Unregulated Power Supply

[jbolt]

Will an unregulated power supply ever go over the rated output voltage?

I have a 77vdc PS that puts out 78vdc under no load. I'm concerned if it can exceed 80vdc, the upper limit of my stepper drivers.

Jay

 

[brino]

Hi Jay,

Unfortunately, without more details for the power supply and or motor drivers it is very hard to know.

If it is simply a transformer, full- or half-wave rectifiers and some decoupling "smoothing" capacitors, then the output voltage really is at the whim of the input AC voltage. A brown-out could give you a low voltage, and a spike could give you an over-voltage.

Even if it has some "smarts" the question is how it would react to a changing load. If you have a couple motors running then suddenly stop (or reverse them) does the output voltage overshoot?

You really need specs for the supply to be sure....
You could perhaps find a regulator to use after the unregulated supply, it would have to be "low-drop out" and rated for the max current you would need.

-brino

 

[JimDawson]

It probably won't go over the no load voltage. That sounds about right for a 75V supply. The drives will actually take better than 85volts, I've been there.

 

[jbolt]

It is a basic unregulated supply with a toroidal transformer, bridge rectifier and a couple of capacitors. 110vac in 78vdc out.

I ask because the Wantai DQM860A drive I had on the Z blew two rectifier diodes at the motor terminals. The drives were powered and enabled but not moving when the board blew.

I have decided to replace the Wantai drive and the older G201's with new G201x's. I don't want to damage the Gecko's. Could be the cheap rectifier diodes just weren't up to the task.

Jay

 

[mksj]

Transformer regulation is a measure of the voltage rise on the secondary due to off-load or light-load conditions with the primary input voltage remaining constant. This measure is expressed as a percentage of the secondary voltage; for example, a transformer with 10% regulation and rated at 36v at full load will have an off-load voltage of 39.6v. Regulation varies inversely with power rating (VA) and is approximately linear for any given loading on the secondary. In the above example, if the load was 50% of full load, the voltage would be 5% higher, or 37.8v. A 500VA toroid transformer will hold about a 4% voltage regulation between 0 and 100% load, usually the larger, the better the voltage regulation. Toroid transformers hold better voltage regulation than traditional E-core types. Also beware that the input voltage may be rated at something like 117V AC on the primary, my line voltage is 122V, which adds another 4% to the voltage.

Rectifiers may go because of the large inrush current, and high spike loads. Good quality diodes should not fail if rated correctly.