[sdiy] [semi-OT] Droop on power supply output voltage - when is it normal?

[rsdio at audiobanshee.com]

On Dec 20, 2014, at 1:14 PM, Colin Hinz <lists at meccanion.com> wrote:
> On Sat, 20 Dec 2014, Justin Owen wrote:
>> I marked this as OT because it came out of a conversation about an area of electro-mechanics/magnetics that I'm not at all familiar with.
>> 
>> Are there any examples where something that is connected to a correctly specced power supply causes the power supply output voltage to drop by 2-3 volts under *normal* operation (where, in this case normal operation is 9-10V)?
>> 
>> To me that would generally be a symptom of a problem - at least for SDIY stuff - but I was wondering if there were examples out there where a significant voltage drop would be considered part of normal operation?
> 
> Sure, if it's an unregulated supply.
> 
> The simplest possible power supply (transformer -> bridge rectifier ->
> filter capacitors) will droop 10 to 20% from no load to full load,
> depending on the transformer.

Every output has an impedance. It helps to think of a supply as an ideal voltage source followed by an ideal resistor. Then you can see how the voltage will drop as more current is pulled by the load through the output impedance.

A regulated supply uses feedback to sense the voltage drop and compensate by increasing the voltage (or decreasing, if the load draws less current). This solution is highly dependent upon where the feedback is placed. If the feedback is entirely internal to the regulator, then external impedances cannot be compensated. Many regulated supply chips have a pin dedicated to feedback, and this is connected as close as possible to the output terminal. But, if there are traces, connectors, or wires with resistance after that point, then you'll still see a voltage drop. Typically, those resistances are small, so they only become noticeable at extremely high currents.

So, it all depends upon what you mean by "correctly specced." Is the specification as simple as voltage and current? Is a minimum and maximum given for the voltage? Does the power supply say what voltage it is when the maximum current is drawn, or does the spec merely state the "nominal" voltage over the "typical" current? When you say that "normal" is 9-10V, what amount of current is being drawn by the load?

Many power supplies might provide excessive voltage under light loads, and drop to the specified voltage at the maximum current. Other power supplies might only provide the correct voltage under moderate loads, but droop below the specified voltage when maximum current is drawn. It really depends upon the quality of the power supply, and how detailed the specifications are.

Brian Willoughby