Two regulators from one transformer

Andrew Borg · Aug 1, 2016

spec said:
Hy Andrew,

Does the schematic below depict the essence of your triple power supply circuit? If so it will work fine, subject to a few detailed aspects which we can discuss.

You have not asked this, but my advice would be to just have two supply lines, 12V and -12V. Then you can power the mixer from 12V and -12V and the amplifier from just the 12V supply.

Running the mixer from 12V and -12V will improve its performance. Of course, we would need to check the mixer circuit in detail to ensure that it will take 12V and -12V supply lines, but I would be very surprised if not.

spec

View attachment 100669

Wow Spec you are great so I will try to do this circuit on one trafo. You told me to run mixer on 12v but on your circuit it's ok from +9v/-9v. Regarding 12v grd is 0v yes?
Thanks

spec · Aug 2, 2016

Andrew Borg said:
Wow Spec you are great so I will try to do this circuit on one trafo. You told me to run mixer on 12v but on your circuit it's ok from +9v/-9v. Regarding 12v grd is 0v yes?
Thanks

Hy Andrew,

Thx for the compliment.

Yes, OV is ground (it is not strictly correct to call it ground but I am being pedantic).

spec

{XtremeDry} · Aug 2, 2016

Can I use three output of this PS at the same time or it will cause any anomaly ?
Thanks

spec · Aug 2, 2016

{XtremeDry} said:
Can I use three output of this PS at the same time or it will cause any anomaly ?
Thanks

Hy XD,

No, you can use all of the outputs at the same time without any interaction (to speak of).

Of course, the current drain on each supply line will be limited by:
(1) The current capability of the three terminal regulators.
(2) The power dissipation and heat sinking of the regulators
(3) The voltage and effective impedance of the transformer secondaries.
(4) The current capacity and effective resistance of the bridge rectifier.
(5) The value of the reservoir capacitors, which define the ripple voltage.

So for any application a few simple calculations would need to be worked out, but nothing too difficult.

spec

Andrew Borg · Aug 2, 2016

Hi Spec me again. Regarding Errata note that wrote thos resistors are important?
C2 and C9 1000uf?
Thanks

spec · Aug 2, 2016

Andrew Borg said:
Hi Spec me again. Regarding Errata note that wrote those resistors are important?
C2 and C9 1000uf?
Thanks

Hy Andrew,

The specification sheet for the LM78xx and LM79xx says the minimum load current is 10mA. So provided your load is 10mA, or more, the output voltage will be correct. But if you test the power supply by itself, say for development and fault finding, there is a chance that the output voltage will be incorrect so it is a wise precaution to fit the current drain resistors.

There is also a minor advantage in having a current load physically close to the regulators.

A 1000uF reservoir capacitor will be adequate for the -9V supply but we do not know what the current drain on the positive reservour capacitor will be because I don't think you have defined what the amplifier is that runs off the 12V supply. The value of the reservoir capacitor will also depend on the characteristics of the transformer. If you post this information I will be able to give a definitive answer.

If the amplifier running off the 12V supply is an audio power amplifier I would suggest that a reservoir capacitor of 4,700uF (4m7mF) or larger would be a good choice for the positive output from the bridge rectifier. In case you are concerned that there will be some kind of problem due to an unbalance, with the two different values of reservoir capacitors, don't be; there will be no problem.

spec

kinarfi · Aug 2, 2016

What are the ratings of the transformer your using?

schmitt trigger · Aug 2, 2016

Andrew Borg said:
Hi Schmitt can you post a circuit of yhis charge pump that you mentioned please?
Thanks

Download the datasheet. Check figure 13A....I doesn't get any simpler than this.

**broken link removed**

Andrew Borg · Aug 2, 2016

spec said:
Hy Andrew,

The specification sheet for the LM78xx and LM79xx says the minimum load current is 10mA. So provided your load is 10mA, or more, the output voltage will be correct. But if you test the power supply by itself, say for development and fault finding, there is a chance that the output voltage will be incorrect so it is a wise precaution to fit the current drain resistors.

There is also a minor advantage in having a current load physically close to the regulators.

A 1000uF reservoir capacitor will be adequate for the -9V supply but we do not know what the current drain on the positive reservour capacitor will be because I don't think you have defined what the amplifier is that runs off the 12V supply. The value of the reservoir capacitor will also depend on the characteristics of the transformer. If you post this information I will be able to give a definitive answer.

If the amplifier running of the 12V supply is an audio power amplifier I would suggest that a reservoir capacitor of 4,700uF (4m7mF) would be a good choice for the positive output from the bridge rectifier. In case you are concerned that there will be some kind of problem due to an unbalance, with the two different values of reservoir capacitors, don't be; there will be no problem.

spec

Thanks Spec and the others for help.
Trafo is 240v to 18v ac don't know exact amperage but I think it's 1amp.
This was taken from a video player if I remember rightly.
On it there are these codes:
TPW -12RAD
57PV@6@ @5@
B-220-27H-1

kinarfi · Aug 2, 2016

18 volts AC rectified and filtered (capacitors) yields 25 volts DC, using 7812 and 7809 wastes at least 1/2 of the power from your transformer.
Mini DC Adjustable Power Supply Buck Module Step Down Module are cheap and work well and easy to find on the web

kinarfi · Aug 2, 2016

made a drawing of one possibility for you, quick and easy and adjustable

You just have to wait for the parts to arrive.

MikeMl · Aug 2, 2016

kinarfi said:
..
Mini DC Adjustable Power Supply Buck Module Step Down Module are cheap and work well and easy to find on the web
View attachment 100675

Run, don't walk, to get as far as possible away from these pieces of crap, especially if you are dealing with low-level analog or RF...

Andrew Borg · Aug 2, 2016

MikeMl said:
Run, don't walk, to get as far as possible away from these pieces of crap, especially if you are dealing with low-level analog or RF...

Through the br only 14.5v are passing cause there is another small transformer with it.

spec · Aug 2, 2016

Andrew Borg said:
Thanks Spec and the others for help.
Trafo is 240v to 18v ac don't know exact amperage but I think it's 1amp.
This was taken from a video player if I remember rightly.
On it there are these codes:
TPW -12RAD
57PV@6@ @5@
B-220-27H-1

Hy Andrew,

Does your transformer have just one 18V secondary winding or does it have two.

As you say that the primary of your transformer is 240V, I take it that the mains frequency in your local is 50Hz.

spec

Andrew Borg · Aug 2, 2016

spec said:
Hy Andrew,

Does your transformer have just one 18V secondary winding or does it have two.

As you say that the primary of your transformer is 240V, I take it that the mains frequency in your local is 50Hz.

spec

Hi Spec yes it's 50hz I am in Europe. As I said from the br 14.5v are coming out The transformer is center tapped.
Thanks.

spec · Aug 2, 2016

Andrew Borg said:
Hi Spec yes it's 50hz I am in Europe. As I said from the br 14.5v are coming out The transformer is center tapped.
Thanks.

Hi Andrew.

Can you wire up the transformer bridge and two reservoir capacitors (1000 uF will be OK in both positions) as shown in the circuit of post #20 ?

Then can you connect a 1K, 1W or higher resistor across each reservoir capacitor?

Then can you measure the voltage across each of the reservoir capacitors, both positive and negative?

spec

kinarfi · Aug 2, 2016

MikeMl said:
Run, don't walk, to get as far as possible away from these pieces of crap, especially if you are dealing with low-level analog or RF...

Mike
Mike, Really? I've used a few for USB power, hope I've not made a big mistake, can you elaborate, may in a new thread so a not to hi jack this one or pm me, Please
Jeff

{XtremeDry} · Aug 3, 2016

I tried to simulate a circuit similar to this circuit, when I observe the output on the oscilloscope, I saw the output voltage looks pretty nice with very minimal noise, but when i attach a load like 10k ohms. the output doesn't look well any more. is there any way that i cam improve the circuit, or i can put another small capacitors parallel in with c3 and c4?

spec · Aug 3, 2016

{XtremeDry} said:
I tried to simulate a circuit similar to this circuit, when I observe the output on the oscilloscope, I saw the output voltage looks pretty nice with very minimal noise, but when i attach a load like 10k ohms. the output doesn't look well any more. is there any way that i cam improve the circuit, or i can put another small capacitors parallel in with c3 and c4?

In theory you should connect a 330nF capacitor from the input of each voltage regulator to GND.

The 7812 and 7912 will not regulate for loads less than 10mA so you need to connect a 1.2K resistor (1K resistor will be suitable) from the output of each regulator to GND.

If you simulator accounts for the above characteristics of the 7812 and 7912 that may be why you are getting hash.

In practice the above circuit would probably work fine.

spec

Andrew Borg · Aug 3, 2016

spec said:
Hi Andrew.

Can you wire up the transformer bridge and two reservoir capacitors (1000 uF will be OK in both positions) as shown in the circuit of post #20 ?

Then can you connect a 1K, 1W or higher resistor across each reservoir capacitor?

Then can you measure the voltage across each of the reservoir capacitors, both positive and negative?

spec

Hi Spec I've did this but with 2w resistors cause that I had available, voltage was 11v on both sides.

Two regulators from one transformer

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