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# Reduce DC Volts

#1
09-20-13, 06:05 AM
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Reduce DC Volts

I have a 12 volt DC electrical panel on my boat.
There are 8 12V 1A backlighting lamps that illuminate gauges in that panel.
These are mini incandescent lamps and are on a dimmer.
Over the years, many of these lamps have burnt out and are very difficult to replace because of the location of the panel.
I suspect they burn out when the battery charger goes into its automatic battery equalizing mode.
During this equalizing process, the voltage in the system rises to 13.8 volts for short periods.
I plan to put a resister in the backlighting circuit to reduce the voltage to the lamps.
I am prepared to accept a reduction in lighting brightness for the gauges in order to achieve a longer life of the backlight lamps.
What size of resistor should I use to reduce the voltage by 1.5 volts?
To sum up - Current draw 8 Amps on a 12 Volt DC circuit.
What size resister to reduce voltage to 10.5 on that circuit?
Thanks

#2
09-20-13, 07:51 AM
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Actually the dimmer you mention may be a resistor that is already reducing your voltage. 13.8 volts is in the normal range for 12 vot auto/marine bulbs. Have you considered replacing the incandescent bulbs with LEDs. You probably couldn't dim them but they would have a longer life. If you want to limit the voltage to the existing bulbs a voltage regulator using a zener diode would make more sense than a simple resistor.

Just an example: How to Build a Zener Diode Voltage Regulator

#3
09-20-13, 07:58 AM
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Welcome to the forums.

A 12 watt bulb would be massive for gauge lighting. (1A @ 12v = 12watts)
A 12 watt bulb is brighter than a brake light bulb.

If you're sure they are 12 watts then a single resistor would have to be massive.
Eight individual resistors isn't to practical either.

I would use a PWM type dimmer control. They make them in 6, 8, and 10 amp models.
Now with 12vdv LED lighting there are a million different ones on the market.

Do a quick search thru google and you'll see all the choices. The model in the picture is around 12 bucks and handles 8 amps of power. The picture says to LED strips but your bulbs will work just fine also.

#4
09-20-13, 09:26 AM
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This is the lamp - Incandescent 2194A1-12V mini indicator lamp.
There are 8 of them.
I assumed that the "A1" part on the label meant it was a 1 amp. bulb.
I need the dimmer function for traveling at night.
I thought the dimmer (built into the panel), was just a variable resistor.
I also thought a small resistor placed in series would solve the problem.
Where am I going wrong?

#5
09-20-13, 10:33 AM
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13.8V @ 1A x 8 bulbs is 110 watts. Dropping that approximately 10% (1.4V) is 11-12 watts. A resistor dissipating 12 watts is big & hot and a waste of energy. A dimmer ahead of your dimmer as PJmax suggests could be used to set your maximum brightness while the panel dimmer varies it from there down to minimum.
You could also use 2 regular diodes in series to drop about 1.3V. Half of a 10A bridge rectifier could be used, would be cheap and might be available with 1/4" push-on terminals to make wiring easy.

#6
09-20-13, 10:42 AM
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I Googled that number and found it is a sealed panel light with no replaceable bulb. The A1 refers to the color--red. Inside is a #2162 bulb consuming 1W so that brings you the possibility of using a resistor (or cheap diodes).

#7
09-20-13, 10:45 AM
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You can measure the MA with a multimeter.

#8
09-20-13, 04:09 PM
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If Guy48065 is correct, and I believe he is, each bulb is 0.10A - 14V -1.4W. (X8 bulbs).
Assuming these numbers, does a resistor in series make sense?
If so, what size resistor is needed to reduce the voltage by 1 to 1.5 volts?
!s there another easy way to extend the life of these bulbs?
I want to reduce the frequency of taking the panel apart to change these back-light bulbs.
Thanks all.

#9
09-20-13, 04:25 PM
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A resistor will do the job but dissipate heat.

A single diode in series with the lights will drop the voltage .7volts (with no heat)

I would place three IN-5401 diodes in series to drop the voltage. Those are 50v 3a diodes.
I buy them by the 100's for .06 each.

Any electronic store should have them as well as amazon and ebay.

#10
09-20-13, 09:26 PM
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ray2047 suggested using a zener diode to limit the voltage.
I'm not familiar with what that is but the idea of limiting voltage spikes has a ring to it.
It sounds like this method actually caps the voltage as opposed to reducing it by some degree.
The boat has large DC loads that are being turned on and off so I can imagine sharp voltage spikes and that may be why the backlight bulbs burn out in the first place.
I'm not sure if I'm up for creating something too complicated though.
On further inspection, it turns out there are 9 of these backlight bulbs in the panel.
More thanks

#11
09-20-13, 10:48 PM
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A zener diode would need to be used with a resistor. Back to the resistor again.

#12
09-21-13, 01:02 AM
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Just get an adjustable DC to DC converter module. Here is one with a five ampere output for less than \$10.

DC DC Converter Step Down Module 5 32V to 1 27V 12V 24V Power Voltage Regulator | eBay

I have three similar units powering my Internet connection, a 12 volt for the optical interface and second router, a five volt for the primary router and VoIP telephone adapter and one of the adjustable set for 9 volts to power a pair of Ethernet switches.

#13
09-21-13, 04:35 AM
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Do they make DC dimmers that do not consist of rheostats which incidentally are resistors that, for that number of amperes, are big and hot and a waste of energy?

With several small bulbs (perhaps totalling 12 watts) you could use small resistors, one at each light location so no one place gets all the heat, although the energy dissipated in all the resistors is the same as with one large resistor.

You might expect that the bulb itself would limit the amount of current so a diode could be used in place of a resistor, but the inrush current as the bulb is first turned on might fry the diode.

As late as the 1960's some of NYC's Broadway theaters had 120 volt DC power (as opposed to 120/240V, or 120/208V 3 phase, AC) and visiting theater groups has to put aside their state of the art dimmers and bring in rheostats some of which controlled over 2000 watts of lighting apiece.

Last edited by AllanJ; 09-21-13 at 04:55 AM.
#14
09-21-13, 04:44 AM
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My boat also has a dimmer for the gauge panel lighting. The solution is already in place: Don't turn the dimmer all the way up.

#15
09-21-13, 04:54 AM
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Might as well give it up Rick. No one is listening. In the first reply I wrote:
Actually the dimmer you mention may be a resistor that is already reducing your voltage. 1
but it was ignored.

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