The AC adapter with my computer speakers is broken/dead and I want to replace it with a new one. The specifications on the dead adapter are as follows.
INPUT: 120VAC 60Hz OUTPUT : 13VAC 800mA
If I have to replace it with a new one what are my options. Should I look for exactly the same specifications or something other will also work? Do I have to replace it with an AC to AC adapter or can I replace it with a DC adapter? If I can use a AC to DC adapter what specifications should I remember.
The old 8 bit Nintendo comes to mind, the original adapter was AC but DC worked fine as it had a rectifier and regulator in it. Many things that use AC output adapters do so for a reason though and will not work correctly on DC, though it's unlikely to cause damage right away.
Well, at first: if DC adapters were for 'universal use' no one would bother to design / produce / stock / sell AC types :-)
If you supply AC to a device, you can do all sorts of nice things with the input power:
- rectify it
- rectify it with a doubler (giving + and - for the same voltage)
- rectify it and pull some mains frequency for timing. (Old times clocks used to do that. Before crystals and digital deviders became cheap.)
- some loads do not allow dc at all. As several switches don't. Some motors don't either. In general: if the device is some kind of coil, don't even think of replacing an AC adapter with a DC type.
What also counts is the voltage. If an adapter is labeled 10 Volt AC it is quite easy to pull some 14 volts out of it. (Rectify it, which yields root 2 times the ac voltage.) You cannot do the same with DC. (And you cannot replace a 10 volt AC type by a 14 volt DC type either. :-) (Please note, that also the load applied may play a rather important role in the actual voltage an adapter supplies!)
James Sweet mentioned a special case: if the only thing done on the input is a rectifier, you can connect a DC adapter. That is: if the device is designed to handle a rather big range of input voltages and if the rectifier is overdesigned, as in the DC situation the current through half the rectifier is more than double that of the AC situation. (Again: with cheap diodes plentiful in supply, a modern device is likely to handle this situation. Old devices maybe not. I've seen many 1N4148 'bridge rectifiers :-)
Err, AC wallwarts are cheaper and because of slightly smaller size like for like may be the answer for some. And of course AC is fine for things like low voltage lighting. Might also be that a device produced for using throughout the world makes sense to have the DC conversion and regulation within it.
I think you're clutching at straws. ;-)
It would be a very poor description and specification of transformer which supplied 14 volts while being labelled as 10 volts. ;-)
You're missing the point that most electronic devices which use an AC wallwart will have internal regulation. And the only common one is most external modems. Why - I've got no idea.
With any DC device designed to be fed with an AC source you can feed it with DC - provided you take into account the voltage drop through the rectifier. And given the poor regulation of most AC wallwarts this is rather easy to calculate.
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Dave Plowman dave@davenoise.co.uk London SW
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That's really not always true, I had an external modem years ago that would only work properly from an AC output adapter, and a couple different bubble lamp things and kinetic sculptures that used synchronous motors and vibrating air pumps, those devices all required
12VAC and would not work correctly from DC. I've built a few digital clocks that also require an AC output wall wart because they derive the timing from the line which is much more accurate long term than a crystal though since those are not a commercial product I can't really count them.
Yes, but they're not true DC devices if they include an AC motor etc. The modem should have worked properly - I've used a DC supply for one with success. But if say its internal electronics have a 12 volt rail the DC supply will need to be higher for the internal regulator to work, and regulators fed with a lower voltage than they require can output nasties.
Yes again, but then it's not a DC device. The OP's speakers will be.
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Dave Plowman dave@davenoise.co.uk London SW
To e-mail, change noise into sound.
Many of these shitty supplies are marked with RMS voltage. The PEAK voltage however is RMS X 1.414 (X2 for full wave) which is what appears after being rectified and smoothed. Best to check (if possible) the output from the original with a 'scope or meter (a meter measures RMS) or better still get a direct replacement.
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