LM7809 Power Supply Issues

[Dr Tony Balls]

Hi All,

Having a problem I cant solve and I'm not sure what the issue is. Basically I have small tube amp and i'm trying to add a buffered rangemaster circuit to it. To get 9V for the circuit, I tapped off one of the preamp tube supply nodes which sits at 130V. I put that into a voltage divider (R14 and R15) to drop it to 23V, and then put that into a LM7809 ic to provide a regulated 9V. The problem is with that last part, the LM7809.

- with the 7809 installed I get 130V in to the board but measure like 1V on the 7809's input pin, and 1V on the output pin.
- with the 7809 removed I get 130V in to the board, and 23V at the junction of the voltage divider (row C) as expected.
- with the 7809 partially installed (input and ground pin only), the same results occur as in bullet 1 (trying to remove the rest of the circuit as a culprit)

Here's the board layout:



Any ideas? Is there something about using the 7809 that i'm missing?? Can you use a voltage divider before the regulator, or is that causing problems?

[mozz]

Schematic?

EDIT: was looking for the whole schematic, as i hate vero.

[Dr Tony Balls]

Schematic?

There's no much to it:



The voltage divider on its own is doing its thing, its just when its connected to the regulator that it doesnt produce the right input voltage (23V), so its making me think there's an interaction between the divider an the regulator. Im gonna do some googling.

[Manfred]

This cannot work because the 7809 requires an average quiescent current of 5 milliamps.
The current through the voltage divider is 0.49 milliamperes without load.
The thump rule states that the current through the voltage divider should be at least five times as high as the load current.
The load current at the tap of the voltage divider is then the quiescent current plus the load current at the output of the voltage controller.
This means that with the quiescent current alone, the cross current through the voltage divider should be at least 50 milliamps
and that the resistances of the voltage divider would then be 2.2k and 470R.
So the 2.2k resistor would already have at least 5W.
With a load at the output, the resistor would be even smaller and its power value would have to be selected even higher.
For this application, a simple standard voltage regulation using a transistor and Zener diode is recommended.

[deltafred]

Manfred replied while I was slowly typing.

I would look to see if it is possible to wind enough extra turns of insulated wire on your mains transformer to create an isolated low voltage AC supply.

Or if not add a small low voltage transformer with a bridge rectifier and smoothing caps to give you enough voltage to supply your 7809.

Also add some capacitors close to the 7809 as per the datasheet recommendations (pins 1-2 0.33uF, pins 2-3 0.1uF) .

[Dr Tony Balls]

This cannot work because the 7809 requires an average quiescent current of 5 milliamps.
The current through the voltage divider is 0.49 milliamperes without load.
The thump rule states that the current through the voltage divider should be at least five times as high as the load current.
The load current at the tap of the voltage divider is then the quiescent current plus the load current at the output of the voltage controller.
This means that with the quiescent current alone, the cross current through the voltage divider should be at least 50 milliamps
and that the resistances of the voltage divider would then be 2.2k and 470R.
So the 2.2k resistor would already have at least 5W.
With a load at the output, the resistor would be even smaller and its power value would have to be selected even higher.
For this application, a simple standard voltage regulation using a transistor and Zener diode is recommended.

THANK YOU for this explanation!!!!

And yes, in the meantime I began experimenting with Zeners and just went that way.

[plush]

I would look to see if it is possible to wind enough extra turns of insulated wire on your mains transformer to create an isolated low voltage AC supply.

His amp has heater winding, just add voltage multiplier and decent filter stage and done.

But in all honesty, separate small transformer is way better solution.

[EddieTavares]

Does it not work at higher voltages? Something like 18... 24v?
What about a zenner?