12Vdc relay help (simple circuit)

Thread Starter

earthtodan

Joined Dec 3, 2008
24
I'm powering a sub in my garage with a car amp attached to a car battery. The battery is charged by a trickle charger. The charging circuit is controlled by a relay. I have a switch that turns the charging circuit on and off. (see diagram). When I turn on the switch, the red LED turns on but the relay makes a loud buzzing noise. It sounds like the relay is rapidly switching on and off- like what would happen if you connected power to pole 87 instead of 87a. But it's hooked up properly. What could be the matter?



Dan
 

tdubliture

Joined Sep 3, 2009
13
Usually relays will do that if they drop below like 9v depending on the relay. I don't think a trickle charger will keep up the higher demands of a sub amp. U need a power supply. Like out of an old computer or something. Those are like 300 to 400 watts
 

SgtWookie

Joined Jul 17, 2007
22,201
The battery is dead, and it doesn't have enough power to energize the relay to charge itself and run the amp at the same time.

If you are running a subwoofer in your garage, your neighbors probably don't like you very much.
 

Thread Starter

earthtodan

Joined Dec 3, 2008
24
From your schematic, the leds are shorting the relay coil and the Amp supply.

You should place a 1.2K resistor in series to each led and very likely the "loud buzzing noise" will disappear.

Alberto
Those are all good ideas, but not the answer.
The battery tests at 12.3v, and I hooked it up to the amp and made sure it would power the sub before I started the switching project.
The schematic doesn't show resistors inline with the LEDs but they are there. I bought LEDs with integrated resistors made for a 12V supply at Radio Shack.

I tested continuity of the relay leads with a multimeter and found that the diagram drawn on the side of the relay is wrong: 87 and 87a are switched. That's why it was rapidly switching on and off. So I switched them around. Now instead of turning on and off, it does nothing... time to check the wiring again I suppose.

The 1A trickle charger is not meant to power the amp during operation. It's meant to charge the battery when it's not operating. As the diagram shows, play and charge are two separate circuits. The reason is that a trickle charger is meant to see the resistance of a battery; I'm not sure it will handle the extra current path of a working amplifier, and I don't want to find out through experiment. :rolleyes:

Aside from the resistors in the diagram, it should work as shown, right?
 

SgtWookie

Joined Jul 17, 2007
22,201
I'm wondering why you're using the relay in the first place? It'll just use power.

Disconnect the charger +.
Remove the relay from the circuit.
Connect the charger + to the lower switch connection.
Connect the charger - to battery -.

See the attached
 

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Thread Starter

earthtodan

Joined Dec 3, 2008
24
"M" is a voltmeter, so I can monitor the condition of the battery while it's being used. I don't have a datasheet for the relay, but it's a standard Bosch/Tyco 12v automotive relay.

Sgt Wookie,
I might do that. The reason I didn't want to run it through the switch is that the charge current would be running through my soldered connections, and I don't know how good they are, or what it takes to conduct 1A continuous. I'm sure it will be fine, but I'd also like to figure out why plan A isn't working!

Dan
 

SgtWookie

Joined Jul 17, 2007
22,201
The reason I didn't want to run it through the switch is that the charge current would be running through my soldered connections, and I don't know how good they are, or what it takes to conduct 1A continuous. I'm sure it will be fine, but I'd also like to figure out why plan A isn't working!
Well Dan,
If you have any doubts about how good your soldered connections are, you should re-do them.

It can help a good bit to start off by pre-tinning the wire right after you strip the insulation. Then make a good physical connection between the wire and the terminal. Often this consists of wrapping the wire around the terminal, and squeezing it gently with pliers or the like. Then apply a bit of flux, heat it well and apply a bit of solder.

Good solder joints using 63/37 or 60/40 electronic solder look shiny, have very little excess solder, and look like the solder has "flowed" across the surface. If you have big gray balls of solder, start over. If you move the parts while they are cooling, you will get what is known as a "cold" solder joint, which is physically weak and electrically high in resistance.

1A current isn't very much.

Don't let your battery discharge lower than 12.5v @ 77°F; that is where plate sulphation begins. Once your battery plates become heavily sulphated, your battery will no longer be able to accept or release a charge.

Leaving the relay in the circuit will simply cause problems. If the battery is heavily discharged, it won't have the power to energize the relay coil. If the coil IS energized, it will reduce the charge current by it's current draw. You already have some load on the battery by the current the amplifier requires when in standby mode.
 
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