Hello all,

In the picture attached below, you can see that this circuit uses a transistors with the uln2003 to drive a 5-Lead unipolar stepper motor, but isn't the ULN2003 capable of driving the stepper without the extra transistors and diodes!? as it can output a current of 500 mA per driver and the 24byj48 stepper motor requires only 40mA per phase! and it already has the diodes to protect from inductive spiking..

can anyone explain what's the purpose of using the transistors with the ULN2003?


Thanks,

 

hi YT,
A ULN2003 cannot source current, it is open collector output, so it can only sink upto 500mA.
The circuit uses the ULN as a level shifter from 5V to 12V , in order to control the Darlington Bases

E
Corrected MOS error > BJT

 

hi YT,
A ULN2003 cannot source current, it is open collector output, so it can only sink upto 500mA.
The circuit uses the ULN as a level shifter from 5V to 12V , in order to control the MOSFET Gates

E
View attachment 229435

Got it,
in this case can I remove the ULN2003 and keep only the transistors (PDTB123YT) and drive them directly from mcu pins. is this gonna cause problems with mcu?

Thanks again,

 

hi,
If you only use a 5V in order to control the Darlington Bases will never switch OFF, as the Source will always be at 12V -5V.
ie VB to Vs ,, either -12v or -7v

E
Corrected MOS > BJT

 

The mcu outputs can only get up to 5V and so the transistors will never turn off. Also the current from the transistor bias resistors will flow through the protection diodes in the mcu. Depending on the mcu, this may cause latch-up and damage the mcu. Also this current may disrupt the operation of the mcu.

 

I do not intend to derail this thread.

Puzzled by how the diodes work.

 

hola at,
You mean the external diodes in his diagram or the inbuilt ones.?

E

 

hola at,
You mean the external diodes in his diagram or the inbuilt ones.?

E

The external ones, Eric.

 

hi at,
Two PNP inductive drivers, one with clamp diode, one without, note the -HV transient on the unclamp Collector.
E

 

I do not intend to derail this thread.

Puzzled by how the diodes work.

The motor is a 5-lead unipolar stepper. The fifth lead is not shown in the diagram. Eric has added the connection to GND on the fifth lead.

 

Puzzled by how the diodes work.

The motor inductance tries to keep the current moving in the same direction when the transistor switches off.
The path for that current is the diode.
Without the diode there would be no current path, generating a large negative transient.

 

Wouldn't that chip drive a 5 wire unipolar this way?
I have done it with just MOSFETs.

 

While a ULN type can sink current, the specs for it have to read carefully. Total dissipation is a limiting factor, you cannot drive all lines 100% at the same time at full pin rating. Same goes for the UDN series (they source current). Most are rated at about 400 to 500mA per pin, but only if one pin is active. If using 2 pins, current per pin drops and so on until all 7 (or 8) pins are on, each is limited to about 100-120mA each.
So, application and design dictate how you can use the ULN or UDN series of buffer chips. I use them to drive relays, and if more currecnt is needed I either pair up a set of inputs and outputs for double the current, or use external transistors/mosfets to drive even higher currents.

 

The motor is a 5-lead unipolar stepper. The fifth lead is not shown in the diagram. Eric has added the connection to GND on the fifth lead.

And that would be wrong. The fifth lead is a voltage supply, not ground. So the chip in the original post would/should make the motor rotate, it just wouldn't be possible to have any load of the shaft.

 

Thanks. I failed to recognize them as flyback diodes. My bad!

 

And that would be wrong. The fifth lead is a voltage supply, not ground. So the chip in the original post would/should make the motor rotate, it just wouldn't be possible to have any load of the shaft.

The power transistors shown in the original schematic are PNP on the high side.

 

isn't the ULN2003 capable of driving the stepper without the extra transistors and diodes!?

Yes - IF -

IF you can connect the motor's common lead to the +12 V instead of GND

AND IF

the peak current at any time through any transistor does not exceed 500 mA

AND IF the total current through all transistors does not exceed 1 A at any time.

If all of that is true, then the four external transistors and four suppression diodes can be eliminated. Note that 2003 pin 16 must be connected to the same supply as the motor.

ak

 

The power transistors shown in the original schematic are PNP on the high side.

That may be so in the original schematic, but that wasn't his question. It was,

but isn't the ULN2003 capable of driving the stepper without the extra transistors and diodes?

And then someone else said the ULN2003 couldn't source current, only sink current. The way a 5 lead stepper is USUALLY wired is with the fifth lead sourcing the power and the other 4 sinking it.

 

I know nothing about steppers (other than basic theory) but I'm curious...does the polarity of the feeds affect the direction of the stepper?

 

I know nothing about steppers (other than basic theory) but I'm curious...does the polarity of the feeds affect the direction of the stepper?

Not for a unipolar stepper. The stepper moves based on the phase of the four coils. Reverse the phase and the motor moves in the reverse direction.