If I tell a wall wart that has a 110/220 input switch that it's getting 220 and then plug it into a 110 source, is that enough to break/fry the wall wart?
(Details I have a wall wart that's adjustable in two ways: it has a 110/220 input switch and a voltage out switch that runs from 12V to 3V - it's actually the supply that comes w/ the Make: Electronics book kit #1. When I plugged it in the first time, it was outputting a lot higher voltages - i.e, on the '3' setting, it was outputting 5 or 6+. I tried switching the input switch to 220. Now, regardless of the input setting OR the output setting, the wall wart only outputs around 22V.
Thanks for the reply - I think it did work the first time (I think it was outputting a higher voltage because I didn't have any load on it, which sounds right from reading about wall wart behavior elsewhere).
But I think it's fixed - I think it was user error :-). It looks like the wall wart holds a charge if it has no load - it'll say 22V even after I unplug it from the wall. Then if I short the wires, it'll go down to zero. If I then change the output voltage and plug it back in, it'll correctly output the lower voltage.
I was testing while plugged in, then unplugging and switching to a lower output voltage and then plugging in and testing again. But I didn't have any load on it and didn't short it when unplugged, so it stayed at a higher voltage even though I'd moved the output setting lower.
It appears to work now, but if what I wrote above doesn't sound logical, then I'm probably still confused :-). Thanks for setting me straight if that's the case.
Yeah, cheaper wall warts can do things like this. If you're planning on using it to power sensitive electronics, I would use it to power a voltage regulator (such as the lm7805), and then use the voltage regulator to power your parts. It's not perfect, but will provide some measure of protection for your parts. If you're powering some big motor or something, that is probably not necessary :-).
Most wall warts are nothing more than a transformer feeding a full wave rectifier that in turn feeds a smoothing capacitor. These will give a much higher reading than the rating when unloaded. It will be the peak voltage of the secondary minus the drop across the diode bridge. A small load of 1K or so should get you a close to proper reading. There may be some rudimentary regulator circuit in the thing but obviously it requires some sort of a load to function properly. My guess is a Zener driving a power pass transistor and a small filter cap.
You can do as Matt suggests and use the wall wart to feed a regulator but you need to feed it with a voltage greater than the output of the regulator. IE: if you want to get 5 Volts out you will likely need to feed it with at least 8 Volts.
I actually almost caused an engineering disaster once by not spec'ing a high enough supply voltage for a voltage regulator in a design once. Fortunately, one of my co-workers suggested swapping in a low dropout regulator (http://en.wikipedia.org/wiki/Low-dropout_regulator), which use a different circuit design that can tolerate slightly lower input voltages.
In general, though, I think the regular voltage regulators are a good way to go, making sure to feed it a high enough voltage to let it do its job.
Yep, The low drop out regs are a great thing. they generally have good regulation but are a bit more pricey. The plus is that they drop less voltage and so dissipate less less heat. Another option is to use a switcher, not much more to implementing them and they can provide buck or boost function. The chips are cheap. I find them on old boards all the time and simply pull them, the inductors, and the low ESR caps.
Actually, it will output only half the voltage if it is a simple rectifier with a filter cap. The current is determined by the winding wire sizes and saturation point of the transformer. Let us not confuse the difference between the two.
On the other hand if one were to plug it in an outlet that is 220 when it is set for 110 then you will end up with a fried primary and a blown output filter cap. You may even get a good fire rolling, ABS burns really well.
Could be a bad unit. I don't know what the circuit is inside of the thing so it is hard to tell you where to go from here. If you can open it up and post a pic then it will tell us allot.
The reason why you got that result was because you used a cheap wall wart. When it comes to usage with electronics which are power sensitive, what you would need is a power voltage regulator which would help you power up the parts without any short circuit or other voltage issues.