Stupidly simple go/no-go DC voltage tester

SoundGuyAndy

Joined Jan 27, 2006
6
Hey guys,
I'm trying to put together a brainlessly simple test circuit that simply checks a PSU's output for DC from 24-30 V and lights up an LED to tell you if you're good, and another to tell you if you're over-voltage. Sort of a stupidly oversimplified bargraph, if you will. My initial thought was just to use an LED with an appropriate resistor in front of it so that it'll take 24 V to hit the turn-on voltage for the first LED, and 30 for the second, but my worry there is what damage might be caused if the voltage hits significantly higher than that range. Can I just do something as simple as this and then add a diode that's above the turn-on voltage but below the "whoah, you don't really want to do that to the LED, buddy!" voltage to protect it?

Again, I'm not going for precision, just need to be able to tell the end user if the voltage is too high or low. In fact, it really doesn't even need to be able to differentiate between high vs. low, it can truly be a "light on if it's 24-30 V, light off if it's outside that range" thing.

The other thought I had was two relays in series, the first being NO and taking 24 V to open, and then the second NC and opening at 30 V, but I feel like there might be a better way?

Andy

thingmaker3

Joined May 16, 2005
5,084
Would it work to put appropriately rated zener diodes in parallel with the indicator circuits?

mrmeval

Joined Jun 30, 2006
833
Put a 30 volt zener in the circuit in series with a transistor base so that when enough voltage trips the zener it will turn on the transistor and light a red LED.
You can put another zener on the base to protect the transistor from over voltage.

A duplicate circuit with a 24volt zener can be used for the green LED, it will still be on when the red lights.

beenthere

Joined Apr 20, 2004
15,819
Hi,

Two zeners and a red plus a green LED are the simplest way to go.

Mouser is a good source - www.mouser.com

Red indicator LED - 645-558-0101-007F @ $1.74 Green indicator - 645-558-0201-007F @$1.74
24 volt zener - 512-1N5252B @ $.50 30 volt zener - 512-1N5256B @$.50
3K resistor (get 2)- 291-3K-RC @ \$.08