centre tapped transformer

Thread Starter

craig harrison

Joined Mar 20, 2022
22
Hello everyone....new member here in need of guidance:
I am trying to replace a dual output transformer, 230 vac primary with two independent outputs 24v and 6v, The original transformer is quite small and I', having difficulty sourcing a replacement.
I am trying to install a toroidal transformer 230vac primary with two independent secondary outputs of 12v. I have connected both secondary windings together to provide one 24v secondary output that feeds a voltage regulator circuit to give me 24vdc, and I am using this connection point (centre tap) for one off 12v output.....this output I am feeding to a voltage regulator to drop down to the required 6vdc. However these two dc outputs share a common negative on the dc side and I'm finding I get 24vac at the input to the 6vdc voltage regulator, the regulator has to work hard and gets hot.....I suspect I cant join the two negatives on the dc side and this is giving me the 24vac.........confused? I am ;¬)
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oh....and this is the original configuration...
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crutschow

Joined Mar 14, 2008
34,842
Yes, using two separate bridges is not possible with a common ground, as it generates a short through some of the rectifiers.

Below is an interesting way to get dual DC voltages from a center-tapped transformer using only one bridge-rectifier.
The rectifiers act as a full-wave bridge for the 24V output (≈ 31Vdc), with the bottom two rectifiers acting as full-wave rectifiers for the center-tap 12V output (≈15Vdc).
This configuration also means both transformers carry the current from both DC outputs equally.

Note that the rectifiers must be able to handle the sum of the two output currents.

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Thread Starter

craig harrison

Joined Mar 20, 2022
22
Thanks for that.....I didnt think it was possible but needed someone to confirm it for me.......I will mark up my diagram above to represent what I think you advise, using one bridge rectifier instead of the two currently wired, the 24vdc supply just as it currently is wired and take the centre tap 12vac directly to the input of my new voltage regulator which will regulate the to the 6vdc required voltage....if you could look it over when I'm done I would appreciate it
 

Thread Starter

craig harrison

Joined Mar 20, 2022
22
Ok.....marked up drg, the bridge rectifier im keeping is rated at 3.7A which im thinking is fine for this record deck (bang and olufsen BG4000)....
supply33.png
 

Thread Starter

craig harrison

Joined Mar 20, 2022
22
and finally the toroidal transformer im using, this has been chosen mostly because of its size, needs to be no larger than 72mm diameter to fit into the footprint of the original
818By9byH9L._SL1500_.jpgI selected the VTX-146-030-112
 

Thread Starter

craig harrison

Joined Mar 20, 2022
22
I did measure them at the time I purchased the transformer, dont remember the readings right now...when i wired up both bridges as per the first diagram I posted everything worked fine with the exception of the new voltage regulator getting hot after a very short time, nothing else was heating up. I will wire up this evening and take measurements when finished.
 

Thread Starter

craig harrison

Joined Mar 20, 2022
22
I measured the current output from the 6vdc regulator and got 160ma........the regulator gets warm to the touch, I have a heatsink fitted right now which is probably helping, still not sure if the sums add up to a healthy design? I will measure the current on the 24vdc side tomorrow......
 

crutschow

Joined Mar 14, 2008
34,842
still not sure if the sums add up to a healthy design? I will measure the current on the 24vdc side tomorrow......
What sums?
The 24Vdc measurement will tell if the total current is acceptable.
The total DC current should be no more than about 50% of the transformers AC 1.25Arms rating.
 

Thread Starter

craig harrison

Joined Mar 20, 2022
22
I do have concerns about the voltage regulators temperature and if a different component would be better suited to the conditions in the 6vdc circuit?
 
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sagor

Joined Mar 10, 2019
927
Your 6V regulator may still get hot as it has to regulate the V difference times current. That is, with a source of +15V and 6V output, you are generating 9V drop x 0.160A = 1.44W of heat.
One trick is to put a small resistor in series with the source 15V, so the resistor generates some of the heat due to voltage drop through it. Resistors can handle heat as long as it is within its ratings.
For example, since you have a fixed 0.160A load (assuming it is constant and does not change much), a 33 ohm 2W (3W would be better) resistor will drop the voltage to the input of the regulator by about 5V(5.28V), making it closer to a 10V input to the regulator. The resistor will dissipate almost 1W (0.844W) of all that heat, leaving just above 1/2W for the regulator. Thus the regulator may not need a heatsink, maybe.
 

Thread Starter

craig harrison

Joined Mar 20, 2022
22
Ok....I have measured the current on the 24vdc circuit and get 440ma under normal operation, I have also connected a 22ohm resistor into the input for the voltage regulator, I dont think its 3w as it has no markings to identify it as such....I have ordered a couple of 33ohm 3w resistors and should get them this week.......P1016748 (Copy).jpg
 

Thread Starter

craig harrison

Joined Mar 20, 2022
22
The 22ohm resistor gets hot....not so as one cant touch it so I'm guessing its below 60 degC, The voltage regulator is markedly cooler to the touch and I'm expecting the 33ohm resistor will improve on that once fitted.....I will of course keep this thread going until I finish the excercise.
 
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