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Rheostat help??!??

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Unread postAuthor: frozebyte » Sat Aug 01, 2009 12:37 am

Im not good in electronics, but would a potentiometer work? Potentiometers can be connected to work as a rheostat. These can salvaged easily.
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Unread postAuthor: Technician1002 » Sat Aug 01, 2009 2:07 am

frozebyte wrote:Im not good in electronics, but would a potentiometer work? Potentiometers can be connected to work as a rheostat. These can salvaged easily.


If you study Ohm's law and look at the voltage drop required at full load current, you will find most potentiometers won't be able to handle the heat generated by the Current squared times the resistance. Most potentiometers are unable to handle more and a few thousandths of an Amp. The requirement is several orders of magnitude too big. A 1/2 ohm 25 Watt rheostat would be a closer match.
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Unread postAuthor: frozebyte » Sat Aug 01, 2009 2:36 am

Alright i didn't understand any of that, but its just telling me that its too weak to handle the voltage right, i get it :)
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Unread postAuthor: Davidvaini » Sat Aug 01, 2009 3:02 am

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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Unread postAuthor: dewey-1 » Sat Aug 01, 2009 8:26 am

Davidvaini wrote: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.


That 30 amps is probably the peak or surge current.
I am guessing the nominal continous current is about 4 to 5 amps.

You could always change the MOSFET to IRFZ42 for higher surge current.

I could design a custom speed control for this use but that is time restrictive and a little more pricey.

I am sure someone has already done this. Example,Why reinvent the wheel;

http://unconventional-airsoft.com/

User manual;
http://unconventional-airsoft.com/

Other sites;

http://www.awsairsoft.com/

http://extreme-fire.com/

There are several options for you to pursue.
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Unread postAuthor: Davidvaini » Sat Aug 01, 2009 12:38 pm

only problem is.. that mosfet is not at all what I want right now....

That has options for 3 round burst, semi, etc..

I want a device that lowers and raises voltage via the turn of a Knob. That can handle the high amperage that the gun draws from the battery.

I want to be able to purchase a circuit or an item that has the circuit in it for cheap($20 or less)


I want the voltage range to be 0-100% and to be able to accept batteries up to 12v. If for whatever reason 12v cant be done, I will settle with 10.8v being the maximum voltage battery the circuit will accept.


Now I keep reading on multiple airsoft forums that the average amperage the gun draws is in fact 15 amps - 35 amps.
http://www.airsoftforum.com/board/Amps- ... p=17890155

Airsoft Gun's draw a lot of power, a lot of people like to use Lipo batteries because of this:
http://www.amazon.com/11-1V-2000mAh-LiP ... B0020NVYG4

As you can see that lipo battery says:
"15C continuous discharge rate of 30 amps with 40 amp bursts"

Now I realize that is the batteries capability, but again I keep seeing posts that say average amperage is around 15-35amps.
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Last edited by Davidvaini on Sat Aug 01, 2009 12:47 pm, edited 1 time in total.
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Unread postAuthor: TurboSuper » Sat Aug 01, 2009 12:39 pm

I'm no airsoft expert, but I've been reading a bit, and apparently an electric airsoft gun uses a motor to drive a piston/spring system. If that's true, I'm not surprized it needs a healthy amount of current to fire. Luckily, adjusting the speed of an electric motor is quite easy with a simple PWM system.

Here's a nice kit that can handle 30A:

http://www.bakatronics.com/shop/item.aspx?itemid=581

Keep in mind that if the airsoft gun has any logic circuitry that requires a regulated voltage, you'd want that to be powered seperately.

Edit: Here's a 50A kit:
http://www.bakatronics.com/shop/item.aspx?itemid=688

Increasing the power capacity of one of these guys is simple: Just add more MOSFETs in parallel. Unfortunately, this also makes them more difficult to drive.
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Unread postAuthor: Davidvaini » Sat Aug 01, 2009 12:49 pm

thank you turbosuper, that is exactly what I want.. now only to find it cheaper....

And here is a dumb question:
Can I replace those heatsinks with better heatsinks to increase said amperage?

As far as the airsoft gun.. just has two wires going to the battery from the gun.
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Last edited by Davidvaini on Sat Aug 01, 2009 12:54 pm, edited 1 time in total.
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Unread postAuthor: jimmy101 » Sat Aug 01, 2009 12:53 pm

Looks like PWM is really about the only way to get the power required. The device Turbo linked to will work but it is pretty pricey and pretty large. You might be able to use a smaller/cheaper one with an added power transistor and heat sink to handle the higher load.

BTW: Take a 12V DC motor and feed it a volt or two and it will just sit there. DC motors usually need to be within a factor of 2 to 4 of their rated voltage in order to run. The big advantage of PWM is that you can run at the correct voltage but because of the pulsing, get the motor to actually turn at much lower duty cycles (effective voltages) than it will with straight DC.
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Unread postAuthor: TurboSuper » Sat Aug 01, 2009 1:02 pm

Davidvaini wrote:And here is a dumb question:
Can I replace those heatsinks with better heatsinks to increase said amperage?


According to the datasheet for those MOSFETs, they can handle up to 80A each...that just might work.
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Unread postAuthor: Davidvaini » Sat Aug 01, 2009 1:06 pm

Also just so im clear...

If I use a 8.4v battery, this circuit will be able to do 0-100% of the 8.4?

I just dont want someone to throw in a 8.4v, crank it up to max and they get 12v.


TurboSuper wrote:According to the datasheet for those MOSFETs, they can handle up to 80A each...that just might work.

Time to take out the thermal paste :D
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Unread postAuthor: dewey-1 » Sat Aug 01, 2009 1:18 pm

Davidvaini wrote:only problem is.. that mosfet is not at all what I want right now....

That has options for 3 round burst, semi, etc..

I want a device that lowers and raises voltage via the turn of a Knob. That can handle the high amperage that the gun draws from the battery.

I want to be able to purchase a circuit or an item that has the circuit in it for cheap($20 or less)


I want the voltage range to be 0-100% and to be able to accept batteries up to 12v. If for whatever reason 12v cant be done, I will settle with 10.8v being the maximum voltage battery the circuit will accept.



Well I want a lot of things too, but I am trying to give some suggestions and ideas.

Changing the MOSFET in one of the PWM kits is the easiesr method.

I can tell you are not going to find much for that price unless you build it from scratch and have it be efficient enough for your desired requirements.

Size is another factor to be concerned with. All this electronics has to as close as possible to the motor.

Those with electronics experience could do it for that price.

Why 0 volts? I am sure most these motors won't operate at 4 volts or less under load.
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Unread postAuthor: TurboSuper » Sat Aug 01, 2009 1:36 pm

Oh, I should mention: It's perfectly normal for the controller to cause the motor to emit an ungodly whine :D
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