Help on Voltage Reduction Resistor

[foamcutter]

I'm having brain lock right now and need help. I have a 65dc volt power supply and want to use it with 50vdc servo motors and Gecko 320's. The motors should run about 5amp at full power. I know there is a formula for figuring the resistor size to drop the voltage from 65v to 50v but I can't seem to remember it or find it. Can anyone help me? Thanks for the help! Ron

[Al_The_Man]

Ron, A plain resistor is probabally not going to work for you, if the current remains constant at 5a then it might work, but as the current fluctuates so will the voltage.
For a constant 5a at 15v drop will be R = 15/5 = 3ohm, wattage will be 15v x 5a = 75watts!!
Al

[foamcutter]

Al,
Thanks for the reply. Ok your probably right. here's what I have, I have a power supply that once rectified ( torroid coil puts out 48vac before rectified) puts out 65 volts DC I am going to be running 48volt servos ( 3 of them pulling about 5amp each at full power) with Gecko 320's. Everything says not to run the voltage over 5 volts more than the rated servo voltage. So I need to drop the 65 vdc down to around 48-50vdc so as not to fry the servos, at least that's how I understand it, or am I way off? Do you have any suggestions? Thanks for the help Ron

[ESjaavik]

You can Restrict your speed.
Voltage = K * Speed, so if you restrict your speed you also restrict your motor voltage.
This method can only be used if the no-load voltage out of the rectified and smoothed (capacitors) supply is below the voltage rating of the drive. Check with the Gecko manual. And the chance of frying your servos with a too high voltage is slim. I would not worry about 65V, that should still be far below the insulation breakdown voltage. And a 20% overspeed would not worry me much either. It's more like overclocking your PC. You need to watch the motor temperature if you use +20% votage without lowering the current. But how often will you run the motor at max speed for extended time?

Or, you can remove windings from the secondary of the transformer. Make a temporary winding of a few turns on the transformer (I usually just use the test leads of the VOM). Then connect primary and measure the voltage over the temp winding. Divide this by number of turns. Now you know how many volt/turn you have, so just remove enough turns from the primary to get down where you want to be. Another bonus of this method is that it may bring you down to where you can use a lower voltage rating for the cap's (cheaper).

[Al_The_Man]

Originally Posted by foamcutter Al, Thanks for the reply. Ok your probably right. here's what I have, I have a power supply that once rectified ( torroid coil puts out 48vac before rectified) puts out 65 volts DC I am going to be running 48volt servos ( 3 of them pulling about 5amp each at full power) with Gecko 320's. Everything says not to run the voltage over 5 volts more than the rated servo voltage. So I need to drop the 65 vdc down to around 48-50vdc so as not to fry the servos,

Ron, I had not noticed you were running servo's not steppers, there should be no problem with the higher voltage to the motors, unless the Gecko's do not like the higher voltage, it is high current that fries servo's so unless the Gecko's have current limit setting, you would have to watch the loading or fuse fairly tight to protect them. Also as Esjaavik mentioned the winding on the toroid can often be reduced, if it is a bifilar (parallel) winding make sure you remove both together.
Al

[Mariss Freimanis]

NEVER put a resistor in series with a power supply lead!! Switching type drives recirculate power supply current. That means they alternately draw current for a time, then turn that current around and send it back to the power supply for a shorter period of time. This happens 25,000 times a second.

When it is returning current, it has to pass thru the same resistor that dropped the voltage. Now, instead it ADDS that voltage drop to the power supply instead of subtracting it. For that period of time, the drive will see a voltage in excess of its max rated.

You will kill the drive.

Mariss

[lerman]

Originally Posted by ESjaavik ... Or, you can remove windings from the secondary of the transformer. Make a temporary winding of a few turns on the transformer (I usually just use the test leads of the VOM). Then connect primary and measure the voltage over the temp winding. Divide this by number of turns. Now you know how many volt/turn you have, so just remove enough turns from the primary to get down where you want to be. Another bonus of this method is that it may bring you down to where you can use a lower voltage rating for the cap's (cheaper).

Another thing you can do instead of removing turns if there is room in the core is to add turns and then wire them backwards so that the voltage subtracts from the original voltage. That's particularly handy if the original turns are potted, varnished, or otherwised gooped together.

Ken

[foamcutter]

Thanks for all the input. Glad I asked first before turning everything on, letting the magic smoke out of new drives would not be fun. I think since the voltage is under the Gecko limit and won't hurt the servos I'm going to leave it alone. This is a power supply I bought already put together and I'm sure the coil was wound several years ago so I think unwinding some turns would probably flake off the enamel wire coating and could cause even more magic smoke to appear. Thanks again to those who posted. Ron

[Coogrrr]

I do need help. on this also. I have a xylotex controller that wants 24vdc and all i have is a 35vdc power supply what can I do about this?

TIA
Coog

[vtxstar]

Coogrr, what is the brand/type of 35V supply you have?

Some linear supplies can be adjusted down - such as a "frame" type made by several companies.