StepStick BOM mysteries

I found these mystery items in the StepStick BOM ( [github.com] ) that I'm not sure about:

0,2	RESISTOR        	0805	 SparkFun

Is it simply a 0.2 Ohm resistor?

TRIM          	 TRIMPOTSMD      	 TRIMPOT-3MM 	SparkFun

What are the specs of this trimpot?

And then the biggest mystery of them all:

 CLONOLU       	 CLONOLU         	 CLONOLU     	 pololu-allegro

I have no clue what component this is, I can't even find it on the board schematics: [reprap.org]

Any help kindly appreciated.

Quote

What are the specs of this trimpot?

It's a 10 kOhms one. See the wiki page.

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Generation 7 Electronics	Teacup Firmware	RepRap DIY

Yes, the 0.2 resistors 0805 is a 0.2 ohm SMD resistor in package size 0805.

If you zoom in on the pictures in the wiki, you can see the two resistors above the 4988 are both labelled R200 and the silk screen next to them says S1X and S2X. R200 = 0.2 ohm. wiki StepStick That verifies it.

These are the current sense resistors. Usually, current sense resistors are precision, but in this case the current is set by the trim pot, so I'm not sure how much precision is really required. How repeatable does the pot setting need to be from one of these to the next? How interchangeable are the pots themselves? I'd go with 5% for the precision, like Mouser part number 774-73L3R20J, for example. It does cost $0.11 each but why not go for it?

That part is a 1/8 watt. I can't figure out from the 4988 specs and the circuit what its rating SHOULD be. The same package could also be 1/4 watt.

Perhaps someone else can suggest what power rating this resistor needs? We can't run these things any higher than about 1 amp, because of cooling, and 1 amp though 0.2 ohms is 0.2 watts. Does the whole current flow through here? It looks like it from the 4988's functional block diagram.

Mouser SR732ATTDR200F is 1/4 watt but costs $0.43 each. My money says THIS is the right part.

Can someone who knows something more than I chime in?

How many programmers does it take to spec a resistor? CAN'T - Resistors are HARDWARE! (but we can guess)

Yes it will dissipate 0.2W when the motor stops on the one coil on position. The Pololus use 0.05R so they can get away with smaller resistors but that makes the current measurement more susceptible to noise and track impedance.

You want them well matched so that the two coil currents are matched to give accurate micro-stepping.

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[www.hydraraptor.blogspot.com]

Thanks guys! For my understanding, which one will be more reliable: 1/4 or 1/8 watts? I think 1/4 watts because of the nominal load (0.2 watts.)

@nophead which components do you imply when you say:

Quote
nophead
You want them well matched so that the two coil currents are matched to give accurate micro-stepping.

I guess you mean both 0.2 ohm resistors but maybe you mean something else.

Yes you need 1/4 W resistors.

JED implied the tolerance of the resistors did not matter because you can just change VREF to compensate. I was pointing out that although the absolute value does not matter much you do want both channels to be same as they share VREF. That implies close tolerance resistors or manual matching.

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[www.hydraraptor.blogspot.com]

@nophead do you mean both resistors need to have the same tolerance, or do you mean both resistors should have low tolerance? I guess you mean both of these statements need to be true. Would 1% be an acceptable tolerance?

The values of the resistors need to close to each other so that means unless you hand select them you need close tolerance resistors. Yes 1% will be fine.

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[www.hydraraptor.blogspot.com]

Quote
nophead
The values of the resistors need to close to each other so that means unless you hand select them you need close tolerance resistors. Yes 1% will be fine.

Thanks, just one more question: in general, what should the tolerances be for the other resistors and capacitors on the StepStick board? Should they all be 1% or are only these two resistors critical. In other words, is 5% okay for the other resistors and capacitors?

nophead is absolutely correct - the two current sense resistors should be balanced. I had not thought of that. Great explanation, btw.

1% and 1/4 watt 0.2 ohm 0805 for the S resistors.

In looking at the other resistors, 10% is fine.
R1 - sets limit for Ref voltage 10% is fine
R2,R3 set default levels for Enable and Sleep 10% is fine
R4 - sets signals timing choice - 10% is fine

C4 is a "ceramic capacitor [that] will be responsible for delivering the high frequency current components" - again, value isn't critical but placement is. The spec doesn't say more, but the following type should work here, too.

C5 is a charge pump, C6 is "a reservoir for operating the high-side FET gates", C7 is "is used to operate the sink-side FET outputs". For all of these, the 4988 spec says: "Capacitor values should be Class 2 dielectric ±15% maximum, or tolerance R, according to EIA specifications."

C1,C2, and C3 are decoupling the VMOT - the 4988 spec says nothing directly about these.

Hi JED -

Asked this question on another thread - hopefully someone can give me a heads up.

Do you happen to know what the following parts are:

J1E CLONOLU CLONOLU CLONOLU pololu-allegro 1
J1X CLONOLU CLONOLU CLONOLU pololu-allegro 1
J1Y CLONOLU CLONOLU CLONOLU pololu-allegro 1
J1Z CLONOLU CLONOLU CLONOLU pololu-allegro 1

Thanks!