24VDC Power Supply Output is 40VDC !!! 1

[ataslaki]

As my subject header suggest, I have a "regulted" power supply 110VAC in 24VDC out, but measuring the + and - terminals on the output, I am getting 40VDC.

Does this mean that my power supply has gone bad? What could cause this?

Thanks

 

[felixc]

It should not be the case if this is a regulated P/S. Does it output this voltage at the rated load, or with no load? Some regulated supplies need a minimal load to work fine.
If it does so at rated load, well, it's broken.

 

[buzzp]

Felix hit the nail on the head. Often they need a load to regulate first. And make sure your measuring across the output directly and not using ground in your measurements.

 

[ataslaki]

Thanks guys.

I confirmed:

Voltage under load: 40VDC
Voltage no load: 44VDC

It's gone bad, and am replacing it already.

However, what kind of safety regulator or switch can I add to the output to make sure that the switch will shut off if it delivers more than let's say 30VDC.

Are there any such safety switches or sensors?

Thanks.

 

[eemotor]

You can use a "Crow Bar Circuit". All you need is a resistor, capacitor, SCR and a Zener diode rated for the desired voltage protection. The SCR should be sized such that it can handle the maximum current output of your power supply. Then get a 30V Zener diode. When your power supply exceeds 30V, the Zener diode will conduct, thus turning on the SCR which in turn short circuits the output of your power supply blowing out the fuse or tripping a circuit breaker. It is best if you use a fuse of a minimum rating (for example, if your load is 4 Amps, and your power supply can output 10 Amps, it's better if you use a 5 Amp fuse). Also, use fast blow fuses for most protection.

 

[sreid]

Most power supplies can be purchased with an over voltage crowbar as an option.

 

[BrianG]

One thing no-one has suggested so far is whether your power supply has "remote sense" terminals (usually next to the power terminals and marked +S and -S, or similar).

If you have not come across these before, they are usually provided on high current PSUs to overcome any voltage drop in the connecting wires between the power output terminals and the load. The sense terminals allow the PSU's control amplifier to be connected by separate low-current wires so that it can measure the voltage at the point of delivery to the load. It can then adjust the regulator output to compensate for voltage drop as required.

Usually - but not always - the sense terminals are connected locally inside the PSU to their respective power terminals by 100R resistors(+O/P to +S, -O/P to -S). Although this provides a safety feature in case the user forgets, or does not make, either full remote sense connections, or shorting link connections across to the respective power terminals, many older model PSUs did not have the resistors and relied on the user to connect up correctly.

The point of all this is that unless your PSU has a definite fault, the effect you are describing of over-voltage at the output, and no load regulation is exactly the kind of result you can get if the sense circuit is not connected properly.

Worth a check?

 

[ataslaki]

Brian,

Very true. The Power supply I have is a 7.2A unit, and has +S and -S next the output terminals. However, I can see that the +S and -S are simply wired to their respective output terminals without any resistors.

WOW!

 

[BrianG]

OK, it was worth a try! The PSU must have a fault.

If you want to try and fix it yourself, if ithe PSU is a simple linear supply, it may be worth checking first to see if one or more of the power transistors on the heatsink has failed short cicuit / or leaky (the most common type is a 2N3055 - they cost a few pence/cents each these days).

Otherwise it will be a case of detailed fault finding.

Good Luck!

 

[ScottyUK]

A minor addition to eemotor's crowbar suggestion: heavy current crowbars tend to use the zener to trigger the fire gate of a thyristor across the positive and negative terminals. This gives a very rapid response to an overvoltage condition by tying the positive and negative rails together until the fuse blows. It may be fast enough to protect the load if you are lucky. A zener coupled across a supply as a crowbar will burn up unless it is a very high power device and the fuse is a fast type, and this configuration allows the overvoltage to remain until the fuse blows, increasing the likelihood of damaging the load.


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