# Where can I get a 12V timer switch?



## MedSailor (Mar 30, 2008)

I want to install an engine oil pan heater and it would be most convenient if I could have it on a timer switch. I have other items that would be amenable to a timer switch such as the propane solenoid.

Can I use one of these household ones or does it have to be a 12V switch?










I've owned cars that had the defroster on a timer (that was not adjustable) so I suppose this could be an option, though I'm not sure how to find one of those either.

MedSailor


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## asdf38 (Jul 7, 2010)

I don't see why not. Mind the current rating of course. When it comes to relays switching power DC is harder than AC but 12V is so much less than 120V that it should basically cancel. In-fact as a reference point the Bluesea switches I just put in my panel have this rating:

[email protected] AC, [email protected] AC, [email protected] DC

Note the 15A rating is the same for 125V AC as it is for 12V DC. That should roughly hold across other switches. The challenge with DC is that the voltage is constant whereas AC always crosses 0. If the switch arcs after breaking current the AC will extinguish when it crosses 0.


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## Waltthesalt (Sep 22, 2009)

I agree. It's a spring timer and contacts. Having a lower voltage only means you've got extra margin on the insulation. I think if there's any issue its using the switch in a humid marine environment where it's internals may corrode.


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## mitiempo (Sep 19, 2008)

Lower voltage but higher current probably.


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## MedSailor (Mar 30, 2008)

Waltthesalt said:


> I agree. It's a spring timer and contacts. Having a lower voltage only means you've got extra margin on the insulation. I think if there's any issue its using the switch in a humid marine environment where it's internals may corrode.


A spring with contacts. That makes sense.

The second part you mention, that you have an extra margin, makes me think otherwise though. It takes a lot more copper to transmit amps without heat than it does at 120V. That's why boat cables are so big. If it's rated for 20A at 120V it may not take 20A at 12V without melting, right?

It's late, I'm not sure I have that last bit right. Might have to run it by Ohm's law after coffee in the morning and see how it stands up to the light of day...

MedSailor


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## tap (Apr 1, 2009)

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.


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## ron_hudson (Dec 17, 2011)

Could you not use an electronic timer or at least hook it up to see if it works . Could you go through an inverter?


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## Tanley (Aug 20, 2009)

LED lighting, soundproof, Sailor's Solutions Inc.


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## hellosailor (Apr 11, 2006)

The spring timers are almost impossible to find these days, but Ace hardware still carries them, and their web site sells them for about 1/3 less than the stores--with free delivery to your local store. (Great logic, huh?)

Electronic timers could be a problem if they are all solid state (no relay) and intended for use with AC. You'd have to check with the maker for specifics.


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## bljones (Oct 13, 2008)

Why would you want it on a timer?
Unless you are seriously sub-zero for extended periods, there really is no need for proplonged or timed use on a pan heater- besides, since most spring timers have only an hour duration, if you will be gone from the boat for any length of time, the timer is almost useless.If you want a warm pan for ease of starting, simply plug your heater in before starting, while you are prepping the boat for departure.
If you are aboartd, and want to periodically warm up the oil for whatever reason, what I would do is set my watch, my alarm clock, the alarm on my laptop, whatever, plug in the heater, and when the alarm goes off unplug it. Done. For free.


Or just solve the whole problem by rolling Baffin Island style- start your motor in October. Don't shut it off until May.


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## MedSailor (Mar 30, 2008)

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. 

MedSailor


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## bljones (Oct 13, 2008)

Ah, okay. what you want is a VW gas heater timer switch.


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## Capt Len (Oct 9, 2011)

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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## asdf38 (Jul 7, 2010)

tap said:


> 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/products/CB1-R-SM-12V/255-2164-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 [email protected]@[email protected]@http://ecx.images-amazon.com/images/I/[email protected]@[email protected]@31gn6sXlLLL


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## MedSailor (Mar 30, 2008)

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

MedSailor


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## mitiempo (Sep 19, 2008)

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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## asdf38 (Jul 7, 2010)

MedSailor said:


> 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,
> 
> 
> ...


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.


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## MedSailor (Mar 30, 2008)

asdf38 said:


> 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
> ...


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 ...:hothead

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


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## mitiempo (Sep 19, 2008)

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.
Relay - Wikipedia, the free encyclopedia


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## asdf38 (Jul 7, 2010)

MedSailor said:


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


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:


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