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Old 10-02-2016, 10:44 AM   #1
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Hot SPST Switch

I’ve discovered that I have two SPST switches on my flybridge console that become real hot to the touch when energized. One services running lights, the second an engine room ventilation blower.

The running lights are standard incandescents in “new” fixtures added about 2 years ago. The lights seem to be at normal intensity and typical heat output. The blower is basic 12v dc that is probably original equipment. The blower is not overheating and functions normally.

My thought is that the switches (likely original equipment) are internally cruddy, creating a high resistance at the contact point, dictating replacement. Does this sound reasonable or am I possibly missing a downstream problem in the circuit?

The second question is: The pricing of replacement SPST switches is all over the place. As are the constituent components. Plastic, stamped brass, with full boots or half boots, screw connections or potted with leads, etc., etc.

Third: The price differential between, say, a 15 amp and a 50 amp switch is slight ($7-8). Any advantage (or disadvantage) to replacement with a higher rated switch?

Appreciate the help.
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Old 10-02-2016, 10:54 AM   #2
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Could be bad switches. Most digital meters, you can measure amp flow inline. See if it is reasonable currents.
Under the load, measure the volts at the powered devices, if it is low could also point to high resistance in the switches. Jumper over the switches and see if the voltage rises.

Up on my flybridge, I have those push-pull round chrome knob switches. I have cleaned them before by using PB Blaster. Take them off and saturate the inside, then work them. It dissolves corrosion off the brass contacts. I have also taken those apart and repaired them, but I like to do things like that.

Lower helm, I have Carling breaker switches. Get a breaker and a switch in one. They are not too expensive on Ebay. Just bought a 20 amp one for $8.95. It was NOS (new old stock).

Breakers are sized to protect the wire, but a switch going with higher amps is ok, if you want to spend more money than needed.
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Old 10-02-2016, 11:50 AM   #3
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Good idea sdowney - I'll check that this afternoon. Due to space/layout, I think I'm stuck with the toggle arrangement.

I see I didn't complete question 2 - The second question is: The pricing of replacement SPST switches is all over the place. As are the constituent components. Plastic, stamped brass, with full boots or half boots, screw connections or potted with leads, etc., etc. Any preferred configuration for this use?
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Old 10-02-2016, 11:53 AM   #4
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The size of the wireing may also be marginal causing low voltage which will increase the current draw. Look at the total length of the circuit run and see if the wireing is adequate for the blower. The running light are a small draw even with incandescent bulbs.
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Old 10-02-2016, 11:57 AM   #5
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I don't have any booted switches on the boat except at the windless.
I prefer screw connections, wire leads can break off. At least it makes for easier repairs having a screw.

I prefer screws over spade lugs too, I think they offer more secure connections, especially if more exposed to the weather.

Any metal handled switches I prefer outside in the weather, I just think they hold up better to the sunlight. Never seen one rust on me.
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Old 10-02-2016, 12:29 PM   #6
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A failing switch can indeed have too much internal resistance and get hot even if everything else is good. Another possibity are the connections AT the switch. A poor connection can also get very hot and be felt in the switch.

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Old 10-02-2016, 12:31 PM   #7
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Stupid question, but do these switches have indicator lights incorporated into them that illuminate when the switch is selected ON?
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Old 10-02-2016, 01:14 PM   #8
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Replace 'em with waterproof toggle switches. I got mine from McMaster-Carr, an over-the-internet supply company. You may find that the waterproof ones are slightly larger behind the mounting, and might have a larger diameter penetration through the mounting. A moment's work with your Dremel.
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Old 10-02-2016, 01:35 PM   #9
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HEXSEAL® Sealing Boots - APM Hexseal | Digi-Key
I have a couple boots which I have not used yet.
These screw on to seal the toggle.
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Old 10-02-2016, 01:36 PM   #10
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Hot to the touch is a warning sign of high resistance. Nav lights draw about one amp each so thats only 4 amps. Blower fans will need a little homework. To make it easy, buy switches with like kind connections and at least 10 amp DC ratings from a reputable manufacturer. A rubber boot keeps the water out.
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Old 10-02-2016, 05:16 PM   #11
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Agreed with above statements. Mostly.
Simplest way to evaluate a switch is either:
1. With the switch made, ON that is, use your volt meter to measure the voltage DROP from one terminal of the switch to the other. Should be .02 VDC or less. If you have a volt or more replace the switch. Higher ampacity at rated voltage for a switch never hurts if it fits. Use ring lugs on screw terminals.
2. Turn off the circuit, Remove the wires measure the resistance of the switch with it ON. If more than one Ohm. Replace as above. 0.2 Ohms would be acceptable but high for my criteria.

Are the wires getting hot also? Away from the switch. Hot wires suggest circuit problems as others have noted.
Good luck, Dave
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Old 10-03-2016, 06:27 AM   #12
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One problem with DC switches is they are liable to fail in the ON position.

This can be hard on the batt if you don't notice.
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Old 10-03-2016, 07:20 AM   #13
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Quote:
Originally Posted by Toolbuddie View Post
Simplest way to evaluate a switch is either:

1. With the switch made, ON that is, use your volt meter to measure the voltage DROP from one terminal of the switch to the other. Should be .02 VDC or less. If you have a volt or more replace the switch. Higher ampacity at rated voltage for a switch never hurts if it fits. Use ring lugs on screw terminals.

2. Turn off the circuit, Remove the wires measure the resistance of the switch with it ON. If more than one Ohm. Replace as above. 0.2 Ohms would be acceptable but high for my criteria.
Straightforward diagnostics with a multi-meter. Excellent!
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Old 10-03-2016, 08:02 AM   #14
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Originally Posted by Comodave View Post
The size of the wireing may also be marginal causing low voltage which will increase the current draw. ............
That is incorrect and bad advice. Any resistance in the wiring will reduce the voltage and reduce the current draw. Honestly, I don't know how this crap keeps getting posted on the internet.

If the switches are getting hot in use, they should be replaced, It a simple and inexpensive job. Make sure the replacement switches are rated for the current draw of whatever they are switching on and off. Or, match the switch capacity to the fuse or circuit breaker that protects the circuit.


kchace is correct that resistance at the connections can cause a switch to overheat so check for this condition.


As for different construction and price, we are talking three or four dollars for a cheap switch and perhaps twenty dollars for a good one. Go to a marine store and buy the good one.
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Old 10-03-2016, 11:02 AM   #15
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Wes K is correct, just replace the switches. Two points- If you are using the same connection points on the wires(i.e. stakons),make sure you clean, or replace with new. Second,most VOM's are only rated for a ten ampere feed thru, at which point you will either blow an internal fuse, or damage the VOM. The correct instrument to use is an amprobe.
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Old 10-03-2016, 03:51 PM   #16
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That is incorrect and bad advice. Any resistance in the wiring will reduce the voltage and reduce the current draw. Honestly, I don't know how this crap keeps getting posted on the internet.
.
Ohm's Law E (voltage) = I * R
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Old 10-03-2016, 07:12 PM   #17
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What Wes is forgetting is Watts Law. P=IxE. Power is equal to I (current) times E (voltage). Every device requires power to operate, in this case a blower. If the voltage is low then the current must go up in order to get the power (watts) that the device needs to operate. Yes Ohms Law does not include watts, but you must include it in the overall picture. You still need so much power to operate the device and in order to get it if there is resistance in the wiring due to undersize wire, the current flow will go up and that is when it gets hot. The device will draw the power it needs, it can get it from higher voltage or higher current. If the voltage is low, the current draw will go up, unless it exceeds the breaker rating. That is why we have breakers and fuses to protect the wiring. The switch can also get hot due to higher current flow.
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Old 10-03-2016, 09:11 PM   #18
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I'm not forgetting anything. I did this stuff for a living.


Forget Ohm's law, forget the math, consider this: When the batteries get weak in your flashlight, does the bulb shine brighter or dimmer?


When the voltage goes down, the power goes down.
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Old 10-03-2016, 09:41 PM   #19
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Ok, this is my last post on this subject since I do not want to get into a contest. Unfortunately you can not ignore Ohms Law, Watts Law, physics and the math because they do tell you what you need to know when you are calculating voltage drop, current draw and power consumption. I guess what you are telling me from my original post is that wire size does not matter in voltage drop and current draw. Ok, I am glad that I never had you do wiring on my boat. I will continue to use adequate size wiring. Comparing a flashlight with a dead battery is apples and oranges to a 12 volt system that can supply plenty of current to undersize wiring.
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Old 10-04-2016, 01:55 AM   #20
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Using Ohm's law (E=I*R) is correct and easy to prove in this case.
Go to your boat and turn on lights one at a time and watch the amp meter go up while the volt meter stays the same provide you have a good charge on your batteries. E = the battery voltage measured by the volt meter, I = the current measured by the amp meter, and R = the lights (resistance).
If you leave the lights on long enough your battery voltage will begin to drop (assuming your battery charger is off) the current will also go down and the lights will begin to dim. This is also the case of the flashlight getting dimmer because the batteries get weaker.
So as the resistance goes up, the current goes up..... to a certain point at which time the circuit breaker trips. If the resistance goes up and the current was to go down, which it doesn't, the circuit breaker would never pop.

Using Watt's law P=I*E does not apply in this case. If the voltage is low then the current must go up in order to get the power (watts) that the device needs to operate is not true.
If the voltage is low (or the current is low) then the device will not operate properly or possibly not operate at all. The blower may not work at all, or may blow less air, or may become hot and get highly resistive and pop a circuit breaker (another discussion). A light will just get dimmer.
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