Power supply voltage too high

[spoiledbrat]

I purchased a 75V @ 6 amp power supply. When I plug it in to my wall, it is producing 83.3 VDC. This is under a no load condition. The power coming out of my wall is usually 125 VAC.I am thinking that I should reduce this somehow, but I haven't a clue how.
I am hooking 4 Geckos to this supply, to power 3 servos @ 80 VDC, and 1 @ 24 VDC.
I do not want to cook the geckos.

Rob

[Al_The_Man]

If it is a non-regulated linear supply, you can take windings off the transformer, there have been several threads, one just today on the same subject if you search the forum.
To prevent over reducing, it pays to find out the turns/volt of the transformer before you take too many off, as if it is a square lamination type it is a pain to rewind turns on.
Al.

[spoiledbrat]

Al, it is a square lamination type. I was thinking maybe I could put some kind of potentiometer on it. Nothing is easy, though.

Another question I would like to ask you, Al:

Some people put some kind of resistor between the + and - armature on their Geckos. Is this really necessary? I believe Mariss said that it would require great strength and effort to create back EMF that would damage the drive, and that the drives circuitry would cause the motor to try and hold it's position.
Is their another reason I do not know about that I need to use the resistor?

(ps. I think it is called a resistor, you probably know what I am talking about.)

[Al_The_Man]

Originally Posted by spoiledbrat Al, it is a square lamination type. I was thinking maybe I could put some kind of potentiometer on it. Nothing is easy, though. Another question I would like to ask you, Al: Some people put some kind of resistor between the + and - armature on their Geckos. Is this really necessary? I believe Mariss said that it would require great strength and effort to create back EMF that would damage the drive, and that the drives circuitry would cause the motor to try and hold it's position. Is their another reason I do not know about that I need to use the resistor?

The Transformer turn reduction is just about the only way to go.
On the drives, I have never used the Gecko amps so I am not really the person to ask. If it is a DC motor you are talking about, I would think no.
Al.

[cp8071]

You could use a string of diodes to reduce the voltage. At 0.7 Volts each it would only take 5 to get down below 80 Volts (83.3 Volts - (5 * 0.7 Volts) = 79.8 Volts) But you should probably leave some room and try not to be right at the max supply voltage. It would pretty easy to find diodes that would handle that current, be sure to check that their wattage can handle the continuous current that the steppers will pull.

CP

[gar]

050611-1925 EST USA

spoiledbrat:

(edit 6-14-05)
***************** BIG CAUTION *****************
Do not connect a bridge rectifier directly to a variable autotransformer such as a Variac. Also do not connect any power supply to a Variac unless the power supply includes an input isolation transformer.

My intent in this post was for you to use the Variac at the input to your current existing power supply as an easy way to experiment with lowering your output voltage.

Also you may not want to go above the 120 volt position on the Variac.

Thanks go to sbrpollock for pointing out that this post was not clear.
(end edit)


For experimentation you could use a Variac (General Radio) or Powerstat (Superior Electric) (trade names for variable auto-transformers).

These are typically adjustable from 0 to 140 VAC with a nominal 120 VAC input. A powerstat 3PN116 has a 1 KVA rating and maximum current of 7.5 amps. Their 1 KVA rating probably comes from rounding off 140 * 7.5 = 1050. No matter what output voltage you set the Variac to the maximum current is the rated current because this is determined by the contact brush. The Variac trade name is often used as a generic name.

With one of these variable transformers you could adjust your voltage wherever you wanted.

A more permanent method without rewinding the transformer would be to use an appropriate small filament transformer or rectifier transformer secondary to buck the incoming voltage to your power supply. The buck transformer secondary current rating must be greater than maximum load ac input current to your power supply.

Keep in mind that your power supply output voltage will drop with load.

Let us assume that using this 83 vdc open circuit power supply with 125 vac that you want 70 vdc open circuit output, this (70/83) * 125 = 105 vac input. Rough input AC current is 5 amps or somewhat more for 70 vdc and 6 amps output. Thus you would need a buck transformer of 20 vac secondary and maybe 8 amps rating.

But first I would experiment with a variable input transformer.

Once you really know what you want, then you might rewind or buy a new transformer.

.

[ViperTX]

6 amps for servos....okay....what kind are they? You can run the 80 VDC servos with this supply, get another supply for the 24 volt servos.

[sbrpollock]

Using a variac for a machine power supply is EXTREMELY dangerous and should NOT be done!

[gar]

050613-1021 EST USA

sbrpollock:

Explain what you mean by dangerous relative to the Variac?

It appears I did not make it clear that the Variac would be used to adjust the input voltage to his current power supply. Thanks for pointing out my failure to make clear my intention.

The Variac is an autotransformer and therefore the output does not have DC isolation from the primary or ac line. Is this what you meant?

My point of using a Variac was not to eliminate his power transformer, but to provide an experimental means to adjust the input voltage to his power supply so he could determine what he wants for a power supply output voltage. His power supply has an isolation transformer.

Were you to put a bridge rectifier directly on the output of a Variac that would be very BAD for these kinds of applications.

If your power supply is a switching type or some other type of regulated supply, then a Variac or other means to vary the input AC voltage will not adjust the output voltage.

.