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Old 31st May 2010, 08:29 PM   #11
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It took me awhile to figure out that in the second drawing the bridge to circuit "A" is not a bridge, it's just two diodes.
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Old 1st June 2010, 02:33 AM   #12
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Quote:
Originally Posted by Speedskater View Post
It took me awhile to figure out that in the second drawing the bridge to circuit "A" is not a bridge, it's just two diodes.
yea. actually i should avoid the two diodes.

thanks.
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Old 5th June 2010, 02:55 PM   #13
Kuro is offline Kuro  Hong Kong
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Hi Motbuddy,

I'm trying to build the exact same circuit as yours. I want to have 3 voltages: +5V, +12V and -12V. I'm doing the first design like yours. I want to use 2 transformers as I can use a lower voltage one for the +5V supply so that I don't have to dissipate so much heat with the regulator on this rail. I need 2.6A on the +5V rail.

What I concerned is the ground. There are two separate grounds on the two circuits, one using the center tap from the +/-12V circuit, the other one is not using center tap as ground. Would the two circuits work together by connecting the two different grounds together?
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Old 5th June 2010, 03:36 PM   #14
AndrewT is offline AndrewT  Scotland
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Hi,
you use the word circuits.
A circuit is a circular type loop. What goes out in the flow must come back in the return. If it can't it's not a circuit.
Can the two circuits operate correctly if they are completely independent. Or asked another way, do the circuits need to be connected together for the two circuits to operate?
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Old 5th June 2010, 05:55 PM   #15
Kuro is offline Kuro  Hong Kong
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Quote:
Originally Posted by AndrewT View Post
Hi,
you use the word circuits.
A circuit is a circular type loop. What goes out in the flow must come back in the return. If it can't it's not a circuit.
Can the two circuits operate correctly if they are completely independent. Or asked another way, do the circuits need to be connected together for the two circuits to operate?
Of course I know the 2 circuits will work independently.

I mean I want to use both circuits if (ground) connected together. The device I'm powering requires all 3 voltages (+5, +12, -12) simultaneously.

If not, can I use a center tapped transformer in the top circuit, and use a dual half wave rectification (2 diodes), so both circuit grounds are connected to the CT in each separate transformer?
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Old 5th June 2010, 07:25 PM   #16
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by Kuro View Post
I want to use 2 transformers ................. Would the two circuits work together by connecting the two different grounds together?
will the two circuits work independently?
Quote:
I know the 2 circuits will work independently.
then why do you need to connect them together?

The two transformers will create two isolated supplies that can power two isolated circuits. They don't need to be connected together.
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Old 6th June 2010, 02:12 AM   #17
Kuro is offline Kuro  Hong Kong
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I guess I've not been too clear. The word "circuit" refer to the two rectifying circuits in this topic. The device I'm powering is actually just one physical circuit, which requires +5, +12 and -12V.

My device was powered by a SMPS and I want to change it to Linear PS. But I want to minimize power dissipation on the 5V rail so I use a transformer with 7VAC secondary. Now I've two linear power supplies, one providing 5V and the other providing +/-12V DC. Can I physically connect both supplies to my device as I noticed that the +/-12V one has ground in the CT of the transformer, but the +5V one has ground on one end of the transformer's secondary. The two "grounds" are not in phase during an AC cycle.

What I worry is that these two grounds may have different potential...

BTW, I have the parts, but the supplies have not been built yet.
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Old 6th June 2010, 09:33 AM   #18
AndrewT is offline AndrewT  Scotland
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the two transformers and their PSU each give an isolated output flow and return.
The 12V version could be 0,12,24Vdc or 0,-12,-24Vdc or 12,0,-12Vdc or any other combination that covers a range of 24V,i.e. 100,112,124Vdc.
The 5V version could be 0,5Vdc, or 0,-5Vdc, or 12,17Vdc, etc

If you link any ONE output from the 12 to any ONE output from the 5 then you will have one common voltage and the three other voltages.

I cannot tell you if the 4 power input terminals are designed to work well together with a common 0Vdc.
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Old 6th June 2010, 11:59 AM   #19
Kuro is offline Kuro  Hong Kong
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Quote:
Originally Posted by AndrewT View Post
the two transformers and their PSU each give an isolated output flow and return.
The 12V version could be 0,12,24Vdc or 0,-12,-24Vdc or 12,0,-12Vdc or any other combination that covers a range of 24V,i.e. 100,112,124Vdc.
The 5V version could be 0,5Vdc, or 0,-5Vdc, or 12,17Vdc, etc

If you link any ONE output from the 12 to any ONE output from the 5 then you will have one common voltage and the three other voltages.

I cannot tell you if the 4 power input terminals are designed to work well together with a common 0Vdc.
That essentially was my question. So I guess no real answer...
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Old 7th June 2010, 01:40 AM   #20
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hi kuro,
if u are asking about the first circuit that i built which is using two transformers. yes,it works.i connect both circuits' GND together.if u dont connect the GND together it wont work and it will work just like two independent circuit.

Hope this will help.
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