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

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

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and there are a lot of parts that are close, but none that meet my requirements.

Thank You

Reply to
WP
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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

Reply to
Joe McElvenney

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

Reply to
John Larkin

A buck transformer to lower the voltage to the primary would make a lot more sense.

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Michael A. Terrell
Central Florida
Reply to
Michael A. Terrell

In article , cnuk@_at_shaw.ca mentioned...

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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Reply to
Watson A.Name - "Watt Sun

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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Reply to
Watson A.Name - "Watt Sun

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

Reply to
John Fortier

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