24,48 and 125VDC Power Supply ???

Discussion in 'The Projects Forum' started by Steeno, Jan 9, 2009.

  1. Steeno

    Thread Starter New Member

    Jan 9, 2009
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    Hi...I'm new here and see lots of great info. Thanks.

    I'd like to build a portable power supply that provides three DC voltage outputs: 24VDC@2A, 48VDC@2A and 125VDC@1.5-2A. The outputs should be isolated from ground.

    My first thought was simply to get 3 toroid transformers of proper secondary voltage and VA capacity, connect them to bridge rectifiers and filter caps and call it good.

    My concern is that the range of voltage output from no load to full load might be unacceptable, therefore I need at least some regulation.

    I'm having trouble figuring out where to look for information and unsure of possible solutions.

    Any help would be greatly appreciated.
     
  2. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    You are talking about roughly 50 + 100 + 250 Watts, That is a total off 400 Watts.
    The 24 Volt can be easely stabelized.
    The 48 Volts also can be managed.
    The 125 Volts might give some trouble, there are not many stabelizer parts that can manage the 125 Volts, the 2 A is a high rate for this voltage.

    Greetings,
    Bertus

    PS this is my post numer 1000
     
  3. Steeno

    Thread Starter New Member

    Jan 9, 2009
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    Bertus,
    Thank you for the reply. When you say stabelized, what do you mean?
    Steeno
     
  4. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    With stabelized is mean : The output voltage is monitored and corrected when it would change due to load changes.
    There are several types of powersupplies.
    There are "linear" powersupplies that have a pass transistor that controles teh output voltage.
    (there is always a current flowing through the pass transistor, it needs to be cooled very well).
    http://www.educypedia.be/electronics/circuitssupplies.htm
    There are "switched" powersupplies, here the pass transistor is very fast switched on and off to have the correct output voltage.
    (also called switch mode power supply).
    http://www.educypedia.be/electronics/circuitssmps.htm

    Greetings,
    Bertus
     
  5. studiot

    AAC Fanatic!

    Nov 9, 2007
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    Will the three supplies have a common ground or zero volts or are they to be isolated from each other?

    I think you need to consider some form of protection circuitry at these voltage and power levels.

    What are they going to be used for and what physical form is acceptable?
     
  6. Steeno

    Thread Starter New Member

    Jan 9, 2009
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    I'm intending to use this power supply to power modems, PLC's, communication hardware and various microprocessor based protective relays for testing before commissioning. These devices typically reside on panels or on 19" racks in a power utility substation environment where ungrounded 48VDC and 125VDC batteries and chargers supply them. The 24VDC would generally come from a DC-DC stepdown converter.

    The ungrounded DC provides personnel safety and continuity of service in the case of a + or - ground.

    For the power supply I'm contemplating, the DC supplies should be ungrounded. I suppose they could share the same zero volt reference. The AC side of the circuit would need to be grounded of course and fusing on both AC and DC sides of circuit.

    As far as the physical form. I expect at least 6lb. of weight, probably more to get the VA necessary. I envisioned and enclosed aluminum box with banana jacks isolated from the box.

    In the pass I've used the product below, but often it doesn't have enough power.

    http://www.selinc.com/sel-lps.htm

    I'm hoping to educate myself and come up with a working prototype if feasible.

    Thanks for any help...
     
  7. studiot

    AAC Fanatic!

    Nov 9, 2007
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    Well that's put some meat on the bones.

    I don't see you need a very fancy circuitry arrangement. Your transformer/bridge/reservoir capacitr should suffice.

    However I would suggest that instead of a single capacitor reservoir/filter you used a Pi arrangement with two capacitors and a series resistor.

    This could also form part of the protection you will need against shorted equipment being connected. The resistor will limit the current. If you include a breaker as well you will have full protection for the supply.

    When you are considering the case make sure you make good ventilation arrangements.

    Consider also extra terminals on the front panel for measuremet or what have you . you would be suprised how useful these can turn out to be.

    Finally since these are separate supplies they may be more useful in individual cases as positioning/ racking or moving about will be easier. Also if you need say just the 48V one somewhere else pro temp ..
     
  8. Steeno

    Thread Starter New Member

    Jan 9, 2009
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    I appreciate the advice. There will be times when I put only say 200mA of load and others when I put 1+A of load on the supply. I guess it's a matter of picking a transformer secondary voltage and VA rating that will give me acceptable voltage level output over an expected range of load.

    Thanks again for the help!
     
  9. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Here are a couple of recommendations...
    24V supply:
    http://www.mpja.com/prodinfo.asp?number=16008+PS
    48V supply:
    http://www.mpja.com/prodinfo.asp?number=16016+PS

    You'd be hard-pressed to build one from scratch for that kind of price; and these are regulated switching supplies.

    I've bought a number of items from that company; they ship very quickly, and I've never been dissapointed with any of my purchases from them.

    The 125vdc supply is going to be a challenge. At a minimum, you'll need a 120V isolation transformer capable of supplying 2A; that in itself won't be cheap.

    However, here is a starting place; a variac:
    http://www.mpja.com/prodinfo.asp?number=15162+TR
    You could plug a bridge rectifier with a cap on the output into the rectifier, and adjust the voltage manually using the variable tap on the variac output. This would work best with a load that is relatively constant.
     
  10. Steeno

    Thread Starter New Member

    Jan 9, 2009
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    I stumbled across the powersupplydepot site and came to the same conclusion with regard to the 24 and 48vdc supplies. A guy could put them both in a relatively small vented aluminum box, bring the supply and load terminals out appropriately and have a real cost effective supply with plenty of oomph.

    I like the idea of using a variac for the 125vdc. I already carry one for ratioing and saturation testing CT's. I could build a small enclosure with bridge, caps and a digital vdc display. It could be quite versatile.

    Thanks for the advice!
     
  11. leftyretro

    Active Member

    Nov 25, 2008
    394
    2
    Keep in mind when using a Variac that it is a auto-transformer (one winding) and is not isolated, the common winding terminal is wired to AC neutral which in turn is wired to power ground at the service panel. If you use a 1:1 isolation transformer in front of the Variac then you can use it in applications requiring ground isolation.

    Lefty
     
  12. mik3

    Senior Member

    Feb 4, 2008
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    He said he is going to power PLCs, modems etc, these devices need regulated power supplies and not just filtered otherwise you will destroy them. The mains voltage varies and this variation is enough for an electronic device to be destroyed. I suggest use switch mode power supply method to make your power supply.
     
  13. Steeno

    Thread Starter New Member

    Jan 9, 2009
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    Good point. I require ground isolation on the DC side of things for personnel safety because in addition to powering devices, I'm using this DC to power control circuits that I'll be making and breaking for testing (ie. I've got my fingers in it). Normally I simply treat the AC side of things with "kid gloves", but an isolation transformer would offer greater safety.

    As a funny aside, often times I'm using my variac in a substation to ratio current transformers in breakers in the yard. There may be 3 or more 115kV and 3 or more 230kV lines into the sub. In this case, there will be a 125 or 250mva autotransformer connecting the two grids. This autotransformer has the only tie to earth and therefore is the neutral return for faults on the lines. The funny (or warped) part is I'm standing on the neutral return for upwards of 30,000 amp ground faults. We minimize hazardous voltage step potential by analyzing the soil and creating a copper grid a few feet down that spans the area of the yard within the fence.
     
  14. Steeno

    Thread Starter New Member

    Jan 9, 2009
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    Another good point. When I originally posted, I naively thought I could make a simple power supply with all 3 voltages. Now I realize, like you suggest, it's probably best to buy simple regulated 24 & 48vdc supplies. But it doesn't seem so easy to find 125vdc supplies with 2A+ capability. SgtWookie pointed out the variac, bridge, filter idea. I think this should work fine for my needs as long as I incorporate an output voltage display and remain mindful of how I'm loading it. I looked, and many of the devices I'm powering that are advertized as 125vdc, can accept quite a wide range of input, some 90-300vdc! I suppose that has to do with the "substation hardened" requirement. Thanks!
     
  15. mik3

    Senior Member

    Feb 4, 2008
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    You can regulate 125VDC with a LM317 voltage regulator which can supply up to 1.5Amps as long as the input-output differential voltage does not exceed 40V.
     
  16. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    The TL783 is designed to operate upto 125 Volts.
    It can only deliver 700 mA, but the datasheet shows how to get more.

    Greetings,
    Bertus
     
  17. studiot

    AAC Fanatic!

    Nov 9, 2007
    5,005
    513

    The majority of ac adapters (wall warts) available with interface/control boxes (modems switches etc etc) of all sorts are simple transformer/rectifier arrangements with generally inadequate reservoir capacitors. They have the most horrendous waveshapes and regulation eg I have seen 9volt supplies at 16 volts off load.

    10 accumulators (or 5 x 24volt lorry ones) in a row plus a trickle charger would get you all the voltages you require and more at all the currents you require and more in a very stable and pretty bomb proof setup.
     
  18. Plotzke

    New Member

    Nov 11, 2013
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    If Steeno is still monitoring this thread, I'd be interested to hear how the 125 VDC supply turned out. I have almost an identical application with similar control equipment in a training rack scenario.
     
  19. MaxHeadRoom

    Expert

    Jul 18, 2013
    10,526
    2,369
    I need a variable 0-280dc supply a while ago, I used a 120v/240 transformer with the Variac on the 120v primary, the 240v secondary fed a bridge and a smoothing cap.
    This also provided isolation.
    Max.
     
  20. inwo

    Well-Known Member

    Nov 7, 2013
    2,433
    315
    Industrial 24 volt 50 watt power supplies are inexpensive, reliable, and common. 2.1 amps @ 24vdc

    I've got a shelf full of them.

    5 in series with diodes across outputs should let them all fire up together.
    It will help if they are the same or can be inhibited for a few ms on start-up.

    There would be a common with taps at each 24 volts making very versatile.

    Ive only done this with pairs. Some research would be required on taking it to 5. The question would be voltage to ground. Even though isolated.

    If nothing comes up online I could easily test it with your loads.

    These also work with DC input.
     
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