Relays for circuit to switch on/off when the car starts

[runtime]

Relay design?

Hello All,

I have a small circuit in my car (it's a tracking device), it has a wire for main power (connected to main vehicle's battery) and also has a wire connected to a backup battery (small 12V 4aH battery).

Due to spikes or noise, the circuit sometimes stops working, I believe the oscilator stops working and the processor stops also, If I circle the power, everything works again, so I would like to have the circuit switched off/on when the starter is switched on, like the radio or lights goes off when you switch the starter on.

So the idea is to circle power to the circuit only when you start your vehicle...
is this possible with a couple of relays? or cut-out relays?

Thank you in advance
Peter

 

[aghielan]

Re: Relay design?

runtime,

Y not u use car cigarate power to turn on/off a relay, which can attach to ur circuit?

 

[Darrell]

Re: Relay design?

You can try to put a suppressor or capasitor on the input power supply of your tracker board.

 

[runtime]

Re: Relay design?

The power supply of my circuit has several inductors, capacitors, etc to filter the battery power, I don't think the power supply is causing the noise.

The circuit has a wireless modem, several relays, etc. For some reason I haven't discovered yet, it stops working (although we redesigned the power supply to include more filters), but if I circle power it works ok, so maybe the other components are the cause of the noise or electromagnetic pulses that make the processor to go nuts.

If the board is tested in the lab with a regulated power it can work for months, but when placed in a vehicle, randomly it stops working. So I need an easy solution, like puting some relays to circle power when engine is started...the relays should cut the main power and the backup batt power...

Please advice
Peter

 

[Sal]

Relay design?

Hi

Let me give you my opinion
As you said, you haven't found the real problem, then applying a practical solution to this problem could be risky, the circuit may not work when it should.

In order to isolate the problem (discard the possibility of being the car's power system causing the failure), you can let your circuit running only on the backup battery, if it doesnt fail, you can blame the car's power system. Otherwise, you can think about other factors which are present in a car environment, not in the lab, i.e. movement.
If your problem is the car's power system, you can include a 12v regulator CI, it may even mean that the car you are using have a problem in its power system, but lets not go that far.

About the circuit you want to implement, there are many possibilities. One simple one may be to include a relay on the power line to your circuit, which may be switched off controlled by monostable circuit triggered by the start of the motor or on a more rough option, controlled by the start signal itself

cheers

Sal

 

[runtime]

Re: Relay design?

Sal,

Thank you for your feedbak, as I mentioned, the circuit has its own power supply and regulator (12V to 5v), the power supply is well designed, the 5V output is stable but still, the circuit stops randomly.

Would you please elaborate the idea of the relay and monostable circuit?

Thank you
Peter

 

[A.Anand Srinivasan]

Re: Relay design?

try using transient suppressors like capacitor, varistor etc.....
if u want it to work only when the car is on then try switching a MOSFET or relay switch using the main supply...

 

[Jepeto]

Re: Relay design?

If yours circuit has voltage switch-over like switching diode
and the main battery has voltage drop while starting the car the backup battery voltage will drop also.
Check the main battery under car starting load
Try to find 12V node that disconnected while starting the car like main lights, air condition, radio... for the backup battery instead the connection to main battery.
If cables connected to the circuit use shielded cables and connect the cable shield via 1M Ohm resistors parallel to 0.1uF 500v capacitor to the car chassis for ESD discharge

 

[bmwcoupe]

Re: Relay design?

you should design new power supply mechanism for you application
that will include TVS diode and of course better regulation .

 

[aghielan]

Re: Relay design?

This is my basic idea to solve this problem.. this circuit will trigger and on/off the relay when there is a change in power supply at cigarate power. U can use this relay to cycle ur power.

 

[Sal]

Relay design?

Hi again

It was said your circuit has its own power supply, but you also said it has a connection to the cars power. You also suspect that some oscilations (in the car power supply terminal?) maybe causing the problem. Then we can assume that this connection is causing the problem, I think my approach to isolating the failure is valid, if you ignore the connection to the car power and let your circuit work on its own, then if it continues failing the problem is inside your circuit.

About the circuit I talked about you need 2 things. 1) Identify the cable which goes out from the START switch (key). Now you are on your own here, depending in your the car you have, there are many versions of these switch, and you need the ability to identify this cable under the dashboard. 2)Identify the power line form the car to your circuit, which must be easy since you already have it installed.

For the circuit let me refer to http://www.kpsec.freeuk.com/555timer.htm For the connection of the relay coil look under "Relay coils and other inductive loads" and the figure which says "relay on when the input high". The type of relay you should use is normally closed, which only will open at the start time.

For the block "555 or 556 output" refer to the circuit under the title "555/556 Monostable", there you can also find how to calculate the ON time time period, T = 1.1 × R1 × C1.

Although I wish you good luck with the project, you should know that this is not the right way to deal with the problem which still exists and this trick may not solve the real problem since it is not fully identified. Technically, this way of dealing with the problem is wrong

cheers

Sal

 

[aghielan]

Re: Relay design?

i agree with Sal's oppinion... y not u try to findout the actual root cause & fix it?.....