5 second 'slow on' circuit

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.

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.

Karklebapper said:

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

Click to expand...

What type of loads will not work?

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....

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

danadak said:

crutschow, do electrical appliances in US have to meet FCC, ANSI, ......standards, not sure.
Ignition systems still incorporating noise suppression or is EMI a thing of the past.....

PWM are excellent noise generators, especially when involved in driving high currents
thru wires (antennas).

Of course this is a one off, and the standards police are few and far between.


Knight

Click to expand...

I've not known any riding mower or electric mower can generate EMI when near a home or garage to interfere with home appliances, car audio or test equipment. AGCB's initial post lacks a timeline for the delay to reach full voltage, so crutschow's circuit is a starting point; an RC circuit with a relay is the simplest experiment.