stereo stays on until door is opened

I'm considering using a latching relay to add the subject function to my car, but was wondering how much current I should expect from the +12V (IGN) wire of my aftermarket head unit.  Is this just a signal wire and the current in miliamps?

The example relay showing the use of two relays to provide "Stereo Stays On Until Door Is Opened" is great, but I want to use a latching relay instead.  The 12V latching relay I found provides all the requirements, but it only has a contact rating of 2 amps.

Thanks,  borland

Use the output of the latching relay to drive a standard automotive relay.

The other option would be to use an ammeter to measure the actual current, then double that number.

Thanks KP, but I've already got the circuit designed with just one latching relay, I just need to know how much current I should expect on the +12V (Ignition) wire.

If I had a spare head unit around, I'd power it up and measure it with a meter.   I'm trying to find out what to expect without having to tear into my car for the hack measurement. 

One other comment.... According to the factory wiring diagram on this car, the +12V (Ignition) to the stereo head unit comes directly from the ignition switch, unfused, and only supplies the stereo head unit (no other sources).

So the factory must have assumed this feed as a signal current and very low probability of a short or high current failure.

You should be able to easily drive a standard automotive relay off the latching relay, so you can still use the circuit and the latching relay, you will just be amplifying the current of the latching resistor by driving a high current relay with the latched output.

I have never seen a car that had an ignition feed that went right to the radio with no fuse and no other accessories on the wire.  What kind of car is it?

The assumption that the factory must have assumed no load because there is no fuse is the exact opposite as what I would think.  If there is no secondary fuse they must have felt that the primary fuse, before the key, would be sufficent for protecting the wiring.  If the radio is in fact the only device on that circuit then the fuse size that is before the key is presumably the amount of current you need to have for the radio.

Remember that main fuses are there to protect the wiring, not the device.  Even if the device has 0% chance of failing or shorting out there is still the high probabilty that on some car some where during the course of the cars life something will happen to that wire and short it directly to ground.

Kevin,

It's a 1990 Porsche. The +12V (ignition) wire from the ignition switch to the radio is not fused, it's directly from the battery.  There are other ignition wires from the ignition switch that are fused, but they are not internally on the same switch wiper.  I can insert the key and turn the key slightly which makes the radio turn on, but not any other accessories, but turning the key more makes them come alive. Those accessory wires do have fuses downsteam of the ignition switch.

What is fused on the stereo head unit is the +12V (constant). That wire comes from a 15 amp fuse at the fuse panel.  The factory external amplifier is powered by a fused +12V (Constant).

The two-coil DPDT latching relay is all that's required for adding this function. No other relays are need to do the job.  I just need to make sure it's correct amp rating for the application.

https://www.the12volt.com/relays/relaydiagram30.html

https://www.the12volt.com/relays/relaydiagram31.html

I did not look to see what type of door trigger system your car has, but one of the above diagrams should help you.

IAAI,

Thanks for the replies.  I will assume the 2A rated relay is sufficent in this application.  The advantage of the 2A relay is that they use less power, are very quiet, and comes in a very small package.

My car is the more common (-) triggered door.  Those example diagrams are using THREE common DPDT relays.  The same circuit can be made with just ONE 2-coil Latching relay. Since my Accessory Wire to the Stereo head unit from the ignition switch is isolated from the other Accessory wires (as mention in my prior post), I won't need the isolation diodes shown in those 12volt.com diagrams.

I wouldn't assume anything on a 20 year old German car....

I guess worst case scenario put a 1A fuse inline with the contacts of the 2A relay and then the issue becomes irrelevent.

If the fuse doesn't blow then you know you are at a safe level, if the fuse does blow you can assume you need a larger relay on the output.

I see now that the The12volt.com example answers my original question, as the example uses a 1A diode.

Here's what I have worked up for a schematic using the single DPDT latching relay...

and with a delay circuit to keep the circuit from immediately latching before engine starter cranking...

Since my car's accessary wire is isolated from other accessories, this circuit should work..

And on my car with the delay circuit..

Maybe, but my car has a newer replacement ignition switch.  The old Porsche part number has been superseded by a Porsche one with a 944 prefix. Anyway, the ignition switch currently in my car does interupt the stereo accessory wire during starter cranking.  BTW, those old ceramic fuses were replaced by Porsche starting in 1985.

Here's a view of the prototype circuit I built.

Component list

RL1 – DPDT Latching Relay, 12VDC, two coil, 2A contact rating, eBay, about $7 with shipping
Q1 – NPN transistor, general purpose, 2N4401
R1 – Resistor, 52K ohms, 1/4 watt
C1 – Electrolytic Capacitor, 100 uF, 35 volt
D1, D2, D3 – Diode, 1N4005, 1A, 600V

The time delay can be adjusted by selection of the capacitor and resistor values. The resistor’s value is also a function of the relay coil’s current draw and the transistors gain rating.