wall wart not rated supply voltage?

[Bobby Ironsights]

I measured my wall wart just before plugging it into my led array, and I got 19.3 volts, not the 12 volts it's supposed to deliver according to the label.

Is this a matter of RMS, and do I now have to figure out different resistors to use in my array?

Thanks for your time,
Bobby.

P.S.

I found this
https://www.horrorseek.com/home/hal...al_WallWarts.html#WallWartsPutOutWrongVoltage
that says that if I'm not using the wall wart at or near the rated capacity, then it will deliver substantially more voltage than rated.

So, I'm guessing the multimeter uses almost nothing when measuring the voltage, and so gives a much higher voltage than the wall wart would actually supply me under full load.

I'm concerned that if I go over by a little, I'll burn out the array, but I don't want to just keep blindly substituting resistors randomly, and I don't have a variable resistor.

My question now, is how do I find out what voltage my wart will give me when I'm running the array, which is eating about 240 ma, and the wart is rated for 500 ma.

 

[MikeMl]

No, wall-warts are very poor transformers. They use loosely-coupled windings to create a transformer which is supposed not to catch fire if you overload it. They spec the output voltage at a specified load current. If you lightly load them, they put out a much higher voltage. If you overload them, the output voltage drops very abruptly.

If you have a fixed load, you might be able to resize your resistors; if you have a varying load, use an electronic voltage regulator like an LM7812 or 7815.

 

[Sceadwian]

No, wallwarts aren't generally regulated or precise. But typically their rated voltage is at it's rated current. No load voltage will always be higher. Find out what the rated current of that 12 volt supply is, apply a dummy resistor load (that can burn off the power) and you should get pretty close to 12 volts, and quiet a bit of ripple.

 

[Sceadwian]

Jeez Mike, you'd think we were in a race Our posts were so close in content it's kind of creepy!

 

[MikeMl]

Yeah, but I put more useful info into my post

 

[Sceadwian]

Says you =)

 

[Bobby Ironsights]

Thank you, I'll look around for a small 12v car interior lamp that draws a quarter amp or so to test.

oops, brain fart, I should make up a dummy load with resistors. (Sorry, I'm new here)

 

[Sceadwian]

A car lamp will work just fine if you have one, they work great as dummy loads. 12 volts at 250ma is 3 watts, you'd need 48ohms of resistance able to dissipate 3 watts for a resistor bank. Either way it'll be fine. If you can test your transformer at a few different load currents you'll be able to draw a graph that will approximate what the voltage will be for any load. Two points or three will give you a decent idea.

 

[smanches]

All an incandescent light bulb is, is a resistor.

 

[Sceadwian]

They're not thermally stable resistors though smanches.

 

[crutschow]

A very non-linear resistor.

 

[smanches]

Thank you, I'll look around for a small 12v car interior lamp that draws a quarter amp or so to test.

oops, brain fart, I should make up a dummy load with resistors. (Sorry, I'm new here)

They're not thermally stable resistors though smanches.

I was clarifying a point that he can use the bulb and doesn't need a "dummy load resistor." specifically. There are many other ways a light bulb isn't exactly like a resistor too, but in this context they are essentially the same. Now quite being so nit-picky.

 

[Sceadwian]

All an incandescent light bulb is, is a resistor.

Neither me nor crut are picking nits here, it's a very important point to be aware of. General real world resistors are designed to be relativly linear through their voltage and wattage ratings. Lightbulbs are not. If you say a lightbulb is a resistor and then use one and expect it to be linear like a typical resistor you would be wrong. In this case you're correct it doesn't matter, but it isn't true for all cases so at least mentioning the non linearity thermal instability of lightbulbs is a good idea, which is what we did, not pick nits =)

 

[smanches]

Neither me nor crut are picking nits here, it's a very important point to be aware of. General real world resistors are designed to be relativly linear through their voltage and wattage ratings. Lightbulbs are not. If you say a lightbulb is a resistor and then use one and expect it to be linear like a typical resistor you would be wrong. In this case you're correct it doesn't matter, but it isn't true for all cases so at least mentioning the non linearity thermal instability of lightbulbs is a good idea, which is what we did, not pick nits =)

But then we'd be explaining every point all the time. By using the context of the conversation, we can make some assumptions and save some time.

 

[Sceadwian]

No smanches, not every point all the time, just the important ones. A simple statement of the non-linearity of a lightbulb compared to a typical resistor is very important. Intentionally not saying anything is not doing the person any favors for simplicity as it at some point it could be important to them, it wasted no one's time to add an extra sentence which the poster might not even respond to, it is wasting people's time to argue about it.