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  #11  
Old 03-31-2012
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Re: Where can I get a 12V timer switch?

Why do I want a timer for the heater? Well, the short answer is because I'm absent-minded. I WILL forget it and leave it on all day while motoring, and then it will drain my batteries while I'm sleeping.

I don't see adding the switch to be too burdensome. I need some kind of switch anyway (this will be 12V so I won't be plugging it in). The setting of an alarm is a good idea too but my watch is analogue and setting an egg timer from the galley is a 2 step process. Besides, over the rattle-bang-clank of my perkins I'd never actually hear the timer.

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Old 03-31-2012
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Re: Where can I get a 12V timer switch?

Ah, okay. what you want is a VW gas heater timer switch.
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Old 04-01-2012
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Re: Where can I get a 12V timer switch?

Many older washing machines and other appliances( MWs) used mechanical timers.If it goes click click it's what you want. I use them for filling day tank and fans in heads .They are made for 15 amp at 110 ac so plenty good .
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Old 04-01-2012
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Re: Where can I get a 12V timer switch?

Quote:
Originally Posted by tap View Post
If it's rated for 20A at 120V then it will do 20A at 12V too. The issue is that a 1kW heater that uses 120V would draw about 10A or so. If it used 12V, it would be need near 100A.
Exactly. Amps are amps from the conductors perspective. The voltage matters to the load, the insulation and any switches in the path.

You havn't said how much current the heater needs but as someone said, heaters will tend to use a lot and it might be necessary to wire the timer to a relay. That would be easy to do if you can find the right relay. Luckily automotive 12V relays are cheap and there are lots of them. Here is an example of a fully sealed chassis mount 12V relay with a resistor built in.

http://search.digikey.com/us/en/prod...2164-ND/646993

Also here is a 5-star timer switch on amazon. It appears to be mechanical although I'm not sure.
Amazon.com: Intermatic FD15MWC 15-Minute Spring Loaded Wall Timer, White: Home Improvement Amazon.com: Intermatic FD15MWC 15-Minute Spring Loaded Wall Timer, White: Home Improvement



Last edited by asdf38; 04-01-2012 at 12:37 AM.
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Old 04-01-2012
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Re: Where can I get a 12V timer switch?

The pad heaters I'm looking at are from Titan:
Titan Engine oil pan heaters, filter heaters and battery warmers. Designed for cars, diesel engines, more effective than block heaters, magnetic heaters. Suitable for Volkswagen vw beetle, Audi, Land Rover, Ford, Chrysler, GM,



I was thinking either the 12A or 20A (12V) models would work. That equates to 125W or 250W.

I'm not sure I believe that "an amp is an amp" in this application. I'm more worried about resistance and voltage. The wall light switch timer is meant for 120V. It will have some resistance based on the size and material used in the connections. Since 120V overcomes reisistance quite easily, there is normally minimal heat generated at the switch. If one were to run 15A at 12V through the same metal, if it is not appropriately sized it might get hot from the resistance.

If this were not the case we could all be using 14g wire (what's used for a 15A house circuit) for extremely long wire runs on the boat without fear of setting fire to the wire....

Is this correct? Is there a way to convert from the 120V safe rating to a 12V safe amp rating?

This website sells LED 12v lights. They say (without references) that the standard 15A wall timer switch is only good for 4amps of 12V....
http://www.backwoodssolar.com/catalog/lights.htm


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Last edited by MedSailor; 04-01-2012 at 01:03 AM.
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Old 04-01-2012
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Re: Where can I get a 12V timer switch?

I don't think so. It depends how it is built. For example some switches can handle the same amperage @12 volts as 120 volts, or nearly as much. But some only handle a fraction of the 120 volt amperage @ 12 volts.

Use a relay and it isn't an issue.
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Old 04-01-2012
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Re: Where can I get a 12V timer switch?

Quote:
Originally Posted by MedSailor View Post
The pad heaters I'm looking at are from Titan:
Titan Engine oil pan heaters, filter heaters and battery warmers. Designed for cars, diesel engines, more effective than block heaters, magnetic heaters. Suitable for Volkswagen vw beetle, Audi, Land Rover, Ford, Chrysler, GM,



I was thinking either the 12A or 20A (12V) models would work. That equates to 125W or 250W.

I'm not sure I believe that "an amp is an amp" in this application. I'm more worried about resistance and voltage. The wall light switch timer is meant for 120V. It will have some resistance based on the size and material used in the connections. Since 120V overcomes reisistance quite easily, there is normally minimal heat generated at the switch. If one were to run 15A at 12V through the same metal, if it is not appropriately sized it might get hot from the resistance. Likewise, the resistance in the switch would have the same drop regardless of the voltage it's passing.

The challenge for the switch, where voltage DOES matter, is when it has to make and break connections. That's why you see the voltage rating on the switch and I described the difference between AC and DC in my first post.

If this were not the case we could all be using 14g wire (what's used for a 15A house circuit) for extremely long wire runs on the boat without fear of setting fire to the wire....

Is this correct? Is there a way to convert from the 120V safe rating to a 12V safe amp rating?

This website sells LED 12v lights. They say (without references) that the standard 15A wall timer switch is only good for 4amps of 12V....
Lighting


MedSailor
Med, as far as ohms law and conductors an amp is exactly one amp. Ohms law tells you the voltage drop across a resistance due to current. V=I*R where V is the voltage drop. Here is the calculation for voltage drop across a 100' 14awg wire carrying 15A.

14AWG wire is 2.52 ohms per 1000' so 100' would be 0.252 ohms
0.252*15A=3.78V drop

So a 12V run drops to 8.22V
A 120V run drops to 116.2V

See what's happening here? The voltage drop is exactly the same but you just can't tolerate a 4V drop on a 12V bus. On 120V line you just don't care - 116V is just fine. The wire in both cases is dissipating exactly 3.78V*15A=56.7W which it can do because that 56W is being dissipated evenly over 100'.

"Is this correct? Is there a way to convert from the 120V safe rating to a 12V safe amp rating?"

It's up to switch and relay companies to rate their devices at different voltages but I gave you a reference point in the first post and I'm surprised by this 4A number. That said it seems that you're pretty safely into the external relay territory if you're drawing 20A.

Last edited by asdf38; 04-01-2012 at 12:20 PM.
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Re: Where can I get a 12V timer switch?

Quote:
Originally Posted by asdf38 View Post
Med, as far as ohms law and conductors an amp is exactly one amp. Ohms law tells you the voltage drop across a resistance due to current. V=I*R where V is the voltage drop. Here is the calculation for voltage drop across a 100' 14awg wire carrying 15A.

14AWG wire is 2.52 ohms per 1000' so 100' would be 0.252 ohms
0.252*15A=3.78V drop

So a 12V run drops to 8.22V
A 120V run drops to 116.2V

See what's happening here? The voltage drop is exactly the same but you just can't tolerate a 4V drop on a 12V bus. On 120V line you just don't care - 116V is just fine. The wire in both cases is dissipating exactly 3.78V*15A=56.7W which it can do because that 56W is being dissipated evenly over 100'.

"Is this correct? Is there a way to convert from the 120V safe rating to a 12V safe amp rating?"

It's up to switch and relay companies to rate their devices at different voltages but I gave you a reference point in the first post and I'm surprised by this 4A number. That said it seems that you're pretty safely into the external relay territory if you're drawing 20A.
This makes sense. The part I'm worried about is the possibility low conductivity parts inside the switch. After all, it's 120V right? Who cares about a little resistance. If the contacts are small and the resistance causes you to dissapate that 56.7W in a 1/8" piece of metal inside the switch you could be seeing some ...

One experiment is to just hook it up and watch it with an IR thermometer.

So forgive my electrical ignorance. How does one go about setting up a relay? (I have a feeling Bjones's egg timer is going to suddenly look more attractive...).

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Re: Where can I get a 12V timer switch?

A relay is like a solenoid. The switch controls low amperage to the relay which then switches internally to control whether the higher amperage line is isolated or connected.
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Old 04-01-2012
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Re: Where can I get a 12V timer switch?

Quote:
Originally Posted by MedSailor View Post
This makes sense. The part I'm worried about is the possibility low conductivity parts inside the switch. After all, it's 120V right? Who cares about a little resistance. If the contacts are small and the resistance causes you to dissapate that 56.7W in a 1/8" piece of metal inside the switch you could be seeing some ...

One experiment is to just hook it up and watch it with an IR thermometer.

So forgive my electrical ignorance. How does one go about setting up a relay? (I have a feeling Bjones's egg timer is going to suddenly look more attractive...).

MedSailor
You're on the right track but two things:
-If they rated the switch for 15A then it can handle 15A (if they rated it properly). No IR thermometer needed.
-I can tell you from experience that a switch that size can't dissipate more than a couple to a few watts at most (1-6ish). 50W in that small a size is on the order of a computer CPU which needs a large heat sink and powerful fan. So the wattage dissipated in a switch like that isn't going to add up to anything significant.

Adding a relay is really easy. You want one with an built in diode or resistor (the magnetic coil in a relay is inductive and when it's turned off it's current needs somewhere to go. A diode or resistor dumps this energy). The one I linked takes quick connect contacts and will add a total of two wires to your setup. The timer switch switches 12V to the relay coil, the relay switches 12V to the heater.

This also has the advantage that the timer switch is now only switching ~0.1A. The wires to/from the timer switch can be minimum size and the timer can easily be far away from the relay which should stay close to the high current heater to keep the heavy wire runs to a minimum.


Here is a schematic:
Attached Thumbnails
Where can I get a 12V timer switch?-img_0743.jpg  

Last edited by asdf38; 04-01-2012 at 05:24 PM.
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