5 second 'slow on' circuit

[AGCB]

I'm trying to build a soft start circuit for an electric clutch (riding mower type). So far, I can only get about 1 second or less to full power. I tried this circuit less the Zener and LED.

Voltage is 12 volts DC, load is 5 amps and using a IRFZ48N transistor.
I've increased the capacitor to 15,000 and played with resistors but nothing seems to help much.

I know I could do it with a MCU (and have) but would like to keep it simple if possible.

Would appreciate if someone could point me to a solution.

Thanks
Aaron

 

[Tony Stewart]

Karklebapper 's linear driver uses a voltage doubler to boost the gate voltage following the source.
The FET reaches 24W on startup until Vout reaches 12V.
The clutch load could be 2 ohms but in series with a large inductance if it draws 6 Amps steady state.
The inverter may be eliminated by swapping the inputs on the LM339.
This does not satisfy the 5 s ramp expectation.

Crutchow's PWM solution is a better way to go.

However a 5 second clutch is a curious requirement to me.

There are some interesting conversion kit video's with some critical mechanical requirements.

I suspect the coil R/L ratio is over 50.

 

[Karklebapper]

I will not argue about peak power in the FET in my design. Other than to say that PWMing the load will not work with a wide variety of loads. My circuit is more of a general recommendation which can work with a wide variety of loads. I did not attempt to set the soft start time to 5 seconds, but the ramp time can be slowed several ways:
1) Slowing the oscillator.
2) Smaller value for C3.
3) Larger value for C5.

 

[crutschow]

PWMing the load will not work with a wide variety of loads.

What type of loads will not work?

 

[Karklebapper]

Well, as I say: If your load is an electronic circuit which has bypass capacitors across it (so it has low impedance at high frequency) you will not want to be hard switching that to a similarly low impedance source voltage. You need a current limiting impedance between the two.

Just so we understand each other, I set up a quick example "not recommended 01".asc. The load is R8 and C4.

The FET is driven with a fixed 2% duty cycle. 50 uSec period and 1 uSec "on" time. Peak current at startup exceeds 700 Amps. I don't think you want to do this....

 

[Tony Stewart]

True but irrelevant. A mower clutch is electromagnetic.
Driver burnout is relevant.