AC/DC Power Adaptors - Rated Outputs

[terryt]

When I believed one of my car jump starters was losing some of its internal battery performance I tested its power adaptor at the “pin”.  The adaptor is rated 15V DC output and my multimeter read 18V. 

Ignorant of much things “electronic” and concerned I may be overcharging the battery I talked to the supplier.  They advised 18V output was too high (with 16V the highest acceptable output) and they would find me a replacement adapter.

I own some 30 AC/DC power adaptors with various rated DC outputs (all AC inputs are 240V, Australia).  Being a curious type I checked all my power adaptors and found only 12 tested at, or very close to, their rated DC output.  The other 18 adaptors all produced “high” results ranging from +3V to +8V above their rated DC outputs. 

While a lot of my adaptors are quite old (retained as spares after their devices were discarded etc) I was still surprised at the percentage of “duds” I seemed to have and this made me question the validity of what I was doing.

Below is a representative sampling of my results:

Device            Rated Output   DMM Tested
15 LED worklight         12V               20V
Hand torch, 5W           12V               18V
Jump starter, 1900A    15V               18V
Sanyo cordless phone    9V               14.5V

My question is: should a AC/DC power adapter always output its rated DC output or be say, within 1V…or is a larger variation acceptable.  Is there a general rule-of-thumb e.g. adaptor output should be within say, 10% of its rated output?

I'd appreciate any comments/advice to be in Simple English.  Thanks

[digsys]

It all depends on how well it's designed. There's often not a lot of room in the plugpack, so designers may take shortcuts. eg Rely on a 20-50% load to pull it down to the
rated voltage, or they pre-compensate for the cable loss, and hope to lose the extra under load. In any case, you must apply a light load, say ~5-10%, as there reading may be
quite meaningless due to S/Mode noise.

[terryt]

@ digsys.  Thank you for your prompt reply.

Perhaps my post did not properly explain that my understanding of electronics is so limited I cannot grasp the concepts you mention. For example, you said: "In any case, you must apply a light load, say ~5-10%, as there reading may be quite meaningless due to S/Mode noise."  I simply don't know what that means in the context of what I was asking.

Essentially I am trying to determine whether I have a box full of plugpacks that are outside acceptable output +/- specs, and should be binned.

If a plugpack for a 15 LED worklight is rated at 12V DC output and, when tested with a multimeter, it shows 20V DC output (because of poor design, or whatever) is that "acceptable" in your opinion, or not? 

   

 





   

[Avacee]

Here's an example: I have a very cheap adapter that has a sticker saying 5V and 4A.
With no load (ie only multimeter attached) it reads 5.4V and its a steady 5.4V
When connected to a DC Load:
1A = 4.8V - peaks of 0.1V
2A = 4.3V - peaks of 0.3V <-- gets a bit warm
3A = 3.7V - peaks of 0.6V <-- gets very warm
4V = 3.2V - peaks of 0.8V <-- hot enough I'm concerned about fire risk

It's garbage - it's a 1A (tops) adapter with a new sticker on it.
It has no feedback to try and maintain a steady 5V - it's hoped that somewhere between 5.4V and 4.8V is acceptable.

Even a 5V phone charger from a reputable company will be more than 5V with no load but it will be matched so that when the phone draws its charging current the voltage will have dropped to around 5V.
Eg my LG charger is 5.2V with no load but 5.0V at the 0.85A my phone charges at.

The point is that your adapters aren't garbage (like my example) but that they'll put out more than the rated voltage with no load and as you add more load the voltage will drop into the expected range.

[Rerouter]

to produce a cheap plugpack they build them to only output a close voltage under some amount of load, e.g. if its a 500mA plugpack, try loading it with a resistor that will pull atleast 10mA. many phone chargers are the same, without a load they can shoot up to 7V on the cheap ones, they assume the device that connects to it will use a regulator to step it down to what it needs, so its better to be higher than lower.

Higher quality ones tend to use a hiccup mode (they only switch on time to time with no load) to maintain a well regulated output with no load, but these are rarer now.

as for what is a good one and a bad one, you will need to apply some kind of load to it, e.g. if that worklight plugpack (which by its nature is likely the cheapest of the bunch) is 13V with a tiny load, and still 12V at full current, i would call it usable if i need a 12V supply,

I am assuming your hoping to use plugpacks as a work around to buying a more expensive dedicated variable power supply, if so on ebay, grab some dirt cheap DC/DC step down converter modules and use them to make sure the voltage never exceeds 12V,

[mariush]

If that adapter uses a linear transformer inside (which wouldn't be uncommon at such low maximum power, 6w) , then with no load applied the transformer inside may output quite a lot more than the desired value. As you increase the load, the voltage will drop towards the actual 12v value.
Such "unregulated" adapters are often used with power drills, by cordless drill has one of these. It says 22.6v 5.6VA on it but the no-load voltage goes much higher.

Another hint that the adapter you have may not be the "switching" kind, would be that it lists only 240v as input, not a wide input voltage range. Switching adapters usually say something like 100-240v AC 50/60Hz 

A simple fix would be to add a small linear regulator that would output a constant 12v regardless of input voltage, maybe slightly more to account for the voltage drop on the thin wires up to the connector.
For 200mA, there are tiny LDOs that only need about 100-200mV above the set output voltage to output a consistent voltage, and even if the input voltage is lower, they can typically follow the voltage (so you have 12v in, they'll output 11.9v on the output or input voltage minus dropdown voltage)
 

[terryt]

@ Avacee, Rerouter, mariush. 

Thank you for your replies which have helped me better understand the issues involved. 

[Shock]

You have regulated and unregulated power supplies, you can find out when they are loaded if the voltage drops it's unregulated and is fine as long as it's used on it's intended device.

Then you have linear (transformer at mains frequency) vs switching supplies.

In my opinion old or quality linear power supplies are keepers (people throw these away) especially ones that you can easily open, good for little projects. The good switching supplies generally came with expensive older laptops etc. You can tell the difference by weight and size, a linear supply hefty and normally box shaped enough for the transformer to squeeze in.

Cheap switching supplies are likely to fail faster and buying any made in China gear can be dangerous because they don't give a shit (killer LED bulbs, phone and ipad chargers etc). Old power supplies (actually anything with capacitors) that has been sitting around needs to be carefully brought back to life (or even better serviced) before using, mainly due to capacitor internal or external leakage.

To test power supplies you need a constant load, multimeter or better an oscilloscope (a cheap power monitor is handy as well). I have a cheap security camera that has failed recently, not looked at it yet but can almost guarantee the made to fail in China supply took it out.