Best way to detect a voltage change

Thread Starter

futureroads

Joined Oct 9, 2011
3
I am trying to put together a "simple" circuit that will control a relay/transistor when the voltage from a source changes from 12.5vdc to 13.5vdc. Was thinking zener, MOSFET, etc .. what would work best/be simple?
 

Thread Starter

futureroads

Joined Oct 9, 2011
3
I "think" the problem I would have with a comparator is that I only have the ONE voltage to look at .. the one that changes from 12.5 to 13.5. I could see using one if I had two different voltages .. a steady 12.5 and the the one that varies to compare. But maybe I don't understand comparators well enough.
 

Adjuster

Joined Dec 26, 2010
2,148
The comparator method is valid. As you say a reference is required, but almost certainly much less than 12.5V.

Due to the doubt over the intended application it may be best not to say more pending moderator's decision.
 

BJT_user

Joined Oct 9, 2011
35
A comparitor is best to monitor voltage levels and trigger when voltages change, but for fine thresholds like you are suggesting, you really need a stable reference. Typically a zener diode connected in series with a resistor would suffice, but alternately you can use a low voltage regulator IC such as 7805 (I recommend the T03 case style) which would work marvelously with a 12v input.
 

Adjuster

Joined Dec 26, 2010
2,148
Since others are giving advice, so far without management intervention, I would point out that the charging circuits of such things as self-starting generators are prone to horrid surges. These could easily fry a 7805, even in a big case style. Additional anti-surge protection would be required before the regulator.

Note that it is also much, much safer to use a low reference voltage, so that the battery input can be well attenuated to feed the comparator. Additional clamping may be needed part-way along the input voltage potential dividing chain.

All of this may not be easy, especially when just beginning to get to grips with the basics.
 
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