# Circuit protection

#### cnp602

Joined Jul 4, 2015
9
Hi

I am working on a circuit for my car to turn on a relay when the voltage is above ~13V (engine running) and keep it on even if the voltage drops.
It also has a input to turn the relay off,
so far I have working circuit on a breadboard mostly by guessing and looking for similar circuits in the internet
but I'm not sure what protection is needed as it's for an old car with very simple electrics (so a pretty "dirty" 12V)
I'm hoping I can get away without too much as the PCB have to be small.
I would really appreciate any suggestions.

#### ericgibbs

Joined Jan 29, 2010
9,060
hi 602,
I see on the top of your circuit < = 12v turns Off the relay.
Is it required that line is manually controlled or could it be a electronic <=12v drops out the relay.?

Can you say what is the purpose of the circuit.?
E

#### MikeML

Joined Oct 2, 2009
5,444
Since the only time that the voltage in a car's electrical system is >13V is when the engine is running, it sounds like you want a circuit that turns on after you start, and then stays on while the car is parked until you do something like push a switch?

A TL431 makes a much simpler voltage detector than the circuit you posted.

#### cnp602

Joined Jul 4, 2015
9
Since the only time that the voltage in a car's electrical system is >13V is when the engine is running, it sounds like you want a circuit that turns on after you start, and then stays on while the car is parked until you do something like push a switch?

A TL431 makes a much simpler voltage detector than the circuit you posted.
That is exactly what I need, how would a circuit with a TL431 look?

I only have basic electronics knowledge (supplemented with Google )
and the circuit is the only way I could think of to do it, but simpler is most definitely better.

#### MikeML

Joined Oct 2, 2009
5,444
I do a similar thing with the ham radio in my car. I devised a circuit that watches the accessory circuit output controlled by the key switch. The acc circuit gets powered when the key is first moved from off to run, then loses power while the key is in the start position, and finally it comes back when the key is released back to run after the motor starts.

If you use the acc circuit to power the radio directly, it goes on, off, and back on again (has to reboot twice). To avoid this, I just build a timer that delays applying power to the radio until after about 10sec from the first time the key is moved. This avoids the radio from rebooting twice.

If you want to run the radio without the engine running, you can turn the key switch to the acc position, and wait 10sec for the radio to boot up. I do this when I want to use the radio while the wife is in the store, and I dont want to run the engine while parked.

I also added a timing function at the other end. The radio remains powered about 15min after the engine is shut off. My radio does APRS beaconing, so needs to be powered for a while after the car is parked to transmit the last couple of packets. This way, if using the radio while the car is parked, I put in the key, turn to acc, wait 10sec for the radio to boot, turn off the key switch, and then I can sit a use the radio for 15min without having anything else in the car on (that would otherwise come on with the key in the acc position).

#### MikeML

Joined Oct 2, 2009
5,444
That is exactly what I need, how would a circuit with a TL431 look?
...
I'll whip it up and post it in a couple of hours. The computer with my LTSpice library is in ham shack in the other building, and I am sitting here in front my wife's computer in my pj...

#### cnp602

Joined Jul 4, 2015
9
I do a similar thing with the ham radio in my car. I devised a circuit that watches the accessory circuit output controlled by the key switch. The acc circuit gets powered when the key is first moved from off to run, then loses power while the key is in the start position, and finally it comes back when the key is released back to run after the motor starts.
Unfortunately my car doesn't have an acc position.
But I need a relay to turn on when the engine is running and remain on until I cut power to it or there is 12v on another line.
the comparator is the only way I could think off.

#### MikeML

Joined Oct 2, 2009
5,444
Here it is: You will need a DPDT 12Vdc Relay with a coil resistance greater than about 250Ω and a Normally-Closed Momentary PushButton.

The TL431 is biased to turn on at 13.4V. If you want to change the cutin voltage, trim R7. As the voltage increases (red trace), the current into the cathode of the TL431 increases rapidly as Vin passes 13.36V (yellow trace).

That draws current through the relay coil, causing it to latch on its own contact. To reset the circuit, momentarily open the PushButton. Use the other contacts on the relay to control your load.

#### cnp602

Joined Jul 4, 2015
9
Here it is: You will need a DPDT 12Vdc Relay with a coil resistance greater than about 250Ω and a Normally-Closed Momentary PushButton.

The TL431 is biased to turn on at 13.4V. If you want to change the cutin voltage, trim R7. As the voltage increases (red trace), the current into the cathode of the TL431 increases rapidly as Vin passes 13.36V (yellow trace).

That draws current through the relay coil, causing it to latch on its own contact. To reset the circuit, momentarily open the PushButton. Use the other contacts on the relay to control your load.

View attachment 88102
Unfortunately I can't use a button, It needs to turn off the relay whenever there is 12v on the 2nd input.
I also cant use a DPDT relay as the board space is really small

#### cnp602

Joined Jul 4, 2015
9
I have been studying the TL431 and I think this circuit should work.
It's a bit simpler then my original.

#### MikeML

Joined Oct 2, 2009
5,444
What is the nature of the Reset input? Is it a contact closure to the same voltage as +12V? Is there any other load connected between it and Gnd. If so, that load current will turn on Q1. You might need some steering diode?

#### ian field

Joined Oct 27, 2012
6,539
Hi

I am working on a circuit for my car to turn on a relay when the voltage is above ~13V (engine running) and keep it on even if the voltage drops.
It also has a input to turn the relay off,
so far I have working circuit on a breadboard mostly by guessing and looking for similar circuits in the internet
but I'm not sure what protection is needed as it's for an old car with very simple electrics (so a pretty "dirty" 12V)
I'm hoping I can get away without too much as the PCB have to be small.
I would really appreciate any suggestions.

View attachment 88086
A basic SCR crowbar type circuit would make a good starting point - obviously you don't want to actually crowbar the supply on over voltage, so break the circuit in the anode lead and insert your relay there.

When you want to turn the indicator off, there are various ways of commutating the SCR, you can simply break the anode current for a short period or shunt the anode out to bypass its holding current.

A neat little 2 SCR commutating switch has a second SCR with just a basic anode load resistor, a commutating capacitor is strung between the 2 anodes.

When you trigger the indicator SCR, its anode pulls down to 0V and takes its end of the commutating capacitor with it. The other SCR is off, so its anode load charges that end of the cap up to the positive voltage - if you now trigger the second SCR, it pulls the positive end down to 0V and the negative end tries to go lower than 0V - that definitely cuts off the indicator SCR.

#### cnp602

Joined Jul 4, 2015
9
Ther
What is the nature of the Reset input? Is it a contact closure to the same voltage as +12V? Is there any other load connected between it and Gnd. If so, that load current will turn on Q1. You might need some steering diode?
There is a load between reset and Gnd, How should I prevent that from turning Q1 on?

#### MikeML

Joined Oct 2, 2009
5,444
Here is what I had to do to your circuit to make it reset, give it some modicum of noise immunity, and make Q1 and Q2 saturate.

#### cnp602

Joined Jul 4, 2015
9
Here is what I had to do to your circuit to make it reset, give it some modicum of noise immunity, and make Q1 and Q2 saturate.

View attachment 88113
Thank you very much that looks perfect.
How would you recommend I protect it from transients etc.
would a TVS diode from the input to Gnd be enough?

#### MikeML

Joined Oct 2, 2009
5,444
Does your car have an alternator?

The worst transients occur during engine starting. If this circuit doesn't glitch on during cranking, or as the key is released, then you are likely ok, because the load that this thing controls hasn't turned on yet...

#### cnp602

Joined Jul 4, 2015
9
Does your car have an alternator?

The worst transients occur during engine starting. If this circuit doesn't glitch on during cranking, or as the key is released, then you are likely ok, because the load that this thing controls hasn't turned on yet...
Yes it has an alternator, with a simple mechanical voltage regulator.
I just don't want to find it not working a year from now if a 1\$ part can help protect it.

#### MikeML

Joined Oct 2, 2009
5,444
Getting rid of the vibrating-point regulator and replacing it with an electronic one would go a long way in de-glitching the car's electrical system. Keeping the battery connections shiny and clean will do a lot, too. Most damage to car's electronics happens if the battery becomes disconnected with the alternator spooled up, or doing the d.s. thing and connecting a battery backwards.

I find that connecting a 10 to 100uF low-esr capacitor across the DC power into something like a ham radio protects it against short transients like come from motors and such.

#### cnp602

Joined Jul 4, 2015
9
Getting rid of the vibrating-point regulator and replacing it with an electronic one would go a long way in de-glitching the car's electrical system. Keeping the battery connections shiny and clean will do a lot, too. Most damage to car's electronics happens if the battery becomes disconnected with the alternator spooled up, or doing the d.s. thing and connecting a battery backwards.

I find that connecting a 10 to 100uF low-esr capacitor across the DC power into something like a ham radio protects it against short transients like come from motors and such.
I may replace the regulator.
Now I just need to start designing the PCB (always a fun puzzle )

Thanks again for all your help.

#### ian field

Joined Oct 27, 2012
6,539
Getting rid of the vibrating-point regulator and replacing it with an electronic one would go a long way in de-glitching the car's electrical system. Keeping the battery connections shiny and clean will do a lot, too. Most damage to car's electronics happens if the battery becomes disconnected with the alternator spooled up, or doing the d.s. thing and connecting a battery backwards.

I find that connecting a 10 to 100uF low-esr capacitor across the DC power into something like a ham radio protects it against short transients like come from motors and such.
The electromechanical regulators usually have a voltage sensing solenoid to stop the battery discharging back into the generator when its voltage is less.

When I get a vehicle with one of these, I usually strap a Shottky-barrier diode across its contacts and insulate the contacts with a wedge of cardboard.

In normal charge maintaining; the field contacts are usually pulsing continuously, and look to the eye as a continuous arc. A linear transistorised field control is much better, and I'm not convinced the transistor dissipation is all that much of a hardship. PWM is an option, but there will still be switching transients, albeit not as bad as with mechanical switching.