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will this work? Answered




Best Answer 4 years ago

Yes .... you have designed a +-24VDC supply suitable for an analog power audio amplifier.

Do not neglect the frame ground.

Yes, power supply for 120W audio amplifier... But this will create about 192W. Will it be safe? I am using LM4780.
And why frame ground necessary?

IF the 24V power supplies are transformer isolated
you can tie the reference ground to the AC frames ie enclosures
This is done to prevent audio noise intrusion into your desired sound
at a single electrical-mechanical point in the system

4×48 = 192 available Watts does not force the LM4780 to use that power.

Assuming the module is properly heat sinked, ie cooled, for the inevitable
waste heat energy.  The power output is determined by the audio, speakers
and the volume setting.

It looks OK to me, with a few caveats.

I think for this trick to work, each power adapter brick must have its AC side completely isolated from its DC side, and that is typically true for these kind of adapters.

If this adapter is in a metal case, then that metal case is often connected to the "ground" side of the DC output.  Then you have to keep the metal cases from touching each other, so to prevent a short.

Also if these power bricks have a "ground" wire on the AC side (i.e the power cord has three wires, and the plug has three prongs) , it might be the case that the DC ground from each power supply is wired to that ground wire in the AC cord.  Again you'll want to check for that to make sure you don't get a short. 

That is to say, if the "ground" of both those DC supplies are somehow connected to each other, then this is going to place a short across one of them, specifically the bottom one, as I see it in the diagram you drew there.

What if i just cut AC grounds of both adapters and just connect the live and neutral? what will happen then?

Yeah. Cutting off both ground wires on the AC side should prevent those wires from being connected to each other.

Line and neutral from both adapters get connected to each other, and connected to the mains power when you power the thing up.

Regarding this possible short that I am worried about, if the short is present you should be able to detect it using a ohmmeter to measure the resistance between the your DC output terminals, the ones you've labeled +24V, GND, -24V, doing this while the power supply is turned off.

You can see this from the diagram.  If gnd1 and gnd2 were connected somehow, then there would be short at your new outputs between GND and -24V, since GND=gnd1 and -24V = gnd2.

BTW, when  probing into the DC terminals of your power bricks, while these are turned off, assume there might be an electrolytic capacitor there,
which looks like infinity ohms (i.e open) when charged in the right direction, and maybe less than infinity that when being charged in its wrong direction. 

Also note, charging an electrolytic capacitor in its wrong direction is usually a bad idea, but the sensing current the ohmmeter uses is very tiny, and it probably won't hurt the electrolytic cap, and any damage done will be instantly reversed when the power supply is turned on, and it charges up that cap in the right direction.

Another circumspect way to do this is to just plug in your power bricks, at first without connecting +24V of the bottom one to gnd1 of the top one.  Then you briefly connect  +24V of the bottom one and gnd1 of the top one.  If there are big sparks, or lots of heat coming from one of the bricks, then that means there's probably a short.  If everything stays cool, then you know, everything's cool.  So to speak.

Thank you very much for the help... Now i got it...

What you have there is 2 power sources wired together in series giving you 48V.

Yes, but if i connect the my multimeter ground to the "reference ground" and connect the red terminal of milultimeter to black wire of adapter 2, will it show -(minus)24v?
I want to clear this before connecting it to the mains...

Even though the meter reads a negative voltage doesn't mean you're getting a negative voltage. It's just seeing the current flow moving in the opposite direction than what it expects and reports it as a negative voltage. You would get the same result if you connect the wrong leads of the meter to the wrong leads of one of those power sources.

If you need a negative voltage then check this out. You need a voltage regulator designed to output a negative voltage. 

Actually i want to make supply for 120W audio amplifier, will this work?