Need help designing an AC/DC circuit

[ryan256]

The goal here is to power an 80 watt car stereo amp off of 110 AC current. I'm planning to build a circuit similar to the one shown on this page. I'm planning on using this rectifier and also constructing the filter circuit. Need some help with a few things though.


1. Do I need something to step down the voltage from 110V to 12V before I apply it to the rectifier?

2. The "choke" mentioned near the end of the article I'm guessing is just an inductor? Where could I find one of these?

3. Would these recommended capacitors be ok for my project? Would it make much difference if I used larger capacitors?

 

[Born2bwire]

It's a fairly crude power source, I would add a resister and a zener diode as your voltage regulator after the filter stage. Obviously you need to step down the voltage to your rectifier since they are not rated for 220*sqrt(2) V PIV. As for your filter cap, what's your desired ripple voltage specification?

EDIT: In fact, you can't really say anything until we know what specs you need. Output voltage, output current loads, desired ripple voltages, etc.

 

[BrownTown]

Yeah, you need a transformer to step the voltage down to the desired voltage (also remember that AC voltages are given in RMS, so 1.414 times higher than 120 on the peaks). Might wanna look into maybe a linear voltage regulator to give you a much more stable voltage, I see on digikey a 12V 7.5A regulator here, although thats alot of watts to be using a linear voltage regulator at least imo, you would need some considerable heatsinking too obviously.

 

[ryan256]

Originally posted by: Born2bwire
It's a fairly crude power source, I would add a resister and a zener diode as your voltage regulator after the filter stage. Obviously you need to step down the voltage to your rectifier since they are not rated for 220*sqrt(2) V PIV. As for your filter cap, what's your desired ripple voltage specification?

EDIT: In fact, you can't really say anything until we know what specs you need. Output voltage, output current loads, desired ripple voltages, etc.

Sorry about that.
I need this circuit to accept standard 110AC current from a wall socket and step it down to 12V DC. But I also need it to have enough power to put out about 7 amps. If I'm correct in that 80Watts = 12V * Amps, Amps = 6.667.
As far as ripple voltage that I really can't say. I'm not sure just how clean the power needs to be for the car amplifier but I guess it needs to be as clean as possible since it is going to be used for music. This whole thing is going to be used for the audio system in the MAME/Jukebox cabinet I'm building. I have some nice 6x9 speakers and an old kenwood 80watt amp that I was going to use. I need this circuit to provide power for the amplifier.

Looking closer I think this is the transformer I'll need to step down the 110AC before putting the current into the rectifier and also another one to use for the second winding mentioned for the choke.

If someone else has a better idea for a circuit to run this thing I'm open to suggestions.

 

[Born2bwire]

Originally posted by: ryan256

Originally posted by: Born2bwire
It's a fairly crude power source, I would add a resister and a zener diode as your voltage regulator after the filter stage. Obviously you need to step down the voltage to your rectifier since they are not rated for 220*sqrt(2) V PIV. As for your filter cap, what's your desired ripple voltage specification?

EDIT: In fact, you can't really say anything until we know what specs you need. Output voltage, output current loads, desired ripple voltages, etc.

Sorry about that.
I need this circuit to accept standard 110AC current from a wall socket and step it down to 12V DC. But I also need it to have enough power to put out about 7 amps. If I'm correct in that 80Watts = 12V * Amps, Amps = 6.667.
As far as ripple voltage that I really can't say. I'm not sure just how clean the power needs to be for the car amplifier but I guess it needs to be as clean as possible since it is going to be used for music. This whole thing is going to be used for the audio system in the MAME/Jukebox cabinet I'm building. I have some nice 6x9 speakers and an old kenwood 80watt amp that I was going to use. I need this circuit to provide power for the amplifier.

Looking closer I think this is the transformer I'll need to step down the 110AC before putting the current into the rectifier and also another one to use for the second winding mentioned for the choke.

If someone else has a better idea for a circuit to run this thing I'm open to suggestions.

That transformer is only rated for 3 amps. What you should do is transformer->bridge rectifier -> filter -> voltage regulator. You want a transformer to step down to a voltage higher than 12 Vrms (maybe 15 or 30 Vrms) and then use a regulator to output the desired 12 VDC. Keep in mind though, your recitfier circuit needs to be able to handle TWICE the peak voltage across the transformer's secondary. So for a 30 Vrms transformer, you need a bridge that can handle at least 84V or so Peak Repetitive Reverse Voltage. You could use a voltage regulator component like Browntown linked to (probably most accurate and desired) or you could use a simple resistor in series with a zener diode. The resistor is used to regulate the bias current through the zener while the voltage is taken acros the zener diode. However, the resistor will consume power and you would need a high wattage zener for your purposes since you would dump up to 6.7 amps through it when the load is off. I would imagine that a 100 microfarad capacitor would be a fair start in terms of your ripple voltage. Techincally though, that still depends on the voltage regulating circuit.

 

[PottedMeat]

Are you sure you want to build this? If you still want to build one, I'd go with a transformer->switchmode power supply for efficiency. You can pull up tons of schematics from any of the big semiconductor companies (On Semi, National, Fairchild, etc. ).

I would just get a simple power supply like this one:
http://www.surplusgizmos.com/V...2V-125A_p_115-967.html

It does 120/220AC in, 12V 12A out - just hook it up. There are lots of others available on ebay, etc. for ~30-60 bucks.

 

[BrownTown]

yeah, I was thinking the same thing, if you have an old power supply you can look how many amps it does on the 12V rails and just use that, or buy a power supply, it will be cheaper than buying all the components and putting them together, not to mention MUCH lower ripple and better efficiency as well.

 

[Mark R]

Don't waste time trying to build this, unless you can find a very cheap transformer on ebay. If you can find one a 9V toroidal transformer (120 VA or above) would be my choice, use the rectifier selected and a decent cap. A cheap ass 1 Farad cap off ebay would be total overkill, but would work fine, and would probably work out cheaper than buying more sensible caps. With a cap that big, you won't need any filtering - the cap will just absorb any noise. A suitable choke would be difficult to find, big, and expensive.

Don't even think about a regulator - it's totally impractical, and the amp won't care anyway.

However, if you have a spare PC PSU lying around, I'd just use that. Just hook up the amp to the 12 V rail, and hotwire the PSU to make it power up (connect green to black).

 

[ryan256]

Originally posted by: BrownTown
yeah, I was thinking the same thing, if you have an old power supply you can look how many amps it does on the 12V rails and just use that, or buy a power supply, it will be cheaper than buying all the components and putting them together, not to mention MUCH lower ripple and better efficiency as well.

Actually thats probably a better idea... and I do have an old AT power supply in the back closet I think still works. Problem is I have no idea how many amps the 12v rail is capable of and I don't think its marked.

 

[PottedMeat]

Originally posted by: ryan256

Originally posted by: BrownTown
yeah, I was thinking the same thing, if you have an old power supply you can look how many amps it does on the 12V rails and just use that, or buy a power supply, it will be cheaper than buying all the components and putting them together, not to mention MUCH lower ripple and better efficiency as well.

Actually thats probably a better idea... and I do have an old AT power supply in the back closet I think still works. Problem is I have no idea how many amps the 12v rail is capable of and I don't think its marked.

An AT may be a problem. Do ATs supply most of their current on the +5V line? I had an unlabeled one that would shut down when I tried to power a 12V 5A peltier. You may be better off with a rock bottom ATX supply.

 

[bobsmith1492]

An old computer PSU would be the easiest way if you have one around.

You could grab something like this; I have had one for years and it's great. I ran my 200W car amp off it for a long time as well as a ham radio.

A switching supply like this would be cheaper, smaller, and more efficient though.

 

[RobertL]

Originally posted by: ryan256

Originally posted by: Born2bwire
It's a fairly crude power source, I would add a resister and a zener diode as your voltage regulator after the filter stage. Obviously you need to step down the voltage to your rectifier since they are not rated for 220*sqrt(2) V PIV. As for your filter cap, what's your desired ripple voltage specification?

EDIT: In fact, you can't really say anything until we know what specs you need. Output voltage, output current loads, desired ripple voltages, etc.

Sorry about that.
I need this circuit to accept standard 110AC current from a wall socket and step it down to 12V DC. But I also need it to have enough power to put out about 7 amps. If I'm correct in that 80Watts = 12V * Amps, Amps = 6.667.
As far as ripple voltage that I really can't say. I'm not sure just how clean the power needs to be for the car amplifier but I guess it needs to be as clean as possible since it is going to be used for music. This whole thing is going to be used for the audio system in the MAME/Jukebox cabinet I'm building. I have some nice 6x9 speakers and an old kenwood 80watt amp that I was going to use. I need this circuit to provide power for the amplifier.

Looking closer I think this is the transformer I'll need to step down the 110AC before putting the current into the rectifier and also another one to use for the second winding mentioned for the choke.

If someone else has a better idea for a circuit to run this thing I'm open to suggestions.

Can't you just use a used mini stereo system? It is much cheaper and better sounding than your convoluted plan of using a car stereo driven by a inadequate, expensive power supply.

 

[bobsmith1492]

Originally posted by: RobertL

Originally posted by: ryan256

Originally posted by: Born2bwire
It's a fairly crude power source, I would add a resister and a zener diode as your voltage regulator after the filter stage. Obviously you need to step down the voltage to your rectifier since they are not rated for 220*sqrt(2) V PIV. As for your filter cap, what's your desired ripple voltage specification?

EDIT: In fact, you can't really say anything until we know what specs you need. Output voltage, output current loads, desired ripple voltages, etc.

Sorry about that.
I need this circuit to accept standard 110AC current from a wall socket and step it down to 12V DC. But I also need it to have enough power to put out about 7 amps. If I'm correct in that 80Watts = 12V * Amps, Amps = 6.667.
As far as ripple voltage that I really can't say. I'm not sure just how clean the power needs to be for the car amplifier but I guess it needs to be as clean as possible since it is going to be used for music. This whole thing is going to be used for the audio system in the MAME/Jukebox cabinet I'm building. I have some nice 6x9 speakers and an old kenwood 80watt amp that I was going to use. I need this circuit to provide power for the amplifier.

Looking closer I think this is the transformer I'll need to step down the 110AC before putting the current into the rectifier and also another one to use for the second winding mentioned for the choke.

If someone else has a better idea for a circuit to run this thing I'm open to suggestions.

Can't you just use a used mini stereo system? It is much cheaper and better sounding than your convoluted plan of using a car stereo driven by a inadequate, expensive power supply.

It's not that convoluted; if you have an amp, you might as well make use of it.

A couple of more notes:

1. You'll want more than 7A and likely more than 12V, too. Amplifiers (especially class A-AB amps like most car amps) are nowhere close to 100% efficient; more like 60% max. So, if you want 80W out of it, you need more like (80/0.6)/12 ~= 11A. Also, car systems run around 14V while operating and due to the internal power supply in most car amps, an unregulated switching supply, you get more available power with higher input voltage. Because of this, you likely won't need 11A at 12V; probably 10A would do it. I would recommend a higher-voltage output, though, say 14-15V at the same current level.

2. You probably won't get 80W out of it unless you're driving 4-ohm speakers since most car amps are rated into 4-ohm loads. Hence, depending on you speakers, you may not need that current.

3. For B2B: please say you didn't recommend a resistor/zener regulator for a 100W/7A power supply. That's way too much current; the resistor and zener would be wasting tens of watts. Zeners are usually only practical for circuits under than a watt or two.

 

[Born2bwire]

Originally posted by: bobsmith1492

Originally posted by: RobertL

Originally posted by: ryan256

Originally posted by: Born2bwire
It's a fairly crude power source, I would add a resister and a zener diode as your voltage regulator after the filter stage. Obviously you need to step down the voltage to your rectifier since they are not rated for 220*sqrt(2) V PIV. As for your filter cap, what's your desired ripple voltage specification?

EDIT: In fact, you can't really say anything until we know what specs you need. Output voltage, output current loads, desired ripple voltages, etc.

Sorry about that.
I need this circuit to accept standard 110AC current from a wall socket and step it down to 12V DC. But I also need it to have enough power to put out about 7 amps. If I'm correct in that 80Watts = 12V * Amps, Amps = 6.667.
As far as ripple voltage that I really can't say. I'm not sure just how clean the power needs to be for the car amplifier but I guess it needs to be as clean as possible since it is going to be used for music. This whole thing is going to be used for the audio system in the MAME/Jukebox cabinet I'm building. I have some nice 6x9 speakers and an old kenwood 80watt amp that I was going to use. I need this circuit to provide power for the amplifier.

Looking closer I think this is the transformer I'll need to step down the 110AC before putting the current into the rectifier and also another one to use for the second winding mentioned for the choke.

If someone else has a better idea for a circuit to run this thing I'm open to suggestions.

Can't you just use a used mini stereo system? It is much cheaper and better sounding than your convoluted plan of using a car stereo driven by a inadequate, expensive power supply.

It's not that convoluted; if you have an amp, you might as well make use of it.

A couple of more notes:

1. You'll want more than 7A and likely more than 12V, too. Amplifiers (especially class A-AB amps like most car amps) are nowhere close to 100% efficient; more like 60% max. So, if you want 80W out of it, you need more like (80/0.6)/12 ~= 11A. Also, car systems run around 14V while operating and due to the internal power supply in most car amps, an unregulated switching supply, you get more available power with higher input voltage. Because of this, you likely won't need 11A at 12V; probably 10A would do it. I would recommend a higher-voltage output, though, say 14-15V at the same current level.

2. You probably won't get 80W out of it unless you're driving 4-ohm speakers since most car amps are rated into 4-ohm loads. Hence, depending on you speakers, you may not need that current.

3. For B2B: please say you didn't recommend a resistor/zener regulator for a 100W/7A power supply. That's way too much current; the resistor and zener would be wasting tens of watts. Zeners are usually only practical for circuits under than a watt or two.

That was before he gave the power specifics.

 

[Modelworks]

Originally posted by: Mark R
Don't waste time trying to build this, unless you can find a very cheap transformer on ebay. If you can find one a 9V toroidal transformer (120 VA or above) would be my choice, use the rectifier selected and a decent cap. A cheap ass 1 Farad cap off ebay would be total overkill, but would work fine, and would probably work out cheaper than buying more sensible caps. With a cap that big, you won't need any filtering - the cap will just absorb any noise. A suitable choke would be difficult to find, big, and expensive.

Don't even think about a regulator - it's totally impractical, and the amp won't care anyway.

However, if you have a spare PC PSU lying around, I'd just use that. Just hook up the amp to the 12 V rail, and hotwire the PSU to make it power up (connect green to black).

I agree.
A 9v secondary transformer would be perfect for this.
Regulation really isn't a big deal with something like a car amp.

 

[futuristicmonkey]

Originally posted by: bobsmith1492

3. For B2B: please say you didn't recommend a resistor/zener regulator for a 100W/7A power supply. That's way too much current; the resistor and zener would be wasting tens of watts. Zeners are usually only practical for circuits under than a watt or two.

Not unless you use the resistor/zener to maintain the base voltage of a large, bipolar junction transistor, anyway.

-ben

 

[bobsmith1492]

Originally posted by: futuristicmonkey

Originally posted by: bobsmith1492

3. For B2B: please say you didn't recommend a resistor/zener regulator for a 100W/7A power supply. That's way too much current; the resistor and zener would be wasting tens of watts. Zeners are usually only practical for circuits under than a watt or two.

Not unless you use the resistor/zener to maintain the base voltage of a large, bipolar junction transistor, anyway.

-ben

Touche. Still, linear power supplies are going the way of the dodo. Switching supplies are cheaper AND more efficient AND can provide cleaner power.