4 wire stepper motor control (ladder logic)

[CasualKilla]

As Spyman nicely showed, resistors come in various standard power ratings. Most often you will find 1/4W and 1/2W (every electronics store)... It is also good practice, as Sensei mentioned, to have some power margin (not necessarily double, but some 50% would be nice).

 

For your simple drive circuit you don't need Schottky diodes... just use some that are faster than typical diodes used in 50/60Hz rectifiers. (If you made a high performance PWM driver then Schottky might be recommended, but in your case is probably overkill).... Anyway diodes are must and if you don't have them you risk damaging your transistors (even motor insulation, maybe).

 

I am not sure what kind of current spike do you see on R1 (during switch-on or during switch-off)? Do you use oscilloscope to see these spikes? BTW, if you don't use diodes all kind of spikes are possible, but I cannot tell for sure if this is the reason.

 

I am a bit unsure about the diode selection, what are the different applications for silcon vs schotty diodes, and what kind of current rating must the diodes be speced for? 2.5A since that is the max motor current?

 

And no, that is from a LTSpice simulation, I have not built the circuit yet.

Edited April 24, 2015 by CasualKilla

[Danijel Gorupec]

I am using, for small drivers (about 2A), BYV27 diodes because I have them in the local store. These work fine. BYV28 are larger... Anyway, there is large selection of fast silicon diodes.

 

Schottky I never use (cannot find them here).

 

 

Only now I see your picture with spikes on R1.... This is simulation.... can you see the width of the spike? This could be only parasitic capacity of the transistor (base-emiter junction).

 

Edit: Yes, if possible find diodes with 2.5A, but this is not necessary because normally even smaller diode can handle 2.5A for small fraction of time. So, my guess is that even 1.5A diodes would work without problem in your case (but, because in electronics we always add considerable margin, using 2.5A diodes is a smart move).

Edited April 24, 2015 by Danijel Gorupec

[CasualKilla]

I am using, for small drivers (about 2A), BYV27 diodes because I have them in the local store. These work fine. BYV28 are larger... Anyway, there is large selection of fast silicon diodes.

 

Schottky I never use (cannot find them here).

 

 

Only now I see your picture with spikes on R1.... This is simulation.... can you see the width of the spike? This could be only parasitic capacity of the transistor (base-emiter junction).

 

Edit: Yes, if possible find diodes with 2.5A, but this is not necessary because normally even smaller diode can handle 2.5A for small fraction of time. So, my guess is that even 1.5A diodes would work without problem in your case (but, because in electronics we always add considerable margin, using 2.5A diodes is a smart move).

it is about 200ns, I think you are right, it must be current discharging across the CE junction

 

 

It seems when Q1 and Q4 are switch on with the input switch switch, there is a pulse of current that flows out the base of Q2 and Q3, weird. I can increase the value of R3 to reduce this current spike though... The pulse seems to be caused by a 4.5V spike at the base, does not seems to be effected by the addition of the diodes or changing the resistors.

Edited April 24, 2015 by CasualKilla

[Danijel Gorupec]

I suppose the simulator tool shows all those spikes because of parasitic capacities in transistors (while I don't know how detailed transistor model is used). If so, then by increasing R3 resistance the spike will become shorter, as you said, but also wider - is this what is happening?

 

Note that in switching applications you usually deal with those parasitic capacities by, funny enough, decreasing resistances (usually by decreasing resistor value that leads the current to/from transistor base). This way you make spikes higher but thinner - making your transistor switch faster.

 

If indeed these spikes are from parasitic capacities, then you don't need to worry. A 200ns-wide spike is not a problem in your application.

[CasualKilla]

I suppose the simulator tool shows all those spikes because of parasitic capacities in transistors (while I don't know how detailed transistor model is used). If so, then by increasing R3 resistance the spike will become shorter, as you said, but also wider - is this what is happening?

 

Note that in switching applications you usually deal with those parasitic capacities by, funny enough, decreasing resistances (usually by decreasing resistor value that leads the current to/from transistor base). This way you make spikes higher but thinner - making your transistor switch faster.

 

If indeed these spikes are from parasitic capacities, then you don't need to worry. A 200ns-wide spike is not a problem in your application.

Thankyou, very good insight, I noticed the voltage actually does reduce slightly when the resistance of the base circuit increase, I was looking at the current, which increases slightly with decreasing resistance. You are 100% right, reducing the resistance drastically reduces the pulse width and pulse voltage.

[Mordred]

Which PLC model are you using? If it's AB it may support the SQO function this will avoid scantime and switch all outputs simulaneous.

 

By the way I would have just used relays to interface the 24 to 5 volt.

(Though industrial interface methods usually include using a stepper motor control card and simply outputting the direction command or a serial to stepper controller card.)

 

In The SLC series for AB here is the manual

 

http://literature.rockwellautomation.com/idc/groups/literature/documents/um/1746-um121_-en-p.pdf

Cost naturally being a factor and application..

 

I've programmed AB, Omron, Zelios and other numerous smart relays. If you download pdf995.exe (printer to pdf emulator, you can print your code in pdf format) then post the pdf for programming assistance.

[THX-1138]

I can recommend the book Easy Steppin' from Square 1 if you can find a copy. Maybe at ABEbooks.Com or a used bookstore.