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I would like to use a 555 timer to operate a 12V heater which draws about 13 amperes.

Would using two MOSFETs in parallel connected to the 555 output be a good solution to improve output power compared to one MOSFET? I have a couple of 2SK1941 spare with a rating of 16A.

I was looking at this earlier.

  • Would simply using one resistor from each MOSFET to the 555 with maximum 200mA output be ok?
  • Should I have the 555 output trigger a transistor with a higher power rating to drive 2 MOSFETs?
  • Is it going to be very complicated to split the load evenly between the MOSFETs?
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  • \$\begingroup\$ The FET has Rds(on) of about 0.4 ohms. Your load is about 0.9 ohms or about 150W. Combining them in series makes a 1.3 ohm load, so you get about 9A curret. At 10A current, there is about 4V over the FET and the rest over your load. The FET would heat at about 40W. Even if you parallel two FETs, it does not help much. \$\endgroup\$
    – Justme
    Commented Jul 29, 2020 at 11:51
  • \$\begingroup\$ You can parallel mosfets and the 555 can drive both, at a low switching frequency to keep switching losses low. But your mosfet is a poor choice for this application. The commonly available IRFZ44 is a much better choice (even a single mosfet will give less than 10 watt dissipation). \$\endgroup\$
    – Indraneel
    Commented Jul 29, 2020 at 12:10
  • \$\begingroup\$ Thanks for your help \$\endgroup\$
    – Hobbyist
    Commented Jul 29, 2020 at 12:11
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    \$\begingroup\$ Parallel fully on MOSFETs do play nice together, in the way that bipolars, diodes, or current limiting MOSFETs don't. As the answers have pointed out, far better to start with a good MOSFET, paralleling several lousy ones is a waste of time and space. \$\endgroup\$
    – Neil_UK
    Commented Jul 29, 2020 at 12:18
  • \$\begingroup\$ How about one of these: PSMN0R9-25YLC alltransistors.com/mosfet/transistor.php?transistor=10323 Looks like it's much better suited for my purpose, and that just one should be capable of much more than what I need, and the power loss will be almost nothing! Would it need much of a heatsink to run at around 40-50c with a 13amp constant load? \$\endgroup\$
    – Hobbyist
    Commented Jul 29, 2020 at 13:14

2 Answers 2

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No, the 2SK1941 would be a poor choice due to it's high on-resistance. If you paralleled two devices and assumed that both would take 6.5 amps each then the forward conduction characteristic would be like this: -

enter image description here

I added the red lines.

With 6.5 amps through one device, the volt drop would be about 2.2 volts and the power dissipation would be 6.5 x 2.2 = 14.3 watts. Using two devices means a total power dissipation of over 28 watts.

That's a lot of heat to get rid of and 28 watts represents nearly an 18% waste of heat and that's pretty poor for this type of circuit. I would certainly choose a better MOSFET and you'll probably only need one device.

You should be able to locate one with an on-resistance around 5 milli ohm and that means a power loss of 13² x 0.005 = 0.9 watts.

Is it going to be very complicated to split the load evenly between the MOSFETs

That's always something to consider and can be problematic hence why I suggest one MOSFET and much higher efficiency.

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    \$\begingroup\$ Thanks for your help \$\endgroup\$
    – Hobbyist
    Commented Jul 29, 2020 at 12:11
  • \$\begingroup\$ @Hobbyist no problem. \$\endgroup\$
    – Andy aka
    Commented Jul 29, 2020 at 12:17
  • \$\begingroup\$ Andy is a big helper. \$\endgroup\$
    – Helena Wells
    Commented Jul 29, 2020 at 15:22
  • \$\begingroup\$ How about one of these: PSMN0R9-25YLC alltransistors.com/mosfet/transistor.php?transistor=10323 Looks like it's much better suited for my purpose, and that just one should be capable of much more than what I need, and the power loss will be almost nothing! Would it need much of a heatsink to run at around 40-50c with a 13amp constant load? \$\endgroup\$
    – Hobbyist
    Commented Jul 29, 2020 at 20:17
  • \$\begingroup\$ Yeah, that's a beast of a device and should work fine without a heatsink but on the PCB, give it as much copper surface area as you can. It should have recommendations in the data sheet for the copper area but it looks good @Hobbyist \$\endgroup\$
    – Andy aka
    Commented Jul 29, 2020 at 20:23
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If you want to draw that much current continuosly you will defintely need at least two of those MOSFET and also very good heat dissipation on the MOSFETs. Using only the MOSTFET without a heatsink will burn them. Even so you should look for a MOSFET with less Rds

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  • \$\begingroup\$ Thanks for your help \$\endgroup\$
    – Hobbyist
    Commented Jul 29, 2020 at 12:12

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