Help with Motor Driver

[Souper man]

I wanted to drive 2 motors that run on their own power supply. I was wondering if i could use 2907A PNP transistors to run them, or would I need a MOSFET? The control current would run to base, and it would be isolated. But i need to know how to control it on its own voltage/power supply. I have tried this with transistors, but it doesnt work. Anybody have any ideas or suggestions?

 

[ericgibbs]

hi sm,
It really depends on the motor spec and the supply voltage/currents.

Are you also wanting motor speed control, any reversing?

Like to help, got any sketches we could look at.?

 

[audioguru]

The datasheet for the old 2N2907A PNP transistor shows that its max current allowed is only 600mA. When your motor starts running or when it is stalled then its current might be much more then the old transistor will break.

 

[Souper man]

ok here are the values for the motors, but please read the note below too.

Voltage- 6v

mA use unloaded at 6v- 35mA

Stall current at 6v - 639mA

*BUT*

I am also willing to use MOSFET, or whatever cheap solution i can use to power my motors. also, it must be isolated from the circuit; the motors must be on their seprate and very own power supply.

ALSO: i can supply these motors with voltage between 3v and 12v. the 3v specs are listed below:

Voltage- 3v

mA use unloaded at 3v- 23mA

Stall current at 3v- 389mA


AND

I require no motor reversing, speed contol, or other fancy smancy stuff. I simply require some straight moving motors!

I also want to use them with AA size batteries, and I think thats all I need to suggest.

 

[audioguru]

If you use four AA alkaline battery cells then they won't be 6V for long. They will quickly drop to about 4.8V.
If you use five Ni-MH rechargable cells then the voltage will be about 6V for a fairly long time.

Ordinary Mosfets need 10V on the gate to turn on.
"Logic-level" Mosfets work with only 4.5V on the gate.

 

[Souper man]

Sounds like i will use the Logic level mosfets, since they will work on lower voltages.

also, I will use AA rechargeable energizer 2.5 aH batteries for the motors

AND what is the schematic that i should use?

 

[ericgibbs]

hi,
This should be useful for you project, also some good background info on MOSFET drivers.

 

[Hero999]

The datasheet for the old 2N2907A PNP transistor shows that its max current allowed is only 600mA. When your motor starts running or when it is stalled then its current might be much more then the old transistor will break.

That's an over simplification.
The transisitor doesn't need to be continously rated for the stall current unless you're using a specially designed motor that will be operated at stall for long periods of time.

For example if the stall current is 2.4A for 100ms and your wimpy 600mA transistor can handle that current for a that short period of time then it'll be fine.

Ordinary Mosfets need 10V on the gate to turn on.
"Logic-level" Mosfets work with only 4.5V on the gate.

Another gross over simplification.

Most MOSFETs can pass small currents at low gate voltages. For example the IRF740 can pass 1.25A if the gate voltage is 3V. The moral of the story is look at the datasheet.

 

[audioguru]

That's an over simplification.
The transisitor doesn't need to be continously rated for the stall current unless you're using a specially designed motor that will be operated at stall for long periods of time.

For example if the stall current is 2.4A for 100ms and your wimpy 600mA transistor can handle that current for a that short period of time then it'll be fine.

It is a small transistor, not a power transistor.
I found its Safe-Operating-Area info on a datasheet from ON Semi. It can pass 2A for only 1ms with 10V across it. 1ms is a very short time.

Another gross over simplification.

Most MOSFETs can pass small currents at low gate voltages. For example the IRF740 can pass 1.25A if the gate voltage is 3V. The moral of the story is look at the datasheet.

The datasheet shows its max threshold voltage is 4.0V where it conducts only 250uA. They have a range of threshold voltages, you can'y buy "a very sensistive one".

International Rectifier invented the IRF740 Mosfet and their graph starts with a gate voltage of 4.5V where it typically conducts only 0.6A with a whopping 0.6V saturation voltage.
With a 10V gate voltage it typically conducts 1.4A with a 0.6V saturation voltage.

It is a lousy Mosfet, maybe because it is rated at 400V.

 

[Souper man]

Well, the torque on my robot is very high for its small size (a whopping 80 ounces for such a small motor!). Also, they dont seem to stall, and when they do the transistors take the brunt very well. Still my question seems to be unanswered: How do i power my motors on seprate power supply!!!

 

[audioguru]

Well, the torque on my robot is very high for its small size (a whopping 80 ounces for such a small motor!). Also, they dont seem to stall, and when they do the transistors take the brunt very well. Still my question seems to be unanswered: How do i power my motors on seprate power supply!!!

When an electric motor starts to run, it is stalled at the beginning with an extremely high current. When the inertia of the motor and its load are finished then the current drops.

For separate power supplies then simply connect together all the grounds and use a separate positive supply for each section.

 

[Souper man]

Oh, that makes sense, lol