6V DC motor control with transistors

[audioguru]

would be something like this?

The input voltages +1.2V and +4.5V are wrong. They should be +0.1V and +5.9V from the microcontroller since the L293 is not needed. Then the transistors that are supposed to be turned off will be turned off.

Your voltmeter shows -2.4V at the base-emitter of Q2 which is impossible.

The base-emitter of Q1 and Q3 do not have a current-limiting resistor in series.

There is no current-limiting resistor between the emitter of Q5 and the base of Q6 because R5 is in the wrong location. R6 is correct.

 

[MrAl]

Hi there,


Just a note, that you can not expect to get 6.00v across the motor using
bipolar transistors for the switches. The bipolars always have some voltage
drop when driven correctly, like 0.2 volts or more. Some have much more
voltage drop.
The solution is to either use MOSFETs for the transistors, or raise the supply
voltage from 6v to maybe 7 or even 8 volts.

 

[case-sensitive]

@MrAl: would be just 4 AA batteries, so I am stuck with 5-6 V. Just don't want to loose too much current somewhere along the line to my motors. And there would be 4 instead 2 motors on the L293, so I am concerned, if it can handle this.

Later more,

Case.

 

[case-sensitive]

arr, strange simulation that. Tired, giving up for today. See picture.

 

[audioguru]

On your new schematic, you have the emitter of Q5 shorted to the emitter of Q6.

You have the PNP transistors connected as a darlington with a 1V loss.
R7 and R8 should be wires.

You have the NPN transistors wired as emitter-followers with a 2.2v loss.
If you replace R1 and R4 with wires then the loss will be "only" 1.7V.

Do it like this:

 

[case-sensitive]

thanks, looks good!

 

[MrAl]

Hi again,

In the real world it wont be that good though. There will be more voltage
drop and some drop in the battery cells themselves.
You should build a prototype and see how well it works before you
commit to this design. If it works to your liking, then go with it, but
if not you may wish to switch to MOSFETs.

 

[audioguru]

you may wish to switch to MOSFETs.

It is a school project. Mosfets might be allowed next year.

 

[case-sensitive]

-well, the program is from 2000, should get a newer one...
-Was not meant to do the transistor thing, do it for myself. My as well try MOSFETs.

Case

 

[case-sensitive]

-ok, went for new software, quite different result.
-tried MOSFETs, got 1 to work, but not the H-Bridgr, tried several from the **broken link removed**.

 

[case-sensitive]

there we go. Again.

 

[audioguru]

You got poor results because you used overloaded wimpy little transistors that have a problem with a current more than only 100mA. The motor is trying to draw 2A.

In my circuit I used TIP power transistors that work well with current up to 2A.

 

[case-sensitive]

the program does not have the TIPs....
Used the good old BD329/330 instead and some other exotic ones...
For MOSFETS: don't I need 15 or so V to control them?

 

[audioguru]

Most Mosfets are spec'd with 10V between the gate and source to turn them on.
A few are "logic level" and are spec'd wirh 4.5V to turn them on.

 

[case-sensitive]

Arr, sounds troublesome, is it really worth it (MOSFETS, better still logic level), some things are rather hard to get down here?

Going away until Sunday,

Case

 

[audioguru]

Don't bother shopping at Dick Smith war surplus (which war?) electronic parts and used car parts. Shop online at Farnell.

 

[case-sensitive]

which war?
war against
terrorism ,-) ?
Korea?
Iran?
plenty of work to do, luckily ,-)
Yes, the ZX or so MOSFETs EDIT: sound like they are military ones.
Anyway, is it worth it (MOSFETS), feels like the others do the job.

Case.

 

[audioguru]

Transistors have a voltage loss. Modern Mosfets don't.

 

[case-sensitive]

ok, so be it, will try to find some suitable logic ones...

Case

 

[blueroomelectronics]

The fellow on Robotroom used a TC4424 as a small motor driver. Handy little 8pin IC.
https://www.robotroom.com/HBridge.html