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