Giday from Aussie land
I need a 30vDC PWM schematic, any one got any ideas?

 

Something like this? http://www.pcsilencioso.com/cpemma/pwm.html
Lm324 has a max single supply voltage of 32V.

Ken

 

This is about as simple as it gets. Both the frequency and the duty cycle of the PWM are adjustable. And by choosing the right mosfet to match the voltage and amperage you need, it can be used for about any motor.

 

But the mosfet choice is limited to logic-level mosfets. Also I don´t like the D1-D2-R5 combination, I think that choosing a chip suited to driving a single mosfet would be better.

 

The D1,D2, are needed because the outputs don't go the full range of duty cycle independently. The diodes act as a "diode OR" This is according to the SG3525 data sheet as a normal thing to do.

R5 is a gate pull down.

If you supply pin 1 on the chip with 12V it will drive any normal N mosfet. Why do you say it will only drive a logic mosfet?

 

If you supply pin 1 on the chip with 12V it will drive any normal N mosfet. Why do you say it will only drive a logic mosfet?

Because pin 13 is now connected to pin 16, which is a 5.1V reference supply. Look at the datasheet, the output stage is supplied from pin 13, so if you don´t connect this pin to at least +10V you can´t use standard mosfets.

R5 limits the turn-off speed, which is why I would like there a single push-pull stage better than a diode-resistor one.

 

Something like this? http://www.pcsilencioso.com/cpemma/pwm.html
Lm324 has a max single supply voltage of 32V.

Ken

alternating between fully on (nearly 12V) and zero
i need it to be switching 30 vDC

 

But the mosfet choice is limited to logic-level mosfets. Also I don´t like the D1-D2-R5 combination, I think that choosing a chip suited to driving a single mosfet would be better.

I plan to use IRF1405 x 4

 

You could try a simple sigma delta modulator. Patch a simple triangle wave (Dual opamp) into one leg of a comparator and use a pot as a voltage divider (or any variable voltage source) as input to the other leg. The resulting modulation is quite accurate. The voltage divider controls the duty cycle and the period is determined by the tri osc frequency. then the output can be bumped up by a standard BJT or MOSFET to whatever amplitude you desire. Just google class D amplifiers.

 

Currently I was using a simple NE555 on 12 volts and using a IRF1405 FET to switch the 30V , but it did not last any more than 30 minutes. The problem is the leading edge of the square wave, it has a 12 v spike of a few nano seconds. i want that spike to stay. the current drain during the spike is as high as 40 amp.the frequency I want to use is 42Khz.
The IRF1405 FETs are great for 12 volt DC operation as long as the heat sink is big enough

 

I need to switch 30vDC and run the PWM also off 30 v