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Thread: 72 VDC Linear Power Supply Questions

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    Default 72 VDC Linear Power Supply Questions

    Hi, this application is lighting related, although lighting is only part of the project. This power supply is to drive stepper motors so I can add cnc automation to a lathe (to make flashlights).

    I need to obtain or build 2 power supplies:
    - 48 VDC x 30 - 40 amps
    - 72 VDC x 30 - 40 amps

    Incoming power is just standard household 110V 60 hz line power. If it can be made to work internationally that is fine, but not a requirement (home project, not a critical professional level reliability)

    Due to the nature of the load, perfect voltage stability is not a requirement, but the primary pull is from drivers that call out a linear PS rather than a switch mode supply.

    I had not really thought through what it means to operate in these power ranges vs the normal 10 - 20 watt LED projects I have done, but it gets pretty serious quickly.

    The most common approach I am seeing is to start with a toroidal transformer to bring AC to 38 volts, a bridge rectifier to sinusoidal DC, and then Al electrolyte capacitors to smooth out the voltage. Some supplies have methods to dump back EMF from reversing motor loads. (which this also has)

    The biggest challenge is specifying the components, especially the capacitors, and that is what I need help with. There are wide ranges of cap pricing with what appear to be small difference in specs. Please check my thinking.

    It probably would be easier and cheaper to buy, but I would like to do this as a father + teens summer project if practical.
    Last edited by HarryN; 07-07-2012 at 09:34 AM.
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    Default Re: 48 VDC Linear Power Supply Questions

    Question 1 toroid
    - Is this design just more efficient than other transformers or does this really matter which is used ?
    - I assume I spec by VA, just multiply the secondary coil ( 38 volts x 40 amps ) ?
    - Does it matter if I use two coils in parallel vs one larger one ? ( size placement issue)
    - There are always losses in a device. How much "over design" do you put in the the transformer stage compared to what you think the real peak load / normal load will be ?
    Homebuilts - "BREEZE" RCR2 sidexside, "Tornado" 4 x 18650 side x side, Streamlight SL20x LED module (Custom BST Feeler Thread)

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    Default Re: 48 VDC Linear Power Supply Questions

    Capacitors
    - What voltage do I pick for them ?
    - The peak voltage in the waveform, spikes, ?
    - I used dramatically higher voltage caps in LED projects than were needed since they would be abused, but do I use 2x the peak voltage expected ? The price rises quickly above 63 and 100 volt ratings.
    - The linear supplies I have seen used 63 volt caps for 48 vdc supplies. Is this reasonable ?

    In switch mode supplies running in the mhz range, it is common to use multiple cap sizes to filter out noise. Is this needed when the supply is AC 60 hz ?

    Thanks

    Harry
    Homebuilts - "BREEZE" RCR2 sidexside, "Tornado" 4 x 18650 side x side, Streamlight SL20x LED module (Custom BST Feeler Thread)

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    Default Re: 48 VDC Linear Power Supply Questions

    Back EMF dump circuit
    - When the motors go into reverse, they can create significant back emf.
    - Some supplies dump this power to a resistor.
    - One approach I guess is to put in some kind of simple linear voltage regulator that dumps the excess (I need to look up this kind of part)


    Battery Final Stage Option
    - I looked at using 40 cells in the final stage to smooth out the 48 VDC
    - I assume it would take some kind of NiCd cell to deal with this kind of abuse of short charge / discharge cycles, and 30 ish amps ?

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    Default Re: 48 VDC Linear Power Supply Questions

    Quote Originally Posted by HarryN View Post

    I need to obtain or build 2 power supplies:
    - 48 VDC x 30 - 40 amps
    - 72 VDC x 30 - 40 amps

    Incoming power is just standard household 110V 60 hz line power.
    Hi Harry. I'm afraid I'm not going to be much help here at all, as most of this is way over my head. One thing that stands out though, you're going to have quite a bit of trouble pulling that much current out of a 110V AC line. The 48V requirements might work out, but the 72V would be over the top for a typical 110V 20A line. I would think you need to consider 220V for your AC mains for this project.

    Dave

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    Default Re: 48 VDC Linear Power Supply Questions

    Quote Originally Posted by 45/70 View Post
    Hi Harry. I'm afraid I'm not going to be much help here at all, as most of this is way over my head. One thing that stands out though, you're going to have quite a bit of trouble pulling that much current out of a 110V AC line. The 48V requirements might work out, but the 72V would be over the top for a typical 110V 20A line. I would think you need to consider 220V for your AC mains for this project.

    Dave
    Hi Dave, thanks for catching that. You are right, at 48 volts it is a lot easier. I could go to 220V since this is in the garage.

    The average draw is much lower, possibly only 8- 10 amps at 110v, the headroom requirement if all of the motors kick in full power at once is where the peak draw problem comes in, which might only last a few seconds to perhaps 10 seconds.

    That is one of the reasons for considering to put a battery pack as the final output stage - to cover those occassional surges. Even a 1 amp-hour ( 1 000 mah) NiCd cell might be enough to cover that short term surge, but it is a matter of trading off the cost of a pack vs a pure electronics solution.

    From a pure battery / cell perspective, is there a cell out there that could delivery 30 amps for 30 seconds, then accept rapid (10 amp) charge 5X per hour in a long term way? NiCd / NiMH / Pb ? I am not ready to attempt this with Li based cells.

    I guess in theory, a super cap could do this, but I have not really looked at those yet at all.
    Last edited by HarryN; 07-02-2012 at 11:02 AM.

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    Default Re: 48 VDC Linear Power Supply Questions

    Quote Originally Posted by HarryN View Post
    From a pure battery / cell perspective, is there a cell out there that could delivery 30 amps for 30 seconds, then accept rapid (10 amp) charge 5X per hour in a long term way? NiCd / NiMH / Pb ? I am not ready to attempt this with Li based cells.
    You should be looking at LiFePO4's (especially ones manufactured by A123Systems). The 2.3Ah/3.3V ANR26650M1B, for example, can deliver up to 120A burst (for 10 seconds) or 70A continuous.
    They're also very safe compared to Li-Cobalt / Li-Manganese cells.

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    Default Re: 48 VDC Linear Power Supply Questions

    Quote Originally Posted by Shadowww View Post
    You should be looking at LiFePO4's (especially ones manufactured by A123Systems). The 2.3Ah/3.3V ANR26650M1B, for example, can deliver up to 120A burst (for 10 seconds) or 70A continuous.
    They're also very safe compared to Li-Cobalt / Li-Manganese cells.
    Thanks for that reminder, and you are right, they are impressive cells. I suspect that the price point they are at is higher than the electronic solution though, but I will look around. Then again, it would take fewer cells to do the job than NiCd. Have any good sources for those cells ? (non ebay sources)

    I am looking for cells that do not require regular charge blalancing. For some reason, I was thinking that all of the Li cell based packs need charge balancing, which is not possible for a setup like this.

    That is one reason I am considering NiCd vs the others - in spite of their challenges, they tend to be pretty abuse tolerant, and leak enough charge to keep the pack somewhat balanced.
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    Default Re: 48 VDC Linear Power Supply Questions

    Instead of constant balancing you can just add a BMS circuit which'll do all the battery management tasks for you. (:

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    Default Re: 48 VDC Linear Power Supply Questions

    Google: "48 volt bicycle battery" and you will find both Pb and Li batteries. However, they are expensive.
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    Default Re: 48 VDC Linear Power Supply Questions

    Thanks for the tips, esp. the 48 volt bike battery one.

    I managed to get the power requirements down to 1 each supply of 72 volts x 20 amps max, and continuous perhaps as low as 15 amps. This will help simplify things a lot.

    Anyone have an comments on the capacitors needed for such a supply ?

    Thanks

    Harry
    Homebuilts - "BREEZE" RCR2 sidexside, "Tornado" 4 x 18650 side x side, Streamlight SL20x LED module (Custom BST Feeler Thread)

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    Default Re: 48 VDC Linear Power Supply Questions

    some random comments regarding the capacitance required.

    How much ripple is tolerated by the stepper motor drive circuit that you are employing?

    Micro-stepper motors and drives require a lower ripple factor than ordinary stepper motors. The problem is that to smooth out the ripple from a full-wave rectifier from 60hz, you will need very large capacitance for brute-force ripple filtration, or you could cut the capacitance need down by inserting a couple stages of filtering using chokes and capacitors. In any case, filtering down the ripple from a full-wave rectifier on 60hz at this current level will be much more difficult than filtering ripple from a full-wave rectifier on a switching power supply operating at maybe 400hz or higher.

    and then there will be the problem of voltage regulation for the input to the stepper drives.

    You might make more than a couple trips to a surplus store like the Axeman to keep your costs down.

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    Default Re: 48 VDC Linear Power Supply Questions

    48VDC at 40A is relatively easier than 72V 40A, off the shelf supplies are available too
    http://www.mouser.com/ProductDetail/...252brwGQ%3d%3d

    72V is a bit...
    Last edited by Illum; 07-08-2012 at 03:07 PM.

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    Default Re: 48 VDC Linear Power Supply Questions

    I would recommend AGAINST such large linear power supplies. Large Linear supplies throw away a substantial portion of the difference between actual power consumed and rated power output. That means you are likely to be dissipating several hundred watts most of the time. No supply is 100% efficient, and very few have a power factor of 1.0, so even 48V@40A is problematic even for a 20A/115V circuit. In addition most large supplies have very large inrush current, often many times the rated power. This can cause the circuit breakers to trip, it is also very hard on the rectifiers and caps for a linear supply unless you use a choke between the rectifiers and the caps.

    As far as the size of the cap goes, you can figure it out based upon the time constant. 48V at 40Amps is about 1.2 ohms, and if you want low ripple, the time constant must be a significant multiple of 8.33ms for 60hz power. So if you go for a 40ms time consant, RC=.040, since R is 1.2 ohms, you are going to need about 35,000uF for the 48V cap, and about 20,000 for the 72V cap. These are going to be big and expensive. Another good reason to use switching power supplies. Caps that large at those voltages are significant energy hazards, and often have surprisingly short operating lives (a few thousand hours).

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