What's Up With These Wall Wort Power Supplies?

[timd]

I was going through some boxes today and for 3 wall wort power supplies I was going to test for use in some projects. I tested them and found some interesting results:

1. 6V DC 200 mA - registered - 10.4 V

2. 6V DC 240 mA - registered - 11.3 V

3. 5.4 V DC 500mA - registered - 7.5 V

Is there something wrong with these? The 2nd power supply was almost double the printed voltage. 

[John Lyons]

Wall Wart

They are either unregulated supplies and or unloaded. With a load
applied the voltage will drop.

[armdnrdy]

As I recall, R.G. stated that this is sort of a manufacturers "guarantee" that the transformer will meet it's mark.

The first one for example: It reads 10.4 volts with no load. With a 200ma load it "should read at least 6 volts.

The third one is kind of odd. Unless it's a far superior transformer with great efficiency.... it seems like it would drop below 5.4 at 500ma.

[smallbearelec]

This is normal behavior for unregulated consumer-grade wall warts. Were you planning to power 9 Volt effects with those? I don't think you'll be happy even with the first two, because the smallest load will poop the output voltage. Consumer types also have almost no filtering, and you need some "headroom" in the output voltage to accommodate the filter and regulator. Suggestions:

--Find a 12 Volt or so consumer wall wart, add filtering and regulate it down. There are numerous tips out there for doing this.

--I was thinking about having a wall wart made to my specs with extra filtering and a three-terminal regulator built in. But I just got a piece of sales poop from Morley recently, and they already offer one. It has an in-use LED, and it really does pump 9 Volts, both no load and under load. It's rated for 300 ma. and it's quiet...we tested it with a BOSS HM-2 today. It's twice the price of regular consumer types, but hey...ya sometimes get what ya pay for. Put three or four on an outlet strip and you have an instant supply for a board. I'm gonna order for stock and will have them posted in a week.

Regards
SD

[duck_arse]

string a 100R, 1W resistor across the output of each of the first 2 warts, see what the output voltage falls to at half rated current.

33R 2W on the third.

[Mark Hammer]

The frustrating reality is that those little black boxes provide no clues about what's inside, based on superficial visual inspection.  If they had more detailed specs, or if they were made of transparent plastic so you could see what was inside, or were opaque but easily disassembled by unscrewing them, life would be different.  But they remain sealed little mysteries.

Apart from purchasing higher end power bricks, sometimes I think the best choice is to get a 12VDC wall wart and down-regulate, with a 1A/9v regulator, just "to be sure".

[amptramp]

I did get a surprise looking at one wall wart that I opened up.  There was a transformer, two diodes connected to the push-pull transformer output and a filter capacitor, but the filter capacitor had a resistor in series with it to avoid excess inrush current through the diodes.  Needless to say, the ripple was quite high because the output was negative from the transformer centre tap and positive from the diode junction.  The resistor was not in series with the output, it was only in series with the capacitor.   It used to be that capacitor ESR (equivalent series resistance) was high enough that you could put an electrolytic cap across a rectified DC output and not worry about the current surge when the capacitor was charging up.  Evidently, not any more - capacitors are getting too good.

[LucifersTrip]

1. 6V DC 200 mA - registered - 10.4 V

2. 6V DC 240 mA - registered - 11.3 V

3. 5.4 V DC 500mA - registered - 7.5 V

This is normal behavior for unregulated consumer-grade wall warts.

...and the common 9V unregulated ones are usually around 14V.  to op, also note that the common wallwarts are center pos, not center neg like used in pedals.