Drive a MOSFET is harder to drive car

Discussion in 'General Electronics Chat' started by oookey, May 25, 2010.

  1. oookey

    Thread Starter Member

    May 24, 2010
    Hellooo everyone! I'm first time here.:)

    I got a problem in MOSFET, hope to have some advice & guidance.

    Please refer to the attached schematic(zip file), I’m using a IC (555 timer) pin 6 signal & an Op amp (LM358) to produce PWM signal hence to control the brightness of strings of 1W white(color) LEDs, through a MOSFET (BUZ11), the components are mounted on breadboard, except the LEDs & its branch resistors are soldered on Vero board.

    I’m expecting total 1.7A of current flow thru the entire branch of LEDs (0.34A per branch x 5 branches), however only 0.16A of total current flowing thru, at 100% duty cycle.

    So far I’ve tried using MOSFET IRF510 & IRF540, could not get the expected current, & the frequencies generated by 555 timer tried are 140 kHz; 100 kHz; 10 kHz 5.2 kHz.; & now 200Hz I attached a snap shot of waveform signals at pin 6 of 555 timer & at MOSFET gate, oscilloscope sweep time/div: 1msec, volt/div: 2V. The total branch current measured was 0.08A at this instant. :(

    Thank you for your time.:)
  2. SgtWookie


    Jul 17, 2007
    Sorry, but an LM358 will make a terrible MOSFET gate driver. It is a very old and slow opamp, and has limited current output. Thankfully, that 30 Ohm resistor has kept the gate held low so your MOSFETs haven't been killed yet.

    Do it like the attached instead.

    R3 can be reduced down to around 20 Ohms if a faster gate switch time is desired.
  3. oookey

    Thread Starter Member

    May 24, 2010
    Thanks for the info! :)

    I patched up the ckt, it still did not fetch 1.7A of total current, even at 100% duty cycle, I merely got 0.11A. Both BUZ11 & IRF510 produced similar result.

    One observation I noticed when hookup the LEDs to the IRF510 without connect the gate, there still a total 20mA passing thru it (LEDs light up) when power was on, but the BUZ11 no current flow, I must have cooked the IRF510. :mad:

    Am I too greedy to drive so many 1W LEDs at one go? Please advice.

    Thank you for your time. ;)
  4. John P

    AAC Fanatic!

    Oct 14, 2008
    I don't like the look of those 0.5 ohm resistors, R9 in the diagram. The actual voltage drop that you get through an LED is very hard to predict, and 0.5 ohm won't do much in the way of limiting current. Meanwhile the IRF510 itself has over .5 ohm on-state resistance, and you're expecting it to drive multiple LED strings in parallel. By the way, the 78S12 can supply 2 amps max, assuming it doesn't overheat. It's not possible that you're overloading it, is it?

    An LM358 ought to be able to drive the MOSFET gate, but you can't expect it to switch very fast. Just keep the chopping rate low, if you want to use it.

    Here's a question: what is the current through the LED strings if you just connect them across your 12V power supply (with the resistor, of course!) and no MOSFET at all? How consistent is one string with another? The total current there is what your FET must carry, and you can calculate its power dissipation.
  5. SgtWookie


    Jul 17, 2007
    Your 555 pin 3 waveform is 8v peak-peak. If there is no load on the 555 output, that indicates you have about 9.3v available from Vcc to GND at the 555.

    However, you're only getting 5.4v peak at the gate. Are you certain that you have a 10 Ohm resistor from the 555 out to the gate, and a 10k Ohm resistor to ground? I think that you may have used different values than you show on the schematic.

    Let's back up a bit.

    What is the rating of your transformer? Output VAC @ current, or voltage and VA or Watt rating?

    Measure the voltage across your filter cap on the output of the bridge. What do you measure there?

    1,000 uF is a rather small cap for a supply filter.

    The 78xx series regulators are rated for 1A continuous, 2A absolute maximum peak. They get hot and shut down if they dissipate too much power. They are not efficient.

    PWM alone is not sufficient to regulate current delivered to LEDs. The maximum current must be limited to keep current within the LED specifications.

    What is the specified typical Vf @ current for your LEDs?
    The 1W specification is important. However, that causes the allowable current to change with the LEDs' Vf.

    I=P/E, or Current(Amperes) = Power(Watts) / Voltage(Volts)
    Conversely, P=EI, or Power(Watts) = Volts * Amperes
    So, in order to not exceed the 1W power rating, you need to know what the voltage across the LED is at a certain current.

    You will need to have your LEDs mounted on a heat sink to dissipate the 1W power, or they will overheat and burn up.
  6. oookey

    Thread Starter Member

    May 24, 2010
    I have changed the 78S12 to 78S15, because I realized that the 4 LEDs in series would be 13.6V.(3.4V x 4pcs) I choose 0.5 Ω resistance is to provide low drop in voltage & maintain constant forward current thru the LED strings. I ref to the datasheet, this regulator’s short circuit peck current is 3A , I suppose when it’s overdrawing the max current, couple with proper heat sink & cooling, the thermal overload protection will not activate.

    When directly connect the string of LED to the 15V supply, the total current is 1.723A.(without going 555 timer & MOSFET.), so obviously the branch current is ~0.34A.

    I hooked up again to the timer & MOSFET, power up adjust the total current thru LEDs to 1.0A, the voltage drop across the 0.5Ω is 0.1V ~ 0.14V, across LED is 2.4V~ 2.7V.

    For the transformer, originally it’s a center-tap model, I used half of the winding to get 18Vac, if measure across u would get 36Vac. The transformer current rating is 2A.
    The resistor from pin 3 to gate its confirm a 10Ω, but now I changed to 27 Ω, I worry to cook another MOSFET. I’m using the veroboard surface as the heat dissipation media, so far it’s working fine.

    As pointed out by John, there is an “on-state” resistance at the drain terminal, will this play apart in restricting the MOSFET from fetching the max current? Perhaps t should use smaller series resistors in the string circuit. Please ref to the file for attached snap shot of waveforms.

    Thanks for the advices & time. ;)