[cueshark]

i have a 80 volt 17.5 amp ps. can someone tell me what size resisitor to use & how to hook it up? thanks greg

[acondit]

Greg,

I did a spreadsheet to calculate the power rating, resistance and bleed time for a bleeder resistor for my power supply. I edited the voltage, capacitance and current for yours. If you have excel, then you can see that the power rating for the resistor, and the resistance are determined by the capacitance that you need to discharge and the time in which you want to discharge it.

Alan

[cueshark]

alan
i can read it but i am ignorant about electronics. i would like it to bleed off in a max of 3 to 4 sec. if you could tell mewhat i need & how to wire it i will be indebited to you. thanks greg

[acondit]

Greg,

I need to know what capacitance you are using in your power supply.

For example, if you were using 18,000uf then a 150 ohm resistor would bleed it in 2.7 seconds and it would have to dissipate 42.7 watts of power. This would suggest using a 50 watt power resistor. However, if you chose a 270 ohm resistor, it would bleed the same capacitor in 4.86 seconds and only have to dissipate 23.7 watts (a 25 watt power resistor).

The bleed time equals resistance in ohms x capacitance in farads.

Alan

[Khalid]

In 3 to 4 second ur resistor may bleed about 1liter of blood...

[cueshark]

i am usieng a 21000 cap.so what do you recomend? how do you wire it? thanks greg

[acondit]

i am using a 21000 cap.so what do you recommend?

With 21,000uf then a 150 ohm resistor would bleed it in 3.15 seconds and it would have to dissipate 42.7 watts of power. This would suggest using a 50 watt power resistor. If you chose a 220 ohm resistor, it would bleed the same capacitor in 4.62 seconds and only have to dissipate 29.09 watts (a 30 watt power resistor). If you chose a 270 ohm resistor, it would bleed the same capacitor in 5.67 seconds and only have to dissipate 23.7 watts (a 25 watt power resistor). There is both the safety issue and how much do you want to spend issue. A 25 watt resistor is a cheaper than a 50 watt resistor.

how do you wire it? thanks greg

To bleed the capacitor the resistor has to be placed between the positive side of the capacitor and ground. You don't want it in the circuit all the time so you need some kind of switching circuit that will place it in the circuit only when the power is shut off. Someone else is going to need to help out with this part of your question.

Alan

[acondit]

Greg,

What are you driving with this PS?

Alan

[cueshark]

[4] 4.6 volt steppers

[acondit]

[4] 4.6 volt steppers

What drive(s) are you using?

[cueshark]

gecko 203s greg

[lvittori]

After one RC Time Constant period the capacitor will still have 36.8% of the original voltage. Typically 5 RC Time Constant periods are considered the time required to fully charge or discharge a capacitor.

[epineh]

Does it HAVE to drain in 3-4 seconds ?

The only reason I ask is that on the DC systems I used to work on (800Volt, anything up to and around 500 Amps) it was the norm to simply put the bleed resistors across the DC bus and leave them on all the time, of course they were rated for this, and had bleed times around 10 minutes, so the power dissapation was nice and slow, this also meant there was no need for extra switching in and out of the resistors.

The main reason for a bleed resistor is to get rid of the charge in the DC caps, to allow to work on the system safely, if it takes 10 mins then that allows for a cup of coffee before starting work

Unless there is a specific need to get rid of the charge then you could probably size the resistor for a 5 min discharge, which will still give you enough time for a caffeine fix !

Russell.

[epineh]

Just a thought, if you are driving steppers, why not just let the drives bleed the DC away ? On my system when I had steppers with a 30V suppy and 80,000 uF cap it took only seconds for the drives to drain the cap after switching off the power.

[cueshark]

epineh: are you saying , turn the power off to the transformer only & let the motors hold until the power is gone? this sounds like an easy way out. thanks greg

[epineh]

Yup that's it, it worked for me. I don't know about your drivers, but mine had seperate constant current supplies, which in turn had little cooling fans, you could tell when the cap had discharged when the fans stopped spinning, or I guess if you have LED's on board they will go out when the power is gone.

Cheers.

Russell

[acondit]

Just a thought, if you are driving steppers, why not just let the drives bleed the DC away ? On my system when I had steppers with a 30V suppy and 80,000 uF cap it took only seconds for the drives to drain the cap after switching off the power.

Russell,

I put a bleeder in mine because in testing I have been known to turn on the power supply without drives attached and one time playing around I used a small screwdriver to short the leads of a capacitor. As a result, I had a much shorter screwdriver.

Alan

[cueshark]

that is why i was interested in the bleeder. i am worried i will foreget & unplug a drive, at least with the gecko 203s it would just be a fuse. thanks to all who responded. greg