9v ac to dc converter

[aalauzier]

Hello everyone, i am a young electronics geek, i love to build things for myself and friends. I recently built a small audio amp that I am hoping to sell, it runs off of two LM386 IC's. I am using a single 9v battery to power the whole thing, but i also have a 9v AC wall adapter that i want to use, however, it has an output of around 1 amp, and it cause the 100uf smoothing capacitor to get very hot very quickly. So i was wondering if i could get some help with a schematic to make an ac to dc converter, and maybe even bring it down to around .5 amps. i have searched the web for a circuit like that but most of them involve making an entire converter from 120VAC to 9V, but since i already have a wall adapter, i only need a smaller converter. Help would be greatly appreciated.

 

[cowana]

You just need a bridge rectifier and a slightly larger reservoir capacitor.

The 1A rating is simply the maximum, not the current that is flowing.

Andrew

 

[aalauzier]

Thank you, so a simple Full-Wave Bridge Rectifier is all i need, besides a bigger cap?

 

[Vizier87]

are you sure the output is 9V AC? Sounds pretty funny if it's a wall adapter, usually wall-warts are DC-output from m y experience. Probably I'm slower in discovering things now.

 

[aalauzier]

Yeah, almost all wall adapters have an AC output, all electricity running through a house is AC, Batteries are DC

 

[MikeMl]

Yeah, almost all wall adapters have an AC output, all electricity running through a house is AC, Batteries are DC

NO! Most wall warts are DC output. Almost all have a full-wave bridge rectifier inside. Some have a filter capacitor; some don't. A few contain an IC voltage regulator. Most modern ones are switchers to reduce the cost of a transformer. Wall-warts with a pure AC output are quite rare.

 

[audioguru]

The wall adapter will have its output printed on it and will say if it is AC or DC.
If it is AC then it will have 9VAC only when it is fully loaded. With less load than 1A then its output is higher, maybe as high as 12VAC or 15VAC.

If its actual output is 12VAC when it is powering your amplifiers then its peak voltage is 17V and the full wave bridge rectifier will drop it to 15VDC. The LM386 amplifiers will get extremely hot when the supply is 15VDC and they will probably be fried. Then they will not work anymore.

With a 9VDC supply, an LM386 amplifier has an output at clipping of only 0.45W into an 8 ohm speaker. At a horrible sounding clipping distortion of 10% its output is 0.7W. Its max heating will be 0.5W so its total power from the power supply is 0.7W + 0.5W= 1.2W. Then the max 9V power supply current is 1.2W/9V= 133mA for each amplifier.

 

[aalauzier]

So I should not use that wall wart or i should get a different one? Or what else could i use besides a bridge rectifier that would bring the power down to a good amount for the 386's because i dont want to fry those.

 

[audioguru]

You can buy a digital voltmeter for $10.00 or less and use it to measure if the output of your wall wart is 9VAC, 12VAC, 9VDC or 12VDC.
Why don't you use a regulated 9VDC/300mA wall wart?

 

[aalauzier]

I have a Voltmeter, it is an old one, and for some reason will not give me the voltage of it, so I will look it up to make sure i am using it right. I would like to use a different wall wart, but I already have this one so i wanted to see if i could make it work first. Thank you for all of your advice, I know that I am a newbie to not only this site, but also to electronics. I am not even 16 yet, but i have been working with electronics for almost 2 years, I know that that is not long, but i have been trying to teach myself many things and learn from experiments, books, and the occasional youtube video. The help is greatly appreciated, and I know that I have a hard time understanding some of the things that you are talking about, but I am still learning. That and i don't have much money to go out and buy a lot of parts so i try to scavenge and make the things that I have work.

 

[MikeMl]

Are you using the correct range on your voltmeter? For example, if you have a wall-wart which has a built-in rectifier and a built-in filter capacitor, then measuring with the voltmeter in AC mode should read nearly zero, while reading in DC mode should show ~10V.

If you have a wall-wart with a rectifier, but no internal filtering, then it will show AC ripple in the AC mode, and DC average value in DC mode...

A pure AC output wall-wart will show AC, but zero DC on the voltmeter.

 

[aalauzier]

My Voltmeter is not that advanced, it is more of a simple electricians voltmeter, not an electronics voltmeter, i cannot change it to read AC or DC, it seems to only read DC because it can read a battery, but no AC wall wart. I found a very low cost but effective voltmeter that i hope to purchase in the near future. Thank you for your help, i need all that i can get

 

[Reloadron]

Aren't there any markings or identifying features on the unit you have? Maybe some symbols?

Ron

 

[aalauzier]

It is a Sperry DM 350A, it seems to be having some problems now, but I dont mind, because i can borrow an oscilloscope