Resistors
 

Hi guys, I was looking for some information on resistors and cant find any.
There is something on everything including transformer, rectifier and capacitor. What about a resistor? Where do I put one in the power supply box? I cant figure that out. Can you please help?

Bleeder Resistors
Resistors are not necessary in the power supply itself (when powering multiple stepper motors). Many power supplies used for non-stepper applications have a "bleeder resistor" whose function is to "bleed" or drain the electricity from the large filter capacitor. It is usually a large capacitor that is sized to burn off 1% to 5% of the current that the power supply produces. Because of that, it is usually a large wattage resistor, otherwise it will literally fry. Because the stepper motors act as bleeder resistors when the power is turned off, you don't need an additional bleeder resistor for the voltage used to power the stepper motors; however, if your power supply has an auxiliary voltage, such as 5V, 12V or 24V, to power other devices, it is usually a good idea to add a bleeder resistor in parallel with the auxiliary load.

Ohms law states that Voltage / Resistance = Amps, so you could use this modified formula to determine the proper size of bleeder resistor to use with the auxiliary voltages: Voltage / (Rated Amps X 5%) = Resistance. That will give you the value of the resistor.

To solve for the wattage of the resistor, use this formula: Voltage X Amps = Watts. So, Voltage X (Rated Amps X 5%) = Watts. That will give you a wattage that is guaranteed to burn your fingers if you touch it. I use a wattage at least 4X higher than the wattage determined by that formula. For instance, 5V X (3A X 5%) = 0.75W. I would use a 5W resistor, or I would revise the formula to use 2% or even 1% as the constant. That would require more time for the bleeder resistor to drain off the voltage, but given the choice between waiting a few seconds or heating up the shop from a high wattage resistor, I would choose to wait.

So, if you substitute 1% as the constant, 5V / (3A X 1%) = 166-ohms [use standard values of 150-ohms or 220-ohms] and 5V X (3A X 1%) = 0.15W. I would use a 1/2W. In this case, a 220-ohm resistor wouldn't be of much value with a 5V power supply, because any load on that 5V would probably bleed off the 5V faster than a 220-ohm resistor. I just used 1% to show the process of selecting a low wattage resistor. Let heat be your guide. For practical purposes there is no need to use components that are hot to the touch.

Current Limit Resistors
The current limit resistor that you will need if you use the Gecko G201, G202, or G203v is a small wattage resistor (1/8 to 1/2 watt - whatever is available locally) that is connected directly to terminals #11 and #12 on the Gecko. The value of that resistor is determined by using the formula: 47 X Motor Amps / (7 - Motor Amps). So, for a motor rated at 3A, plug in the numbers: 47 X 3A / (7 - 3A) = 35.25K resistance. A standard value resistor of 30K or 33K is perfectly fine. A value LOWER than the computed value will work, since it limits the current to a level LESS than the motor is rated to handle. A resistor value SLIGHTLY higher will usually work, depending on how liberal the manufacturer was with the current rating for the motor. If the motor seems to be excessively hot, replace the resistor with one having a lower resistance.

The G540, the G250 and the G251 use the formula: 1000 X Motor Amps. So, a 2A motor would require 1000 X 2A = 2K resistor. Check the data sheet for the correct way to attach the current limit resistor for the G250, G251 or the G540.

To summarise, you only need to buy resistors for the Geckos, if you are using them. You do not need bleed resistors, because the drives have enough resistance and do good "bleeding".

Some people with oversized toroidal transformers (typically over 1000VA) will need a inrush current limiting resistor, bypassed by a relay. The time it takes the relay to switch is enough time to limit the inrush and keep the resistor cool.

Another reason not use bleeders here is the low voltage - it machines with more volts and currents there are large capacitance values ( like I have on my other machine 25000uf total) the charge stored in them can be lethal while servicing - so its always better to have a bleeder resistor on machines with high voltage and currents and not on contollers like Mechmates - though if I am operating the G203 at their max volts and currents I would add em just to be safe enough :D

Irfan,
You could always use a manual bleeder when you go to service the box. Just have a power resistor with some leads so you can discharge the caps before you touch. (After you have turned off the box.) I have to do this all the time for work when we are working with plus 150VDC and sometimes in the neighbourhood of 2 Farads of capacitance. :)

Heath, I have just automated that for me - as soon as power is off - the caps are automatically shorted via the relay.

Heath & Irfan, once you get to the stage where you have 4 Gecko's connected directly to your capacitors (as they should be), you will wonder why this talk about bleeder resistors. It is a fun discussion, but confusing to those that are not really into their electrics.

Let's keep it simple: Anyone building a control box with 4 Geckos will only need to purchase 4 resistors to fit to the Geckos to limit the current going to the stepper motors.

Gerald,
Yes you are right, My point was just some shop talk with Irfan. Mechmate builders do not need the complexity of bleeders. :)

I Agree :)

Some how I always get mixed up with both of my machines :) and forget that we are mechmate specific here.

G' kindly delete the confusing post .

Regards
Irfan

No need to delete - readers will see now that we all agree about the bleed resistor subject.

Standard (typical) resistor values:

Very common (10% tolerance):
10, 12, 15,18, 22, 27, 33, 39, 47, 56, 68, 82

Also Common (2 or 5% tolerance):
10, 11, 12, 13, 15, 16, 18, 20, 22, 24, 27, 30, 33, 36, 39, 43, 47, 51, 56, 62, 68, 75, 82, 91

Also for 1% resistors the EIA table is
the E96 series. 1% resistors are becoming
more common. (Note any value in the following table
can be multiplied by 10, 100, 1000 etc to get the real
value you are looking for.

E96 series:
1.00 1.02 1.05
1.07 1.10 1.13
1.15 1.18 1.21
1.24 1.27 1.30
1.33 1.37 1.40
1.43 1.47 1.50
1.54 1,58 1.62
1.65 1.69 1.74
1.78 1.82 1.87
1.91 1.96 2.00
2.05 2.10 2.16
2.21 2.36 2.32
2.37 2.43 2.49
2.55 2.61 2.67
2.74 2.80 2.87
2.94 3.01 3.09
3.16 3.24 3.32
3.40 3.48 3.57
3.65 3.74 3.83
3.92 4.02 4.12
4.22 4.32 4.42
4.53 4.64 4.75
4.87 4.91 5.11
5.23 5.36 5.49
5.62 5.76 5.90
6.04 6.19 6.34
6.49 6.65 6,81
6.98 7.15 7.32
7.50 7.68 7.87
8.06 8.25 8.45
8.66 8.87 9.09
9.31 9.59 9.76