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12 volt amp power circuit

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mike03 View Drop Down
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    Posted: 09 March 2010 at 2:44pm

I have this old ppi450 4channel amp that i would like to build a power supply for my home purpose I have a 11volt ac output transformer and a bridge rectifier to change it to dc the problem is I would like to know what size capacitor to use to smooth the dc voltage and the size fuse to use on the amp as the amp does not have a builtin fuse holder nor markers indicating the size fuse to use?

dc
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sheffield-junglist View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote sheffield-junglist Quote  Post ReplyReply Direct Link To This Post Posted: 09 March 2010 at 6:20pm
Yeah I've just got into building power supplies also. I understand them and so far have just been modding other people's designs to suit my needs and to try and learn more but how you decide on the values of the smoothing caps is still one of the mystery's I've yet to solve.

I know that a higher capacitance makes for better smoothing due to it being able to absorb/release more amounts of power to accomodate larger variations in the current but surely there is some sort of upper limit as to how big a cap you can use?

I would have assumed that the only thing that limits it is when the cap is able to hold such a large amount of charge that you have to wait a significant amount of time for them to charge up (But even then they would have to be HUGE?)




Edited by sheffield-junglist - 09 March 2010 at 6:21pm

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mike03 View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote mike03 Quote  Post ReplyReply Direct Link To This Post Posted: 09 March 2010 at 7:28pm
need some recommendations please
dc
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Post Options Post Options   Thanks (0) Thanks(0)   Quote haloweenhamster Quote  Post ReplyReply Direct Link To This Post Posted: 09 March 2010 at 8:14pm
the biggest u can fiscally fix into what ur building

u do know an 11vac supply rectified will give u 15ish vdc

and also i hope u have a chunky transformer as i would take a guess that, that amp is 200w or 450w
so u will need a 250/500va transformer to be safe

if u want a figure i would use anythin between 47mF-1F
start with 47mF and if u get hum break through the audio then increase till hum is no longer noticeable 
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Deej View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Deej Quote  Post ReplyReply Direct Link To This Post Posted: 09 March 2010 at 9:04pm
You can use this basic formula to calculate the required capacitance based on the amount of voltage ripple you can tolerate on your supply. As haloweenhamster says an 11Vrms AC voltage once rectified will give you 14-15V DC depending on the type of diodes in the rectifier.


C = Load Current / (2 * mains freq. (50hz or 60hz) * desired ripple voltage)

For example if you are trying to power a 200W amp at 14V, your load current could be up to ~14A. For a tolerable ripple voltage of 0.5V you would need 290mF of capacitance. That's a heck of a lot of capacitance and won't be small or cheap, but that is the problem of using such a low voltage.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote rich_gale Quote  Post ReplyReply Direct Link To This Post Posted: 09 March 2010 at 9:30pm
oldschool ppi amps will handle 14-15v.  might get warmer than normal so a fan might be a good idea. why not just whack a 1farad automotive capacitor into the chain and be done with it?
REFLEX ALL THE WAY.... (however, im playing with horns again...) That ok Mister Valiant? :)
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mike03 View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote mike03 Quote  Post ReplyReply Direct Link To This Post Posted: 09 March 2010 at 10:06pm

thnx

dc
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haloweenhamster View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote haloweenhamster Quote  Post ReplyReply Direct Link To This Post Posted: 09 March 2010 at 11:38pm
the main problem with high value eg 1F caps is, to charge them u will probably blow the fuses/bridge rectifier which ever is weakest
unless u charge the cap on a battery and dont leave the supply off for to long between uses
but i think ur main problem will be getting a transformer big enough
might be easier and cheaper to buy a 12v switchmode supply it will be smaller, regulated and need less smoothing which will be built in anyway
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Post Options Post Options   Thanks (0) Thanks(0)   Quote slaz Quote  Post ReplyReply Direct Link To This Post Posted: 10 March 2010 at 12:20am
Look in a couple of skips .. fish out an old PC - use its PSU ?
Typically a 300W PC SMPSU would be able to supply 20A or so on the 12V rail ....
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sheffield-junglist View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote sheffield-junglist Quote  Post ReplyReply Direct Link To This Post Posted: 10 March 2010 at 11:02am
Originally posted by Deej Deej wrote:

You can use this basic formula to calculate the required capacitance based on the amount of voltage ripple you can tolerate on your supply. As haloweenhamster says an 11Vrms AC voltage once rectified will give you 14-15V DC depending on the type of diodes in the rectifier.


C = Load Current / (2 * mains freq. (50hz or 60hz) * desired ripple voltage)

For example if you are trying to power a 200W amp at 14V, your load current could be up to ~14A. For a tolerable ripple voltage of 0.5V you would need 290mF of capacitance. That's a heck of a lot of capacitance and won't be small or cheap, but that is the problem of using such a low voltage.


Couple of questions,
why will the voltage be higher after its been rectified? I knew that the voltage will be higher after the transformer under a light load (Still don't fully understand why though) but I didn't know it was higher after the rectifier. Also, why do the different types of diodes make a difference to how much higher the voltage will be?

Also, how do you work out the max current draw of the supply?
Lastly, how do you know what will be an acceptable maximum ripple voltage?

Like I said I've only been modding other people's designs to the best of my ability but I really want to start designing my own from scratch. (Obviously getting someone who knows what they are doing (A certain forum member) to check the design before I try and build it)


Bet you wish you'd never have posted that now LOL

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Deej View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Deej Quote  Post ReplyReply Direct Link To This Post Posted: 10 March 2010 at 7:03pm
The reason for the voltage increase is that the AC output from the transformer is an RMS value, whereas the rectified DC voltage seen after the diode bridge is equal to the peak (RMS x sqrt 2) of the AC voltage, assuming you have a large smoothing capacitor.

The output voltage from the transformer reduces slightly with load as the resistive and reactive (complex frequency dependant) losses in the secondary winding increase with current.

Diodes always have a certain amount of forward voltage drop. Depending on their structure this can vary from around 0.3V for schottky devices up to a few volts for very large PIN power diodes. You will most likely dealing with diodes with a forward voltage drop of around 0.5-0.8V. Also bear in mind that in a bridge/full wave rectifier there are two diodes conducting in series at any given time, so the total voltage drop will be twice that of a single diode. You will probably see around 1-1.6V.

The acceptable ripple voltage will depend on how tolerant the amp's smps is to input voltage variation, and how much additional distortion you can tolerate in its output. It's difficult for me to answer this but I would have thought you would want to keep it under half a volt.

Just as a side note, the system that you are building is quite inefficient because you are converting the large AC mains voltage into a small DC voltage, and then the amp will convert this into a larger DC voltage internally. There's no reason that this won't work, but ideally you would go from the mains straight to the DC voltage required for the supply rails in one step as with a conventional amplifier.

Hope that helps.
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sheffield-junglist View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote sheffield-junglist Quote  Post ReplyReply Direct Link To This Post Posted: 10 March 2010 at 7:37pm
Originally posted by Deej Deej wrote:

The reason for the voltage increase is that the AC output from the transformer is an RMS value, whereas the rectified DC voltage seen after the diode bridge is equal to the peak (RMS x sqrt 2) of the AC voltage, assuming you have a large smoothing capacitor.

The output voltage from the transformer reduces slightly with load as the resistive and reactive (complex frequency dependant) losses in the secondary winding increase with current.

Diodes always have a certain amount of forward voltage drop. Depending on their structure this can vary from around 0.3V for schottky devices up to a few volts for very large PIN power diodes. You will most likely dealing with diodes with a forward voltage drop of around 0.5-0.8V. Also bear in mind that in a bridge/full wave rectifier there are two diodes conducting in series at any given time, so the total voltage drop will be twice that of a single diode. You will probably see around 1-1.6V.

The acceptable ripple voltage will depend on how tolerant the amp's smps is to input voltage variation, and how much additional distortion you can tolerate in its output. It's difficult for me to answer this but I would have thought you would want to keep it under half a volt.

Just as a side note, the system that you are building is quite inefficient because you are converting the large AC mains voltage into a small DC voltage, and then the amp will convert this into a larger DC voltage internally. There's no reason that this won't work, but ideally you would go from the mains straight to the DC voltage required for the supply rails in one step as with a conventional amplifier.

Hope that helps.


Thanks a lot, thats really useful! I understand it a lot better now.

When you say the system is quite in-efficient, I don't know what mr Mike03 is building but I am building a small, crappy little 70W RMS @ 4 kit amplifier I got for £15 quid from Maplins. Even though the amplifier itself is very easy to build and generally not very good, I wanted to try and make a half decent power supply.

It includes a power supply on the board ( apart from the tranny) but I'm not even going to bother building it and build the whole PSU from scratch myself and have it as an external unit.

To be honest I'm not doing this for any serious reason, just purely as a learning exercise so it doesn't have to be perfect.

Cheers for all the info, really helped.

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