Reverse Polarity Protection for 9VDC input to Basic Stamp 2 Board

Mooneyguy

Thanks to the tremendous help from Tracy Allen, I have a circuit that creates 9 VDC from a 12 VDC motorcycle battery. I was able to plug +9VDC into (+) Vin and hook up the ground in a number of different places. I found that I eliminated the clip on battery terminal, I had to hook (-) Vss manually to a ground since the battery terminals on the board are not used in my circuit any longer.

My concern now is reverse polarity protection. I have seen several cautions about not mixing up the + and - 9VDC inputs. Is there a simple circuit that someone could recommend that provides reverse polarity protection for my case? I am certain one day I will mess it up and switch the leads my accident.

Mike Green

Some regulators, like the one on your Stamp board, may already have reverse polarity protection built in. That would depend on which Stamp board you're using. The easiest way to add additional protection is to put a diode (like a 1N4001) in series with your positive lead that plugs into the Stamp board. I'd just cut the wire, add some heat shrink tubing over the ends, solder the diode between the cut ends, and insulate with the heat shrink tubing.

Mooneyguy

Thanks Mike. Will that diode work on the 12 v input from a motorcycle battery? I am using a 12v input to my 9 v voltage regulator.

Mike Green

The same things apply for the 12V input to a 9V regulator. You can use a regulator that's has built in protection against incorrect polarity or you can use a series diode on the input or both. The LM2940 is one example of a regulator that has built in protection.

Mooneyguy

Thanks Mike, I will change my voltage regulator from the LM7809 to the LM2940. Sounds like a good improvement. I am currently using a 1uF cap on the input to the regulator to Vss and a 0.33uF cap between the 9 VDC regulated output to Vss. Any problem with those capacitor values that you know of?

Mike Green

The problem you run into with a 1uF input capacitor is that it's probably electrolytic and these usually will blow up if you connect them to reversed voltage. A polyester or ceramic 0.47uF capacitor with at least a 25V rating should be ok. Remember that the LM2940, unlike the LM7809, requires a minimum output capacitance. For the LM2940, it's 22uF (see the datasheet for details) and I'd use a 22uF or greater tantalum with a 10V or 15V rating.

4x5n

If you have 1.2 volts to "spare" you could put in a diode bridge. With one of those it wouldn't matter what the input polarity is.