Unknown 78l15 Regulator problem. Very Weird ???? - diyAudio
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Old 5th January 2015, 10:49 AM   #1
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Default Unknown 78l15 Regulator problem. Very Weird ????

So I am building a PSU for my preamp that will 48v and dual 15v supply. I want the power to be easily available so I decided to run the PSU in 9V DC input.

Here is what I am doing. A 9V is stepped up into 48v. It then spited into dual 24v and Regulated by 78L15 and 79L15 to get a dual 15v. I am using a XL6009 module and successfully obtained 48v and dual 24v. Everything work as expected until the last voltage regulating stage. I never got the 78L15 and 79L15 function properly. Here is the schematic of my circuit.

Click the image to open in full size.

The green circuit is working as expected while THE red circled part is not. I am sure my 78L15 and 79L15 are good I have tested them individually. They just don't work when combining the boost converter and the bipolar supply. Can some please suggest a possible explanation to this?
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Old 5th January 2015, 11:02 AM   #2
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Possible cause is the centre voltage or 0v point. It is floating. If you use two 7815's in series, that may work.
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Old 5th January 2015, 11:11 AM   #3
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You can't use the regulators like that. Either use another SMPS to generate negative rail, or use a supply splitter.
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Old 5th January 2015, 11:13 AM   #4
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Originally Posted by JonSnell Electronic View Post
Possible cause is the centre voltage or 0v point. It is floating. If you use two 7815's in series, that may work.
Thanks for your help. I am not a expert on electronic. What do you mean by floating? How to connect two 7815's in series and should I do the same thing with 7915?
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Old 5th January 2015, 11:17 AM   #5
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Originally Posted by richie00boy View Post
You can't use the regulators like that. Either use another SMPS to generate negative rail, or use a supply splitter.
That regulators circuit is taken from the internet and I think it is pretty common for this connection. I just replaced the input source. And I also added a supply splitter into my circuit (the second stage).
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Old 5th January 2015, 11:33 AM   #6
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The regulators are not the problem, it is your floating 0V.
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Old 5th January 2015, 12:48 PM   #7
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Originally Posted by KatieandDad View Post
The regulators are not the problem, it is your floating 0V.
Sorry I don't understand the term floating 0V. Can you explain more on this? Thanks
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Old 5th January 2015, 12:59 PM   #8
sangram is offline sangram  India
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The input supply has a ground referenced to 0V. (Node connected to GND).

The 0V reference to the two regulators is being split by two 10k resistors.

When you connect the two circuits together, the lower resistor is effectively removed from the circuit by the two 'grounds'.

By itself the circuit would work but the current available would be very small, limited by the current available through the 10k resistors. In that sense it is NOT 'dual' 24V, but a single 48V with a virtual ground formed by the potential divider of 10k+10k.

What will not work is the interconnection of the two downstream circuits.

The maximum current will be limited to 2.4mA. If you want more current you will have to use a virtual ground generator, but cannot then connect a microphone to the input of the circuit as you will damage this generator.

I would advise generating one 30V rail and using single-supply applications if possible.
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Old 5th January 2015, 01:17 PM   #9
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Originally Posted by sangram View Post
The input supply has a ground referenced to 0V. (Node connected to GND).

The 0V reference to the two regulators is being split by two 10k resistors.

When you connect the two circuits together, the lower resistor is effectively removed from the circuit by the two 'grounds'.

By itself the circuit would work but the current available would be very small, limited by the current available through the 10k resistors. In that sense it is NOT 'dual' 24V, but a single 48V with a virtual ground formed by the potential divider of 10k+10k.

What will not work is the interconnection of the two downstream circuits.

The maximum current will be limited to 2.4mA. If you want more current you will have to use a virtual ground generator, but cannot then connect a microphone to the input of the circuit as you will damage this generator.

I would advise generating one 30V rail and using single-supply applications if possible.
Is it mean that there will be no proper way to generate bipolar supply from DC ?
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Old 5th January 2015, 01:21 PM   #10
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See this article on virtual grounds -- you can use BUF634 or LME49600:
Virtual Ground Circuits
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