6V DC motor control with transistors

[case-sensitive]

Hello,

its for a model car. There is 2 inputs per motor. 1 1 or 0 0 is stop. 0 1 is forward, 0 1 is backward. Normally it is done by a L3 somewhat, but won't be enough for two motors. So I came up with a transistor design. Basically lika a H, motor would be in the beam of the H, each leg is a transistor. Current from left upper to right lower corner is one direction, right upper to left lower the other direction. Works in a simulation, but from 6V the motor gets 5, the other 1V heats the transistor. Will post a picture later today. Guess/hope there is a better way for the small wallet (school project)

Case.

 

[audioguru]

A transistor used as a switch has a high voltage loss if it doesn't have enough base current for its load current.
Post your schematic with part numbers and currents.

 

[case-sensitive]

will do in a couple of hours.
Thanks for the quick reply!

Case.

 

[case-sensitive]

Howdy,

there they are (the pictures). The switches would be the outputs of the L293D.
The schematic with the instruments is a bit messy, so I created another one without.

Later,

Case

 

[ericgibbs]

Caution when opening the above attachments, I picked up a screen blanking virus.

EDIT: Just by chance as I opened these attachments my LCD monitor flickered and blanked out.. Now using a standby monitor.

Checked the first monitor on a spare PC, its appears to be a power supply heating problem.?

 

[case-sensitive]

Virus??

in that a small picture?? Anyway, am scanning my HDD, just in case...

Case.

Edit: Can post the .ct files (circuit maker)
PS: the motors are for a school project, Killbot, while programming it, my laptop reset...

 

[ericgibbs]

in that a small picture?? Anyway, am scanning my HDD, just in case...

Case.

Edit: Can post the .ct files (circuit maker)
PS: the motors are for a school project, Killbot, while programming it, my laptop reset...

hi,
As I said in my EDIT, at the moment I opened that right hand image the screen flickered and blank.. I thought it would be wise to warn other users just in case.

Would you believe that the 3 year warranty on my monitor has 3 days left to run. I've contacted the supplier its still covered.!

Ref your problem, the top two transistors are in the emitter follower configuration, so they will never saturate and so will get warm/hot.

 

[case-sensitive]

once I posted it your comment came up (reg virus), anyway. You got lucky with your warranty still valid, designer stuffed up, meant to lase 3 years and 1 day, not 3 year minus 1 day.

Anyway, what to do with my transistor problem, basically its XOR, for 0 1 should be negative, for 1 0 positive on the motor? Needed the two top transistors, so I could lead the voltage from left to right or vice versa. See motor 1 resp motor 3.

Cheers,

Case.

 

[ericgibbs]

Anyway, what to do with my transistor problem, basically its XOR, for 0 1 should be negative, for 1 0 positive on the motor? Needed the two top transistors, so I could lead the voltage from left to right or vice versa. See motor 1 resp motor 3.

Cheers,

Case.

I would suggest using PNP transistors for the top pair.

Modify the switch so that Tleft,Bright say On, Tright,Bleft Off and vice versa.

Drive enough current into the transistor bases to ensure that they saturate when ON.

At present the top pair will always have at least 0.7V across Vce.
So Pwr = Imtr*0.7V watts

EDIT: whats the rated motor current at 6V.?

 

[case-sensitive]

Hi Eric,

there we go, nearly 6 V.
Current: don't know, less than 500mA I reckon, just tiny Tamiya ones, Twin Gear box.
Do I need resistors in front of the transistors at all hope not?

Case.

 

[audioguru]

Your PNP transistors are upside-down.
Why are you using transistors when the L293 is a motor driver IC? Except it has a voltage loss of about 1.4V when its output is high and a voltage loss of about 1.2V when its output is low. So it cannot turn off your NPN and PNP transistors.

Your transistors need a resistor in series with the base to limit the current. Now you have only 1 ohm.

 

[audioguru]

Double-post deleted.

 

[case-sensitive]

Hello,

don't know, if the L293 can manage to drive 4 motors in a worst case scenario.

If I increase the value of the resistors the voltage on the motor goes down, of course.

Does not look that bad (see pictures)

Case

 

[Grossel]

That looks ok.

 

[audioguru]

The L293 is too small and weak to drive four of your 500mA motors. The L298 will work fine but it has the same saturation voltage losses as the L293.

Your 1st schematic is wrong because it has an input at 0V instead of at 1.2V from the output of an L293. Then the NPN transistor that is supposed to be turned off is actually turned on a little and is wasting power by getting hot.

Your 2nd schematic is correct but it shows that the PNP transistor that is supposed to be turned off is actually turned on a little and is wasting power by getting hot.

The value of the 470 ohm base resistors limits the base current to only 8mA. Power transistors need 50mA of base current to saturate well when their collector current is 500mA.
If you reduce the value of the base resistors then the transistors that are turned on a little will be extremely hot.

You should use transistors to drive your power transistors instead of an L293.

 

[case-sensitive]

Howdy,

thanks again.
Next one down the track is an Atmel AtMega8, the one who is controlling the L293. Seems it has about 10mA output current, but should be enough for the first set of transistors. Try to come up with a design later on.
The motors are just as big as half a thumb, no idea if I can find some values on their consumption. won´t be that much on a batterie driven vehicle.

Case.

 

[audioguru]

Electric model racing cars draw 10A to 30A from their battery.
My electric model airplane draws 8A at full power from its seven AAA Ni-MH cells (8.4V/850mAh).

I have a cheap portable fan that is powered from one AAA battery cell. It draws 1A.

 

[case-sensitive]

its that one:
**broken link removed**

twin gear box item no. 70097**700.

Hope, the 4.5V one, the others won't be happy with 6 V....

 

[case-sensitive]

its just for a school project, rather a robot which pushes other from the table, tiny car. Not a racing one. Want to put 4 motors/gear boxes in instread 2.

 

[case-sensitive]

would be something like this? (See picture)