How could I produce a small AC voltage? - Page 1
Forum section: electronics / beginners

author: CodyW
author_rank: Contributor
timestamp: September 14, 2013, 06:24:54 pm
content: Hi Everyone,I am currently doing a project for university. I have to show that four diodes (1N4007) in a bridge configuration change AC to DC. I am having a hard time tracking down a AC to AC power pack and I don't have time to order it online and wait for it to arrive. I also don't want to be mucking around with wiring a mains step down transformer.How could I produce 5 volt AC from DC?

author: ivan747
author_rank: Super Contributor
timestamp: September 14, 2013, 06:30:40 pm
content: You can, but basically, it will be 10 times more complicated than putting a transformer with a switch and a fuse inside a box.Since I sense some urgency in this, a good source for a decent transformer is a stereo. Also try looking for them inside computer speakers or the heaviest power brick you can find. Do not confuse a standard 50Hz/60Hz transformer with the transformers used in a switch-mode power supply (SMPS). If you use a SMPS transformer directly into the mains, you will have a very bad time.Linear wall wart with a typical 240V/50Hz transformer:Absolutely crappy SMPS wall wart:Never leave electronics projects for the last week. Shit happens.

author: FrankBuss
author_rank: Unknown
timestamp: September 14, 2013, 06:38:18 pm
content: Simpler than a sine wave generator is a square wave generator, which is AC nevertheless. You can use anything you have for it, like the classic two transistor astable multivibrator, or a 555, or a resistor, a capacitor and a 7414 etc. Add a capacitor at the output for positive/negative voltage, relative to ground.Or create a sine wave file with Audacity and output it with a big audio amplifier

author: edavid
author_rank: Super Contributor
timestamp: September 14, 2013, 07:16:41 pm
content: Quote from: CodyW on September 14, 2013, 06:24:54 pmGo to a thrift store/second hand store/junk shop/boot sale/tip/whatever it's called in Australia and look for any power pack that is marked 240VAC only (not 100-240VAC). Then cut it open and remove the diodes (or just use the whole thing as your project).

author: 6502nop
author_rank: Regular Contributor
timestamp: September 14, 2013, 07:48:58 pm
content: Not too sure how things are in Aus, but over here I have many transformers that I can use:Rechargeable power tools: Most use an AC-AC adaptor (wall wart) so they can use a longer lead to the charging base (it gets rectified inside the base). Since AC has less drop over distance than DC, they tend to do things this way.Doorbell: Most US doorbell transformers are about 18-24VAC (and even have handy screw terminals!).Thermostat/HVAC control: Most use ~25VAC transformers. Since we just replaced ours last month, I have a couple of them (and a nifty high-voltage transformer from the electronic air cleaner!).Model car/train set: My old Tyco railway transformer had AC "Accessory" terminals.SOMEolder PC power supplies have standard transformers for their 5VSB rail. If you're not comfortable poking around in one, then don't.So, if you don't have anything around the house, just head over to your local hardware store. Don't worry about the voltage output too much - those 1N4007s can handle 240V mains without sweating (keep it below an amp on current draw, though)If you're using a cap to filter, double check the voltage rating. It should be 1.5-2x AC output voltage of the transformer used (e.g., 18VAC transformer would need a 35V cap, 25V transformer = 50V cap).nop

author: Zero999
author_rank: Super Contributor
timestamp: September 14, 2013, 08:32:25 pm
content: What do you need it for? Any how much current do you need?Doe the output need to be isolated from the input?There are thee parts to this:1) 5V AC means 5V RMS which has a peak voltage of just over 7V so you need to increase the voltage first using a boost converter. If you need isolation, this is a good place to put it.2) You need to construct an oscillator to generate a sine wave.3) The oscillator only generates a small signal so you need an amplifier. If efficiency isn't important and the power output is small, class AB will do but if you're worried about power losses, it needs to be class D.The boost converter I mentioned will need a higher voltage output than 7V to cover for the voltages losses in the amplifier which will not be able to output the full supply voltage, 9V will probably do. You can probably buy a ready made boost converter but the rest is more tricky.

author: peter.mitchell
author_rank: Super Contributor
timestamp: September 15, 2013, 04:02:57 am
content: if you need to show 4 diodes can rectify ac , why don't you just get a little stepper motor or make a "h bridge" from some multipole switches, then you can either make your own ac or demonstrate that no matter what direction you put the current in, it comes out the same way.put some LEDs in either direction with resistors across the "ac" input and one on the output to show what the polarity of the input is and that the output remains the same.

author: amyk
author_rank: Super Contributor
timestamp: September 15, 2013, 09:53:50 am
content: Quote from: 6502nop on September 14, 2013, 07:48:58 pmSource on this? 5VSB implies ATX and everyone moved to SMPS by that time already.

author: 6502nop
author_rank: Regular Contributor
timestamp: September 16, 2013, 01:12:23 am
content: QuoteThe prestigious School of Hard Knocks. And here:http://electronics.stackexchange.com/questions/64680/atx-psu-5vsb-5v(scroll down to the answer part),QuoteYes. For themainsupply - not necessarily the 5vsb! I've opened lots of PSUs that had a standard ~1A transformer feeding a typical 7805 regulator. You'll find these on cheaper and older ATX1.x PSUs, typically rated at only 1-1.5A on the 5vsb rail. If it's rated at 2 or more amps, it's a full switcher. I can even recall one PSU (forget the maker) that had a 7805 for the 5vsb, a 7912 for the -12V (with a 7905 slaved for the -5), and a 7812 to run the fan inside. I'm thinking it was a Doorman? Deerfield? Something with a "D" in it...?nop

author: Stonent
author_rank: Super Contributor
timestamp: September 16, 2013, 03:28:33 am
content: Original Nintendo (NES) power supplies were 9V-ACOr just take another wall-wart power supply and bypass the rectifier.

author: Corporate666
author_rank: Unknown
timestamp: September 16, 2013, 05:17:36 am
content: Quote from: CodyW on September 14, 2013, 06:24:54 pmLow voltage home lighting runs on 12V AC. You should be able to pick up 12VAC transformers at any home improvement store for short money.

author: johnboxall
author_rank: Unknown
timestamp: September 16, 2013, 06:25:00 am
content: Quote from: CodyW on September 14, 2013, 06:24:54 pmWhere are you?http://www.altronics.com.au/index.asp?search=AC+plugpack&area=srch&Submit=SEARCHhttp://search.jaycar.com.au/search?w=ac%20plugpack&view=list

author: amyk
author_rank: Super Contributor
timestamp: September 16, 2013, 11:10:51 am
content: Quote from: 6502nop on September 16, 2013, 01:12:23 amI think we're using different meanings here? I took "standard transformer" to mean line-frequency, and that's definitely something you won't find in an ATX PSU; the older cheaper ones (I think you're referring to Deer) did use a 7805 for +5VSB but the transformer is SMPS type being fed from a free-running oscillator. Seehere,here, and everythinghere...