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# Power transistor help

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#### guitardenver

##### New Member
Motor current = 8amps
V supply = 12v (car battery)
base voltage = 9v

I need a transistor that will be able to handle this current (8A). And open with a 9v. PNP and NPN

I would really like to now how to calculate what resistance value I would need so the transistor is as open as possible with least amount of resistance. Also how do I calculate the watts of the transistor? Would like a easy to understand math lesson. So I can find what transistor to use.

Also I might need like 12 of these so the least expensive the better.

Can anybody give me a link to High torque 12v motors they have to move like 80-100 or more pounds around 3.5mph. Needs horizontal mounting braket and around 11 tooth #25 chain gear on it. Specific I know.

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Motor current = 8amps
V supply = 12v (car battery)
base voltage = 9v

I need a transistor that will be able to handle this current (8A). And open with a 9v. PNP and NPN

I would really like to now how to calculate what resistance value I would need so the transistor is as open as possible with least amount of resistance. Also how do I calculate the watts of the transistor? Would like a easy to understand math lesson. So I can find what transistor to use.

Also I might need like 12 of these so the least expensive the better.

Can anybody give me a link to High torque 12v motors they have to move like 80-100 or more pounds around 3.5mph. Needs horizontal mounting braket and around 11 tooth #25 chain gear on it. Specific I know.

I need a transistor that will be able to handle this current (8A). And open with a 9v. PNP and NPN (?)

I would really like to know how to calculate what resistance value I would need so the transistor is as open as possible with least amount of resistance. (?)

I don't know what that means.

But, you should go to the Motorola site and look at power transistors and FEts, they make the most.

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It means I want to amplify the 9v to 12 volt to drive a motor. And just what base resistor should I use. I don't want things to get hot.

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Things will get very hot. Since your design is already done please post a schematic so we can find out if I guessed correctly how you plan to wire it. Please also note that in electrical terminology, a switch is "open" when it is NOT conducting. You cannot get NPN and PNP transistors that will "open" when a control current is applied to the base.

Your final transistor will probably have 0.5V from emitter to collector when it is conducting, meaning it will dissipate 4W at 8A. Depending on the transistor's gain, you might need around 0.4A into the base. If we guess that your driving circuit is a single resistor to the base from 9V, that would be a 20 ohm resistor which will be dissipating an additional 3.2 watts. This should be at least a 5W resistor and should be protected from contact with anything that could burn. The base-emitter voltage is probably going to be about 1.0 volts. This adds about 0.4 watts to the transistor due to this base power. This is what you get with just a base resistor on a single output transistor. It's too bad your design is already done. It's extremely wasteful and there is no base resistor value that can repair this.

I will not suggest a transistor. Since you have concealed your location or at least failed to list it in your UserCP I will not waste my time suggesting a transistor available in my country. Too many people have come back and said how stupid it is to suggest a transistor that they can't get in their country.

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In a circuit the power dissipation is one of:
Code:
P = V * I

P = I^2 * R

P = V^2 / R

These three relations all say the same thing and are obtained from each other by the application of Ohm's Law. What are the implications?
1. Anything with a voltage across it has current going through it.
2. Anything with current going through it will dissipate power

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Things will get very hot. Since your design is already done please post a schematic so we can find out if I guessed correctly how you plan to wire it. Please also note that in electrical terminology, a switch is "open" when it is NOT conducting. You cannot get NPN and PNP transistors that will "open" when a control current is applied to the base.

Your final transistor will probably have 0.5V from emitter to collector when it is conducting, meaning it will dissipate 4W at 8A. Depending on the transistor's gain, you might need around 0.4A into the base. If we guess that your driving circuit is a single resistor to the base from 9V, that would be a 20 ohm resistor which will be dissipating an additional 3.2 watts. This should be at least a 5W resistor and should be protected from contact with anything that could burn. The base-emitter voltage is probably going to be about 1.0 volts. This adds about 0.4 watts to the transistor due to this base power. This is what you get with just a base resistor on a single output transistor. It's too bad your design is already done. It's extremely wasteful and there is no base resistor value that can repair this.

I will not suggest a transistor. Since you have concealed your location or at least failed to list it in your UserCP I will not waste my time suggesting a transistor available in my country. Too many people have come back and said how stupid it is to suggest a transistor that they can't get in their country.

I guess I was refering to "open" as how much current the transistor is freely letting through. Kinda like a gate. Just the way I think of transistors i guess.

What would you suggest a MOSFET? Arn't they expensive? I have never worked with them before but always good to learn.
I am useing two H-Bridges. Ones that look like this.
Robot Room: Bipolar Transistor HBridge Motor Driver
12v supply (car battery) One motor for each H bridge. 9v controlling transistors.

I will take any suggestions. I have not bought or built anything yet. I live in the US.

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you are very confusing.
Transistors do not open and close like a door.
They turn on and turn off.

The extremely simple H-bridge circuit drives a motor with only 200mA or less. Its control voltage must be the same as the power supply voltage. If its input is 9V and its supply is 12V then the PNP transistors will always be turned on.
The 1k base resistors limit the motor current to only 112mA when the control voltage and supply voltage are 12V.

You didn't say you wanted an H-bridge. My answer was wrong; sorry.I thought you wanted to drive one transistor, and run the motor in one direction. What I wrote only applies to unidirectional motors where you can connect the output transistor's emitter (there would only be one transistor) to the common of the controller.

I would suggest that you read as much as you can from RobotRoom. He is much more thorough than I could be.

Unfortunately RobotRoom seems to only cover intermediate projects, up to about 2A and 9.2V. An 8A 12V H-bridge is an advanced project. I have seen experienced designers use inadequate safety margins, and see their project melt as a result. If your motor has a running current of 8A, then your H-bridge must be capable of handling 40A peaks (starting torque). A "12V" design will probably be powered by a Gel Cell and might see 14.4V if it's on a charger, that's even more margin needed.

We haven't even broached the topic of control (or will it always burn rubber up to 3.5MPH till you reverse it?), or current limiting and overload protection.

I am sorry if I wasted your time with my misleading answer to the wrong question. My mind reader is working over time and still can't keep up.

Totally ok man. That was the wrong question I agree lol

The thing is I do have speed controll. Sorta home made. I use a 555 timer and a trimpot to adjust the fequency to a couple of transistors and the frequency will adjust the input going to the H-Bridges. (worked great as speed control on a small motor.) So the input for the H-Bridges will be pulsed. But once I get the motors to the speed that I want it will be that speed either on or off. Except if i want a power boost or something.

But Thought haveing transistors to handle the full amps would be enough. The motor will see the 8amps though. But I need to have a 40amp circuit? wow

The motors I might use till I find a better one is a 24volt 15Amp 250W scooter motor. which will be 7.5 amps at 12v.
So I think it will be 8amps without the voltage drop when the motor is running. (I think).

But am I kinda on the right track at least?

I guess this project might be way over my head right now till I go through much more school. But I still want to try if I can.

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A DC motor speed controller uses pulse-width-modulation to adjust the speed, not the frequency. Pulse-width-modulation adjusts the duty-cycle of the power to the motor. Full power for most of the time results in full speed. Full power for half the time (switching on and off quickly) results in half power but a lot of torque is available. A low duty-cycle results in a slow speed but still with plenty of torque if it is needed.

Motors don't run all the time; they have to start too.

I am guessing once again, but it sounds like you're quoting the steady current draw of the motor under normal load. It draws many times that current while it is starting from 0 towards your desired speed. Observe that RobotRoom is recommending 4A transistors (ZTX1047 and ZTX1147) to drive a 300mA motor. (He says a motor suitable for his H-drive should read 5 ohms or more when stopped. This probably means after the controller losses it would have a start/stall current of about 1A on a 7.2V battery).

To avoid overheating at specified load, a 24V 15A motor would normally have a resistance of around 0.1 ohm. That motor will therefore have a starting current of 120A at "only" 12V. If your motor controller isn't robust enough to routinely deal with this peak current, you should keep a big pile of spares on hand.

Since H-bridges tend to fail dramatically, keep spare fuses on hand, too. Don't even think of applying an H-bridge to a car battery without a proper fuse or breaker. A fuse cannot save the transistors, but the resulting fire should be smaller.

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I don't see where you got .1ohms from? 24/15 is 1.6Ohm which would have a starting current at 7.5amps at 12v. Or am I doing totally different math.

So is an h-bridge sutible to drive an 8 amp motor. Or would it work with MOSFETS. Or is something else needed entirly.

24/V15A would be 1.6 ohm if you want all the power of the motor to go into resistive losses leaving nothing to do mechanical work. This would make the motor a motionless 360W heater.

If the motor has 0.1 ohm resistive losses, then this means it robs "only" 22.5W for heating and the motor still can move something.

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A good MOSFET H-bridge is almost mandatory. For an 8A motor, I would make sure it can handle 8A continuous, and surges up to 150A.

It's a lot easier if you only reverse the motor when stopped. You would only use one transistor instead of four, and reverse with a heavy duty DPDT relay or toggle switch. Everything would still need to handle 150A, but a bunch of N-channel MOSFETs in parallel can be used to get the power handling you need. But the design still isn't foolproof. If you reverse while in powered motion, the current skyrockets.

Wow, I did not know that. So all motors have that? If I have a drill motor that has 2 Ohm resistance (I measured it) what would be the peak current or starting current be?

The data sheet for the p60nf06 that is on the drill motor control says starting current is 8A (but thats not for the motor) I dont know what that it. below the first chart. Also it can handle a 16A continuous.
STP60NF06FP pdf, STP60NF06FP description, STP60NF06FP datasheets, STP60NF06FP view ::: ALLDATASHEET :::

All this sounds very expensive and inefficient in respect to the amount of components. A bunch of MOSFETS in parallel sounds like someone would have found a different way.

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