Unknown 78l15 Regulator problem. Very Weird ????

[nguitar12]

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.



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?

[slacker]

In what way do they not function properly?  What happens when you connect them? Voltages?

[antonis]

In think that the problem is on the small "bipolar supply" circuit at the end of the green square..

As you've connected the GND of the first circuit you should read +48/+24/0 V respectively and not +24/0/-24...

[gtudoran]

That spiting technique is used on low power applications and it's very dependent on the load. So all in all NO you will not make it work, at least not with a decent amount of parts.

What you can do is this, use another boost module. So you will have 2 x XL6009 modules that are using the same 9V supply and you can tie the:

- output + pad of 1st module with the - pad of the 2nd module - this will represent the ground, and the remaining + and - will represent your bipolar supply... so this way you will have a dual PSU that you can use.

Regards,
Gabriel

[R.G.]

I believe that it would work if you do two things.
1. Change from a 7815+7915 to two stacked 7815 modules. Your circuit as shown has to source the differential currents in the ground lead through the two 10K resistors. Changing to stacked 7815s lets the 7815s (or two 7915s, equally) source/sink the ground currents to the load instead.
2. Do not use the 9V input supply for anything else. Especially do not use the negative side as "ground" for other circuits. That side is tied to the -15V output. Or, if you use the 9V input negative as ground for other things, this forces the outputs of your bipolar supply to be 0V/15V/30V. In either case, you cannot tie both the middle of the bipolar output and the 9V minus terminal together.

[nguitar12]

I believe that it would work if you do two things.
1. Change from a 7815+7915 to two stacked 7815 modules. Your circuit as shown has to source the differential currents in the ground lead through the two 10K resistors. Changing to stacked 7815s lets the 7815s (or two 7915s, equally) source/sink the ground currents to the load instead.
2. Do not use the 9V input supply for anything else. Especially do not use the negative side as "ground" for other circuits. That side is tied to the -15V output. Or, if you use the 9V input negative as ground for other things, this forces the outputs of your bipolar supply to be 0V/15V/30V. In either case, you cannot tie both the middle of the bipolar output and the 9V minus terminal together.

I finally get them working. The problem is I test the 7815 and 7915 one by one. When I hook them up at the same time they work. A precise +15v and -15v.

Someone at other forum say that if I split the voltage like this the maximum current will be limited to 2.4mA. Is it true? If so what is the principle behind this?

Anyway I test the supply on my preamp and everything is working. However I find that the preamp will distort when the output hit around -2db. Usually I won't record the vocal at this level but I don't think distortion is normal. Is it relate to the psu I use? Or due to the current issue mentioned?

[R.G.]

Someone at other forum say that if I split the voltage like this the maximum current will be limited to 2.4mA. Is it true? If so what is the principle behind this?

I believe that is the same issue I referred to about the 10K's generating "ground". The 7815 can only let current flow out of its output pin, and the 7915 can only let current flow in through its input pin. In real circuits, ground can let current flow in and out. In this circuit, there are in effect, two 10K resistors limiting the currents into and out of ground. So current that is from +15V to ground, not to -15V, will flow through those 10K resistors. Actually, if you map the current flow, a current from +15 to "ground" will flow through the 7815 and then through the bottom 10K. This means that the "ground" will shift because the 7915 can't eat the ground current. Same thing happens from the -15V side.

Anyway I test the supply on my preamp and everything is working. However I find that the preamp will distort when the output hit around -2db. Usually I won't record the vocal at this level but I don't think distortion is normal. Is it relate to the psu I use? Or due to the current issue mentioned?

Maybe. Maybe.

Time to get out the oscilloscope. Could be power supply shift, could be ground shift. Could be the circuit. Hard to say without actually looking.

[slacker]

Even if it works correctly, will it do what you want? It can't be +48 Volts and +-24 Volts at the same time can it? You can only have one ground, if you say ground is the middle of the 10k divider then you can only get +-24 Volts out of it, or you can say ground is the bottom of the 10k divider, then it's +48 Volts. I'd be tempted to treat it as +48 Volts and run the preamp off a 30 Volt regulator.

[nguitar12]

That spiting technique is used on low power applications and it's very dependent on the load. So all in all NO you will not make it work, at least not with a decent amount of parts.

What you can do is this, use another boost module. So you will have 2 x XL6009 modules that are using the same 9V supply and you can tie the:

- output + pad of 1st module with the - pad of the 2nd module - this will represent the ground, and the remaining + and - will represent your bipolar supply... so this way you will have a dual PSU that you can use.

Regards,
Gabriel

Tried today with no sucess. There is no output immediately when I connect two module. Why is that?

[nguitar12]

Even if it works correctly, will it do what you want? It can't be +48 Volts and +-24 Volts at the same time can it?

I later on find that the 48v will actually become 24v as the GND point is changed.

I'd be tempted to treat it as +48 Volts and run the preamp off a 30 Volt regulator.

How do you run the preamp off a 30 Volt regulator while it is supposed to take +-15v at different pins?

[slacker]

How do you run the preamp off a 30 Volt regulator while it is supposed to take +-15v at different pins?

Your preamp wants three Voltages, a positive supply, a negative supply and a Voltage in the middle of them. In this case  +15 Volts = positive supply, -15 Volts = negative supply, 0 Volts = Middle. This is 30 Volts in total -15 to +15. Instead of this you can use 30 Volts =positive supply, 0 Volts = negative supply and generate 15 Volts to get a supply in the middle of the other two.

That's essentially what your bipolar supply is doing, generating 48 Volts and splitting it in half with the 10k resistors. if you connect ground to the middle of the 10k resistors, how you have it now and measure from the middle to the top and bottom you get +24 and -24. If you measure from the bottom to the middle and top you get +24 in the middle and +48 at the top. If you now move ground to the bottom, and do the same measurements you'll get the same answers, the only difference is where you're deciding to put ground.

You might need to make a few small changes to your pre amp circuit to get it to work but most circuits can be run like this.