Adapters with More Voltage than Advertised

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roadtrip

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Hi I have purchase a guitar effects rack off eBay and it came without an adapter. It requires 12 volts DC, (sleeve negative, 700 mA). I purchased an adapter from an online electronics retailer, 12 volts 800 mA, along with a 9 volt DC adapter for a midi controller pedal also from eBay. (The midi pedal requires 9 to 12 volts DC, mA unknown, it's not in the manual online either, so I got a 500 mA adapter, I figured that would be enough to power a basic midi pedal). I tested the adapters before I connected them, and the 9 volt adapter outputs 13.02 volts, and the 12 volt adapter outputs 16.54 volts! Are pieces of equipment designed to accommodate this much increase in output from the adapter? Do adapters usually output this much more than they are rated for? Thanks in advance!
 
A cheap adapter has no voltage regulator. But it has resistance in the windings of miles of thin wire in its transformer so its voltage drops to its rating when it has its rated load current.
But you measured its voltage with almost no load current so of course its voltage is higher.

The higher is its current rating then the less increased voltage at a lower current. A low current adapter usually has horrible voltage regulation, it might be double its rated voltage with no load current.
 
Some adapters give a higher-than-rated output if they have no load.
Try connecting a 470 Ohm (or thereabouts) resistor at the output and measure the volts again. What do you get?
 
Thanks for your prompt replies, audioguru and alec_t. Audioguru, think I got what you are saying (I'm better at music than electronics, I have no formal training). are you saying that with an adapter, especially a cheap one, that the current should be close to the current that is needed by the piece of equipment, and not too much higher? (I'm wondering if 500 mA is too much for a midi controller pedal. It's just switches, leds, and serial data).

alec_t, I did have a 470 ohm resistor, that came with a 105 piece led kit with voltage dropping resistors. I retested the adapters, the 12 volt was outputting 16.53 volts (instead of 16.54) and with the 470 Ohm resistor (in series) I got 16.54/16.55 volts. Pretty much the same. For the 9 volt adapter I retested it and got 13.14 volts (instead of 13.02, maybe a fluctuation in A/C supply?) and with the 470 Ohm resistor I got 13.08 volts. (Do you mean I should connect it across the positive to negative of the adapter?)
 
"But it has resistance in the windings of miles of thin wire in its transformer so its voltage drops to its rating when it has its rated load current."

This is so incorrect. I thought you had some idea of how electrical things work.

"Try connecting a 470 Ohm (or thereabouts) resistor at the output and measure the volts again. What do you get?"

This is another piece of useless advice from a non technical person.
 
Your best bet is to look for a switch mode adapter, as most of these have a regulated output. A supply might be rated at 12 volts @3 amps, but its output voltage will only change a few tenths of a voltage from no load to full losd.

A non-switch-mode supply will have a large transformer and be close to a cube with equal dimensions in all three directions. A switch mode supply doesn't have a large transformer and they tend to ne lighter and lass of a regular cube shape.

AC - DC Power Supplies - Using Wall Warts may be useful reading.
 
So for the 9 volt adapter I get 12.63 volts (down from 13.14), and for the 12 volt adapter I get around 15.6 to 15.8 volts (it keeps moving), (down from 16.54).

Colin, are you suggesting that this isn't an accurate test?
 
Please!! Furnish us with your resolve....

The one thing always gets to me about your answers... You are quick to rubbish all others whilst never divulging why!!
 
JonSea I didn't see your post before I posted. I read the link and actually understood almost all of it. I had opened the midi pedal, I wanted to see if I could put a diode in line with the positive lead to protect against accidentally connecting a reverse polarity adapter. After the DC in jack there is a resistor then a diode. I'm pretty sure I also saw a voltage regulator on the power input board. I'm going to open it up again to make sure. I will need to open the rack to see if there is a voltage regulator in there, but it sounds like the voltage of the adapter drops under load anyways, close to the rated voltage, at 100% load. Thanks for the info!
 

I suspect you have older style transformer based adapters of which yes they tend to run quite a bit over their rated voltage at light to no loads and yes most properly designed electronics device that rely on such adapters for power are more than capable of dealing with the higher inputs.

I've seen many old 12 volt non SMPS (Switch Mode Power Supply) adapters run as high as 20 volts at no load but pull down to their rated voltages when loaded to the currents they were rated to handle.

As for the 470 ohm load test I would say that was off by a factor of 10 or even 20 too high and it should have been a 47 or even half as much lower ohm resistor.

Personally for a load test on a 9 - 12 volt adapter would have used something in the 15 - 25 ohm range myself. Odds are your adapters would have read much closer to their values with that load.
 
roadtrip said:
So for the 9 volt adapter I get 12.63 volts (down from 13.14), and for the 12 volt adapter I get around 15.6 to 15.8 volts (it keeps moving), (down from 16.54).

Colin, are you suggesting that this isn't an accurate test?
Click to expand...

I did a quick calculation using the numbers provided by roadtrip and found that the 9v wallwart has an apparent internal resistance of 19 Ohms, and the 12v wallwart has an apparent internal resistance of 25 Ohms.

These numbers are rather high, using these figures the 12v unit could never deliver 800mA.
There may of course be other effects which are not taken into account in my simple calculations.

As I type, I see that tcmtech has suggested testing using a lower value of resistance, this is a good idea.
What may be interesting is to use a range of load resistances, say 470, 330, 220, 150, 100, 68, and 47 Ohms.
Measure the voltage with value of load and plot a graph.

Or alternatively we could adopt Colins approach and just call each other all the useless idiots under the sun.
What does every one think?

JimB
 
Yes.
But you did not measure the current of the thing you want to power so we are just guessing. I suspect it draws 30mA, not 500mA then a 500mA adapter voltage output will be much higher than its rating.
I have a 12VDC/1A adapter that measures 16.9VDC with no load. But it is rated with a 120VAC input and my electricity measures 123V so with 120VAC input its output will be 16.5V.

A load must be a load. Your load was the 470 ohm resistor in series with the millions of ohms of your meter so it was not a load. Connect the 470 ohm resistor to the adapter as its load (12V/470 ohms= 26mA which is almost nothing, use 47 ohms for 260mA). But the 47 ohm resistor powered by 12V will heat with (12V squared)/47 ohms= 3.1w so an ordinary 1/4W resistor will be smoking in a couple of seconds.
 
Thanks again for replying, audioguru. You are correct I will not know the current until I hook them up. I will need to assemble a 2.1 mm jack to 2.1 mm plug to tap in to test the current. I just didn't want to blow the rack, it's a vintage piece, not too many around online, and it was shown as working with the leds powered on so I took a chance and bought it. The midi pedal was demoed for me in a you tube video and was also fully functional so I bought it too. (The midi pedal runs on batteries as well, so I have tested it now and it does work).

I have a limited supply of resistors, I am just beginning to build some pedal switchers and bought some resistors for the leds if I needed them. I have a few that are rated 1 watt, but I think the lowest I have in 1 watt is 160 Ohm. I don't have any resistors rated for 3+ watts, sorry. (I think the 470 Ohm resistor I used was 1/4 watt or 1/8 watt, so thankfully it didn't burn up!)
 
Colin said:
"Try connecting a 470 Ohm (or thereabouts) resistor at the output and measure the volts again. What do you get?"
This is another piece of useless advice from a non technical person.
Click to expand...
The purpose of the test was merely to establish, using a low wattage resistor, the power supply output impedance so as to assess the current draw needed (and hence a possible shunt resistor value) to pull the volts down to the desired level. JimB has kindly done the sums.
 
Last edited:
Colin said I have no idea of how electrical things work. Then the cheap adapter is made to accurately produce its rated voltage when it has no load. It is wound with thick silver wire instead of thin copper wire so its resistance is very low then its voltage does not drop much when loaded. Right?
 
audioguru said:
Colin said I have no idea of how electrical things work.
Click to expand...
AG, it is well known that you are a crusty argumentative old devil,
but when it comes to electronicals, you do know your stuff.

JimB
 
Colin is in Down Under where everything is upside down?
We are having Winter but Colin is having Summer.
I guess Ohm's Law also works differently there.
 
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