# Voltage coverters



## captaintom (Jun 15, 2016)

I bought some really good looking street lights on eBay.
Trouble is that they require 3v DC at 20mAmps.
I can find 12V to 3V converters but none at 20mAmps.

Any ideas where I can find one or how I can build one?

Thanks.

Capt Tom


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## wvgca (Jan 21, 2013)

you can use an ordinary 12v to 3v dc converter, even if the converter is, say a one amp, each light will only draw the 20ma that it needs .. multiply 20ma by the number of lights that you have, or intent to have, to get a minimum current rating for the converter, more is okay


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## captaintom (Jun 15, 2016)

*Voltage regulators*

Really appreciate the fast reply.
Your response makes my life easier.
This forum has always been a big help to me.

Capt Tom


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## captaintom (Jun 15, 2016)

*Voltage converters*

I found a few voltage converters on ebay. Since they were from China I bought two from different manufacturers - was hoping one would work. Both have similar markings, i.e. input and out put leads are marked and the item indicates that it would convert anything from 23 to 12 volt DC to 3 volt DC.

As it turns out when I hooked up the voltage converter to the lights my transformer shorted out. I had wired the input leads to my transformer and the output leads to the street lights. I did it the same way for both converters with the same results.
The lights themselves don't even flicker.

Any ideas as to what I am doing wrong? Any way to test whether the converters are working at all.

Thanks.

Capt Tom


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## gunrunnerjohn (Nov 10, 2010)

The DC<->DC converters are polarity sensitive on the inputs, so if you connected them backwards, you release the _*magic smoke*_.

It would help to know exactly what you bought and exactly how you connected it.


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## DonR (Oct 18, 2012)

You can likely find the exact power that you need 
with one or more wall warts from a good flea market. 
Each one has a label on it that shows AC or DC, the
voltage and m amps capability. Usually, 5.00 or so.
They'll bargain.

Don


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## T-Man (May 16, 2008)

Lemax, Halloween has 3 or 4.5 volt regulators for their lighted accessories. Ac Moore or Michaels. It is a local source.


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## captaintom (Jun 15, 2016)

*Voltage Regulator*

I tested my controller and found which is the positive side and which is the negative.
The voltage regulator(12vDC to 3vDC) (picture attached) shows the red wire is positive and the black negative. The output side also shows pos and neg leads. I hooked up the regulator to the controller (pos to pos and neg to neg) and connected the light. I got a series of sparks and the controller was shorting out. The light itself never flickered. I tried the same hook up with the other converter and it did the same thing.

The lights themselves do not indicate pos and neg.

What am I doing wrong?

Thanks for any help.

Capt Tom


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## Brakeman Jake (Mar 8, 2009)

Could you post a picture of the other components?Though possible,having two defective regulator doesn't seem likely.
What do the instructions with the regulators say?


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## wvgca (Jan 21, 2013)

You referred to a 'controller' ... if by that you mean a train set transformer, the DC out may be higher than 12V, depending on where the dial is, if you hooked up to the accessory terminals, those are quite likely AC, not DC , and are normally higher than 12V .. . best power source may be an ordinary wall wart, just make sure it's 12V dc output, and meter the output to find which is positive / negative ..the picture that you showed indicated 12V DC input only, not higher than that .. 

However, if you got sparks, that's not a good sign ..and replacement converters may be the next step??

On the LED itself, the positive lead is normally the longer of the two


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## bhabbott (Sep 28, 2016)

captaintom said:


> they require 3v DC at 20mAmps.
> I can find 12V to 3V converters but none at 20mAmps.


You don't need or want a 3V power supply. The LED itself drops (approximately) 3V, but current rises dramatically with even a tiny voltage increase so you need a higher voltage which is _limited_ to 20mA or less. 

Current can be limited with a resistor wired in series (between the LED and either power supply lead). The resistance value is calculated using Ohm's Law, resistance = voltage / current. For example if your supply is 12V then the resistor has to absorb 12V-3V = 9V. Applying Ohm's Law, 9V / 0.02A = 450 Ohms. The nearest standard value is 470 Ohms, which is close enough.

If the power supply voltage changes then the voltage across the resistor will vary and this will change the LED current, but _much_ less than it would without a resistor. Using 470 Ohms it would rise to ~23mA at 14V, and drop to ~6mA at 6V. Without the resistor, even a little over 3V would burn it out instantly.



> The lights themselves do not indicate pos and neg.


This could be a problem because LEDs can be destroyed if they get too much reverse voltage. However you should be safe testing the polarity with it connected through a current limiting resistor. Just try it one way and if the LED doesn't light you have it reversed (or it's blown up). Use lower voltage for testing (eg. 9V or 6V) if you are worried that 12V might be too much.


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## captaintom (Jun 15, 2016)

*Voltage Converters*

The controller is in fact my train transformer. I am hooking up to the DC terminals not AC. The out put is 14,5 volts at all dial settings. All my other lighted DC accessories work fine, although they are grain of wheat bulbs. 

But this is no linger worth the effort. They look good so I'll just use the fixtures as unlighted. I hate to give up but I am in the middle of doing my scenery and time is scarce as it is

Thanks for your help and suggestions.

Capt Tom


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## wvgca (Jan 21, 2013)

You can always look at this again when you have completed the portions of your layout that are a higher current priority .. most of us [hopefully] will still be around


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## bhabbott (Sep 28, 2016)

captaintom said:


> The controller is in fact my train transformer. I am hooking up to the DC terminals not AC.


DC is what you need. 

To get 20mA at 14.5V the correct resistor value is 725 Ohms, but anything close to that (eg. 1k) will work, and even 470 Ohms would be OK (current would only be slightly higher than the LED's rating). 



> All my other lighted DC accessories work fine, although they are grain of wheat bulbs.


Sure, because incandescent bulbs work more like a resistor. LEDs are different. An LED light which works on 12V already has a resistor built in. Yours don't, so you need to add an external resistor valued to suit the voltage you are using. 

BTW if your grain-of-wheat bulbs are rated at 12V then 14.5V will make them run hotter and burn out sooner than normal. LEDs last practically forever and produce almost no heat.



> But this is no linger worth the effort. They look good so I'll just use the fixtures as unlighted. I hate to give up but I am in the middle of doing my scenery and time is scarce as it is


I understand your frustration. Perhaps you could bring the wires out for use at a later date if you feel up to it...


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