Two regulators from one transformer

Andrew Borg · Aug 4, 2016

JimB said:
You should be able to, unless they are excessively large.
Images etc are attached to a post using the Upload a File button, located at the bottom right hand side of the reply box.

JimB

Yes I tried that but maybe on tablet not compatible.

MikeMl said:
Is it**broken link removed**
It says it needs only 2A at 12V, which means that the 2X26Wrms and 500W??? is total BS!

If y0u are producing 30W of audio power from a 12V supply, allowing for a bit of heating in the amplifier, the 12V power supply will need to deliver at least I=P/E = 36/12 = 3A. No way is your existing transformer going to do that (for very long). A LM7812 is not suitable at this power level, either.

It's not the same but looks similar.

audioguru said:
Ebay spelled the 500 Watts wrong. It should be spelled 500 Whats. The details say R. M. S. 2x26W because it has two channels. Its case looks like a pretty good heatsink so it must waste a lot of power making heat even though it is "digital" (class-D?).
A single-ended fairly efficient amplifier powered from 12V produces 1.9 real Watts into 8 ohms or about 3.6W into 4 ohms. If the amplifier is bridged then the powers are about 3.5 times higher at 6.7W into 8 ohms or 12.6W into 4 ohms.
Then the 26W per channel listed is probably the "peak power" which is simply double the average RMS power and at a horrible 10% of clipping distortion.

It sounds very good when I tried it with batteries to mixer.

spec · Aug 4, 2016

kinarfi said:
I haven't dealt too much with RF or audio, so I can not say just how much noise is created in audio circuit, but it would seem that the frequency of the switch mode step down would be well above the audio frequency. As for RF, I know nothing!!
If any one would like to enlighten me, I'm definitely listening. One example of where switch mode is very common is the 12 to USB.
Jeff

Hy kinarfi,

Don't get me wrong, SMPS are used with both audio circuits and RF circuits. In fact one of the best audio amplifiers uses off-line switch mode power supplies, but the design, filtering and layout are pretty complex. **broken link removed**

By the way SMPS not only generate conducted interference but also radiated interference which couples to equipment by both inductive and capacitive coupling. There is a good book, 'EMC for Product Design' which covers the arena of ElectroMagnetic Compatibility (EMC) without getting too heavy. https://www.amazon.co.uk/EMC-Product-Designers-Tim-Williams/dp/0750681705

It is true that USB power is universally generated by SMPs but the designers of mobile phones, PCs, etc have many resources to design out any problems. Also USB supplies are not up to HiFi standards. Compared to HiFi, the sound on my Android phone is full of distortion and hash and the sound on my laptop is even worse.

Audio, as you know, is generally taken to span the frequency range, 20Hz to 20Khz and RF covers the frequency range, 3KHz (VLF) to say 1TeraHz (microwave). Just in case you mistake 3Khz for an audio signal, remember that radio waves are electromagnetic and audio waves are compression and expansion of air.

You make a very good point that if the switching frequency of a SMPS is higher than the highest audio frequency any interference would not be audible, but the fact that the switching signal can get into the circuitry is not good and can cause problems.

Very often a high frequency signal will get rectified by the input semiconductors of an amplifier and produce an audio signal and a DC component, both of which can cause problems. For this reason audio amps often have a low pass filter on the input to prevent high frequencies from getting into the amplifier. These high frequency signals get picked up by the leads connecting to the amplifier for example. One of my home made audio amplifiers was an excellent broadband radio receiver.

With RF circuits the situation is much more complex and difficult to sort.

spec

PS: another link which may be of interest: https://electronics.stackexchange.c...it-be-powered-by-a-switched-mode-power-supply

audioguru · Aug 4, 2016

Andrew Borg said:
It sounds very good when I tried it with batteries to mixer.

True amplifier power determines the maximum loudness, not the sound quality. I modified my clock radio so its output is 1W. It sounds great on the vented speaker I made with a 6.5" woofer and a dome tweeter.

Andrew Borg · Aug 4, 2016

audioguru said:
True amplifier power determines the maximum loudness, not the sound quality. I modified my clock radio so its output is 1W. It sounds great on the vented speaker I made with a 6.5" woofer and a dome tweeter.

Hi Audioguru you're right but quality plays a good role too. I have an old amplifier Nad to my stereo and was expensive because it has very low distortion hum.

kinarfi · Aug 4, 2016

spec said:
Hy kinarfi,

Don't get me wrong, SMPS are used with both audio circuits and RF circuits. In fact one of the best audio amplifiers uses off-line switch mode power supplies, but the design, filtering and layout are pretty complex. **broken link removed**

By the way SMPS not only generate conducted interference but also radiated interference which couples to equipment by both inductive and capacitive coupling. There is a good book, 'EMC for Product Design' which covers the arena of ElectroMagnetic Compatibility (EMC) without getting too heavy. https://www.amazon.co.uk/EMC-Product-Designers-Tim-Williams/dp/0750681705

It is true that USB power is universally generated by SMPs but the designers of mobile phones, PCs, etc have many resources to design out any problems. Also USB supplies are not up to HiFi standards. Compared to HiFi, the sound on my Android phone is full of distortion and hash and the sound on my laptop is even worse.

Audio, as you know, is generally taken to span the frequency range, 20Hz to 20Khz and RF covers the frequency range, 3KHz (VLF) to say 1TeraHz (microwave). Just in case you mistake 3Khz for an audio signal, remember that radio waves are electromagnetic and audio waves are compression and expansion of air.

You make a very good point that if the switching frequency of a SMPS is higher than the highest audio frequency any interference would not be audible, but the fact that the switching signal can get into the circuitry is not good and can cause problems.

Very often a high frequency signal will get rectified by the input semiconductors of an amplifier and produce an audio signal and a DC component, both of which can cause problems. For this reason audio amps often have a low pass filter on the input to prevent high frequencies from getting into the amplifier. These high frequency signals get picked up by the leads connecting to the amplifier for example. One of my home made audio amplifiers was an excellent broadband radio receiver.

With RF circuits the situation is much more complex and difficult to sort.

spec

PS: another link which may be of interest: https://electronics.stackexchange.c...it-be-powered-by-a-switched-mode-power-supply

Thank you
Jeff

spec · Aug 4, 2016

kinarfi said:
Thank you
Jeff

My pleasure.

spec

Andrew Borg · Aug 5, 2016

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

Hi Spec I am building this. Those 330nf if polytene caps ok? Cause I don't have ceramic.
Thanks for your help.

spec · Aug 5, 2016

Andrew Borg said:
Hi Spec I am building this. Those 330nf if polytene caps ok? Cause I don't have ceramic.
Thanks for your help.

Yes, any metal film capacitors should be OK too.

spec

Andrew Borg · Aug 7, 2016

spec said:
Yes, any metal film capacitors should be OK too.

spec

Today I finished the circuit +9v/-9v were ok but on the 12v side only 4.6v is coming.
Is this maybe cause I used a 1000uf cap instead 4700uf?
Any help please?
Thanks

spec · Aug 7, 2016

Andrew Borg said:
Today I finished the circuit +9v/-9v were ok but on the 12v side only 4.6v is coming.
Is this maybe cause I used a 1000uf cap instead 4700uf?
Any help please?
Thanks

Is that with the amplifier connected to the 12V supply?

Does the 12V regulator produce 12V off load?

spec

Andrew Borg · Aug 7, 2016

spec said:
Is that with the amplifier connected to the 12V supply?

Does the 12V regulator produce 12V off load?

spec

There is something in the transformer cause when I tested voltage only 2v was coming from red to blue wire and from 9v the center wire.

spec · Aug 7, 2016

Andrew Borg said:
There is something in the transformer cause when I tested voltage only 2v was coming from red to blue wire and from 9v the center wire.

Time for a different transformer I think. Get a nice toroid.

spec

Andrew Borg · Aug 8, 2016

spec said:
Time for a different transformer I think. Get a nice toroid.

spec

To correct myself from the rectifier 2v are coming out cause when I tried on trafo I read 16v ac . To connect one leg to br should give full voltage too?

spec · Aug 8, 2016

Hy Andrew,

Can you give an exact definition of your transformer with a picture so we can establish exactly what is going on. When you mention colors of leads it does not mean much to us.

spec

Andrew Borg · Aug 8, 2016

spec said:
Hy Andrew,

Can you give an exact definition of your transformer with a picture so we can establish exactly what is going on. When you mention colors of leads it does not mean much to us.

spec

Today I connected the circuit as it should be and at one time it worked ok but after a while the 1000uf cap got very hot and got swallen releasing a very nasty smell.

kinarfi · Aug 8, 2016

did you happen to check the temperature of you lm78xx chips

spec · Aug 8, 2016

Andrew Borg said:
Today I connected the circuit as it should be and at one time it worked ok but after a while the 1000uf cap got very hot and got swallen releasing a very nasty smell.

You need a high ripple current reservoir capacitor. Also change the value of the 12V capacitor to at least 4,700 uF and at least 28V. This would be a good choice: https://www.digikey.co.uk/product-d...onics-cde/SLPX682M035A7P3/338-1645-ND/1882122

spec

Andrew Borg · Aug 8, 2016

kinarfi said:
did you happen to check the temperature of you lm78xx chips

Hi kinarfi I did not notice that 78xx were getting hot.

spec said:
You need a high ripple current reservoir capacitor. Also change the value of the 12V capacitor to at least 4,700 uF and at least 28V. This would be a good choice: https://www.digikey.co.uk/product-d...onics-cde/SLPX682M035A7P3/338-1645-ND/1882122

spec

I changed the cap to a 50v one but when I switched on no voltage was coming to 12v now maybe 7812 burned. I try to change the chip and as you told me put a 4700uf instead of 1000uf.
Thanks

spec · Aug 9, 2016

As well as the working voltage, the ripple current rating of the 12V reservoir capacitor is also very important . Special heavy duty (big and heavy) capacitors are made to handle the high ripple currents found in power supplies. The ripple current rating of the -9V reservoir capacitor is not a problem because the current drain on the -9V supply is very low.

I suspect that there may be a thermal fuse built into your transformer and that the fuse is cutting in and out. Is the transformer getting hot?

The 7812 will not get damaged by over-temperature because it has built-in temperature protection. It will merely limit its output current. I would get the power supply working without a load to start with.

In the final power supply the 12V regulator will need to be mounted on a substantial heatsink using a TO220 mounting kit (but use an aluminum oxide insulating washer rather than mica or plastic) to keep the 7812 cooler. You do not necessarily need a dedicated heatsink: often the metal case of an equipment can be used.

Also, a power resistor may need to be connected between the positive terminal of the 12V reservoir capacitor and the input to the 7812.

spec

Andrew Borg · Aug 9, 2016

spec said:
As well as the working voltage, the ripple current rating of the 12V reservoir capacitor is also very important . Special heavy duty (big and heavy) capacitors are made to handle the high ripple currents found in power supplies. The ripple current rating of the -9V reservoir capacitor is not a problem because the current drain on the -9V supply is very low.

I suspect that there may be a thermal fuse built into your transformer and that the fuse is cutting in and out. Is the transformer getting hot?

The 7812 will not get damaged by over-temperature because it has built-in temperature protection. It will merely limit its output current. I would get the power supply working without a load to start with.

In the final power supply the 12V regulator will need to be mounted on a substantial heatsink using a TO220 mounting kit (but use an aluminum oxide insulating washer rather than mica or plastic) to keep the 7812 cooler. You do not necessarily need a dedicated heatsink: often the metal case of an equipment can be used.

Also, a power resistor may need to be connected between the positive terminal of the 12V reservoir capacitor and the input to the 7812.

spec

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

Two regulators from one transformer

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