bd0nalds0n

I have a switch that I have tapped into from the door chime that is currently hot when the car is on. There is also a small 12V battery inline so that 1) the battery will charge when the car is running and 2) the switch will continue to receive power after the car is shut off, until the battery runs out.

The problem, of course, is that when I switch off the car, the circuit I have tapped into also receives current from the small battery. This is undesired.

I want to put a diode inline between the hot wire and the battery/my switch, so that I do not send battery current along the hot-switched circuit when the car is shut off--just to my switch/device.

Can someone offer a brief tutorial about the different kinds of diodes and where I might source something that would fit my need?

jeff jackson

ASK Alan...my best option towards all things electrical...

Dave Howerdel

Not fully understanding what you're looking to do but I would use a normally open relay that closes when the car is running and opens when it is turned off(coil powered by the ignition switch).

Bob Rouleau

The relauy is the best solution. You could use a diode in the hot lead to the battery ---->|--- battery. That way the current can flow to the battery but not back towards the door chime power feed. The diode will cause a voltage drop of around 1.5 volts. When the engine is running, the alternator puts out about 13.7 volts, so even with the drop of the diode the battery will see 12 volts on charge. When the engine is off, your battery will see only about 10.5- 11 volts.

The type of diode is not critical as long as it can handle the current. I don't know what the current drawn by the switch is. A visit to a radio shack will find you plenty of diodes - do not use a small signal diaode. You need something that can handle 1 or 2 amps I suppose.

regards,

Alan

Brian,
Look for a rectifier type diode in whatever current level you will need for whatever you are powering + the max charge current of your battery. The cost differential between 1A and 5A is small so if in doubt do 5A.

All diodes behave similarly so it makes no difference what rating except to cost.

The voltage drop will actually be about 0.7v across the diode. The diode will have a white band at one end - this is the negative end. Current will flow only from the positive end to the negative end so connect the positive end to your power source in the car. Incidentally you should not charge a small battery from the car directly without a current limiting device - the battery could get damaged with a low impedance 13-14V supply (assuming its a nicad or similar nominal 12V battery).... you should probably measure the current flow using a series resistance to the battery to limit this to no more than say 300mA charging current (maybe less - check the battery specs). Battery chargers use a constant current supply - but this is hard to do. Assuming 14v and 12v nominal for your battery maybe you need a series resistance of about 4 ohms with a power capability of about 2W.

Maybe you can tell us more about he application and it may make more sense or we may have some other ideas...

Couldn't you just use a direct battery (fused) connection for this circuit so its always on and forget the extra battery....?

Alan

dr bob

If you want to use the same door switch to operate two or more circuits, and you want to isolate the circuits so power doesn't backflow from one to the other, you'll want to use isolation diodes.

Diodes are usually illustrated with an arrow/triangle pointing opposite the direction of electron flow for some reason. A little ASCII art would look like this:

SystemA -----|<|------ to pin switch ------| |----- to ground


SystemB -----|<|------ to pin switch ------| |----- to ground

When you wire the diodes, the silver band should point away from the wire going to the pin switch. The pin switch connection is common to both/all circuits attached. Use some small diodes like 1N1001 that you can buy cheap at Radio Shack. These are rated for 1 amp service and 100 volts, plenty for just alarm and similar circuits. Fuse the circuit you are adding at something slightly less than the rating on the diode and you'll be fine.

bd0nalds0n

Thanks for the advice, and sorry I wasn't clearer on what I've done or what I was trying to accomplish. It was in a related post.

I have been bothered by the fact that when you shut off the car, the climate control cuts off all power to the vacuum manifold solenoids, which causes all the systems to basically go into failure mode (i.e., heater valve open, defrost and heater vents open).

This is a pain in the *** when you are running short errands and repeatedly shutting off the car.

Some time ago, I had an errant mixing motor, so I pulled a switched-hot lead from the door chime relay, and connected it via a switch to the heater control valve solenoid. This way, I could manually but electrically close the heater control valve when the car was on, because the wonky mixing motor wouldn't energize the heater control valve solenoid no matter where the thermostat slider was set. It was a way to eliminate heat in the summertime while I isolated the problem.

Anyway, after seeing Dave Lomas's post regarding hooking up a cooler in the trunk, which is run off battery power (which charges when the car is on, but powers the cooler when the car is off), I thought it would be similarly feasible to have the same kind of battery setup, but to power my heater control valve "override switch" without draining the main battery when the car was shut off. This way, I can keep the heater control valve off across multiple short trips, eliminating the hot blast of air.

So basically I just plugged the small battery into the lead I had pulled off of the door chime relay, bringing it into my new heater control valve solenoid circuit. Except it naturally sends power TO the door chime relay when the car is off (which I don't want) which causes weird things, like the "P" (park) to be illuminated in the pod constantly, plus the door chimes any time you open the door, like you left the lights on.

So I figured a diode between the door chime and the battery/switch would allow the power to flow to the two things I want it to (the battery, and the switch/solenoid, when the key is on), without battery power flowing to the door chime relay when the car is turned off.

Sounds like if I get a 5A (I'm sure much less is needed but I don't have a problem with overkill) diode I should be okay, so long as I keep it pointed in the right direction.

This sound basically right?

Assuming so, I can then entertain the notion of limiting the charging current so as to not damage the small battery, if it's necessary.

Alan

Brian,
Yes - what you said. Check the battery specs for charging current - unless its a small lead acid type it will have a lowish limit (<<1A).

In Phoenix in the summer - I always get a hot blast - I don't think the heater valve will help me all that much! Sounds like what you really need is a heater valve that defaults the other way right ?

(of course its mostly cold in Germany...!)

Alan

the flyin' scotsman

Brian...........to avoid the symtoms you describe its far easier to change the vacuum direction. If the hot water valve is 'wired' closed and the respective acutators are held in the 'cool' position your set. There's enough reserve vacuum to hold the flaps in the desired position for shart trips. See Tonys web site for complete details.

bd0nalds0n

One last thing:

http://www.batteriesandbutter.com/Me...Category_Code=

Here's the link to the battery I bought--it's a 12V sealed lead acid battery. I figured it could be in the charging circuit the same as the main battery, but perhaps I'm way off base.

Alan

Brian,
Its a lead acid type so you can just connect it - I think will be OK with no limiting resistance or you could add a 2 ohm 1 W resistor to play it safer.

Alan

bd0nalds0n

Thanks everyone and Alan and Dr. Bob in particular for the advice, I appreciate it!

dr bob

Hey Brian--

I just bought that exact battery at Fry's for about $20, as a replacement for the battery in the UPS my TV sits on. Look there if you need another.

The size of the current-limiting resistor is based on the total current draw of the battery plus the load on the battery, and that will be a lot bigger than 1 watt. Consider that the battery alone will draw a couple amps if it's discharged half way. Phineas Ohm says that your whole demand would need to be less than 70 milliamps, including the charge and all the HVAC solenoids, for a 1 watt resistor to survive in current-limiting duty. I guess it really would limit the current...

Consider instead that you can protect the wiring and the battery by using a 1A thermal circuit breaker. The issue with the battery charging is overheating the plates in the battery. Get 'em too hot and they warp. Using a self-resetting thermal breaker means that the battery will recharge fine under most conditions, unless it has discharged to the point where the charging current exceeds 1 amp. Then the breaker will 'cycle' off and back on. The higher the current demand, the shorter the time the breaker stays closed.

So your charging circuit would have a diode between the fused/switched supply line and the thermal breaker to the battery + terminal. Another diode from the battery + to the load you want to maintain. A third diode could carry current around the battery by installing it between the fusded/switched supply and the load itself. When the supply power is on, current flows through the third diode to your load, and through the first diode and the thermal breaker to charge the battery. When the primary power is switched off, current flow only through the second diode from the battery to the load. Using this method, with the three diodes, means that the load can be used immediately on availability of the primary supply if the battery has run down. Should you use just one diode and a current-limiting resistor, your load would see reduced supply voltage until the battery was sufficiently recharged.

PM me if you'd like a diagram showing how to make this work.

There are self-resetting thermal breakers used in many auto circuits on modern cars for such things as headlight circuits. The physical format has the same pin size and spacing as an ATA fuse, so they plug in to common inline fuseholders too. FWIW, many cheap battery chargers use this method to try to protect the rectifiers in the chargers from overheating.

HTH!

bd0nalds0n

Dr. Bob, I think I get the gist of what you're saying, it's really not that complicated, the way I read it.

Can I source the 5A diodes at Radio Shack + the 1A thermal circuit breaker?

Alan

Bob,
You are right about the total load (I really have no idea what the load of the vacuum actuator is?). However for the battery alone I think the resistor is probably still a better limiter (maybe in series with the thermal fuse idea - good one).

I'm now thinking a relay is the way to go. Have battery charging only when the ignition is on and supply current to the actuator from the battery only when the ignition is off. The reason is: if this circuit drives the actuator all the time its going to conflict with the AC conroller at least some of the time.
(say in the winter when you have heating on...)

I think you mentioned having a switch.... but this is not really a very good solution to this problem.

Have a relay with coil activated by the ignition, term 30 goes to your new battery+ , term 87 (normally open) contacts go via charge current limiting resistor and/or thermal fuse to the car battery or ignition connection. The 87a (normally closed contacts) go to your vacuum actuator. This way its only ever active with car off.

In that case calculate the charging resistor as ~13.8v - ~12v- ~0.7v = 1.1v
Charging current of ~ 1A => resisitance ~ 1.1ohms

Power = 1.1v x 1A = 1.1W So even for charging you are right that probably 2W would be better. I was assuming the actuator was never powered by the battery circuit when the car was on - which is still the idea I think - but only achived with the manual switch - so no protection = bad assumption.

Brian I really think you need the relay here... clean funtional install - no swicth to remember.

Alan