I have a small set of speakers i'm using for a project that run on 9v (normaly a 9v AC transformer) but I want to run them from a 12v DC source. What would be the easyest way to reduce the 12v to 9v? thanks!
Use a voltage regulator (i think) it depends on how much current the speakers are drawing through the transformer. if its under 2 amps i >think< that you can use a L78S09CV (maplin part code UJ55K) with a suitable heatsink. why do you want to do this? what will it be powered from? a PC power supply?
Have you checked if it realy uses 9V? There is a possibility that the current goes to an internal regulator circuit first time. If so (and that regulator can handle 12V), you don't have to reduce the 12V, just wire it up. If it realy needs 9V, and that unit is low-power, a 7809 should work as Double_C said.
Ummm - the OP said 9V AC. AC != DC! Converting from 12V DC to 9V AC would be a pain though. I think your best bet would be to see if you could modify the speakers to take a DC input.
Crap, just noticed that! Are you sure it's not a 9v AC-DC convertor? One chance I get to help someone and I am proved wrong! JOBBIES!
It probably is DC, look a the context, a 9v output AC transformer. But remember, even if it is regulated, and you start feeding it a higher voltage, it will have to dissipate more heat. You probably could give it regulated 12vdc, since my 9V transformer puts out 12V under no load, so during silence it probably recieves close to 12V Get a meter and check the output of the transformer. If it is ~12VDC under no load, a regulated 12v source will work, but an unregulated one can put out up to 15v under no load.
you sir are right on both accounts boy I sure feel like a fool, and a fool with no more problems thanks! I ran my multimeter on the transformer (9v DC and it reads around 13v)
Well lets see...if a circuit accepts 9 V AC input and it powers an audio amplifier, it probably means that the circuit contains a diode bridge rectifier, filtering capacitors and a linear regulator. So I don't see any problems if you feed 9 - 12 V DC into it. The DC current will pass through the bridge just fine, and the bridge will ensure proper polarity... Under the above circumstances, it's safe to feed DC into the AC - designed circuit. Just draw the schematic and think what happens if you feed DC into a bridge rectifier ! Second issue...feeding the DC into the bridge rectifier will result in a small voltage drop across 2 diodes (2 x 0.7 V ~= 1.4 V). If the rectifier is fed with 9 AC voltage, the DC output after rectifying and filtering will increase by sqrt (2) = 1.41: 9 VAC => bridge+filter caps => 12.7 VDC, and the regulator must be able to handle this.
now more problems I pluged the speakers into a 12v regulated source and nothing. I'm trying to plug them into my cars cigerette lighter, I measured the plug directly and it varies around 14v with the regulator I get a solid 12.81v but the speakers do not turn on. I looked at the original 9v transformer and it says output 9v 600ma's so I think the cigerette lighter must be putting out more then 600ma's so wat should I do to lower it? thanks!
Are you sure the positive & negative were the right way round? As said, most 9v amplifiers will be OK on 12v providing you don't run it full volume for a long time (the heatsinking may not cope with the extra power). Don't worry about the current ratings, the amp will take what it needs, whatever the source. As it's only 600mA max, a 7809 regulator (1A rating) would be OK from a 12V source if you want to play safe.
