voltage regulation

Discussion in 'General Electronics Chat' started by gremlin, Jul 22, 2010.

  1. gremlin

    Thread Starter New Member

    Jul 22, 2010
    19
    2
    hello all
    Wow what a great forum

    does any one have any info on a power supply circuit that will give me a constant 24 v dc at about 8 amps 10 amps max from a variable dc voltage in of 30 - 70
    this is to work a mig welder wire feed motor from a generator welder where the voltage goes up with the welding curent selected !

    as i want to use the welding circuit voltage to run the 24v wire feed and gas solenoids

    many thanks in advance

    gremlin :)
     
  2. tom66

    Senior Member

    May 9, 2009
    2,613
    214
    30 volts to 70 volts is quite a wide variation.

    First you must understand that you cannot use a linear regulator (such as a high voltage 78xx series IC) for this project. Why?

    Because, if you work out how much power dissipation you will be getting at 70 volts dropped to 24 volts, with a max output of 10 amps, that is well over 460 watts!! Which is going to be very difficult to get rid off, not to mention the inefficiencies of such a system.

    So it looks like the only way forwards in this is to use one type of switch mode power supply; a buck converter to be specific.

    A quick look on the 'net suggests you may want to look into the LM5116, an integrated circuit for switching up to 100V into 24V. It requires quite a few external components, including power MOSFETs.

    But building such a circuit is not necessarily for the novice, so you must think if there is any other solution you can consider, like figuring out how to reduce the supply voltage.
     
    gremlin likes this.
  3. bountyhunter

    Well-Known Member

    Sep 7, 2009
    2,498
    507
    10A @ 24V is 240W of load power. This is not a trivial design. 30 - 70V puts it in a range where the higher VIN is too high for a push-pull converter which would be feasible at 30VIN. However, 30V is too low for a half bridge or full bridge to be very efficient. You would be better off running a half bridge converter off the 115VAC line.
     
  4. gremlin

    Thread Starter New Member

    Jul 22, 2010
    19
    2
    thank you veru much for the info
    it for use on a mobile diesel welding set using the welding circuit voltage to get a supply for the 24v dc to run a wire feed motor and gas solenoid, so no 110 or 230 ac avalable when mobile,
    the welding voltage changes with the welding current set ie at 30 amps the open circuit voltage is about 30 Vdc and at 300 amp setting the open circuit voltage is 70 Vdc,
    there are comercialy available wire feed units for these that work of the welding voltage but are much too expensive at about £2000.00 and i have 3 24v DC wire feed units here to use

    once again thanks

    regards

    gremlin:)
     
  5. tom66

    Senior Member

    May 9, 2009
    2,613
    214
    In this case you might consider an inverter unit to provide 115V/230V a.c., and then use a transformer to step it down to 24V.

    So the voltage goes UP as the range increases? And you have to get 24V / 10A out of this varying 30V - 70V?

    What power source is the welder using... why not use that?
     
  6. gremlin

    Thread Starter New Member

    Jul 22, 2010
    19
    2
    welder is diesel engine driven and has a switch on the front to switch from weld to generate so i am unable to use the ac power while welding
    the machine is wired to split down the ac windings further and put them through rectifier diodes in weld mode
    generate mode links back the windings and bypasses the diodes
    hope that makes sense
    i may be able to get away with 5 amps on a smaller wire feed motor i have
    does any one know of a swich mode psu that would do what i want ?

    regards

    gremlin
     
  7. tom66

    Senior Member

    May 9, 2009
    2,613
    214
    If it's switching multiple windings from the generator you can either:
    - take one of the windings (presumably it's 30V) rectify it and use a normal switch-mode buck converter or even a high power LM337 circuit.
    - or, take the AC from the generator BEFORE it is rectified (assuming it produces AC) and use a transformer to produce a lower voltage (say 18 volts at 70 volts input), this can then be stepped up to 24 volts.

    24 volts at 5 amps is more manageable, you could probably build one of those circuits using a controller IC which has the power devices integrated.
     
  8. gremlin

    Thread Starter New Member

    Jul 22, 2010
    19
    2
    thanks i had thought of that but no matter where i try to get a voltage at the genny it varies from 30 to 70 depending on the welding amp sellected ?
    even at the single winding beffore rectifier
    as the welder alters the excitation voltage to controle current output

    its looking like the LM5116 is the way to go
    just been trying to get there flash application to work on the national semiconductors site but seems not to load on windows 7 or vista

    thank you all for the helpfull input

    regards

    gremlin
     
    Last edited: Jul 24, 2010
  9. gremlin

    Thread Starter New Member

    Jul 22, 2010
    19
    2
    i was thinking allong the lines of zener diodes at 5v increments pulling in relays to put 10 amp diodes in serries, using the natural voltage drop of a 10 amp rectifier diode to my advantage although it would take a lot of diodes and and 8 10 amp relays

    any ideas as to wether or not it would be a working option
     
  10. tom66

    Senior Member

    May 9, 2009
    2,613
    214
    It's quite a complicated circuit... are you able to build one?

    There app seems to be having a problem loading here, using Ubuntu... I'll try again later.

    I haven't checked if there are other ICs.

    When you say 30 volts to 30 volts do you mean 30 volts to 70 volts? In order to reduce the output, you might have to load one of the coils down. How about a 10 ohm 25W resistor? About £0.90 here, that would probably cause the generator to reduce voltage.

    That circuit using zeners and diodes would work, but it would dissipate a lot of power as heat! No better than a linear regulator. We're talking about 460 watts. That is a load of heat which must be removed. The diodes aren't magical; to drop 0.7 volts, they must dissipate heat which is proportional to the current flowing through them.
     
    Last edited: Jul 24, 2010
  11. gremlin

    Thread Starter New Member

    Jul 22, 2010
    19
    2
    yes sorry that was 30 to 70 volts " i have edited post"

    yes it's a lot of heat to get rid of but unless i go switch mode thats my option ?

    if the ajustable of fixed voltage regs woukd take the 70 volt input i would have gone that way and paralleled them up to get my 10 amps "with some spare capacity "

    thank you

    gremlin
     
  12. tom66

    Senior Member

    May 9, 2009
    2,613
    214
    Could you try that resistor idea I suggested? If you don't have a resistor try a 120V or 240V 60W light bulb, it will provide a bit of loading. We are talking about connecting to say the first winding of the generator right?

    High power diodes are very pricey, and difficult to obtain.
     
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