Calculating current limiting resistors (I need serious help)

[Konstantin]

EDIT: my appoligies for posting this in here. This one prolly must go to "General Electronics Discussion" board. My bad. *end EDIT

My power supply unit from a printer has 25V and 5V.
I am using the 25V and the OS discrete one axis driver.
My stepper has the folowing specs: 5.2V/Phase 1.4A/Phase. Supposedly the coil resistance should be 5.2/1.4=3.71, I have measured a1 to a2 and have a reading of 8 Ohms, but maybe it does not matter.

Then calculating the power resistor value
(25-5.2)/1.4= 14.14 Ohm
Calculating wattage
1.4*1.4*14.14=27.71 Watts
So I need a 14.14 Ohm resistor with wattage of 27.71 min. Looks like pretty high wattage requirements.

Now comes the part which I fail to grasp yet.

Some power resistors available from Newark
# 01F9907 50W
# 02F2439 10W
# 01F9753 20W
all 7 Ohm resistors. I cannot find 14 Ohm resistor.
If I put a pair of 7 Ohm resistors in series then I get 14 Ohm resistor but what will be their wattage?
If I use 7 Ohm 10W in series with 7Ohm 20W wil I get a 14 Ohm 30W resistor? I am talking loko here?
Wattage is amout of heat dissipated in the resistor, right? It does not add up if I serie the resistors, of gets lower if I parallel the resistors.

I need 27Watts minimmum. If I use a 30W resistor and my motor ratings are 1.4A/Phase
1.4*1.4*X=30W
1.96*X=30W
X=15.03 Ohm
My motor will draw its current rating 1.4A per phase. If I use a 15 Ohm resistor 30W then the voltage in the stepper will be
(25-X)/1.4=15 Ohm
25-X=15*1.4
25-X=21
25-21=X
X=4 Volt.
And my stepper will be operating under its specfications.

How can I use my stepper at full specs, current limiting resistors are puzzling me.

Using this calculator "2 Resistors in Parallel - Calculate One Resistor from Other and Total" from http://www.electronics2000.co.uk/ I input Total R= 14.14 Ohm and R1=30 Ohm so it computes R2=26.74 Ohm
But resistors come in different increments so it is difficult to me compute the resistors values to use my stepper at its full ratings.

Help anybody.


Konstantin.

[Konstantin]

I guess it would be wiser for me to get another power supply.
Or I will end up spending too much on power resistors.



Konstantin.

[slp_prlzys]

maybe you should also build the chopper converter,
atleast you won't need resistors that large, but I
haven't built one yet, so the proof is yet to come.

[cbcnc]

Balsaman, who is a member on this site, wrote a little program called Steppercalc that will calculate the resistors for you. Do a search and you should be able to find it.
Don't be put off by the values of the resistors. Yes they are big resistors. I just purchased 6 10 ohm, 50w aluminum case resistors for my motors. I purchased them from Mouser Electronics. The Arcol brand was a good price.

Chris

[Konstantin]

Originally Posted by cbcnc Balsaman, who is a member on this site, wrote a little program called Steppercalc that will calculate the resistors for you. Do a search and you should be able to find it. Don't be put off by the values of the resistors. Yes they are big resistors. I just purchased 6 10 ohm, 50w aluminum case resistors for my motors. I purchased them from Mouser Electronics. The Alcol brand was a good price. Chris

Yes I did used the StepperCalc and the result is the same, what the stepper calc wont do for you is to choose the resistor value for your setup. It just tells you the theoretical resistance and wattage you need, but real resistor values do not come in any number or increment I want, so I must use lower or higher rated ones.
Thats my concern, to use the stepper at rated specifications.


Konstantin

[cbcnc]

I understand what you are saying. In my case I had gotten a value of 10.8 ohms and 36 watts. At first I used 2 - 20 ohm 25w resistors in parallel.
These are general values though, as the actual resistance necessary to maintain a constant amperage is always changing as a factor of the inductance and frequency of the operation of the motor.

chris

[Konstantin]

I dug up an old PSU from a 8086 computer.
It has +5V and +10.8V outputs.

I calculated the value of the resistors to be 4 Ohm 7.84 Watts.
Went to buy a couple 3.9 Ohm at 10W resistor.
I measured the voltage for my stepper in idle and it was 5.7 (motor rating is 5.2) I hope it is ok. Measuring voltage on a phase with the motor spinning gave a reading of 4.7V

Boy the resistors do get hot. I used a passive heatsink from an old video card and clamped the resistor to it with a bit of termal paste.
The stepper motor just warms up a bit.

I dont know how to setup Mach2 and used TurboCNC to test my 1 axis. I had to play around with "Motion Setup" with the max speed, accell, and start speed settings and the pulse width to get the motor spinning without stalling.


Konstantin.

[frenchnew]

Dear Konstantin,

the power rating of a resistor is the maximum wattage it can dissipate before exceeding it's maximum operating temperature and fail.

For your calculations below, you want a 4 ohms resistor of at least 15 Watts as you do not want nor is it reccomended to operate at more then 50% of its power rating for reliabilty.

Best regards

frenchnew (a former electronic technician)

Originally Posted by Konstantin I dug up an old PSU from a 8086 computer. It has +5V and +10.8V outputs. I calculated the value of the resistors to be 4 Ohm 7.84 Watts. Went to buy a couple 3.9 Ohm at 10W resistor. I measured the voltage for my stepper in idle and it was 5.7 (motor rating is 5.2) I hope it is ok. Measuring voltage on a phase with the motor spinning gave a reading of 4.7V Boy the resistors do get hot. I used a passive heatsink from an old video card and clamped the resistor to it with a bit of termal paste. The stepper motor just warms up a bit. I dont know how to setup Mach2 and used TurboCNC to test my 1 axis. I had to play around with "Motion Setup" with the max speed, accell, and start speed settings and the pulse width to get the motor spinning without stalling. Konstantin.

[Mariss Freimanis]

You will never find a 14 Ohm, 27 Watt resistor and yes, your math is correct. You measured 8 Ohms correctly as well; the measurement was the end to end winding resistance, the 3.71 Ohms is the correct half-winding value.

Back to your resistor problem. You can use two resistors in series or two in parallel to get the 14 Ohms you want. +/-10% will be close enough; if you find a 15 Ohm, 30W or larger wattage resistor, then that's OK too.

Series: Any combo that adds to 14 Ohms will work. The wattage is only a little more complicated. Let's say you find a 5 Ohm and a 10 Ohm resistor. The 10 Ohm resistor will dissipate twice the power of the 5 Ohm resistor so a 20W 10 Ohm and a 10W 5 Ohm will work nicely.

Parallel: Say you have found a 20W resistor. The value needed in parallel to get 15 Ohms is R = (15 Ohms * 20 Ohms) / (20 Ohms - 15 Ohms) = 60 Ohms.

Wattage: The 20 Ohm resistor carries 3 times more current than the 60 Ohm resistor so it dissipates 3/4 of all the heat. Use a 20 Ohm 20W resistor in parallel with a 60 Ohm 7 Watt resistor.

Hope that helps.

Mariss

[Konstantin]

Thank you very much for the explanations guys!!!

I was troubled by the wattage, I believed that it needed to be exact, no lower not less, thats why I was trying to solve equations with two variables, Ohms and Watts.

So then, if I need at least 17Watt resistance it will be a good idea to use 25Watt rated one.




Konstantin.

[frenchnew]

If you need to do away with 17 Watts of power, it would be better for the resitor to be rated at least 34 Watts or the next standard available which is 50 Watts.

Best regards

frenchnew

Originally Posted by Konstantin Thank you very much for the explanations guys!!! I was troubled by the wattage, I believed that it needed to be exact, no lower not less, thats why I was trying to solve equations with two variables, Ohms and Watts. So then, if I need at least 17Watt resistance it will be a good idea to use 25Watt rated one. Konstantin.

[BCwanderer]

You can use power transistors for current control
If you wire the transistors to limit current, You can heatsink your transistors for up to the transistors power limit. Often 75 to 100 watts for large transistors.
If you use an emitter resistor as a current limiter and bias the transistor with reference to the emitter, the transistor will conduct full current until it approaches the current limit.
This allows the current to build up fast with little resistance losses.
Once the transistor starts to limit the current, the motor is at maximum current and you are at full power.
A resistor limits the power fed to the motor through the whole cycle.

C /
B---]
E \
....... ]Resistor on the emitter limits the current.


The transistor absorbs the heat.
Sometimes a power resistor is cheaper than a power resistor.

http://www.maxim-ic.com/appnotes.cfm...te_number/2044

http://www.allaboutcircuits.com/vol_6/chpt_5/14.html

http://www.cs.uiowa.edu/~jones/step/current.html