bleed resisitor

[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]

Originally Posted by cueshark 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]

Originally Posted by cueshark [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.