I'm planning to make an electronic solder sucker from an old portable vacuum cleaner motor and a 5V switching power supply. Luckily I managed to tweak the supply up to 6V so the motor should in theory run at full speed. The problem was that the supply required a minimum load of 7W and would also current limit if the motor was connected directly to the output so the shaft would just vibrate.

The solution was a soft start, now I did consider a 555 timer or a comparator to monitor the voltage across a series resistor then short circuit it once it's dropped below a certain value but in the end I decided to go with the simple circuit attached because it did the job. I used a 100µF tantalum for the capacitor to minimise leakage but it probably isn't needed and two 1R resistors in parallel for the 0R5 resistor.

By the way the motor draws 6A and the supply powers it fine despite its maximum rating of 30W as it probably just limits the current to 6A and at 5V that's 30W.

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Possibly you could run the current through a FET and generate a ramp voltage to turn it on slowly.

 

That's exactly what this circuit does.

I couldn't just omit the resistor and rely on the MOSFET to slowly ramp up the voltage because the power supply has a minimum load requirement of 7W

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I've made a 24V (2 car batteries, peak amps=70A(starting), average amps = 28A-40A (Cruising speed)) electric car. I'm wanting to make soft start to stop the motor shocking the drive shaft. Is there any way this method could be scaled up or would there be any alternatives?

Would really appreciate the help.

 

Is there a parts list with values and wattages?

 

Hero,

I want to build your circuit possibly using a HUF MOSFET from Maplin (14mOhm, 60Amp).

Could you add a bit more detail please, like where would you place the on/off switch and how does the circuit work - do you short circuit the capacitor with a resistor to switch the FET on over 1 or 2 seconds?

I would also like to have an option to fire a big relay to give more power.

I am using it to drive a 350W 24v motor in a child's go-kart. I had just been using a 40A relay, but the is a massive jolt when you start which puts a lot of strain on the drivetrain and it has also burnt out the relay contacts

Thanks
Ben

 

You can put the on/off switch in series with either the +V or 0V rail, like any other circuit.

The operation of this circuit should be pretty self explanatory. When first turned on the MOSFET's gate is at 0V, causing it to be off so all the motor current has to travel through the 0.5Ω resistor. When the capacitor charges to a high enough level for the MOSFET to turn fully on, the resistor is bypassed and the motor operates at full speed.

Connecting a diode (1N4001) will make the circuit reset more quickly and help to protect the MOSFET against any back EMF generated.

For 24V operation another resistor is required as the maximum gate voltage for most MOSFETs is <20V.

Note that this circuit is not suitable for PWM controllers or speed control.

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Thanks Hero.

That clarifies it.

 

Bought a mosfet from maplin HUF75337P; wired it up on 12 volts and it worked great! Delay was a bit long but that is easily rectified.

Next I wired in the second 12v battery to give 24v. It worked ... but only once! Now it is always on.

I had a diode across the motor terminals 1N5406 but did not include the other diode you put in ur diagram ... maybe it was there for a good reason.

What do you think has happened?

 

Did you include both 47k resistors?

Turning permanently on is a common MOSFET failure mode.

The other diode won't be required, especially if you added a diode in parallel with the motor, but expect to have to wait a little before the circuit will reset.

It it's built as shown it shouldn't fail. The component values should also be fine up to 40V.

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Yes i did include both 47k resistors as drawn

 

How about adding a zener diode between the gate and source; of value 12v?

 

I guess that's the problem with these devices; they are extremely delicate; one brief spike of a few nanoseconds and it's ruined.

A good ol' resistor/big relay combination is inherently rugged; the contacts may burn a bit but it'll still work

 

It's the gate that's the most sensitive part.

A zener would work but you won't need the lower resistor.

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Bought another mosfet and wired it in with a 5.6v zener between the gate and the source. Changed the resistors to 4k7 to ensure at least 5mA would be flowing thru the zener. Kept the second resistor as wanted a rapid discharge.

Worked well for about a dozen tests then failed to permanently on.

Any ideas?