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Date:         Fri, 30 Mar 2007 18:20:53 -0500
Reply-To:     Matt Roberds <mattroberds@COX.NET>
Sender:       Vanagon Mailing List <vanagon@gerry.vanagon.com>
From:         Matt Roberds <mattroberds@COX.NET>
Subject:      Re: 12 volt timer
Comments: cc: Bob Mac <BOBMACDOWELL@WEBTV.NET>
In-Reply-To:  <20070330195556.RXMK9199.fed1rmmtai101.cox.net@fed1rmimpi05.cox.net>
Content-Type: TEXT/PLAIN; charset=US-ASCII; format=flowed

> From: Bob Mac <BOBMACDOWELL@WEBTV.NET> > Date: Fri, 30 Mar 2007 11:34:02 -0400 > > Does anybody know where i can find a 12 volt timer. I would like to > hook one up on my propex heater so it will turn on early in the morning.

I know where you can find one, but some assembly is required. :) It won't run continuously like a 120 V timer - you'll have to reset it each night, for instance, but it will work.

Also, a word of caution. Running something like a light or a fan off of a timer is usually no problem. Running something like a heater might be, depending on what safety gear is built into the heater. If it's smart enough to shut itself down on a no-start or overheat condition, OK, but if it depends on a human to recognize that there's a problem and shut it off, turning it on while unattended could be dangerous.

Here's what to do. At the hardware store, buy a spring-wound timer. These are often used on attic fans, bathroom vent fans, etc. You can get them in different time intervals; 12 hours is fairly common but there are shorter and longer ones. The timer will have two wires coming out of it and has a very simple operation: if the knob on the front is pointing to "off", the wires aren't connected, and if the knob is pointing anywhere else, the wires are connected. You might notice that this is exactly backwards from what you want, but read on.

At the auto parts store, buy a standard automotive relay. This is the one that's about a 1" cube and has FIVE prongs, labeled 86, 85, 30, 87, and 87a. If it doesn't have these markings, it's not the right kind of relay. Also, note that there are two variations commonly found. On one of them, 87 and 87a are NOT connected to each other - this is the one you want. On the other (often sold for fog/driving lights), 87 and 87a ARE connected to each other - this one won't work. On this kind of relay, when the coil is powered, 30 and 87 are connected. When the coil is not powered, 30 and 87a are connected. In other words, 30 and 87a do the opposite of whatever the coil is doing.

Also buy an on-off, two prong, 12 V toggle switch at the auto parts store. This switch needs to be rated for whatever current the heater draws.

Now, to hook it up.

Run a wire from +12 V battery power to one of the wires coming out of the timer - it doesn't matter which one. There should be a fuse in this wire somewhere; this wire will be carrying less than one amp so it doesn't need to be a big fuse or thick wire.

Connect the other wire coming out of the timer to terminal 86 on the relay.

Connect a wire from terminal 85 on the relay to ground (van body). Again, this wire will have less than one amp, so it doesn't need to be thick.

Twist the timer knob away from "off" and you should hear the relay click on. Twist the timer knob back to "off" and the relay should click off. If this doesn't happen, recheck the wiring you've done so far.

Now you need to use terminals 30 and 87a to trigger the heater. *IF* the heater is triggered by turning on the +12 V supply to it, this can be done as follows:

Run a wire from +12 V battery power to one terminal of the toggle switch - it doesn't matter which one. There should be a fuse in this wire somewhere; the fuse and wire need to be sized to whatever current the heater draws.

Run a wire from the other terminal of the toggle switch to terminal 30 on the relay. This wire also needs to be sized to whatever the heater draws.

Run a wire from terminal 87a on the relay to the place on the heater that needs +12 V to work. This wire also needs to be sized to whatever the heater draws.

To use:

1. Make sure the toggle switch is OFF. 2. Turn the timer knob to however many hours you want to wait before turning the heater on. Note that most of these timers have a minimum amount you have to turn the knob to make sure the spring is wound tightly enough - on a 12 hour timer it's often 2 hours. 3. Turn the toggle switch ON. The heater should NOT start.

With the timer on, the relay will also be on, and 30-87a will be open. When the timer runs down, the relay will turn off, and 30-87a will close, starting the heater.

If at any point you want to turn the heater off, flip the toggle switch OFF and/or turn the timer knob to zero.

When you're not using the heater, make sure the toggle switch is OFF. If it is turned on while the timer knob is at zero, the heater will start.

Again, be careful when setting a heater up for automatic control. Being a little cold in the morning is better than Vanagon flambe'.

Matt Roberds


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