistor to grnd
    pin 12 connect to +12vdc filtered power ( pwr run circuit used by ECU is already filtered, this would be switched power )
    pin 13 connect to grnd
    pin 14 Vref output to high side of both 10 k trimmer pots
    pin 15 to center conductor of 10 k trimmer pot: one side of pot to grnd and the other to Vref
    pin 16 to Rshunt ( 8" of 12 guage solid conductor wire in grnd path of electrolyzer )

    I us the breadboard from radio shack ( 2 boards in one package )
    I also use there project box and terminal board

    When setting up your pwm adjust pin 15 to about .5vdc and pin 1 to about 2.3vdc. You can adjust pin 6 to ground for 101k ohms for the lowest freq.
    Make sure that pin 15 is a higher voltage than pin 16.
    pin 1 sets your duty cycle and looking at pin 3 you can see a dc output from pins 1 and 2. Pins 15 and 16 make up the current limiting comparator. You will adjust pin 15 once the system is hot.

    If you would like to try one of these but don't want to build it yourself, PM me about building you one. I can also make it work for N-channel mosfets, but it is for P-channel mosfets.
    Last edited by alpha-dog; 05-01-2009 at 10:26 PM.

  6. Default


    I'm new here, and would like to ask whether anyone was able to build this pwm with N-channel mosfets and ask his/her opinion/feedback about that pwm. I have built it with N-channel, and reversed the 15 and 16 pins, I could set .5vdc on pin 16 and 2.3vdc on pin 1 and could also set the frequency, but still I cannot regulate the current as i wish. Neither the freq. nor the duty cycle nor the current can be set or regulated. The HHO generator always works on maximum current,(as it is getting hot, it takes up more and more current) and the current can't be regulated.
    If anyone can help me, please answer soon.

  7. #7
    Join Date
    Jan 2012
    Mississauga, ON, Canada


    The P-ch FET is driven as a high side switch with a Vgs of >10V.
    It goes fully ON and has very little voltage drop to reduce power dissipation.

    This is GOOD

    The problem with just changing the P-ch FET with an N-ch FET is that the N-ch FET would only be a source follower, not being driven to a fully on state.

    This results in a drain to source drop of 3-5V @ whatever current the load would draw.

    Lots of heat.

    (and much lower efficiency)

    This is NOT GOOD

    The correct use of an N-ch FET would be to place it in the return (or minus)
    side of the load.

    The current sense wire would be placed in the N-ch FET's source connection to "gnd"

    This will allow the gate drive to be >10V and it would act as a low side switch.

    It goes fully ON and has very little voltage drop to reduce power dissipation.

    This is GOOD.

    Hope that helps

  8. Default

    Thanks for your help, but the problem isn't that, the N-FET is connected just as you advised. The problem with it is that it is hard to set the current with it, I just set a little on the pot, and that results a big current change. How can I set this much accurately?
    Thanks for your help in advance.

  9. #9

    Default Adjust series resistance

    The error amplifier input ( pin 15 as told) if set at .5 volts will need quite a lot of amperage through the MOSFET. 8" of 12 gage wire will equal about 0.001 ohms. To switch the TL494's internal comparator/error-amp to current limiting mode, you will need 0.5/.001 = 500amps and the 12 gage won't handle this. Maybe a typo, the voltage @pin 15 should have been 0.05V for about 50amps limiting. But again the pot's resolution will be bad in this case.

    One way of tackling is, instead of the 10K pot shown for current limit, a 1k pot in series with 100K will allow better control if you are intending to use lower amperages. Vref from pin14 is 5V, now the 1K pot's wiper-swing will allow 0-0.05V adjustments (better resolution) to be made and hence provides better control/resolution. I just completed a circuit and have used 7" of 20 gage copper wire. This is for an HHO cell meant for a 125 cc scooter and wanted to limit current at about 4~5 amps.


    Hope the concept is clear.

    RTJ Nair

  10. Default

    Thanks a lot, I'll try it.

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