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  1. #1
    Hello All
    I am slowly getting my power supply built (unregulated linear) I wll be using 50 volts after the rectifier 3 x 4700 uf 100v Capacitors. I was just wondering what the correct value for the bleed resistor should be. One post http://www.mycncuk.com/threads/7942-...2015#post62015 gave a formula that gave me about 350 ohms somewhere else on the internet made use of 2.2 k ohms. I was wondering what the pitfalls are using the wrong value resistor. As always any help gratfully accepted.

    Cheers

    Andrew

  2. #2
    m_c's Avatar
    Lives in East Lothian, United Kingdom. Last Activity: 2 Hours Ago Has been a member for 9-10 years. Has a total post count of 2,148. Received thanks 236 times, giving thanks to others 5 times.
    Lower resistance = faster voltage drop after power removal, but more heat generated.
    Higher resistance = slower voltage drop after power removal, but less heat generated.

    Once you have drives attached to the power supply, they'll help reduce the voltage. The bleed resistor is more a safety measure for if there are no drives attached, as otherwise the capacitor(s) could retain quite a high voltage for a considerable amount of time.
    I've just checked what I've used previously, and for my digitiser (around 40VDC IIRC) it's a 270ohm 15W, and the mill a 680ohm 25W (around 55VDC IIRC).
    Avoiding the rubbish customer service from AluminiumWarehouse since July '13.

  3. #3
    Hi
    Does the resitor in parallel with the caps alter the voltage coming into the controller whilst running the motors and controller at all? If so in which way?

    Cheers

    Andrew

  4. #4
    Quote Originally Posted by the great waldo View Post
    Hi
    Does the resitor in parallel with the caps alter the voltage coming into the controller whilst running the motors and controller at all? If so in which way?

    Cheers

    Andrew
    With an unregulated PSU, the bleed resistor will lower the terminal voltage slightly. Provided the PSU is specified sufficiently for the steppers then the resistor effect will be negligible.

    I'd be looking to dissipate 2->5W just to keep the temperature down and keep the costs low.

    Last edited by Doddy; 09-07-2018 at 07:35 PM.

  5. #5
    After switching off primary power supply the drives drain the caps quickly. Even with no steppers attatched.

    You can check it only takes 10 seconds or so.

    Please check your setup with a dvm

    Grts Bert.





    Verstuurd vanaf mijn SM-A320FL met Tapatalk

  6. #6
    Quote Originally Posted by driftspin View Post
    After switching off primary power supply the drives drain the caps quickly. Even with no steppers attatched.

    You can check it only takes 10 seconds or so.

    Please check your setup with a dvm

    Grts Bert.





    Verstuurd vanaf mijn SM-A320FL met Tapatalk
    Hi Bert

    Do you think it's not worth bothering with the bleed resistor ?
    cheers

    Andrew

  7. #7
    Neale's Avatar
    Lives in Plymouth, United Kingdom. Last Activity: 11 Hours Ago Has been a member for 5-6 years. Has a total post count of 1,167. Received thanks 212 times, giving thanks to others 5 times.
    One of the crazy things about specifying a bleed resistor is that it is based on things like heat dissipation at the time it is not doing anything useful, just sitting there wasting energy. The amount of stored energy that it actually needs to bleed if the input voltage fails or is switched off is very small. I chose to forget it altogether - I didn't want all that extra heat and wasted energy when, in practice, the drivers do the job very effectively. The only time a bleed resistor would be useful is under unusual conditions, during testing, for example. I'm happy to be aware of the issue at such times - it's no worse than avoiding the mains connections, for example. I'm not criticising people who do fit them, but for me it falls into the same category as a "cabinet open" switch. Commercially you should fit one of these in case some clown opens the cabinet and puts a finger where they shouldn't. For home workshop use, I want to be able to run with the door open sometimes - for example, to measure the power supply voltage to see what happens when it is turned off!

    I have three e-stop switches around the machine and limit switches on all axes (except lowest position, of course). For me, that is a reasonable set of safety features; there's an MCB where the power enters the cabinet, a safety relay, and a fuse on the motor PSU (because it was fitted when I bought it). The only failure I have had on my machine was that fuse failing - through age, it seems, as there was no overload. Other people fit MCBs, fuses, etc, all over the place. Opinions vary!

  8. #8
    Quote Originally Posted by the great waldo View Post

    Do you think it's not worth bothering with the bleed resistor ?
    Bleed resistors are pointless, unless you have a very high voltage and you must work with the equipment, for example a camera flash, which is potentially dangerous unless you drain the capacitor, or measure the voltage and KNOW that the capacitor is discharged, which can take a very long time in case of a flash. In normal power supplies it will be drained after maximum a few minutes, even without any bleed resistor because you most certainly have some things, like a driver, or other electronics connected to it. Bleed resistors are just waste of energy when the PSU is ON. In your case, 50VDC isn't a risk anyway, unless you are easily scared. You may feel it, but nothing more. Besides, it can't be charged to more than 50V anyway. The fact that you are using 100V capacitors says nothing about the stored voltage, only that the capacitors can be used up to 100V. Your capacitors will never be charged over 50V if the PSU is 50VDC out after the rectifiers.

    What I think is MUCH more important is that you have a slow starter circuit, because toroidal transformers draw a lot of current at start up and they can blow the fuse, or start with a very loud bang. Slow starter circuits are placed on the primary side and they limit the current for a short period (typically about 2 seconds), while the large capacitors are charged. This is necessary because the large capacitors short circuit the secondary side for a short time when they start to charge. The current rush cause the large bang and can also blow your mains circuit breaker, or fuse in the wall plug if you have one.

  9. #9
    Quote Originally Posted by A_Camera View Post
    Bleed resistors are pointless, unless you have a very high voltage and you must work with the equipment, for example a camera flash, which is potentially dangerous unless you drain the capacitor, or measure the voltage and KNOW that the capacitor is discharged, which can take a very long time in case of a flash. In normal power supplies it will be drained after maximum a few minutes, even without any bleed resistor because you most certainly have some things, like a driver, or other electronics connected to it. Bleed resistors are just waste of energy when the PSU is ON. In your case, 50VDC isn't a risk anyway, unless you are easily scared. You may feel it, but nothing more. Besides, it can't be charged to more than 50V anyway. The fact that you are using 100V capacitors says nothing about the stored voltage, only that the capacitors can be used up to 100V. Your capacitors will never be charged over 50V if the PSU is 50VDC out after the rectifiers.

    What I think is MUCH more important is that you have a slow starter circuit, because toroidal transformers draw a lot of current at start up and they can blow the fuse, or start with a very loud bang. Slow starter circuits are placed on the primary side and they limit the current for a short period (typically about 2 seconds), while the large capacitors are charged. This is necessary because the large capacitors short circuit the secondary side for a short time when they start to charge. The current rush cause the large bang and can also blow your mains circuit breaker, or fuse in the wall plug if you have one.
    Hi A camera

    I thought that might be a problem and bought one of these
    https://www.conrad.at/de/renkforce-6...20-622412.html

    I assume that should do the job, although I would like to build a power surge reducer into the case to be a bit more self contained, if anyone has a straight forward circuit diagram for such a device i would like to see it. Thanks in advance.
    Andrew

  10. #10
    Neale's Avatar
    Lives in Plymouth, United Kingdom. Last Activity: 11 Hours Ago Has been a member for 5-6 years. Has a total post count of 1,167. Received thanks 212 times, giving thanks to others 5 times.
    I am using a 650vA toroidal transformer (transformer rating is a good guide to the amount of inrush current you get at startup) and I have a 10A MCB where the mains enters the cabinet. I used a C curve MCB in place of the more usual B curve, which is a bit more tolerant of short-term surges. Never had it blow - unless you have a much larger transformer you probably don't need inrush protection.

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