12-volt light on 24-volts?

[ez-duzit]

A friend has installed a 12-volt VDO clock on his boat, which uses 24-volts for the panel back-lighting circuit. Is there a simple and safe way for him to drop the voltage to the built-in (non-accessible) 12-volt light? Like adding an in-line resistor? Has anyone done this who can recommend a work-around?

This is so he can have the night lighting switch on/off with his panel lights. The clock runs all the time, so he does not want the light on the same 12-volt circuit. And he would rather not have to add another switch just for the clock lighting.

All comments welcome.

 

[iced98lx]

Plenty of options for him, just search DC to DC converter on Amazon. Plenty of options that take 24v in and put 12v out, just need to match the size of the load.

This one takes in 15-40v and puts out 12v up to 5 amps.

 

[mcbane]

The cheapest fix is to use a resistor to drop down to approximately 14 volts and wire a 14v zener diode across the load to make sure you can’t get an overvoltage condition. I have a 24 volt truck but use 12 volt LED marker lights, which are way cheaper than the 24 volt version. All of the lights have the resistor plus diode detail.


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[dogdog]

DC to DC converters, don't use resistor to drop voltage... it can get really hot really quit...dependent on the AMP draw..

basically you are dissipate what ever the power drop as heat using the resistor... not a good thing sometimes...

 

[mcbane]

Backlight in a clock is going to eat milliwatts. So heat dissipated in a resistor will also be milliwatts. DC-DC converter for that application introduces cost and reliability issues that can easily be avoided.

 

[Sevenhills1952]

I'd still go with #2 suggestion. For $12 problem solved. Otherwise you'd have to know the current draw of the clock to calculate resistor value, then a zener.
$12 then you have 12v available anything else you may want to add later up to 5 amps.

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[californiaHank]

Something here just doesn't add up. You're saying that the clock is designed to run on 24V but that its internal backlighting is only rated for 12V? That just sounds weird. I've seen 12V instruments, and 24V instruments, and instruments that could be switched to run on either 12 or 24 volts, but never any that required both voltages at the same time.

 

[ez-duzit]

Thanks to everyone who have posted.

The clock runs on 12-volts and has a separate always-on circuit. The boat's panel illumination is powered by 24-volts.

 

[BillK]

Something here just doesn't add up. You're saying that the clock is designed to run on 24V but that its internal backlighting is only rated for 12V? That just sounds weird. I've seen 12V instruments, and 24V instruments, and instruments that could be switched to run on either 12 or 24 volts, but never any that required both voltages at the same time.

Thats not what he said. The boat panel is 24 volts. The clock is 12v.

 

[californiaHank]

Thats not what he said. The boat panel is 24 volts. The clock is 12v.

Ok. In that case the right answer is probably to buy a correct 24V clock. They're not exppensive, and IMHO, it's a better alternative than kludging up the instument panel on an expensive boat. (What boat isn't expensive? )

 

[slow]

can the lamp be swapped out for a 24 volt lamp?

 

[ez-duzit]

can the lamp be swapped out for a 24 volt lamp?

From original post:

...built-in (non-accessible) 12-volt light...

 

[slow]

missed that, sorry. I would hate to put a resistor voltage divider in a marine environment due to corrosion assuming saltwater based on your location. Is it an option to add a relay behind the dash to drive a 24Volt relay off the backlighting switch and use that to connect the existing 12V to the clock to the backlight circuit. This would be on/off no dimming, but unsure of the clocks or boats backlight system has dimming control.

 

[Sevenhills1952]

Iced gave the $12 solution in the second post. Somehow that's not acceptable?!?
Maybe suggest it to the boat owner letting him decide?

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[nadogail]

A marine supply distributor should be able to point you in the direction of the materials you are looking for.

 

[ez-duzit]

Thanks to all who replied. I have sent this to my friend. Looks like post #2 is how he will go.

 

[TuxThePenguin]

Can also use an adjustable buck (or buck/boost, though you don't need boost) converter, super cheap from Ebay if you don't mind waiting three trillion years for it to arrive https://www.ebay.com/itm/Adjustable...628838?hash=item5dab768166:g:DvMAAOSwLzBe9vKH

The module above is kind of nicer for the specific purpose, though, since it comes in an enclosure, and is higher current than the adjustable that I linked (you can get them in different ratings, though the one I linked is the common cheap one)

 

[iced98lx]

Thanks to all who replied. I have sent this to my friend. Looks like post #2 is how he will go.

Let us know how it works out. Not the most efficient solution power wise but for the situation (low power, KISS) seemed like a fitting choice.

 

[TuxThePenguin]

??? a DC to DC converter (buck converter in this case) IS the most efficient solution here.

 

[iced98lx]

??? a DC to DC converter (buck converter in this case) IS the most efficient solution here.

You are absolutely correct, I was attempting to be as PC as possible regarding other solutions proposed but this is the simplest and most power efficient way of handling this specific need.

 

[Shiftless]

It’s only an ace Demi’s question at this point but does anybody else think like I do that 2 12volt bulbs in series would be the answer?

 

[TuxThePenguin]

It’s only an ace Demi’s question at this point but does anybody else think like I do that 2 12volt bulbs in series would be the answer?

In the ideal case, that'd be fine, but if anyone were to install different bulbs (or even differently aged bulbs), the resistance could vary enough to give them fairly different voltages.

You could perhaps look at paralleling some other stuff to bias them to help in that case.

A buck converter should be around 85-90% efficient though, which is pretty good. A really good quality one can reach 95%+, though a cheap $5 or $10 jobber from a no-name company (likely) won't be quite that good.

 

[Shiftless]

Funny...I thought I typed in “academic” question. Apparently auto correct thought otherwise and I was careless and didn’t proofread.

Why did some logic circuit think Ace Demi’s was correct?