PWM circuit woes

To keep this brief, I was given the job of the electronics portion for a vehicle for an engineering project. We are giving a kit (12V battery so this is all 12V) to be able to build the regular model and then have $20 allotted for modifications, hence why I did a budget controller with a 555 timer.

So basically the left and right motors are hooked to DPDT paddle switches to give the driver tank steering, we needed fine control of these motors so that's where the speed control comes into play.

The 555 drives an N-channel BJT . I have a wire from ground to the emitter side and from there I have the wire from the collector going to both switches.

Here is my problem, with one motor this all works fine and full power and the full range of power is achieved, with both motors on the power to each drops drastically.

Should I a) run a seperate transistor for each motor and have both bases tied together? or B) Put another

Please post a schematic.

Sorry if this is a bit sloppy, I have a busy monday (as i'm sure you do to). That transistor is just a stand-in, the one that I have in the real physical circuit is this one.
FJE5304DTU Fairchild Semiconductor Bipolar Transistors

And I know that this isn't true PWM, as there is also Frequency modulation but for the sake of this project and our budget it is close enough.

Your selected transistor has a low max current and a very low current gain because it is designed for very high voltages. Use a better transistor that is designed for what you need.

EDIT:
Your 555 will probably burn out because nothing is limiting its max output current.

Yes, you need 2 transistors and a resistor in each base as Audio said.

Also, it is an NPN transistor, not an N channel FET.

Insulated gate devices would be better than using transistors or FETs as they have a lower "on resistance" than a FET while retaining the advantage of the high input resistace of a FET gate.

ljcox said:

Insulated gate devices would be better than using transistors or FETs as they have a lower "on resistance" than a FET while retaining the advantage of the high input resistace of a FET gate.

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All the devices you mention are transistors since FET is short for Field-Effect-Transistor. MOSFETS have an insulated gate (looks like a capacitor). Junction FETs have a reverse-biased diode gate which also provides a relatively high input impedance. The base input of a bipolar transistor is a forward biased diode and thus has a low input impedance.

crutschow said:

All the devices you mention are transistors since FET is short for Field-Effect-Transistor. MOSFETS have an insulated gate (looks like a capacitor). Junction FETs have a reverse-biased diode gate which also provides a relatively high input impedance. The base input of a bipolar transistor is a forward biased diode and thus has a low input impedance.

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Yes, I know know all of that.

When I said "transistor" it was short hand for bipolar transistor.

ljcox said:

Yes, I know know all of that.

When I said "transistor" it was short hand for bipolar transistor.

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Well, it wasn't clear that you knew all that. "Transistor" may be your shorthand for a bipolar transistor, but that's not a normally used shorthand. Accurate definitions are the essence of good technical communications.

sounds like the case as others said of needing a more powerful transistor or 2 transistors, why not use a mosfet by the way ?

Try this circuit, just stick it in series with the motor.
https://www.electro-tech-online.com/threads/teo-wire-dimmer.96474/

crutschow said:

Well, it wasn't clear that you knew all that. "Transistor" may be your shorthand for a bipolar transistor, but that's not a normally used shorthand. Accurate definitions are the essence of good technical communications.

Click to expand...

Yes, I agree.

Thunderchild said:

sounds like the case as others said of needing a more powerful transistor or 2 transistors, why not use a mosfet by the way ?

Click to expand...

My recollection is that an IGBT device has a lower saturation voltage than the Vds that will occur with a MOS FET for the same current.

Thus the power dissipation in the device and hence the temperature rise inside the divice is less. It may mean the difference between needing a heat sink or not.