Hello all,I bought this neat bath vanity mirror that has LED lights. It is made for residential homes and uses 110V with a transformer plug, of course.The transformer says the output is 9 volts and 1 amp. I would like to wire this to my RV. Is there a resistor of some kind that can accomplish this for me?Thanks in advance.

You would have to build a regulator circuit taking the 12 volts and regulating it to 9 volts like a lm 39 series regulator or the adjustable regulator circuit.

You can buy a adapter at Radio Shack with a 12 volt plug into a cigarette lighter, and 6, 9 and 12 volt output - adjustable. Check the amp rating, they have some only rated at 300 ma, while other models are rated over 1 amp. Basically they use a internal voltage reducer. . . Good luck,Fred.

Various versions of this product are plentiful online:http://www.amazon.com/RioRand-LM2596-Converter-1-23V-30V-1Pcs-LM2596/dp/B008BHAOQO/ref=sr_1_fkmr0_2?ie=UTF8&qid=1453848779&sr=8-2-fkmr0&keywords=voltage+bucker+lm+2596I use one on a Silverstone AP182 fan which does not like voltages over 13 on highest speed. The hub gets stinky hot.At 11 volts, no worries.

You have the LIGHT in your hand and two wires sticking out of the wall. How much ROOM in inches do you have to stuff in the hole the wires are sticking out of? This is a crucial step 1st step in determining what is correct for your situation.Some things can fit others can't.FOR EXAMPLEhttp://www.ebay.com/itm/9V-DC-Voltage-Regulator-Module-Board-Based-on-7809-/191278889509?hash=item2c891c3a25:g:ExYAAMXQHxVRigpv

There are a few ways you can accomplish this. A regulator like others have posted about would be the best performing way (give the stablest output). A switching regulator will be more efficient than a linear regulator, and so stay cooler.It would be possible to use a simple dropping resistor, too. However, to do that, you need to determine what the actual operating voltage and current of the light is by measuring it in operation with a meter. After that, it's a matter of applying a little math in the form of Ohm's law and related basic electrical principles.Let's say that you measure 9V and 600 mA. (These are, of course, not actual measurements!) Your voltage supply varies between maybe 12V and 14.4V, so perhaps take 13.2 as an average. The resistor needs to drop 13.2V - 9V = 4.2V at 600 mA, which works out to a nominal value of 7 Ohms. It is dissipating 4.2V x 600 mA = about 2.5W; a 5W resistor would be appropriate. (Incidentally, a linear regulator will be dissipating the same amount of power under these conditions.)At 14.4 V supply, this resistor would be dropping 5.4V (the LED's voltage is basically constant, at least for this sort of simple analysis), and the current would be about 770 mA, and the resistor dissipating a bit over 4W. The LED light would be overdriven somewhat as well, but if reasonably designed in terms of heat management should not have too short of a life. If you preferred, you could choose a higher value resistor and instead have it be dimmer under more typical 12V system voltages.It's possible the LED unit may have a built-in regulator and not need any sort of special circuitry to connect to a 12V system. It's impossible to know that without examining and perhaps disassembling it. My hunch would be that it has no such design nicety.

Reducing 12 VDC to 9 VDC | Good Sam Community

22 Replies

LarryJM
Explorer II
Feb 03, 2016
vacuumbed wrote:
I have decided to go with the buck converter, thanks for your help everyone!

Excellent decision:C

Larry
vacuumbed
Explorer
Feb 03, 2016
I have decided to go with the buck converter, thanks for your help everyone!
wa8yxm
Explorer III
Jan 31, 2016
Before you get too far along.. is that wall wart output 9 VDC or 9 VAC?

If it's 9 VAC then the peak voltage will be around 14 and the lights should be quite happy with battery power.. But on the other hand Might not.

Several of circuit boards that are a lot like the one someone pictured above can be had for less than a dollar.
liborko
Explorer
Jan 31, 2016
If you can solder, this may be a solution:

MC 7809 regulator

12V in, 9V out
MEXICOWANDERER
Explorer
Jan 27, 2016
Unless your DW has to stare at it while she's shaving :)
LarryJM
Explorer II
Jan 27, 2016
Shadow Catcher wrote:
I have a number of DC DC buck converters, This one is adjustable and from experience they do a good job of regulating voltage.
Buck Converter

The one I linked to is one of those and I agree they do a good job and for under $9 in this case is IMO probably the best option.

Larry
landyacht318
Explorer
Jan 27, 2016
HTElectrical wrote:
Most cheap transformers run at a higher voltage than they are supposed to.

When tested with zero load, in my experience. The wall warts I have tested read much higher than their output claims unloaded but when loaded even lightly, the voltage fell near to the labels specs.
HTElectrical
Explorer
Jan 27, 2016
Most cheap transformers run at a higher voltage than they are supposed to.
SCVJeff
Explorer
Jan 27, 2016
HTElectrical wrote:
Before you do anything, measure the output of the transformer. More than likely it is going to be at least 12V.
If the spec is 9V, why run it 25% over?
landyacht318
Explorer
Jan 27, 2016

I have two of these voltage regulators as speed controllers for powerful 92 and 80mm muffin fans. They do not fit tightly in a 12v power port and can be tweaked to the side where the Nipple(+) metal retainer comes in contact with the grounded exterior and blows the fuse feeding the 12v power port.

Some adhesive lined heat shrink over most of the tip but for the spring loaded nipple contact makes them usable.

It is also far too easy to reverse polarity when moving the voltage selector switch, which is also too easy to change to a different voltage.

Having lots of experience with this product, I'd not recommend such a device in this application, though it certainly could actually provide 9 volts at upto 2 amps.

The USB output is 500Ma only however.

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Reducing 12 VDC to 9 VDC

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