PWM driver with short circuit protection

[arneansper]

Hi!

I would like to make a simple PWM driver for model railroad. Previously I've done PWM driver for my 12V drill using this circuit:

http://www.geocities.com/tjacodesign/motor/motor.html

It was easy build and works well. But it lacks a short circuit protection and I'm sure that there will lots of shorts in my railroad.

So I'm looking for simple potentiometer driven circuit that has also a short circuit protection.

regards,
Arne

[Acetronics]

Hi, Arnean

Very simple : your MosFet has an internal resistance ( typ .1 ohm ) ... just compare that voltage loss ( D to S ), to a ref voltage with an ordinary comparator ( LM 393 i.e ) and short the gate signal if comparator toggles ...

The pot will be used to trim the comparator ref. voltage ...

Alain

[arneansper]

Thank you. I was doing some googling in order to find some schematics that use this idea and finally came up with this:

http://homepages.which.net/~paul.hills/SpeedControl/SpeedControllersBody.html

It was pretty good read IMHO. Then I found out that there are "smart" transistors that already contain all the protection logic. The price is not so high, so I will first try to replace the FET with "smart" FET.

regards,
Arne

[Phil Cole]

Look carefully - a lot of the smart FETs are intended for DC switching, so have limitations for PWM, depending on the switching frequency you need.

[arneansper]

Thank you for pointing this out. I went through bunch of datasheets. Not all of them mention the switching frequency (e.g. Infineon). International Rectifier and ST mentioned it - 1kHz and 50kHz respectively, without additional measures (IR pointed to application note that describes how to go to higher frequencies). Another problem was input voltage. Most of the devices require logic level input, which is not good in my case. I had three solutions in mind - to bring down the supply voltage of the control chip, to use voltage divider or use separate 5V supply for control chip. I'm using old PC PSU, so I have controlled 5V in additon to 12V that I use for motor.

Finally I found devices from ST that allow input voltage up to 18V. Then I noticed another thing - unlike normal FETs those smart thing drain some current from input pin to run the protection circuitry. Fortunately the 4093 can supply up to 2mA and ST device require no more than 0.5mA. And ST devices had also lowest prices.

So, I'm going to buy VND5N07.

Arne