bleed resistor

[bobleecnc]

Hi Guys can anyone tell me what the rules are for sizing a bleed resistor for my home made power supply ? I have a variac for regulating the ac to the bridge rectifier after the rectifier I have a 39000uf cap 75vdc 90vdc surge .
I think that because I didn't have a bleed resistor across the leads from the cap to the drives that I may have burnt a couple of my stepper drives either that or its because theres no ground in my wiring system to bleed the cap .I noticed that after turning the variac down to 65volts and lower there was still 78 volts present at the drive. Is this a product of no bleed resistor or no ground or both ? Thanks Bob

[Al_The_Man]

It is a bit dangerous to use a variac as a supply, It is an auto-transformer so it is not isolated from the incoming service, therefore it is possible to have both conductors going to the bridge have a potential referenced to ground, depending on wether the 120v socket/plug is wired correctly.
If you then ground one of the conductors you are using for the drive, the results are unpredictable.
Al.

[bobleecnc]

Thanks AL for the help
Would it be possible to ground one side of the variac to ground?
The input and the output share a common terminal couldn't this common terminal be plugged into the receptacle as the neutral side providing a common ground?
And what do you think about the bleed resistors is the lack of one of these and a ground the reason that I'm seeing 78 volts at the drive/ Even when I have the variac turned way down to 24 volts.
Is the cap storing this with no way to dissapate it? Is the voltage remaining 78volts as a product of its design? how would I regulate the voltage in this situation to keep it say below 65 vdc /
Bob

[Al_The_Man]

Thats the problem, One side of the variac input is going to be at ground potential, i.e. the Neutral conductor.
The problem is, is guaranteeing that it is correctly phased, You would have to trace everything through with a meter, your reading could be misleading due to stray paths etc, because of this.
An Electronic multimeter will read very small leakge currents and display a voltage reading due to this.
The voltage on the cap. will take a while to dissapear without a bleed resistor.
Al.

[NC Cams]

Bridgeport used a 3.3K ohm resistor @ 1/2 watt across the filter cap for their DC power supply that fed the servo's. Your particular R/C factor will determine the discharge rate of the cap + resistor combo you choose.

[martinw]

Originally Posted by NC Cams . Your particular R/C factor will determine the discharge rate of the cap + resistor combo you choose.

Dear Bob,


I'm a bit rusty on this, so take it with several pinches of salt, and read other posters' input. Here goes.....

INMVVHO, if you want the power supply to come down from about 90 volts to effectively zero volts in two minutes from the time you turn off the PS, and you have a 39000uF capacitor........

1000 ohm bleed resistor rated at 8 watts continuous.

If I've done the calculations correctly, I would not put the bleed resistor directly across the cap. terminals. You will heat the cap. too much.

Incidentally, an 8 watt resistor is quite a heat generator, and quite big physically. You may well have to go for a much longer "shut-down" time.


My two cents..

Best wishes

Martin

[Lexx0001]

In one of my first days in electronics training they told us to do our testing with one hand in a pocket. One of the first things I learned is that theory sounds a lot better then actual practice. The problem with a auto-transformer is that you aren't isolated from ground, meaning anytime you touch any part of the circuit, you complete the path to ground. That's always a bad thing. Now, having said that, and if you are very careful not to switch the hot and neutral wires coming in to the transformer, you should be ok. Use a three prong plug.
I don't know if you are using a full wave or half wave bridge rectifier, or what the current draw is that you want, or even what voltage you want. However the capacitor is indeed storing the charge. The current can't go backwards against the diodes and has nowhere else to go, except as dissipating as leakage current thru the capacitor's insulation. That's what a bleeder resistor does, act like a controlled short to ground.
Without really knowing what voltage and current you are after I hesitate to go further, but there are several ways to go about it. The problem with the 1 K Ω 8 watt resistor is that it is there all the time, causing heat and sucking energy. Again, I don't know how fancy you want to get but you could put a relay in circuit so that when it's energized, it puts power out to the circuit and when it's de-enerzied, it shorts to ground thru the resistor. Or you could go with a higher resistance, which would take longer to discharge, but a whole lot easier to implement.

My information is free, and usually it's worth every penny

Bill K.

[bobleecnc]

Thanks Guys for the help I think I sized my cap wrong in the first place I'm only running two 5 amp steppers off it .
I read an old post here that mentioned a relay to throw the resistor across the capacitor leads thats probably what I 'll do.
I'm only needing 65 vdc for my PacSci 6410 drives. Should I down size my cap to this voltage and maybe 12-15,000 uf .
Using this formula : C= ( 80,000 * I) / V
I get: 12,307uf for two 5 amp 65v motors if this is right then I don't know how I came up with 39,000uf cap 75vdc
Bob

[vger]

When you initially turn power on the capacitor will begin to charge, and will initially appear as a direct short across the supply. If the cap is too large you might damage the rectifiers. There is a formula for determining the (approximate) cap value based on how little ripple you need.
C = Il/Vr * k * 1000uf
Il is the load current
Vr is the ripple voatage
k is a constant of 6 for 120 hz or 7 for 100 hz

If you are full wave rectifying 60 Hz you will have 120 hz ripple. So if you can deal with 1 volt ripple you would could simplify the formula to be..
C = Il * 6000uf (for 60 Hz line current)
So for a 10 amp load (you must have a decent size variac) and just 1 volt ripple you would need 60,000 uf.
If you can deal with 2 volt ripple then 30,000 uf will do it. For 5 volt ripple, 12,000 uf.
You could get lower ripple voltage with smaller capacitors by adding a choke and a second cap to create the classic Pi filter, or use regulator to really clean it up.

Your bleeder need not completely discharge the cap in 10 seconds to get it to a safe level, only most of the way. The smaller the resistance the faster it will discharge the capacitor and the more current it will draw from the supply and therefore the higher the wattage rating on the resistor. I know, lots of trade-offs. Using the relay option may work for you, just use the normally closed contacts to complete the bleed circuit.

Steve

[bobleecnc]

Hi Steve thanks for the reply its funny you should mention ruining a rectifier thats just what I did last night . It was 35 amps 200v the person I bought them from
has some other ones that are 50 amp 1000v do you think I should opt for these instead what are the rules for sizing the bridge rectifier ?
Should I just go for the biggest and baddest one that I can afford and find ?
Also what effect would a 2volt ripple have on a gecko 201?
Thats where I got the formuls for sizing the cap it was in Marris's pdf file (step motor basics)
Bob

[Al_The_Man]

Blowing a 35amp 200v (7Kw) rectifier takes some doing as they will take alot of abuse via their momentary rating.
I suspect that it may be due to the lack of isolation you have, you mention 'Ground', Ground and Common are terms that are often miss-used.
If in fact you have either deliberately or accidentally a connection of the Common to Earth Ground you will blow the rectifier, using a non-isolated source like a Variac.
I always ground my power supply commons, but I ensure a controlled situation by starting out with the proper isolation.
Al.

[vger]

Bob,
I agree with Al about blowing a 35 amp rectifier would take quite a lot. They will handle quite a surge without blowing. One option you might consider is using an isolation transformer between the outlet and the variac. That would put your power floating and make it safe to touch even (as long as you only touch one wire). I would guess the geco's would handle a couple of volts of ripple on the motor supply with no problem. A comon rule of thumb for capacitors is to always use one with a voltage rating that is about double the operating voltage. If you were rectifying line voltage of 120 VAC you must remember that is the RMS voltage. The peak voltage will be 1.414 times that or about 170 volts. With an unregulated supply the voltage will be higher with no load, and if you have never experienced a capacitor going BOOM, be prepared to check your self for aluminum schrapnel. If you ever hear this faint hissing sound comming from the direction of the cap's kill the power and leave quick (voice of experience ).

Steve