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I have designed a circuit to run a 555 as an astable multivibrator. The values for the oscillator are a 4.7 uF capacitor, a 1k resistor, and a 10k potentiometer. For the capacitor I will use an electrolytic cap, so the value is not exact. This should give an oscillation frequency of no more than 500 Hz maximum, with a duty cycle between 50% and 99%.

The intent of this is to use it to control N-channel MOSFETs for PWM-ing brushed motors. The motors are vacuum cleaner motors, up to 12 amps each. The MOSFETs I have are IRFP460A. I'll need to use rectified mains to do this. This circuit should be testable with a lower voltage, although I'll need to use a dummy load.

The circuit also includes a bridge rectifier constructed from 1N4007s and a 7812 regulator. This will be driven from a small transformer with an output voltage of 20 VAC. This gives a voltage to run the 555, as well as control the gates of the MOSFETs. Since the 555 is operating from +12 VDC, there should be no need for a level shifter. The 555 is not directly operating the gates, I have a 2N3904 that is used to amplify the output of the 555.

Each MOSFET has a 1000 ohm pull down resistor between its gate and source. The gate is driven via a 10 ohm resistor. This means that turn on should be relatively fast but turn off is not as rapid. I don't think this is an issue below 500 Hz. If I have a 99% duty cycle at 500 Hz, the 555 should still be off for 20 microseconds.

Here is my schematic and the circuit board I've designed

enter image description here

enter image description here

There are a number of additional headers on this board, I may use them in the future to take +VDC power from the board.

Here is a quick listing of the parts

  • D1-D4 - 1n4007
  • LED1 - A Red LED
  • R1, R12, R5, R6, R7, R10 - 1000 ohm resistors
  • R3 - 3.3k ohm, used to drive the base of Q1
  • Q1 - 2n3904
  • C1 - Any electrolytic cap I can find to filter the DC from the rectifier
  • C4 - 10 pF
  • C5 - 4.7 uF
  • R2 - 10k potentiometer
  • All four MOSFETs are IRFP460
  • C1, C2 - 0.1 uF capacitors
  • C6 - unsure as to the value

Here are my questions with this circuit:

  1. I moved all the MOSFETs down on the board. Does this help or make the problem of EMF from gate charging? C6 is located down near the MOSFETS, can I size it appropriately to filter out any transients from the gates charging?

  2. Is there anything I can add to protect the MOSFETs from voltage spikes from brushed motors? Is this even necessary?

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    \$\begingroup\$ Christ almightly! You really don't like straight lines. 500 Hz is low, but swithcing 12 A with that layout is still going to be intresting. 1000 ohm turn off can very well be problematic due to miller capacitance. Why not a buffer/MOSFET driver? \$\endgroup\$
    – winny
    Jan 2, 2017 at 9:13
  • \$\begingroup\$ Yes it does .You will eat fets with your circuit . \$\endgroup\$
    – Autistic
    Jan 2, 2017 at 12:09
  • \$\begingroup\$ @winny Can I just use a 2N3904 & 2N3906 to form a simple buffer? That way the gate is driven high/low. I could swap the gate drive resistor for 100 ohm then I believe. \$\endgroup\$
    – Eric Urban
    Jan 2, 2017 at 12:46
  • \$\begingroup\$ Yes, much better. Plan for a turn off gate diode with a resistor in series to be able to fine-tune the turn on and turn off separatly. Also, better is a relative term, not absolute. \$\endgroup\$
    – winny
    Jan 2, 2017 at 12:48
  • \$\begingroup\$ What is a turn off gate diode? \$\endgroup\$
    – Eric Urban
    Jan 2, 2017 at 12:56

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