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Questions relating to designing PCBs
By lutronjim
#172195
I want to control a warning light from a one wire alternator. I have a 12 vdc relay that I want to pick when the voltage goes from static ( about 12.6 volts or lower) to high (13.5 or more). The light would be wired from the battery+ and the relay would ground the circuit form the light to be on when the battery voltage is low. When the voltage goes high, the zener would pick the relay causing the light to go out.

I got a cheap 13v zener diode and hooked to the battery in series with a dvom. In one direction, I get a very low voltage(.1 or .2) in the other I see the 12.6.

I suspect that the zener is not accurate enough for my needs.

Does Sparkfun have a more accurate zener?

Thanks from a newbie.
By lyndon
#172222
First, you should probably post this in the "Projects" forum.
Second, why do you think the zener is not accurate enough? How accurate does it have to be: give numbers e.g.,. +/- 0.5V
By lutronjim
#172289
Sorry about wrong forum. I will try to repost - don't see way to move.

The 13v zener I got shows battery voltage(12.6) one way and very low the other. I want to use the zener to pick a relay when the voltage on the battery goes beyond 13v. I would expect the zener to not conduct until voltage went beyond 13v.

Does this make sense?

Thanks
By InactiveUser001
#172301
Are you trying to build a low battery warning system?
If you use Google for "car battery low voltage warning circuit" and click on images you will get plenty of results for this type of circuit.

When you start your engine the voltage will go above 12v as the alternator will supply 13.8-14.5 ish volts to the battery, depending upon your foot on the accelerator.

However when the engine is not on the battery will settle at about 12v, so the warning light will always be on if you use a 13v zener.

You might be better aiming at just under 12v, say 11.8and to do this you would use a lower voltage zener, perhaps a variable resistor so you can set the exact low voltage threshold.

The googled circuits will show yo this, read and follow how they work to understand what you need to do
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By Ross Robotics
#172303
He has another thread but hasn't replied since Tues. So he probably got his answer and can't even say thanks..