# Simple Voltage Detector Circuit

Discussion in 'The Projects Forum' started by ICBIMProducts, Sep 24, 2016.

1. ### ICBIMProducts Thread Starter New Member

Sep 23, 2016
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Hey folks, I need a simple circuit that will turn on a 12v relay when the input voltage of the circuit reaches 13v. I can't use an op-amp as I will not have a reference voltage. Any ideas?

2. ### AlbertHall Well-Known Member

Jun 4, 2014
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448
How accurate does the switch at 13V need to be?

3. ### ICBIMProducts Thread Starter New Member

Sep 23, 2016
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....Well not super accurate. Maybe from 12.6v to low 13's I suppose....

4. ### blocco a spirale AAC Fanatic!

Jun 18, 2008
1,461
370
Why can't you create a reference voltage, that's usually the procedure?

5. ### ICBIMProducts Thread Starter New Member

Sep 23, 2016
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I am not totally sure I can't however the research I have done so far does not support it. I fly an extensive collection of RC planes and drones from micros to giant scale and have a 125 amp deep cycle marine battery in the rear of my Ford Flex to run my system to recharge all my LiPo batteries while I fly. This battery gets charged from my car however the connector in the car is always live and I fear that at some point I will be at the field all day and kill my car's battery. The voltage of the plug will only be at 13v when the car is running so if I have a circuit that energizes a relay at 13v I will only have the car connected to the marine battery when the car is running...Make sense?

6. ### ci139 Member

Jul 11, 2016
341
39
usually they connect batteries with bridge wires

▲it's for inverter but can be adapted/modified for bridged batteries▲
you must have some sort of current limit from
CAR Battery →relay→incandescent bulb(a current limiting resistor)→ Marine (also ensures your generator coil windings get no excess load)
▲amps pass limited by thermal limiting by W (tungsten) or what they use in over-current protection resistors▲
Not empty → → → Emtpy (100-s of Amperes pass if no limit other than wires heating up when too thin for surge . . . relay contact electro-weld together)

7. ### ci139 Member

Jul 11, 2016
341
39
this is very good value
it ll be around 12.75 - 12.85V for older (more aged) battery

8. ### blocco a spirale AAC Fanatic!

Jun 18, 2008
1,461
370
You can easily create a reference voltage from the battery supply either with a zener diode or even a fixed voltage regulator e.g a 7805. There are plenty of comparator based low-battery-voltage detector circuits on-line that may just require minor changes to suit your application. It is important that the circuit incorporates some hysteresis otherwise it will oscillate around the set-point as the battery voltage will rise a little when the load is disconnected.

9. ### AlbertHall Well-Known Member

Jun 4, 2014
2,270
448
A MAX8212 would do the job, with a transistor or a MOSFET to drive the relay. It provides the reference, comparator and hysteresis in one small chip.

blocco a spirale likes this.
10. ### #12 Expert

Nov 30, 2010
16,703
7,338
Have a look at this:

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11. ### #12 Expert

Nov 30, 2010
16,703
7,338
Here, this will take the jitter out of it.

12. ### ICBIMProducts Thread Starter New Member

Sep 23, 2016
4
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Thanks. I will give this a try and let you know.

13. ### #12 Expert

Nov 30, 2010
16,703
7,338
Obviously, I tried to get that right, but you can ask if something seems off.
I gave it a 1 second time constant to avoid responding to noise.
I gave it 8% hysteresis to stop it from jittering the relay.
These things are judgement calls.
Expect it to have about a second of lag time, 4 or 5 seconds during start-up.
If it jitters, the 330K can be decreased, but you need to be sure the wiring and connections are good for that 8% to be sufficient.
A dirty connection on the power wire will surely cause problems with it cycling in and out when it shouldn't.
(almost) Nothing is carved in stone with analog design.
It's all about uncertainty and how to compensate for those uncertain decision points.
I wouldn't reject the idea of adding a few hundred uf @ 25V right at the input to the circuit so that capacitor can absorb transient voltage spikes in the generator system.
I don't know. The relay coil and the 27k resistor protect the chip from voltage puncture. That might work just fine.
And it might not. Cars are awfully noisy electrical systems.
As you said, try it and find out.