Two regulators from one transformer

Andrew Borg said:

You mean If I don't put a power resistor the readings would be false? What resistance and power should I put? Thanks

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No, the power resistor would only be there, if necessary, to drop some of the volts from the 12V reservoir capacitor so that the 7812 runs a bit cooler. With a good design the 7812 will always output a stabilized 12V. The 7812 is designed to produce a stabilized 12V output, but only when its input voltage is in the range, 15V to 30V.

The dissipation in the 7812 is, (input voltage - output voltage) multiplied by the current that the 7812 is supplying. For example if the input voltage was 18V and the 7812 was passing 0.9A , the 7812 would be dissipating, (18V-12V) * 0.9A = 6.6W (to dissipate this power the 7812 would need to be mounted on a heatsink or it would just reduce its output current to a low value and as a result the output voltage would collapse to a low voltage).

In practical terms, the maximum current that a 7812 can supply is around 1 Amp but this varies from manufacturer to manufacturer. https://www.farnell.com/datasheets/1805459.pdf?_ga=1.4172984.942089101.1451155200

One of the problems is that we do not know what current your 7812 is supplying.

If you could connect 12V to your amplifier and measure the current consumed that would help.

spec

PS: I forgot to mention that in theory, to protect the regulator chips, you should connect three 1N400x (where x is any number between 1 and 7) diodes as follows:
(1) Cathode (+) to 7812 input and anode to 7812 output.
(2) Cathode (+) to 7809 input and anode to 7809 output.
(3) Anode to 7909 input and cathode (+) to 7909 output.

I suspect a serious wiring problem for the big electrolytic capacitor "to got very hot and got swallen releasing a very nasty smell":
1) Its polarity is connected backwards.
2) A rectifier diode is connected backwards or is shorted.

Despite the advice of other, I still suggest the use of one of those DC to DC modules, they don't generate as much heat than the LM7812 and it is far more efficient, especially considering that your transformer may be close to maxed out. Using a DC to DC module, if you need 1.5 amps at 12 v, the transformer only has to supply 1 amp at 18 v, a 6 watt savings, and the ripple will be above the audio freq and probably filtered out by the amplifier, alternatively you could feed the LM7812 with about 15v and let it filter the ripple, as long as you don't draw much more than on amp, the modules say they can handle 3 amps, 8 amps for around $5
I use them, I like them, I havent had any problems with them. I have made both boost and buck devices using switch mode chips.
Jeff

I am still perplexed about this circuit cause at one time when I connected ct to grd 12v came out but after a while 10.10v and nothing from the +9v

Hi Andrew,
I am not clear about which schematic you are referring to. I think at this stage you should post the FULL schematic of your power supply. Not just liknks to application notes. This is so it is clear how the regulators and rectification may interact.

Les.

spec said:

Hi Andrew,

Here is a circuit to overcome the low voltage of the transformer.

The two lower diodes in the bridge rectifier are not used and can be removed if you have a discrete bridge.

What bridge rectifier are you using?

Make sure that the reservoir capacitors are at least 25V working voltage.

You can make C5 1000uF to try the circuit, and it could be that a 1000uF reservoir capacitor will do when your amp is connected to the 12V supply (you still haven't told us what the amplifier is, or is it a secret.)

spec

View attachment 100693
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Hi Les This schematic.

You also need to specify what the load is. The LDOs may bey getting hot and shutting down, if they are dropping too much voltage, try this example from another thread,
V\https://www.electro-tech-online.com/threads/dc-to-dc-conversion.148771/#post-1271757

kinarfi said:

You also need to specify what the load is. The LDOs may bey getting hot and shutting down, if they are dropping too much voltage, try this example from another thread,
V\https://www.electro-tech-online.com/threads/dc-to-dc-conversion.148771/#post-1271757

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Why should I to specify load? On DMM no voltage is coming to +9v and on 12v sometimes works and sometimes goes down to 10v.

Andrew Borg said:

Why should I to specify load? On DMM no voltage is coming to +9v and on 12v sometimes works and sometimes goes down to 10v.

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Sure there is a limit to the power you trafo can deliver.

atferrari said:

Sure there is a limit to the power you trafo can deliver.

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Ok maybe I use two trafos then.