Help with voltage controlled switch


The problem i am trying to solve is i need to build a voltage controlled switch.

I need the output to turn on when the input is above 9.5v and turn off when below 9.5v

I think this could be done with a op amp but im not sure of how.

I am using a supply voltage of 13v.

Thank you


I think that you are looking for a comparator. Here is a page that describes the usage of the LM339 comparator. And the LM339 data sheet.

Thank you that really helped. I now hav a lm393 turning on and off at 9.5v

That was easy !
Too bad all posts aren't such a slam dunk !

Spoke to soon. I got the switch part working but now need to rectify a ac voltage to a dc voltage to feed the comparator.

Have a wire from a alternator which is giving me 8v ac. When i put that to a bridge rectifier i only get 3v dc out.

Something doesn't seam right

So what ? Just change the threshhold for the comparator to 3V. It's just a voltage divider.
Change the resistor values.

What are you using as a rectifier? Is it full wave? If you drop 0.7V across a diode and only do half wave then 3V might not be that low.

Im using a kbpc604 rectifier.

Heres the datasheet

vs-kbpc1series.pdf (117 KB)


Can you please post a copy of your circuit, in CAD or a picture of a hand drawn circuit in jpg, png

If you just rectify the AC you get pulsed DC, not pure DC, you need a smoothing capacitor to get true DC.

8Vac, rectified and smoothed will give 8 x 1.414 = 11.3Vdc smoothed.

It sounds like you have a rectifier connection problem, can you post a picture of the alternator please?

Tom... :slight_smile:

Here is my circuit so far

This diagram shows the alternator and the circuit i am making is replacing the unit labelled 3aw cut out emulator

How are you taking power from the alternator?
Is the case of the alternator one terminal?
If so you will destroy your rectifier.

Please a COMPLETE diagram, including the alternator terminals.

Please post a picture of the alternator, including its terminals and specs/data.

Thanks.. Tom.. :slight_smile:

With 8VAC on the " ~ " terminals you should have around 7VDC from "+ and - " terminals, sure your connections are right? If you're putting pulsating DC on the LM393 input it may not work as expected, you could add a filter cap but that would kick your DCV up to about 10.2, you could still use a voltage divider to get down to 9.5.
Also you need a kickback diode across that relay coil to protect the MOSFET.

OPs Alternator.

You will need to half wave rectify your 8Vac not full wave with the bridge you are using, the bridge is shorting the alternator out on each negative cycle.

Why do you need to replace this part of your car?
Are you sure you know exactly how it works?

Tom.... :slight_smile:

I have now put a 470uf cap across the output of the rectifier and am getting 15.8v outof the rectifier

That wasn't necessary , although it does help to stablize the voltage.
All you needed to do was change the comparator input voltage divider resistor values.