Transistor Question

[mazur50]

I am using a 2n3055 to control a electric motor. I also tryed a 2n6284 The circut works to control the motor but the Varable resistor gets hot i have tried using a 1.2Ohm VR and also a 47 Ohm and a 500ohm.

the 1.2 gets red hot and the 47 gets hot the 500 does not why is this. I am new to this and dont know why. I would really like to be able to use the 1.2 ohm one as it has more bands for adjustment.

[slipstick]

If you look at that circuit you'll see the resistor is connected straight across the battery so the power dissipated in the resistor is V^2 / R.

For the 1.2 ohm that's 144 /1.2 = 120W !!, for 47 ohm it's 144/47 = 3W and for 500 ohm it's only 144/500 = 0.3W. And of course the 1.2 ohm is is taking 10A out of the battery and wasting all that power.

BTW, unless that's a very low current motor the transistor is going to get hot too because when you're running the motor at say half voltage the other half of the 12V is being turned into heat inside the transistor.

Steve

[jeffs555]

It has nothing to do with the transistor.

Connecting a resistor across a voltage will cause power to be lost as heat in the resistor. The power is equal to the voltage squared divided by the resistance. Since 12 volts squared is 144, a 1.2 ohm resistor will dissipate 120 watts as heat(144/1.2=120). With the same voltage, a 500 ohm resistor will only dissipate only about 1/4 watt(144/500=.288). Resistors have a power rating specification, and you need make sure that you do not excede it or the resistor may burn up.

[Malc C]

You would be better off using a darlington pair arrangement. I've used the attached to drive various motors on 9 - 12v DC supplies. It uses PNP transistors (the TIP 2955 is just the equivilent to the NPN 3055 you use) as the output stage, but you can also use the 3055 in such a way, just google "darlington pair" or "model train controllers" and you'll find loads of similar circuits.

[mazur50]

is there any way t use the 1.2 or the 47 ohm. like a different way to lay the circuit out. the 2n6284 is a darlinton pair transistor and when i use that i have the same problem thanks for all the help guys.

[Brandano]

you can add one or two fixed resistors in series with the trimmer, but you'll obviously lose some control range.

[mazur50]

If i wanted to use the 1.2 ohm vr would i have to go the PWM route. if so how would i go about this with the use of a PIC

thanks
Mike

[Malc C]

is there any way t use the 1.2 or the 47 ohm. .

I think this has been well documented in the two posts showing you why the thing gets hot.

In essance sticking a 1.2 ohm resistor across the supply is almost (not quite) like shorting the supply. All the current that is drained from the battery gets turned into heat. That's why in the circuit I suggested, the control stage uses higher value pot, but then amplifies the current before driving the output stage made up of the power tranny and GP tranny in a darlington configuration.

[jeffsr01]

This is a high current circuit similar to high power audio amps. Should always use a driver transistor to control the base voltage of the output device. This will effectively isolate the control section from the output section. You might want to throw a couple of 0.47uf capacitors across the terminals of the motor to control back emf and noise (yes brushless motors can generate electrical noise). BTW, a 1.2ohm resistor across the V+ and V- is essentially a dead short. Good luck...

[ghoti]

One more thing: 12 volts across a 1.2 ohm resister will make the battery hot as well as the resistor since the same current must pass through the battery internal restance. This is a no-win situation.
Bill

[feihu]

I am using a 2n3055 to control a electric motor. I also tryed a 2n6284 The circut works to control the motor but the Varable resistor gets hot i have tried using a 1.2Ohm VR and also a 47 Ohm and a 500ohm.

the 1.2 gets red hot and the 47 gets hot the 500 does not why is this. I am new to this and dont know why. I would really like to be able to use the 1.2 ohm one as it has more bands for adjustment.
The transistor bias is incorrect.
I have a 2n3055 and a 50ohm pot, sp250 motor and a 9.6v battery.
I will connect them together tonight and check out how I think it can work.
feihu

[feihu]

mazur50:

You need to disconnect the wire from the 47 ohm pot from the motor and connect that wire to the emitter which is also the + connection to the motor. I assume that the 47 ohm pot is wirewound or can handle 3 or more watts.
When the pot wiper is fully down (zero resistance between pot and emitter) then the 2n3055 is biased to cutoff. When the wiper is fully up (connected to 12v) then the 2n3055 is fully on.

I used a 50 ohm pot on the circuit and it worked fine.

feihu