# Help with bridge rectifier

#### INJAC

Feb 1, 2017
20
Hello, im using a transformer from an old Hi Fi and i want to build a new one, so this part is related to the power supply. Im a bit confused about the transformer since i cant find its datasheet there is nothing written on it, but i found the service manula from the hi fi and found how it was connected, if it helps. So my idea is to make +21 and -21 V, which i managed, i also want to use the 6 V and 8 V from the transformer, in order to get steady 9V DC from the 6V (with capacitors, voltage regulators etc) and get 12 V nad - 12 V (i dont know if its possible). So here is my circuit how i started it. Im confused how to make the bridge rectifiers after i did the 21V and - 21V, since the ground wont be the same? Thank you in advance

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#### davenn

Moderator
Sep 5, 2009
14,304
So here is my circuit how i started it. Im confused how to make the bridge rectifiers after i did the 21V and - 21V, since the ground wont be the same?

?? don't understand your problem ? using your circuit ( which is good ) on the chosen windings, did you get your ± 21V or something close to it ?

#### INJAC

Feb 1, 2017
20
yes i got 21V, but how from the other pins of transformator do i get the other voltages 9V and 12V, - 12V, how do i design the circuit, how to connect the rectifiers from other part?

#### INJAC

Feb 1, 2017
20
Basicly what i wanted to ask, i managed to get 21 and -21V but can someone help me design it so i can get beside the 21V, -21V also 5 V and 12V, -12V? Cause im not sure what point to use as GND in other rectifier parts (gretz)

#### kellys_eye

Jun 25, 2010
6,514
What is the current requirement for the +/- 12V lines?

If it is very small (and I suspect it's for a basic pre-amp circuit?) then you could simply fit 7812 and 7912 regulators across the +21 and -21V lines. Providing the current consumption is of the order milliamps you may not need heatsinks for the 78(9)12 devices.

#### duke37

Jan 9, 2011
5,364
I think that you have to look at the ground connections. If the current consumption of the +21V is higher than -21V then the votages will not stay similar.

Give details of what windings are connected to ground. Are all windings floating?

#### kellys_eye

Jun 25, 2010
6,514
I think that you have to look at the ground connections. If the current consumption of the +21V is higher than -21V then the votages will not stay similar.

Give details of what windings are connected to ground. Are all windings floating?
If the OP wants to make another hi-fi then the supply rails should remain 'balanced' for power consumption if the output stages are 'classic' (class B) as is required by +/- supply rails.

If they are unbalanced then the circuit is clearly incorrectly biased and/or faulty!

#### INJAC

Feb 1, 2017
20
Ye i need the 12 and - 12 for a preamp. I will have total less then 40w speakers (each channel 20w). The full project i was thinking making a preamp with TL072, and making the amp with TDA2050. I also need the 9V DC, in order to reduce it to 5V to power up a Bluetooth module, and some leds all less then 200mA consumption. I am currently designing a preamp and amp (basicly looking at other projects, and datasheets trying to make it work), so i can post them here and you can tell me how design the circuit for the power supply. If someone can maybe make a scheme just to see how it would work? Thank you

#### kellys_eye

Jun 25, 2010
6,514
The 5V supply can be taken from the 6VAC winding to deliver approximately 9V DC (bridge, smoothing and the 7805 regulator on a small heatsink).

Get the +/-12v from the 21V lines as I previously mentioned.

#### INJAC

Feb 1, 2017
20
Ok, i was thinking of doing that but i am confused about the GND in this image as a circled, so how do i make the gretz (full bridge rectifier). Since its the same point 1,2,3, that part im confused, is this correct how i connected it?

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• ground.jpg
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#### INJAC

Feb 1, 2017
20
And here is the rest of the circuit preamp and amp, i got the schematics some from other projects some from datasheets etc, and some personal modification, any advice what i should change, modify, this is my first time doing this and im not sure if this will work corectly. Thank you

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• AMP.jpg
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#### kellys_eye

Jun 25, 2010
6,514

Measure the AC volts between 10-11 and betwween 10-12 on the transformer and report back.
Measure the AC volts between 8-9 on the transformer and report back.

As it stands, the GND at the junction of C1 and C2 should go to transformer pin 10. You will then get your +/-21V (with the GND being 0V)

The GND of the LOWER bridge rectifier circuit isn't 'GND' - it should connect to pin 8 of the transformer (where you've indicated 8V). Connect a smoothing capacitor across the +/- of the bridge rectifier and then feed the resultant DC into a 5V regulator to get the 5V you want.

#### INJAC

Feb 1, 2017
20
AC voltage between 10-11 is 14.7V and between 10-12 is 14,7V. The voltage between 11-12 is 29.3V, so thats why i used pin 10 so it will be the GND for the whole system. Measured the voltage between 8-9 its 3.5V, and the voltage between 10-8 is 8V AC, betwen 10-9 is 6V AC. And ye i did that junction between C1 and C2 connect to pin 10, i just labeled it GND so is the pin 10. That part about -/+21V im not worried its correct. The thing im not sure is where to connect the other part of the rectifier at lower point. If you mean that i connect it like this?

Where i circled red 1 and 2 im sure i will get 0V, but the part im not sure is if i will get 0V at circle 3 red. Since it wont be the mass of all the circuit and i cannot connect the point 1,2 with point 3, cause there will be a voltage diffrence?

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#### kellys_eye

Jun 25, 2010
6,514
Point 1 will be the common for the +/-21V and needs to be connected to point 2 for the circuit to work - whether you also join it to point 3 depends on whether the circuits have a common return line (usually do).

Other than that, the circuit as you have drawn it is a workable version.

#### INJAC

Feb 1, 2017
20
That was my idea to join all to be common points,so i dont have to worry later on, by connecting something to the same gnd, but they were different voltages. In this case the second rectifier will work, i mean since voltage difference between 8,9 is around 3.5v ac? In your opinion is this a good power supply for a 20W each channel audio amplifier. And those 5 volts i need for some leds, Bluetooth and other low current ICs. Would you add something or modify or you think its good to go?

#### kellys_eye

Jun 25, 2010
6,514
If 8/9 is really 3.5V AC (it's marked as 'fil' usually meaning the filament of a thermionic valve) then it's too low to drive the 7805.

Measure the other secondaries (15/16) and look for at least 6V AC, no more than 12V AC and you'll be ok for the 5V supply.

As for the supply for the 20W amplifier - I can only assume the secondary CURRENT capabilities will be around 1A (or more) to provide sufficient total wattage - but given that the smoothing capacitors are 3300μF then this is likely to be the case.

#### INJAC

Feb 1, 2017
20
Im not sure if im dumb or the transformer has the mind of its own, but the past hour i was looking at the values from the pins every single combination.

Here is the resault i got:

Pins that are on the same voltage point give 0V:
13,14,15,16 (one group)
10,17 (second group).

The voltage that is always the same is:
Pin 10-11 (gives 14,7V) and pin 10-12 (gives 14,7V)

Now whats diffrent everytime i put 1 black probe (if its called from the multimeter) and the other red probe i leave disconnected, when i put it on any pin it shows between 2-3V exept for pin 10 and 17, on them it shows 0V, im not sure maybe because its a cheap multimeter, or interference.

Now the other resaults that i got:
Pin 8/11 gives 6.5V (sometimes 7V it changes sometimes)
Pin 8/12 gives 11.5V (sometimes 12V it changes sometimes)
Pin 9/11 gives 9V (sometimes 8.5V it changes sometimes)
Pin 9/12 gives 9V (sometimes 9.5V it chanes sometimes)

Im not sure anymore, dont have that much knowledge about transformers, i just know theoreticly whats their function and basic transformers that go from 220/110 V to a certain voltage with like 2 pins into 6 pins with diffrent output voltages that are constant. This one is new to me.

Im sorry if i disturbe you to much, thank you for your patience and time

#### kellys_eye

Jun 25, 2010
6,514

Is the transformer disconnected from all other sources other than the AC mains input?

The readings you are getting do not correspond to the diagram shown in your original post. With power disconnected, use your meter set to resistance and check the readings across the terminals according to your own diagram. There should be resistance readings when the diagram shows 'coils' between terminals.

The measurements seem to be consistent when you check 10-11-12 as you're getting the correct AC output but the others are dodgy and not (yet) to be trusted.

If the diagram is correct then you should get steady readings on the following terminals:

15-16
8-9
13-14

Measure then and report back.

#### INJAC

Feb 1, 2017
20
Yes its disconected from any other sources except the main AC. Yes i noticed that something isnt quite the same from the original post, these are now the exact values i got.

When we look at the resistance:
8-9 gives 1.6 Ohm
15-16 gives infinite (gives 1 on multimeter)
13-14 gives infinite (gives 1 on multimeter)

Here are other resistance values:
10-11 gives 1.2 Ohm
10-12 gives 1.2 Ohm
11-12 gives 1.6 Ohm
And all other combinations give infinite (gives 1 on multimeter)

#### kellys_eye

Jun 25, 2010
6,514
Ignoring 10-11-12, do you get any other resistance readings and where are they?

The pinout certainly doesn't match the schematic - or the transformer is defective.......

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