Diode Location in Stepper Driver

[3v0]

I found a little unipolar stepper driver called SNAP that I intend to drive 3 5" floppy drive steppers with. After laying out the PCB I noticed that the schematic seems skitzoid regarding the 2 1N3024 diodes near the motor connectors.

The left diode feeds 12V to both the motor center tap and the transistor array. The right diode feeds 12V to the transistor array but the motor center taps get feed directly from the 12V supply. If the intent is to protect the 12V supply the right diode should be moved ahead of the two motor connectors ?

Note that pin 1 (12V) on the motor connectors go to the center taps because this is a unipolar motor.

Please click the image.

 

[ericgibbs]

hi H,
I have seen zener diodes connected in that way so that the internal clamps diodes of the ULN dont clamp the motor back emf's so 'firmly' to the 12V.
This gives a faster motor response.

Regards

 

[3v0]

Thanks but that did not answer my question. The two diodes are not connected the same. Why are they different ? Should they be ?

 

[ericgibbs]

Thanks but that did not answer my question. The two diodes are not connected the same. Why are they different ? Should they be ?

hi,
The ULN2003 is only a 7 transistor device 16 pin and the ULN2803 is an 8 transistor version 18pin, so the internal diode common is on a different pin

 

[Nigel Goodwin]

The ULN chips have protection diodes built-in, the connecton to the 12V 'rail' is the other end of the diodes, putting them directly across the coils as required. I've no idea why the diodes are there for feeding the 12V through? - I don't see as it's needed?.

 

[ericgibbs]

The ULN chips have protection diodes built-in, the connecton to the 12V 'rail' is the other end of the diodes, putting them directly across the coils as required. I've no idea why the diodes are there for feeding the 12V through? - I don't see as it's needed?.

hi Nigel,
By using a series zener the operating 'release'/field collapse time is reduced.
Commonly used in the old paper tape punch units in order to increase their operating rate.

BTW:
Pleased to see that your BP and eye Op is going well.

 

[Nigel Goodwin]

hi Nigel,
By using a series zener the operating 'release'/field collapse time is reduced.
Commonly used in the old paper tape punch units in order to increase their operating rate.

Except the diode isn't in series with the diode across the coil, it's simply feeding power through it to the top of the stepper and the top of the ULN diodes.

BTW:
Pleased to see that your BP and eye Op is going well.

Thank you - no close vision at the moment though - I'm typing this using a magnifying headset fitted with a x1.8 lens

My wife was going to see if she could find some cheap reading glasses today for me, as they will hopefully only be needed for a few weeks until the second eye is done and healed, so I can get my eyes tested and prescription glasses.

 

[3v0]

I now understand what you are talking about even if I do not fully understand why (been up all night). It really does not matter which side of the diodes the center taps are on. It is only their connection to the ULN's supply that matters.

These are obsolete parts so I thinking of doing without it but found a cross match 1N5362B


it will be interesting to see if one can tell the diff with or without.

 

[ericgibbs]

Except the diode isn't in series with the diode across the coil, it's simply feeding power through it to the top of the stepper and the top of the ULN diodes.

hi,
I have redrawn the circuit to make it clearer, the zener connects the pin on the IC which is the common Anode of the 7 internal clamp diodes to the +12V, so it is effectively in series with all the diodes [ unless I am having a senior moment]

 

[3v0]

Thanks guys. It is sure nice to have people around to set me on the right track.

I have wanted to play with steppers for quite a while.