# Vary Power Supply Voltage



## Grandcheapskate (Jan 5, 2006)

I was recently gifted with a new 18.8v 10 amp power supply by one of our dear friends here on HT. What I would like to do is find a way to vary the voltage.

I went to Radio Shack looking for a variable resistor but came up empty. What I can do however is get some different size resistors and choose which one to insert inline to create the (approximate) voltage I want.

If my basic electrical knowledge is correct, the formula E=IR should tell me what size resistor I need to generate a specific voltage. But I'm not sure I know how to apply that formula. For example, to create a 12v output, is it simply 12=10R and therefore I need a 1.2 ohm resistor? I don't think that's right since the same formula would tell me I need an 1.88 ohm resistor to generate the 18.8 volts I get now.

In this same vein, I want to use a wall wart to power a Tyco lap computer. These older (1989) lap counters require 4 size C batteries to run the computer. I'd like to hook it up to a power source so I don't have to use batteries. So I have the same problem here. What size resistor do I need to lower the 16v wall wart down to 6 volts?

Thanks...Joe


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## rbrunne1 (Sep 22, 2007)

Joe,

The formula is correct, but it calculates the voltage drop across the resistor, not the resulting voltage. Back to your example, a 1.2 ohm resistor at 10A would have a 12V voltage drop; therefore the resulting voltage would be 6V. 

Note that the voltage drop is dependant on the actual current, not the rated maximum of the power supply. You can easily measure the current by placing a .1 ohm, 10W resistor in the circuit, measure the voltage drop across the resistor and multiply by 10.

You need a 6V drop. Most cars draw a couple of amps, so a 3 ohm, 25W resistor would get you close. If you want variablity, you could use a variable resistor or rheostat.

Using a resisitor in this way is not ideal as it converts the unused energy to heat - make sure the resistor is well ventilated and protected from contact as it may burn you if you touch it!

If you want to control the voltage, you can add a zener diode to the circuit to keep the voltage constant regardless of current. See http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/zenereg.html#c1 for an explanation.

Hope this helps,

Bob B.


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## AfxToo (Aug 29, 2003)

Providing linear voltage variability _after the fact_ on a fixed output power supply can be prohibitively expensive because the higher currents at the output require higher power electronics, which are more expensive. Power supplies that are designed to be variable typically provide the variability in the parts of the power supply circuitry that are not operating at full output current and thus can use less expensive components. 

You could use a high power rheostat like these (http://www.ohmite.com/cgi-bin/showpage.cgi?product=rheostats_wirewound). Your power supply is putting out a max of 188 watts so you would want to use the 225 watt unit to give you full range of voltage control. This would be a pretty massive device which would need be mounted in a ventilated enclosure. It would cost at least $150 just for the rheostat. This does not make economic sense. 

If you want a fixed voltage output you could use high power resistors (200 watt) configured as a *voltage divider*. These tend to run $5 - $10 each and you would need two.

Another way to get fixed voltage steps is to use high current semiconductor junctions, like rectifier diodes. Each one would have to be capable of handling say 50V and 10 amps. These are about a buck each. This is a good option for getting small steps. At about 0.5V drop per diode you would need 10 to 12 connected in series to take you from 18V to 12V. If you use a multi-position rotary or slide switch you could use these diodes to give you multiple fixed choices for voltage output. While not continuously linear like a rheostat (or some other electronic voltage controller) this would be an inexpensive way to get a handful of different fixed voltages, say 18V - 15V - 12V - 9V - 6V. The switch contacts must be rated at 10A or greater. Mount it in a project box with binding posts or terminal strips. This is probably the least expensive option that gives you some variability but will still cost you $20 - $30 depending on how fancy you get.

The least expensive option, if you don't need anything above 12V, and don't need 10A, may be to buy one of the cheaper power supplies that give you 12V and maybe a few steps below 12V (like 8V and 6V) at around 2A max. The GJet power supply and its generic equivalents fit into this latter category. 

I run all my cars at 18V and life is good.


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## Grandcheapskate (Jan 5, 2006)

Thanks guys.

No matter how hard I try, electronics just never sink into my rock hard head. This sounds like more trouble than it's worth. I think I'll just run at 18.8v and if I want something less, I'll just convert one of the extra PC power supplies I have.

Thanks...Joe


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## 22tall (Jul 28, 2002)

Joe, If you have a Radio Shack you can probably get a 6 volt wall wort from them. I have gotten a 3v and 9v there so they might have a 6v.


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## NTxSlotCars (May 27, 2008)

Im gonna use this Rheostat by Ohmite. I paid 7 or 8 bucks for it.

















It's pretty heavy duty. I'm using 2 12v deep cycle batteries. (24v)
Isn't the resistance and heat buildup dependent on the current or amp draw?
I may have a heat problem on high powered cars, but I should be okay with stock stuff.


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## AfxToo (Aug 29, 2003)

> Isn't the resistance and heat buildup dependent on the current or amp draw?


The power is proportional to the voltage drop and the current, so yes, if you keep the current low the power generated will be proportionally lower. However good design practice and safety means designing for worst-case and using protection devices like fuses that ensure the current and power will never exceed the specifications of the weakest link in the circuit. 

Rheostats must drop any voltage not delivered to the load, so the worst case occurs at low output voltage. If you want variability down to 6V for example, the rheostat must drop the other 12V and the power dissipated in the rheostat must be able to handle it.

For example, if you used this device:

http://cgi.ebay.com/OHMITE-Rheostat...EW-in-Box_W0QQitemZ140307215039QQcmdZViewItem

you could safely drop the voltage to 6V as long as you limit the current to 4A. Since most stock cars draw around 300 mA to 500 mA maximum at steady state, this would give you 2X current margin with stock cars. 

Having a higher current power supply, even 10A, much less a battery with 500A or more requires a lot more caution and thought to go into what you are doing and the potential safety issues that are involved. If you are going to design with margins that are less than the worst case, make sure you incorporate protection devices.


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## Dslot (Sep 2, 2007)

Joe,

There is some info you might find useful in the *Low Cost Variable Voltage* thread from April. Jerzinferno has picked it up again in the *Hey Dslot* thread in the current *Slot Car General Discussion* section of Hobby Talk. 

The issue is about a $20 router speed control unit, and whether it can provide a good method to vary the input power to powerpacks and wallwarts. If so, it might be a good choice for you.

-- D


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## Grandcheapskate (Jan 5, 2006)

Hi Dslot,
I remember reading that older thread when it was new; I forgot all about it. Now I'll follow the new thread to see if the router control works.

However, I'm beginning to think I may just run at 18.8v with this new power supply and use a computer power supply to give me 12v or 17v

Thanks...Joe


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## resinmonger (Mar 5, 2008)

Wowzers, Rich! Two gnarly 12 volt deep cycle battries beat the stuffing out of my hamster wheel power supply...


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## NTxSlotCars (May 27, 2008)

resinmonger said:


> Wowzers, Rich! Two gnarly 12 volt deep cycle battries beat the stuffing out of my hamster wheel power supply...


I've got em under the track at the front of my garage, so I can jump start my Peterbuilt if I need to.
:tongue:


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## Grandcheapskate (Jan 5, 2006)

Dslot said:


> Joe,
> 
> There is some info you might find useful in the *Low Cost Variable Voltage* thread from April. Jerzinferno has picked it up again in the *Hey Dslot* thread in the current *Slot Car General Discussion* section of Hobby Talk.
> 
> ...


 I ordered two of these the other day at $15 each (about $18 each with shipping) and tried them this morning. They worked great.

I currently still have four Aurora 20v-22v power packs (one per lane) plugged into a power strip. I just plugged the power strip into the router control and played with the setting; it's a rotary dial that you can vary from zero to full power.

Different chassis required different settings to run as desired. I have one JL car (from release 1) that is just too fast to control on 20v; the controller is like an on/off switch (for just this one car). I turned down the voltage and found the sweet spot where this car can now be driven. However, that voltage was too low for almost anything else; an Aurora T-Jet just limped around.

So I would put a car on the track, turn the voltage down low, press the controller as little as possible and then turn up the voltage until the car started to move. This was the sweet spot for that car and it would drive great.

Of course, you can't do this every time you put a car on the track, but it does give you the ability to set the voltage where you want. So now you can save money by not buying a power supply with a voltage regulator as this device seems to do the trick.

Interesting side note. I put the router control on a 22v Aurora pack and tested with a voltmeter. Sure enough, as I turned the dial, the output from the power pack went up and down. But here's the interesting thing. When I hooked the voltmeter to the output of the router control, the reading stayed at 115v AC no matter how I turned the dial; I thought the output voltage from the router control would decrease, but it didn't. I'll never understand electricity.

Thanks...Joe


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