How to tune the linistepper for higher current?

[Niggo]

Hi! I need to drive a 2.1A 1.1ohm unipolar motor with the linistepper but I dont understand which kind of resistor to change and where they are (is the wirewound 1 ohm 5w ? ). Looking to the tune page I calculate the resistor to use : 1 ohm // 1 ohm // 10 ohm for a 0,47 ohm so in theory if I find a resistor of 0,47 ohm can I use it directly instead of the parallel of resistors? How to calculate the watt of this resistor? Last thing: do you think is better to change the transistor or other component to drive my motor ? Thanks in advance!

[jack_live]

Originally Posted by Niggo Hi! I need to drive a 2.1A 1.1ohm unipolar motor with the linistepper but I dont understand which kind of resistor to change and where they are (is the wirewound 1 ohm 5w ? ). Looking to the tune page I calculate the resistor to use : 1 ohm // 1 ohm // 10 ohm for a 0,47 ohm so in theory if I find a resistor of 0,47 ohm can I use it directly instead of the parallel of resistors? How to calculate the watt of this resistor? Last thing: do you think is better to change the transistor or other component to drive my motor ? Thanks in advance!

WHICH THE FEEDING VOLTAGE THAT YOU TO USE IN THE ENGINE? IT HARNESSES IT CALCULATED IN THE ELECTRICAL RESISTANCE HAS OF BEING BASED ON THE CHAIN AND TENSION OF FEEDING. USE THE LINI AND I HAVE GOTTEN GOOD RESULTED. sorry for my english i'am from Brasil.

[Niggo]

HI! the motors are 3.3V but I'd like to use 5V from an ATX PS

[James Newton]

All information on tuning the Linistepper is at

http://www.piclist.com/techref/io/st...p/lini_tun.htm

But in general, you should have at least 2 and more like 3 times the rated motor voltage as a supply voltage. For your motors, it would be best to supply 10V but the 12V supply from your ATX would be ok (assuming it could put out the required amperage.

I don't see how you could find a .47 Ohm resistor... I don't believe those have ever been made. I could be wrong.

The sense resistors supplied with the kit are 3W.

Now, if I remember my basic electronics: Your resistors will dissipate (P = I^2*R) a total of 2.1A^2 * 0.47Ohm = 2.0727W. The Lini will vary the voltage applied to the motor phase coil such that 2.1A is always flowing through the coil / sense resistor circuit. The voltage applied at the coil end will vary greatly based on the state of the coil, but at the resistors, the voltage will be regulated to remain at the same value. In your case, (E=IR) 2.1A * 0.47Ohm = 0.987V. That allows us to calculate the power (P=E^2 / R) dissipation for each resistor individually: 0.987V ^ 2 / 1Ohm = 0.974W for the two 1 Ohm resistors and 0.987V ^ 2 / 10Ohm = 0.0974W for the 10 Ohm resistor. So a pair of 3W 1 Ohm resistors with a half or even quarter watt 10 Ohm resistor should be fine. I would not recommend trying to get away with a pair of 1W 1Ohm resistors.

If you use a 1 Ohm, a 1.5 Ohm, and a 2.2 Ohm set instead, the dissipation will be 0.974W, 0.649W, and 0.443W respectively which would allow a 3W 1Ohm, a 1W 1.5Ohm and a 1W 2.2Ohm. Not much better.

Anyway, I hope that helps.

[Niggo]

Thanks for your answer!! So I can use 1ohm 3W resistor in parallel with a simple 10ohm for each sense resistors, am I right?

I'd like your opinion about choosing between the 3,3v 2.1A 1.1ohm motor of above or a 5v 1A 1ohm motor: what do you think is better for lini in a milling machine made from mdf and about 30x30cm of usable area?

Why is better to overvolt so much the motors?

Thanks again!

[James Newton]

That is TWO 1 Ohm 3W along with the one 10 Ohm 1/4 watt, but yes. I've updated the page at

http://www.piclist.com/techref/io/st...p/lini_tun.htm

to automatically calculate the wattage in each resistor, so if you have a hard time finding those, try that page again for other options.

Between your 3.3v 2.1A and 5v 1A motors: Well, the wattage of the motors is 6.93 vs 5 so the 3.3v motors are a bit stronger, but performance could depend on other things like how the coil is wound, so I guess it's a toss up.

The motor will not be "overvoltaged" since the supply is for both the drive AND the motor. The Linistepper is a linear driver, so it must be supplied with a multiple of the motor drive voltage so that it can regulate what reaches the actual motor coils. It does this by resistive loss through the power transistors, which is why they get hot. If you supply only the exact amount the motor can take, then when each coil is energised, the lini can only drive current into the coil at the rate that the coil would accept when it was totally on. Because of the resistive elements that are in series with the coil (the power transistors and sense resistors, not to mention the internal resistance of the power supply) this results in a slower ramp up of the coils magnetic field. By supplying more voltage, the lini can hit the coil with a burst of voltage when it first turns on, getting the coil up to its operating current very quickly, and then the applied voltage can be reduced (again, by tossing some out as heat in the power transistors) so that the coil is not operated at over its rated current.

I would imagine that a 12v car battery being charged by a 12 volt supply would provide a better power source for your 3.3v motors since the internal resistance of the battery is much lower, but I have not tested that or even thought much about it; it just popped into my head.

If you purchased a kit from me (sorry I don't remember seeing that name) and you try that (12v supply vs 12v battery + supply) and report the results in terms of max speed and torque I will refund $10 of your purchase price via PayPal.

[Niggo]

Great, now is really all clear about resistor adjustment, thanks! Well, probably is easier for me to find the 5v 1A motors so I don't need to add other resistors, but in this case the 12V from ATX can be less then 3 times as you suggested (with a 450W one and 16A for the 12v line and 4 linestepper I can do that...i hope! ).

[jack_live]

thanks a lot all mine you doubt had been cured definitively

[Niggo]

James, one more question: you use the 16f628 in your kit, I found only the 4mhz and 20 mhz version of this pic but the quartz in your project is 16mhz, so if I bought the 20 mhz version of the pic is sufficient the 16mhz quartz or I need the 20mhz one?

Looking around I found this resistor 0.47 ohm 7W : http://export.farnell.com/jsp/endeca...sp?SKU=9504435
do you think it is a good choice instead of the parallel for the 2.1A motor?

Thanks!

[James Newton]

The 20Mhz PIC and resistor are fine.

Of course, by the time you finish purchasing all the components seperatly, you will have spent more than the cost of the kit. And the PCB supplied in the kit is top quality.

[Niggo]

Thanks James, but I will need 4 kit and the pcb are gratis for me (thanks to a friend of mine and his photoplotter), shipping cost for Italy is an added cost and so on, many components of the linistepper are free for me so I can do these with low cost. Thanks again for your explanations and kindness!!