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Rheostat help??!??

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Rheostat help??!??

Unread postAuthor: Davidvaini » Thu Jul 30, 2009 1:32 pm

I've been searching for a device that can control the voltage coming out of a battery.

For example. I have a 9.6v Battery hooked up to a device. Sometimes I want that device to have a lower voltage.. say 8.4v.. but sometimes I want it to be in between. Max voltage would obviously be the max voltage of the battery.

Here is the kicker though, I want it to be able to work with batteries up to 12v. So sometimes a 12v battery will be thrown in, while other times a 9.6v or even a 8.4v battery would be thrown in.

Now I would prefer a turning knob so I can have a full range of adjusting voltages. A slider would work as well, but again it needs a good range of voltage.

I would also like this device to be cheap, easy to build or assemble(I like pre-made circuits), and small.

Now as small as possible would be nice.

The Batteries will be rechargeable air soft batteries as I have plenty of them and they are convenient. I am also building this thing for the use of these batteries and will not change batteries.

I'm purely a beginner when it comes to electronics and don't know a thing. But I can follow step by step instructions very nicely.
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Unread postAuthor: dewey-1 » Thu Jul 30, 2009 1:59 pm

Here is a spec sheet for something easy:
http://www.physics.purdue.edu/~jones105 ... M317-D.PDF

Digi-Key or Radio Shack have them. Use a heat sink for cooling if needed at higher currents. Not knowing your requirements for current.
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Unread postAuthor: Davidvaini » Thu Jul 30, 2009 2:12 pm

well the cells are high amperage..

My 8.4v battery is 5000Mah 18a
Cells in it are ELITE5000SC

Sub C cells.

Again I am a beginner and I cant read that spec sheet. I makes no sense to me at all.

So I guess at this point I would need to purchase one.
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Unread postAuthor: dewey-1 » Thu Jul 30, 2009 2:24 pm

I will try and find a kit for you.
http://www.electronickits.com/kit/compl ... /ck403.htm

http://www.electronickits.com/kit/compl ... /ck402.htm
This kit can use a LM338T which is 5 Amps max.

The current I am referring to is the output current.
Are you attempting a battery pack charger for fast charge, greater than 1500 ma?
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Unread postAuthor: Davidvaini » Thu Jul 30, 2009 2:43 pm

okay this is what im trying to do.

In airsoft sometimes a person wants a lower rate of fire.. sometimes they want a higher rate of fire. Airsoft guns Rate of fire is dependent on the airsoft battery. Higher voltage batteries have a higher rate of fire than lower voltage batteries.

Now in game sometimes people wants to lower their rate of fire for use of support just to keep the enemies heads down. but then sometimes they want to have a higher rate of fire for increased chances of hitting the enemy.

Well swapping out the battery in match can be a hassle and it would be really nice if someone could adjust their rate of fire via a knob.

So I need a device between the airsoft battery and the airsoft gun that changes the voltage the gun receives. It needs to have a turn knob so you can adjust it on the fly very easily without having to fiddle with anything.

It also needs to be able to be small so it can fit on the gun without getting in the way. It also needs to be able to go on any gun.

Now some guns use 8.4v batteries while others use up to 12volts. So I would like something that has 12v max.. but allows people to use lower voltage batteries.

As far as the output to the gun.. I really don't care about the minimum voltage as long as not higher than 4v.(could adjust the ROF down to 0).

Now airsoft batteries output at a high current. My battery which is just a 8.4v says 18a 5000mah. But again I want it to be able to handle any battery up to 12v. Sub C is the size of the cells and these Elite5000SC cells are probably the largest cells people will use.

Some airsoft guns use lipo batteries as well, however I don't want to make a unit for lipo batteries right now.
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Unread postAuthor: Technician1002 » Thu Jul 30, 2009 2:50 pm

Davidvaini wrote:okay this is what im trying to do.

In airsoft sometimes a person wants a lower rate of fire.. sometimes they want a higher rate of fire. Airsoft guns Rate of fire is dependent on the airsoft battery. Higher voltage batteries have a higher rate of fire than lower voltage batteries.

Now in game sometimes people wants to lower their rate of fire for use of support just to keep the enemies heads down. but then sometimes they want to have a higher rate of fire for increased chances of hitting the enemy.

Well swapping out the battery in match can be a hassle and it would be really nice if someone could adjust their rate of fire via a knob.

So I need a device between the airsoft battery and the airsoft gun that changes the voltage the gun receives. It needs to have a turn knob so you can adjust it on the fly very easily without having to fiddle with anything.

It also needs to be able to be small so it can fit on the gun without getting in the way. It also needs to be able to go on any gun.

Now some guns use 8.4v batteries while others use up to 12volts. So I would like something that has 12v max.. but allows people to use lower voltage batteries.

As far as the output to the gun.. I really don't care about the minimum voltage as long as not higher than 4v.(could adjust the ROF down to 0).

Now airsoft batteries output at a high current. My battery which is just a 8.4v says 18a 5000mah. But again I want it to be able to handle any battery up to 12v. Sub C is the size of the cells and these Elite5000SC cells are probably the largest cells people will use.

Some airsoft guns use lipo batteries as well, however I don't want to make a unit for lipo batteries right now.


Due to the nature of his request.. It looks like he would like to lower and raise the voltage. Either a higher starting voltage is needed or a buck/boost configuration of a SMPS is needed.

Linear regulators generate a lot of heat in high current applications. I don't know the current requirements of the gun. A small adjustable DC-DC SMPS supply is what would seem to work best at this point.

Higher power stuff is here;
http://www.vicr.com/products/configurable/flat_pac/
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Unread postAuthor: TurboSuper » Thu Jul 30, 2009 2:54 pm

Here's something more efficient:

http://www.trossenrobotics.com/store/p/ ... lator.aspx

I don't know what current you're pulling, but the above reg. can source 3A. The rating on the battery is just a capacity rating, the actual load is what determines the current draw.
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Unread postAuthor: dewey-1 » Thu Jul 30, 2009 3:07 pm

Tech, I agree with you now that I know the details.

Years ago I designed my own RC race car ESC (electronic speed control)
using N-MOSFETS in a pulse width mode that was fairly independent on input voltage.

I think this type of idea may also work for the airsoft motor.
Rather than control voltage, control pulse width to motor.
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Unread postAuthor: Technician1002 » Thu Jul 30, 2009 3:15 pm

dewey-1 wrote:Tech, I agree with you now that I know the details.

Years ago I designed my own RC race car ESC (electronic speed control)
using N-MOSFETS in a pulse width mode that was fairly independent on input voltage.

I think this type of idea may also work for the airsoft motor.
Rather than control voltage, control pulse width to motor.


Sounds good. I didn't know if it was feeding a electronic valve timer in the gun or a motor. It helps to know what you are running. Electronic motor speed controls are common and may even be adapted from R/C use.
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Unread postAuthor: jimmy101 » Thu Jul 30, 2009 3:20 pm

The LM317 type approach is probably the best way to go. Simple (relatively speaking), can be very small, very cheap, etc.

The LM3xx series though is limited, IIRC, to something like at least a 1.5V (or so) voltage drop. That is, if you put in 12V the most you can get out is 10.5V. I'm sure that is how the fixed voltage reg's work, not 100% sure about the variable voltage version. In any case, with this type of reg you can never get an output voltage higher than the input.

If power is an issue then you can use more than one LM317 in series. They are so cheap and simple that it wouldn't be a big deal.

Size is pretty small, the LM is about the size of a power transistor. You also need the potentiometer and a single low wattage resistor, and perhaps a small heat sink. The input and output capacitors can be omitted in this application since your just running a motor with it so ripple, spikes, RF etc. aren't a problem. You could pretty easily build the whole circuit, even two of them in parrallel, on the LM317(s) and it's heat sink(s).

Another possibility might be to use peak width modulation instead of trying to regulate the voltage. A 555 timer or a real PWM chip controlling a power transistor should do the trick. (I'm assuming that these guns are basically just DC motors without any real electronic controls in them.) The really nice thing about PWM is that you don't waste any power as heat. Most other voltage step downs for DC waste a fair amount of power in the process.
A brief description of PWM.
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Unread postAuthor: POLAND_SPUD » Thu Jul 30, 2009 3:31 pm

wouldn't a PWM circuit work well here?

ohh sorry tech I didn't refresh the page so I couln't see your post... I totally agree with you here
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Unread postAuthor: Davidvaini » Thu Jul 30, 2009 3:51 pm

so are any of these PWM circuits available to purchase?

At this point I'm looking to purchase a circuit.


Here is an idea...

Would a 12v Fan Controller for a computer work?

like this:
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http://www.mountainmods.com/sunbeam-525 ... p-482.html

just take out the unit and put it in a custom enclosure.
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Unread postAuthor: dewey-1 » Thu Jul 30, 2009 4:06 pm

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Unread postAuthor: jimmy101 » Fri Jul 31, 2009 12:25 pm

Davidvaini wrote:so are any of these PWM circuits available to purchase?

At this point I'm looking to purchase a circuit.


Here is an idea...

Would a 12v Fan Controller for a computer work?

like this:
Image
http://www.mountainmods.com/sunbeam-525 ... p-482.html

just take out the unit and put it in a custom enclosure.

That might work, and it sure is cheap. Since it say it uses "rheostats" that suggests it is just a variable voltage divider. That means it'll waste a fair amount of energy from the batteries. But heck, pretty cheap ($15) and simple so it might be worth a try.

Here is a kit for a moderately beefy PWM controller for a DC motor for $26.
Velleman # K8004. Ideal for the accurate control of DC motors, lighting levels and small heaters as well as other applications. The circuit converts DC voltage into a series of pulses, the duration of which is directly proportional to the voltage. Minimizes power loss in the control circuit. Overload and short circuit protection.
Input Voltage: 2.5 - 35 VDC
Maximum output 8-35 Vdc, 6.5 Amps.

Here's the same kit for $22. This page has more info on the controller's specs.

Jameco also sells'm.
DC to Pulse Width Modulator Kit
The circuit converts a DC voltage into a series of pulses, such that the pulse duration is directly proportional to the value of the DC voltage.
# PWM range: 0 to 100%
# PWM frequency: 100Hz to 5kHz (adjustable)
# Operating voltage: 8 to 35VDC
# Maximum output current 6.5A
# Efficiency: better than 90% at full load
# Adjustable sensitivity: 2.5 to 35VDC
# Size: 3.3"L x 1.9"W x 1.8"H
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Unread postAuthor: Davidvaini » Sat Aug 01, 2009 12:09 am

okay I found out the fan controller probably wont work as its rated to 1.6amps and I found out amp draw from an airsoft gun can go up to 30+amps.

So scratch everything said before...


I need a turn knob device that adjusts 0-12v and can handle and pass 50amps.

Now 50 amps is max.. most of the time It wont draw more than 30 amps, but I want a safety margin.
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