Hi Hoping someone can help me. I'm trying to power a pair of amplifier boards for a jukebox that need 20-35vdc vcc to ground. The the original transformer is 22vac 0v 22vac centre tapped transformer. I have tried using just one 22vac and ground onto the rectifier, with a 220uf smoothing capacitor across the dc out of the rectifier. The negative of the DC needs to be grounded to stop hum on the amp output, but when grounded blows the AC 2 amp fuse. Is there anyway I can get a grounded DC supply from this setup. All help appreciated.
Dave

 

Any prior earth grounding point can cause such a problem conflict. The AC secondary of the transformer for e.g..
Check for all points that may be earthed.

 

If you are grounding onto the rectifier (as you imply) with one of the rectifier ac inputs grounded – then connecting the (-) 0V output of the rectifier to ground will result in an effective short circuit (across the bridge rectifier).

You need to ensure that neither of the rectifier ac inputs is referenced/connected to ground.

 

Any prior earth grounding point can cause such a problem conflict. The AC secondary of the transformer for e.g..
Check for all points that may be earthed.

Many thanks for the advice. Much appreciated.

 

If you are grounding onto the rectifier (as you imply) with one of the rectifier ac inputs grounded – then connecting the (-) 0V output of the rectifier to ground will result in an effective short circuit (across the bridge rectifier).

You need to ensure that neither of the rectifier ac inputs is referenced/connected to ground.

Hi Hymie
Many thanks for the advice. I will rewire and try.

 

but when grounded blows the AC 2 amp fuse. Is there anyway I can get a grounded DC supply from this setup.
Dave

BTW, is that chassis GND or Earth GND?

 

BTW, is that chassis GND or Earth GND?

Hi
It is earth ground

 

Please post a wiring diagram of how things are now, how you want them to be, etc. Any little bit of guidance helps, and one schematic really does equal at least 1000 words in posts.

ak

 

Hi Thanks for the reply. I have attached a basic drawing of how it is wired but the DC needs to be at ground potential.

 

I have attached a basic drawing of how it is wired

You need to remove the ground connection at the transformer output

 

You need to remove the ground connection at the transformer output
View attachment 301968

Thanks Dave for your reply. Trouble is that circuit is just part of a large multi tap transformer and I'm not sure if it will affect any other circuits.

 

But you only need to disconnect the secondary winding of that transformer from ground.

As I said, you have one of the ac inputs to the bridge rectifier grounded and the (-) 0V, which is causing a short circuit.

 

Thanks Dave for your reply. Trouble is that circuit is just part of a large multi tap transformer and I'm not sure if it will affect any other circuits.

I think this would work. Hopefully someone else here will tell me if I'm wrong. The size of the capacitor will affect the ripple. I don't know if the amplifiers you want to power need a ripple free DC supply. If they do then you would need a regulated supply. I also don't know if the transformer has the spare power capacity to drive your amplifiers.



Edit: If you don't have diodes and want to use the bridge rectifier, you could use it like this:

 

I think this would work. Hopefully someone else here will tell me if I'm wrong. The size of the capacitor will affect the ripple. I don't know if the amplifiers you want to power need a ripple free DC supply. If they do then you would need a regulated supply. I also don't know if the transformer has the spare power capacity to drive your amplifiers.

View attachment 301983

Edit: If you don't have diodes and want to use the bridge rectifier, you could use it like this:
View attachment 301985

Hi Dave
Thanks for the drawings. So if I get this right. I reinstate the second 22vac do not use the - out of the rectifier and put the - of the capacitor to ground, and that will give me DC output to ground?
Dave

 

So if I get this right. I reinstate the second 22vac do not use the - out of the rectifier and put the - of the capacitor to ground, and that will give me DC output to ground?

What you said is what I intended. To be completely clear, the -ve end of the capacitor should go to the same 'ground' as the centre tap of the transformer. You should get about 31V DC out in theory. I would go for a capacitor rated at 50V or higher. It would be good to check the output voltage with a multimeter before you connect your amplifiers to it.

 

But you only need to disconnect the secondary winding of that transformer from ground.

As I said, you have one of the ac inputs to the bridge rectifier grounded and the (-) 0V, which is causing a short circuit.

Hi Hymie,
Thanks for the reply.
If I disconnect the ground from the transformer will it affect the other circuits? As these circuits power other essential items.
Dave

 

What you said is what I intended. To be completely clear, the -ve end of the capacitor should go to the same 'ground' as the centre tap of the transformer. You should get about 31V DC out in theory. I would go for a capacitor rated at 50V or higher. It would be good to check the output voltage with a multimeter before you connect your amplifiers to it.

Hi Dave,
At present I do have 31vdc across the rectifier output without it earthed so your solution looks exactly what I need. I have attached a pic of the original capacitors uses. I will try this as it is ok and keep you updated.
Thanks again.
Dave

 

There should be no issue with disconnecting a secondary winding from ground – you should be able to work out from simple circuit analysis why you cannot have one of the bridge rectifier ac inputs commoned with one of the bridge rectifier outputs – it is effectively shorting one the bridge rectifier diodes.

 

Please post a wiring diagram of how things are now, how you want them to be, etc. Any little bit of guidance helps, and one schematic really does equal at least 1000 words in posts.

ak

Hello AnalogKid, this is Rick DuRapau from our online conversations over at StackExchange. I cannot figure out any other way to talk to you other than this ham-handed reply to a post here. Forgive me if I'm majorly out of line here. You were so kind to me over on SE, as well as so knowledgeable. I am wondering if I might be able to establish a direct line of communication with you about that product I am working on. Mostly simple little, quick questions like one I just recently had about what happens when one side of a circuit is a thin wire, and the other side is a really thick wire? Or what happens if the device accidently hits a grounded object? Or would it be better to use a heavier gauge wire or is this wire just fine? Mostly just too basic for a forum like this. If you are interested in something like this, I'm<email link deleted, avoid spam>Thanks, and many apologies for butting in here.

 

There should be no issue with disconnecting a secondary winding from ground

I think the TS wants this part of the circuit to continue working as it would with the centre of the secondary winding grounded. I presume the output voltage at the red arrow is intended to be referenced to the centre tap ground. With the ground removed I'm not sure it would continue to work.