resistor choices

[trubleshtr]

Can anyone tell me what type of resistor is better for draining capacitors?

Is there a difference between a cement /ceramic filled one versus a wire wound axial lead one? I'm not sure I even have the terminology correct

I plan on running my power supply at 50vdc/36amps max, and I am going with a 5W 10K resistor set-up across the caps.

[Al_The_Man]

Originally Posted by trubleshtr Can anyone tell me what type of resistor is better for draining capacitors? Is there a difference between a cement /ceramic filled one versus a wire wound axial lead one? I'm not sure I even have the terminology correct I plan on running my power supply at 50vdc/36amps max, and I am going with a 5W 10K resistor set-up across the caps.

Personally for anything above 2w I prefer wire wound, I have found that metalic film type at higher wattages tend to change value when run at max wattage.
Al

[OCNC]

Originally Posted by trubleshtr Can anyone tell me what type of resistor is better for draining capacitors? Is there a difference between a cement /ceramic filled one versus a wire wound axial lead one? I'm not sure I even have the terminology correct I plan on running my power supply at 50vdc/36amps max, and I am going with a 5W 10K resistor set-up across the caps.

You might be interested in this article on discharging capacitors.

Chris

[trubleshtr]

intresting, I 'll remember to turn my power supply off when not using the machine.

[ESjaavik]

Why would you use a 5W resistor of 10K at 50V?

For that use it does not matter what kind of resistor. The ripple frequency is 50Hz/60Hz, so just pick the one closest/cheapest.

[trubleshtr]

ESjaavik, I am afraid I am not too experienced in the electronic dept. I was going with a reccomendation. If you would like to explain how one sizes a resistor to fit an application I would appreciate the learning experience.
As-well, if anyone would like to explain the math behind choosing a capacitor for a power supply I would appreciate that also.
All I know is that my set-up so far will give 50vdc 36 amps max output, and I need 2000 uf/amp.

[fyffe555]

Trubleshtr,

As per your thread 'advice needed' for Cap size the most commonly used calculation is C=(80,000*I)/Vdc

A thought; Do you really need drain resistors on your PS? The problem with draining resistors on smoothing caps in a PS is that they are draining power while the machine is on and running.

If the PS is going to be permanently connected to your drivers the drivers will usually present enough load to drain the PS smoothing caps without the need to put on resistors specifically for draining the caps on power off.

You only really need to drain the caps if there's a risk of you or someone else getting their hands in there while they're still charged, and the driven circuit isn't draining the caps (fast enough) on power off. If that's the case then maybe you should consider wiring the resistors through a switch so you can drain the caps as needed and not have the resistors in circuit all the time.

Andrew

[ESjaavik]

@Troubleshtr: Google turned up with this one:
http://www.allaboutcircuits.com/vol_1/chpt_2/1.html
http://www.allaboutcircuits.com/vol_1/chpt_2/3.html

There you will find P = E^2 / R, in your case (50*50)/10000 = 0.25
So a 1/4W resistor is just enough. Choose the next size up for a lower temperature.

Also check the current flow with your resistor. It will take a while to drain a capacitor designed for a 50V, 36A delivery! Better keep your ring off while poking around inside it.

@Fyffe555: His R only consumes 0.25W. I'll leave it as an excercise to figure out if using a relay with 20mA current draw (guessing) to disable this resistor will save power or waste power.

[fyffe555]

"@Fyffe555: His R only consumes 0.25W. I'll leave it as an excercise to figure out if using a relay with 20mA current draw (guessing) to disable this resistor will save power or waste power."

relay? A simple (manual) switch wouldn't draw power...........

[trubleshtr]

Thanks for your help guys,
I now a guy at work who almost got caught by his cross on his chain necklace while leaning over a lathe... I always take off my ring/necklace when working, you just never know....

[ESjaavik]

Originally Posted by fyffe555 "@Fyffe555: His R only consumes 0.25W. I'll leave it as an excercise to figure out if using a relay with 20mA current draw (guessing) to disable this resistor will save power or waste power." relay? A simple (manual) switch wouldn't draw power...........

Sorry, did'nt read your posting well enough.
It would not. Neither would it be very practical. Do the excercise anyway, it's good for being able to "eyeball" resistance/power etc.

[trubleshtr]

Originally Posted by ESjaavikThere you will find P = E^2 / R, in your case (50*50)/10000 = 0.25 So a 1/4W resistor is [b just[/b] enough. Choose the next size up for a lower temperature.

Where did the value of 10,000 for "R" come from? I read both pages you gave as links, good info, but I think I must be missing something cause I get 1.4 ohm.

If R = E/I
then R= 50vdc/36amps?
then R= 1.4
I'm missing something here arn't I