Why does a transformer have three secondary connections?
Oh my goodness, there was a flash in the back of the power supply. The 1A fast charger was burned out. I put in a new one. The transformer is getting primary power, but nothing is coming out the back. Because I love this power supply (I've had it forever), I want to install a new transformer. The broken one was 13.8 V, but that doesn't matter, 12 will do. It's for my battery charger.
There's nothing written on the transformer, and there's no circuit diagram either, but maybe the mystery can be solved that way. Does anyone know?
(No Ratings Yet)
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A transformer can have several connections not only on the secondary side, but also on the primary side.
If it has several connections on the primary side, the transformer can be operated at different network voltages.
If it has several connections on the secondary side, it can output several voltages or two equal voltages with medium tap.
It is also possible to make 1x 12V from 2 x 12V with mid-reef.
The transformer always supplies only alternating voltage.
Thank you, yes, but why do you do that with 2 x 6 = 12 V? What did the builder think about it?
Before:
The power supply becomes secondary max. 3A, you can see that at the ELKO 3300uF (Faust rule for power supply ELKOs is 100uF / 100mA).
Can you adjust the voltage with a power of externally variable (0-12V or 3-12V)? Or is it a power supply with fixed DC voltage 12V or 12.8V?
From my point of view there are 3 reasons A, B or C why the transformer has secondary 3 lines:
A: Center-two-way rectifier circuit:
It can be that it is an old center-two-way rectifier circuit. This can be seen from the number of rectifier diodes. See how many power diodes (probably 1N540x) of the ELKO 3300uF are loaded on the circuit board. If there are only 2 power diodes, it is very likely to be a center-two-way rectifier circuit. If there are 4 power diodes, then no. Here, compare the circuit to the ELKO: https://www.elektronik-kompendium.de/sites/slt/0201071.htm
B: Power supply supplies 2 voltages (2 times 12V) a +12V and a -12V, i.e. 3 outputs/jacks (+/0/-)
C: Stabilization electronics require a small negative auxiliary voltage
With this negative auxiliary voltage, a regulated power supply can also stabilize extremely small voltage (0….3V…. 12V). Look at the IC whether it is an LM317 (L200). This negative auxiliary voltage is explained here:
https://www.elektronik-kompendium.de/public/schaerer/lm317.htm
Solution:
If it is a midpoint two-way rectifier circuit, you can also use a 12V~ transformer without center, but then you need a bridge rectifier (proposal at least 5A 40V) instead of the 2 power diodes!
Bridge rectifier: https://www.reichlt.de/brueckengleichrichter-100-v-8-a-kbu8b-p9229.html
—- — —
Here is a very similar conventional power supply circuit if an IC LM723 + a power transistor 2N3055 is installed: http://www.schulheft-ottakring.eu/vane/Schulteile/Bauteile_2/Spannungscontrol/LM723/Die%20Electronics%20Hobby%20%20%20Bastelecke.htm
Good luck!
So that’s impressive as you know from a distance what’s in there. 2N3055, the IC is UA723CN.
For all the left I need time, tonight and tomorrow. Sometimes thanks for everything..
Yield: fixed voltage 13.8 V.. CB-Master.
Notes from me for your conversion:
Whether LM723 or UA723 are very compatible.
So you can rebuild this power supply with a 12V transformer (without center tap) as a replacement. 3A-5A then means 40W to 60W at 12V. Depending on the claim, build a similar transformer. Maximum approx. 60W, otherwise the next short circuit will blur the power supply.
5A is already limit-valued for only one 2N3055, it will only be able to perform 3A continuous operation. Because of heat problem, because all over 12V must be “braided” in heat from the 2N3055, which is normal in a power supply with “longitudinal control”.
Instead of the 2 power diodes (probably 540x ?), you need a bridge rectifier for conversion. If you’re with 5A, I’d use an at least 8-10 A rectifier, then the rectifier will survive an accidental short circuit.
for example B70C25A bridge rectifier, 100 V, 25 A
https://www.reichlt.de/brueckengleichrichter-c8527.html?&nbc=1
Here is a schematic that is very similar to your power supply. The only important thing is the correct conversion: 2 diodes out, bridge rectifier in, and properly connect:
“~” to the 12V transformer. “+ and -” from the rectifier go to the ELKO 3300uF!
See schematic in paragraph “What is an LM723 power supply board?” here:
https://www.venture-mfg.com/en/lm723-wireboard
PS:
My nickname is transistor3055 without “2N” this is no coincidence.
https://de.wikipedia.org/wiki/2N3055
A lot of success, in rebuilding!
Here you can see the circuit of a rectifier with divided secondary winding:
https://www.electronics-tutorials.ws/en/diodes/vollwave rectifier.html
The second circuit diagram.
Interesting, thanks for the link.
There are transformers with middle tap, so that, for example, 2 x 6 V can be removed instead of 12 V.
Hello,
the transformer has a middle tap.
So a positive wave for the positive 230V AC and a negative wave for the 230V.
As a replacement, you can take a 12V DC toy folio.
Hansi
However, if this is not the case with the 2-diode rectifier circuit.
If, then 12V AC
Then I have to build a new rectifier board?
Why not, he could also have 10 connections, winding taps with different voltages.
usually there are different tensions you have to choose.messen brings clearness
Don’t go, secondary is dead. I can measure the windings with respect to each other, but this is clear that there are two windings for together …. 13.8 V. Just as on the link of CatsEyes above.