# Voltage dropping resister?



## Old Iron (Feb 3, 2014)

I have a 3 volt Dc power supply and I need to drop the voltage to 1 1/2 volts. Need a little help figuring out what resister to use and at what wattage?

Thanks
Paul


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## John Hasler (Feb 3, 2014)

Old Iron said:


> I have a 3 volt Dc power supply and I need to drop the voltage to 1 1/2 volts. Need a little help figuring out what resister to use and at what wattage?
> 
> Thanks
> Paul



The resistance needed to produce a particular voltage drop depends on the current.  Please tell us exactly what you are trying to do.


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## AlanR (Feb 3, 2014)

Use 2 diodes in series. The voltage drop across a diode is .6 to .7 volts. Very common technique.


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## Codered741 (Feb 3, 2014)

AlanR said:


> Use 2 diodes in series. The voltage drop across a diode is .6 to .7 volts. Very common technique.



If you use this technique, make sure that you take into account the ampacity of the diodes. Of course, it would still help to know exactly what your application is. 

-Cody


Sent from my iPad using Tapatalk HD


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## Wireaddict (Feb 4, 2014)

Resistance=Voltage across it [1.5V in this case]/ the Current that'll flow through it, &

Resistor Power rating in watts=1.25 X the Voltage across it [1.5V] X the current through it.  Hope this helps!


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## tigtorch (Feb 4, 2014)

As us aging EE's have long opined:

"twinkle twinkle little star, E is equal to I R"


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## Old Iron (Feb 4, 2014)

It is a liquid bath and I would guess the more parts I add the more currant it will draw. I really don't want to say what it is till I see if it works or not. The best voltage would be about 1 1/2 volts at 300 milliamp.

I have a variable voltage and currant power supply coming, If and when FedEx delivers it its been on there truck 5 days. They say it is because of the winter weather but the ice has been gone since Wednesday. 

So that problem will be fixed, I still want to lower the voltage on the other one for a friend to use if it works then I'll do a write up on it.

Thanks
Paul


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## John Hasler (Feb 4, 2014)

Old Iron said:


> It is a liquid bath and I would guess the more parts I add the more currant it will draw. I really don't want to say what it is till I see if it works or not. The best voltage would be about 1 1/2 volts at 300 milliamp.
> 
> I have a variable voltage and currant power supply coming, If and when FedEx delivers it its been on there truck 5 days. They say it is because of the winter weather but the ice has been gone since Wednesday.
> 
> ...



At 300 ma a couple of 1 amp diodes will give you approximately the right voltage drop.


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## Pmedic828 (Feb 4, 2014)

From a previous post, I understand that you want to drop the voltage by half.  If half is what you need, use 2 of the same sized resistors in series(from 1 power terminal, to 1 resistor, then to the other resistor, and then to the other terminal) and power your device across 1 leg and the center junction of the 2 resistors in series.  To determine the amperage or current that will be available, note the available amperage on your power supply.  If it is 1 amp, then this equals 1000 ma.  R=P/I squared so if your power supply is 12 volts and can output 1 amp, it would be 12/1 squared = 12 watts  Now determine R=E squared/p  so R=12 squared is 144/12 = 12 ohms so you would need 2 6 ohm resistors in series for 12 volts and 12 watts.
See this site for formulas: http://www.elec-toolbox.com/Formulas/Useful/formulas.htm
Without exact knowledge about power available, voltage and current needs, you can refer to this site.  If this is a LED project, you can use an online calculator found at: http://ledcalc.com/#calc.


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## Old Iron (Feb 4, 2014)

Ok thanks for all the help, I got my new power supply in today the only thing is it doesn't work.

I caller the company and there sending me another one, And they don't want the old one back.

Thanks again
Paul


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## AlanR (Feb 4, 2014)

John Hasler said:


> At 300 ma a couple of 1 amp diodes will give you approximately the right voltage drop.



1N4001, common as dirt, you can probably find some old or broken electronic device in your home with some in it.


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## Tony Wells (Feb 4, 2014)

I have a roll of ~10,000 1N4001 around here somewhere. Haven't seen them in a while. Maybe they are 4004...I think 4001 though.


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## higgite (Feb 4, 2014)

For your 300 ma load at 1.5 volts, your load resistance is 5 ohms. To drop your 3 volt supply to 1.5 volts at the load, you need a 5 ohm, minimum 1/2 watt resistor. This assumes (I know) that your power supply will maintain 3 volts at 300 ma. Be aware that if your load doesn't draw 300 ma all the time, the voltage across it will vary.

Tom


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## AlanR (Feb 4, 2014)

higgite said:


> For your 300 ma load at 1.5 volts, your load resistance is 5 ohms. To drop your 3 volt supply to 1.5 volts at the load, you need a 5 ohm, minimum 1/2 watt resistor. This assumes (I know) that your power supply will maintain 3 volts at 300 ma. Be aware that if your load doesn't draw 300 ma all the time, the voltage across it will vary.



That's why diodes are easier and better. 1N4001 - $1.34 at Radio Shack or a pack of 25 mixed ones for $3.49 (Package of 25 may include: 1N4001, 1N4002, 1N4003, 1N4004, 1N4005, 1N4006 and/or 1N4007).

The X in the 400X family denotes the voltage rating with 1 being the lowest, any will work fine.


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## Pmedic828 (Feb 4, 2014)

*Re: Voltage dropping resistor?*

the 4000 series of diode is the voltage rating for example, the 4001 might be rated for 25 PIV (peak Inverse Voltage) which means that the diode can only block the reverse current under 25 volts
as the numbers increase, if I can remember, the voltage increases, such as 50 V PIV, then 100V PIV, 
If you are using an AC source, the voltage inverses at whatever the Hertz (cycles per second) is.  
The best way to control a dc voltage is to get a standard power supply and use an IC chip voltage regulator to regulate the voltage that you need.  These voltage regulators are available for under $1 usually, and can be programmed with a resistor or purchased for a specific voltage such as +5 volts, -5 volts, + or - 12 volts, 3.3 volts, etc.  Jameco has many voltage regulators that can be purchased - just remember to protect your regulator, you will need some type of heatsink and thermal paste.

  		  				 				 				 				  From Jameco:
http://www.jameco.com/webapp/wcs/st...rch_type=jamecoall&freeText=voltage+regulator



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