Need Help!- Help me understand linistepper circuit

[mmccoo]

Hey all,

First time post, long time lurker.

So I tried building myself a linistepper board and I can't get it to work. Since understanding is the source of all solutions, I'm trying to understand how the thing actually works.

It starts with this schematic: though I'm actually building the variant with aligned devices. It has eagle files that I used to adjust to the parts I have.

Anyway, as I understand it, the diodes and resister form a divider network intended to yield different voltage levels. The BC337 devices are to pick which leg of A or B to activate. The TIP122 are the drive resistors.


ok, in order for the TIP122 to turn on, Vbe must be > 2.5 (per the data sheet). To get the desired current, we want the input to them to be 1 volt higher than that. This automatically turns the devices off when the current drop across the sense resisters reaches 1 volt.

So we want 3.5 volts the input of a TIP122 to get full current.


Looking at the resistor network, I don't see how we attain 3.5 volts unless both BC337s are off. This would counter the idea of selecting one of the two.

At first I tried working the numbers on paper, but then I tried it in LTSpice:


I did not include the TIP122s in the simulation because I assume that the gate draw will be low due to the high beta.

What am I missing?

Thanks in advance for any thoughts anyone may have on this.

Miles

[doorknob]

I don't have an answer or an idea, but do you agree that the schematic appears to have an error in that R4 and R6 are shown in parallel, whereas the intent was probably for the line from R6 to continue down to connect to C6 instead of connecting to the junction of RN3A and RN3B?

Also, my only exposure to spice models was more years ago than I care to admit, but I'm not completely understanding your model. I do get that you are just showing the BC337s and not the TIP122s, so the Q1/Q2 transistor identifications do not correspond to the labels on the main schematic (rather they are Q5/Q6?), but my assumption is that your D1/D2/D3 labels are supposed to correspond to the schematic. In the schematic, the cathode of D3 connects to the anode of D2, but apparently not in the model, and I don't understand the doubled-up series resistor values.

If I'm not mistaken, the originator of the circuit posts on this site, and so maybe you could try sending him a PM to this thread... See http://www.cnczone.com/forums/members/romanlini.html

[James Newton]

That error has been corrected on the /current/ schematic at:
www.piclist.com/images/member/RB-ezy-Q33/LiniV2_sch.gif

[RomanLini]

Darn! Did I make an error? Hmm, just checked all my original schematic GIFs that error did not come from me. Someone must have mdified my original schematic.

I would completely ignore the schematic in post #1 and use the schematic on the official Linistepper page that James linked to in post #3.

Mmccoo- As for your original question, where did you hear that the TIP122 needs 2.5v Vbe to turn on? That is a ridiculous figure it's hard to believe that is listed in the datasheet. If you have a screenshot GIF from the datasheet please post it.

The actual Vbe at the start of turn on is very close to 1.0v and never seems to get much higher than 1.1v. When hot it can even be closer to 0.9v. This is not a guess, it's the actual value from years of testing.

[mmccoo]

The 2.5v I quote was from the electrical characteristics section of the datasheet http://www.fairchildsemi.com/ds/TI/TIP120.pdf:

VBE(on) * Base-Emitter On Voltage VCE = 3V, IC = 3A 2.5 V

but this is the wrong place to look. I should have been looking at graph on the next page. Even there, I'm not entirely sure how to correctly read the sheet.


Mostly I was trying to figure out the expected voltages to see what's broken or not connected properly. Maybe I fried the bc337s during solder. Maybe the tips fried by connected the motors wrong. The voltages at the inputs to the tips don't make it much above a volt.


I'm not that competent at desoldering yet, so swapping them out is not as simple as I'd like.


Either way, I've learned a lot from staring at the schematics. There's some neat stuff in there. In particular, I like how the 120 ohm base resisters trade roles as the pull downs in the voltage divider circuits.

I just need to spend some more time staring at it, and poking and proding with my scope.

[James Newton]

To desolder: first cut all leads just above PCB and remove body of old component. Hold PCB a few inches over table. Apply heat to one pin from the top, then push iron and PCB down quickly so PCB strikes table just as you pull the iron away. Lead and solder removed by gravity. Repeat with farthest (coolest) pin each time or allow cooling to avoid burning board.