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DC voltage step down (58V -> less than 50V)

Discussion in 'Electronic Basics' started by WP, Sep 27, 2003.

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  1. WP

    WP Guest

    I have searched Google and can't find the answer to my question so I'm
    going to ask the experts here.

    I have an old telephone power supply that outputs approx 58V DC under no
    load. I want to use the power supply to drive NEMA 23 stepper motors,
    but the driver circuit uses an L298 who's max rating is 50V. Therefore
    I need to step the voltage down to something less than 50V. It's
    driving motors so it doesn't have to be precise and can fluctuate with
    load. I'm hoping for a device that can handle up to 5 or 6 amps.
    That's 300W so it's going to be big whatever it is, but that's alright.

    Does anyone know what I can use to step down my voltage ? I looked at
    www.linear.com and there are a lot of parts that are close, but none
    that meet my requirements.

    Thank You
     

  2. Hi,

    Just my $0.02.

    As you say that regulation is not an issue, a crude solution I've used
    in the past is low-voltage car headlight bulbs in series with the supply.
    You would need about 60 watts worth and to insure that there is always some
    load current through them.


    Cheers - Joe
     
  3. John Larkin

    John Larkin Guest

    If the supply is unregulated, just load it a bit with a power resistor
    to pull it down. If it's regulated, futz with the regulator feedback
    resistor values to change the output voltage. Actually, you needn't
    even bother to do that: the steppers will load it, and the ICs
    actually won't explode at 58 volts anyhow: they can probably handle a
    lot more than 50 volts in real life.

    John
     
  4. A buck transformer to lower the voltage to the primary would make a lot
    more sense.
     
  5. First off you should look for an adjustment on the power supply. If
    not, you should see if the power supply will drop to 50V under load,
    which would be okay. Standard telephone equipment runs on 48 to 54V,
    so it should drop somewhat.

    Then if you're still not low enough, try putting some 6 amp rectifier
    diodes in series with the power supply output. Each diode should drop
    a minimum of .6V, but closer to 1V under heavy load. 8 or 10 of those
    in series should drop 8V or more.

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  6. Good idea if the transformer is a regular transformer. But my own
    experience is that this type of supply is a high reliability regulated
    type and they usually use a ferroresonant constant voltage
    transformer. You can drop the AC voltage down to 95VAC and the output
    won't change. Then, below that it might not work at all.


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  7. John Fortier

    John Fortier Guest

    If your power supply is anything like the old telephone power supplies I
    remember from the 70s, the output is not regulated at all. In this case you
    are seeing 58 volts with no load, which would be about right for a PSU
    intended to charge a nominal 48 volt battery stack, which actually charged
    up to approximately 51 volts.

    So one, rather bulky and heavy solution, should the PSU be of the old,
    unregulated type, is to use it to charge four car batteries in series and
    use them as both a power backup and for voltage smoothing and regulation.

    On the other hand, that's not exactly a miniaturized solution (!) so you
    need to know what the input resistance of your load is. If it is low
    enough it will, in series with the internal resistance of the PSU, drop the
    voltage from 58 to nearer to the 50 volts specified. If it's still high, a
    power resistor in parallel with both PSU and load will reduce the load
    resistance seen by the PSU and cause the output voltage to decrease further.
    You will need to experiment with resistance values and make sure that the
    resistor is of sufficient wattage to dissipate the power. Remember P=E^2/R.

    I agree that you're probably safe using the PSU as is. especially in light
    of the voltage dropping provided by PSU and load resistances, so the power
    resistor solution is really a last resort. As a resort, the 4 car batteries
    are a bit like Benidorm in august.

    Hope this helps.

    John
     
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