Cheap Unipolar LPT Driver, High Amperes

[Urgundiz]

Ive drawn the schematics for a cheap unipolar driver but capable of driving high ampere steppers, because I got some 1'3V/3'9A steppers and there is no real schematic able to drive that amount of current. I've built and tested with Master5 at a P266 with w98, an old PC-AT PSU and:

- Surplus unipolar Nema23 1V3/3A9 unknown torque
- Surplus unipolar Nema34 1V2/4A8 unknown torque

and variable satisfaction the nema23 has good 'tactile' torque and moderate speed, nema34 very good 'tactile' torque at low rpm.

Credits to the idea to www.cnconabudget.com
and this thread: Upping the capability of a 5804 IC

Next steps will be triying to add some current control IC to avoid RBallast.

Now, for the schematic:
- Wired for LPT pin2 Direction, LPT pin 3 Step, repeat for other axis (pin3&4 for Y, pin 5&6 for Z) or rewire for other pins as needed
- Wired for outputs always enabled
- Use Steper Calc (credits to Eric Van Andel) for Value of Rballast

[ezland00]

What voltage, and amps does it support?

[abasir]

From IRFZ44N datasheet
Voltage: 55V (VDSS)
Current: 47A (ID)

[Urgundiz]

Originally posted by abasir
From IRFZ44N datasheet
Voltage: 55V (VDSS)
Current: 47A (ID)

Instead IRFZ44N I've used IRLZ44N, wich is logic level Mosfet (Vgs=5V) in order to assure full ON when switching. Apart of that, characteristics are the same.

But do not expect to reach 47A because the T0220 package is totally unable to support that level of current. I'm testing it and at 5A/5V the mosfets are cold, and at 10A/5V they are ligthly warm (no heatspreader, no forced air)

These are the safe operating areas (ID/VD/Time) from the datasheet:

[starCNC]

Hey, do you have a better quality picture for schematic?.

[tachus42]

The clamp diodes look like they are in the wrong place. Consider the stepper winding as a auto transfomer when one side has voltage across it the other will be shorted by the diode it will still work to degree buts it's not best. the clamp diode should be accross the Mosfet and the opersite side will clamp the side which has just turned off. This transformer action also means that the voltage rating of the mosfet needs to be twice the power supply voltage.

[Mariss Freimanis]

Couple of design tips:

1) Return the diode cathodes to the +Supply voltage, not across the coils as you have it now. Otherwise you will have excruciatingly slow current decay on "winding current off" time. That will severely limit max speed.

2) If you return the cathodes to where they should go, the peak drain voltage on the MOSFETs will be twice the supply voltage. The MOSFETs you have picked will come apart at 27.5V theoretically, probably at 24V if shoot-thru currents due to diode reverse recovery time are taken into account. Use fast recovery rectifiers (<100nS Trr), a current rating equal to the motor current and a rated voltage at least equal to the supply.

3) Don't pick a fragile MOSFET. The "on" resistance may look real sexy on the data sheet but the number that lets you know how rugged the device is the "Single Pulse Avalanche Energy" rating. The more milli-Joules the better.

4) Use RC snubbers across the ballast resistors. MOSFETs are real fast. Too fast in fact. The rate at which they turn-on, turn-off makes a wire wound resistor's inductance significant. A 200MHz scope would show truly breathtaking voltage spikes you never knew were there. Snub them.

5) Do not breadboard or wire-wrap a MOSFET power circuit. Use a printed-circuit board with big islands of copper pour in the high current sections. Use +Supply to ground capacitors. Reason? Wire is not wire at these speeds; it is an inductor (1nH per inch more or less).

Mariss

[vacpress]

wow. i wish someone would rework that schematic using Mariss's suggestions. the output would probably be suitable for any uniploar indexer circuit - a great output stage i would bet.

[Ferenczyg]

Mariss, tachus, you simply mean the shematic below? And, if possible, could you explain a little bit more about the snubbers? I understand the concept and the utility but I cannot figure how to wire it.

Thanks in advance

Fer

[Mariss Freimanis]

First off, the diode does absolutely nothing as drawn. Move the anode to the MOSFET drain, the cathode to the + terminal of the power supply. Inductors (windings) do not "like" current thru them to change abruptly. They will generate what ever voltage is necessary to keep it flowing until the stored energy (J = L * I squared / 2) is exhausted. This is the way a spark plug coil works; you don't want that on your MOSFET drain.

When the diode is moved the way I suggest, the winding current circulates thru the coil in the original direction, thru the diode (anode to cathode), thru the limit resistor and back into the coil again. This completes the circuit.

The peak voltage on the MOSFET on "turn-off" will be 1 diode drop above the supply voltage. The peak voltage in the circuit will be at the coil to resistor node. That will be twice the supply voltage (assuming a zero-Ohm coil).

On the otherhand, the circuit as drawn will have its peak voltage on the MOSFET drain and will be limited only by the MOSFET's Vds (drain to source breakdown voltage). This is not a good thing.

A snubber is a resistor in series with a capacitor. It is meant to dissipate short voltage spikes. This combo is placed drain to source on the MOSFET. Pick the resistor value to carry an Amp or so. Pick the capacitor so that the RC time constant equals the width of the pulses you want to snub.

Example: 24VDC supply, 2A, 5uS pulse. That should be plenty.

R = 24 / 2 = 12 Ohms
T = RC so C = T / R C= 5uS / 12 Ohms = 0.41666 uF Use a .47uF cap in series with the 12 Ohm resistor.

Mariss

[Ferenczyg]

Originally posted by Mariss Freimani

When the diode is moved the way I suggest, the winding current circulates thru the coil in the original direction, thru the diode (anode to cathode), thru the limit resistor and back into the coil again. This completes the circuit.

Yeah yeah yeah oh yeah, that's the kind of things I love to read here, because is explained a way that is so simply that is almost beautiful. Btw, that's the reason I love electronics too

I've attached a new schematic with the hope to help to the others that were in my own situation.


Fer

PS: Mariss, if you travel some day to Spain, drop a PM before taking the plane and you have assured an invitation to a cofee -as a minimum - at the arrival

[vacpress]

i love the fact that Mr. Freimani not only designs these things for a living, but also is willing to share his hard-won klnolwedge so freely.. i cant begin to say thank you.
(!)