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# 12 VDC transformer to 3 VDC

#1
03-31-13, 08:04 AM
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Join Date: Mar 2013
Posts: 24
12 VDC transformer to 3 VDC

Hi, I have a 12 volt DC transformer but I need a 3 VDC output. What resistance Ohm/watt should I use? I had an electronic lesson 30 years ago so I have some knowledge in DC current. I have a VOM and still have many resistances. It will be used to connect a 3 volt door chime that is batteries hungry. If you also have the time for a little explanation about the calculation I will surely understand it. Thank you

Last edited by Pinotte; 03-31-13 at 08:42 AM.
#2
03-31-13, 08:33 AM
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Join Date: Oct 2012
Posts: 68,820
You can't just use a resistor. A resistor value is calculated by load. When your electronic doorbell is sitting there idle it draws a very small amount of current. Therefore you would need a fairly high value resistance to arrive at the 12 volts.

However..... when you pushed the doorbell button and activated the chime.... the chime would require a lot more current and the resistor you had previously used would be of too high a value.

You will need to regulate the voltage down to 3vdc or purchase a 3-5 vdc adapter.

At the very least ..... to figure a resistance vale......you would need to measure the current draw of the unit. (will be in milliamps)

E=IxR I=E/R R=E/I
E= volts ... I= current ... R= resistance

Using ohms law..... E=12 (for 12 v)

So R=12 x the measured current

#3
03-31-13, 08:55 AM
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Join Date: Mar 2013
Posts: 24
Hi, thank you for the explanation. Is it easy or costly to regulate the voltage down to 3 vdc? I have many 12 volt transformers but I will buy a 3 vdc one if I need too.

#4
03-31-13, 09:07 AM
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Join Date: Oct 2012
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Depending on the load..... a resistor and a zener (regulating) diode may work.
That method is kind of old school and power wasteful but it's inexpensive.

The better way is to use a 3 terminal regulator. That and a small capacitor is all that would be required for an excellent regulator. I left you a link for a 3.3 volt version which is also fine. You can also search for 3 volt positive regulator. They are widely available.

LM2937ET-3.3v : Low Dropout Regulator

#5
03-31-13, 11:33 AM
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Join Date: Mar 2013
Posts: 24
#6
03-31-13, 12:04 PM
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Join Date: Oct 2008
Location: New England
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Hi Pinotte,
Another approach would be to use the zener in series with the 12 vdc. By using a 9v zener and a resister to maintain a small load, you will only see 12v - 9 = 3 vdc where you want it and it will draw very little current when not in use. Not sure what exact zener voltages are available, but something like 8.8 volts would be fine. The resistor can be anything above 1,000 ohms to just keep the diode active.

PJ or I can draw it up for you if needed.
Bud

#7
03-31-13, 01:52 PM
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Join Date: Mar 2013
Posts: 24
Hi Bud

I will use a regulator board but feel free to draw the diagram using the zener diode for learning purpose.

#8
03-31-13, 02:05 PM
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Join Date: Jun 2010
Posts: 723
You can't transform DC via a transformer.

#9
03-31-13, 02:58 PM
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Join Date: Mar 2013
Posts: 24
@2granddaughters

I do not want to transform dc, I want to reduce voltage to 3 volt dc.

M1201 DC 3~40V to 1.25~35V Adjustable Step-Down Converter Voltage Regulator - Blue - Free Shipping - DealExtreme

Last edited by Pinotte; 03-31-13 at 03:56 PM.
#10
03-31-13, 07:29 PM
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Join Date: Oct 2012
Posts: 68,820
That's a pretty impressive regulator board for the price. (and free shipping)
Will be perfect for your application.

#11
03-31-13, 07:33 PM
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Join Date: Mar 2013
Posts: 24
Thank you for the help and the confirmation about the regulator board.