buffered zener power supply

MrDEB · Feb 9, 2009

have 18v (2-9v batteries in series)
powering a cmos version of the 556 timer so we are talking low current needs.
to be safe I was planning on 14v at 10ma min. 7556 requires 3-16 volts supply.
the buffered supply is supposed to be better, more stable.
found 2 versions of circuit
one as pictured and one with another resistor from the vcc to the collector. WHY? and which one to use?
will upload rest of entire circuit later

MrDEB · Feb 9, 2009

Complete circuit with timing diagrams

trigger A is 0-77 seconds
trigger B is .5-2 seconds
A triggers B
not real sure about power supply??

Roff · Feb 9, 2009

You're wasting a lot of current in the zener. Why don't you use an **broken link removed**? It will be more efficient, and has internal short circuit protection.

MrDEB · Feb 9, 2009

I thought about that

space is at a premium.
circuit is battery powered. Of coarse that makes the 7812 a better choice.
the 7556 can run on 18v but don't like going to the limit.
the 7812 will require ?? more caps??

MrDEB · Feb 9, 2009

revised for 7812

decides to go with the 7812

Roff · Feb 9, 2009

MrDEB said:
decides to go with the 7812

I said 78L12. Notice the "L". It's in a TO92 package.
7812 requires 10mA minimum load. 78L12 only requires 1mA.

Oblong · Feb 9, 2009

If your trying to minimize no load current consumption some cheap options could be;

TLVH431 requires worst case 100uA to maintain regulation

https://focus.ti.com/lit/ds/symlink/tlvh431.pdf

Or a low Izt zener like one of the MMSZ4 which specs 50uA Izt.Just be sure the shunt you chose can handle the power dissipation.

https://www.onsemi.com/pub/Collateral/MMSZ4678T1-D.PDF

**broken link removed**

audioguru · Feb 9, 2009

Hi Mr DEB,
You should learn how to draw straight lines then turn off those dots that are all over the place.

Sixteen arrays with 3 or 4 LEDs (without any current-limiting resistors) will kill a huge 18V battery in seconds. Two little 9V batteries in series are not powerful enough to light all those LEDs more than a few blinks.

The reset transistor is connected backwards and the other reset pin must be connected to the positive supply.

MrDEB · Feb 9, 2009

You should learn how to draw straight lines then turn off those dots that are all over the place.
wHAT DOTS?? Could be because the drawings are jpeg??

Sixteen arrays with 3 or 4 LEDs (without any current-limiting resistors) will kill a huge 18V battery in seconds. Two little 9V batteries in series are not powerful enough to light all those LEDs more than a few blinks.

The 100 ohm resistor per 4 LEDs should be enought per array??

The reset transistor is connected backwards and the other reset pin must be connected to the positive supply.
I assume you mean the 3904 connected to the diode D4??
thats a reset upon startup.
confused in Idaho

blueroomelectronics · Feb 9, 2009

Does expressSCH have a snap to grid function? It'll help make a straight line.

audioguru · Feb 9, 2009

MrDEB said:
You should learn how to draw straight lines then turn off those dots that are all over the place.
WHAT DOTS?? Could be because the drawings are jpeg??

The dots are all over the place to help you draw straight lines but you can't see the dots?
The FUZZ is caused by saving the schematic as a JPEG file type.

Sixteen arrays with 3 or 4 LEDs (without any current-limiting resistors) will kill a huge 18V battery in seconds. Two little 9V batteries in series are not powerful enough to light all those LEDs more than a few blinks.

Click to expand...

The 100 ohm resistor per 4 LEDs should be enought per array??

Sorry, I didn't see the 100 ohm resistor hiding in all those dots.
Without knowing the forward voltage of the LEDs then I can guess that the current is 119mA if 3 LEDs are 1.8V each or 101mA if 4 LEDs are 1.8V each.
Sixteen of them is 1.9A or 1.6A. 9V batteries have trouble with a current that is higher than 1/10th.

The reset transistor is connected backwards and the other reset pin must be connected to the positive supply.

Click to expand...

I assume you mean the 3904 connected to the diode D4??
thats a reset upon startup.

Its output is the collector of the transistor that is not connected to anything. You have its base connected to the TS556 reset pin.

MrDEB · Feb 9, 2009

I turned off the grid

Hope this helps.
Each LED array is 4 leds in series with a 100 ohm resistor in series.
there are PLANED 16-20arrays (64 - 80 leds total)
I used this calculator
LED Resistor Calculator
white LEDs = 3.4 forward voltage and 30 ma current
plan is to pulse the LEDs at .5 - 1 second long (one bright flash)
push the current to almost max = 70ma is max so go up to about 50-60ma.
I need to redraw the power up cycle canceling part of the circuit
thanks for catching it.

MrDEB · Feb 9, 2009

power up circuit was almost right

just needed to change the value of R6 from 100k to 10k
connect the base of Q2 to both reset pins on the TS566cn
straighen up all the lines and shut off grid

audioguru · Feb 9, 2009

Horray!
Straight lines.
No dots.

But the reset transistor is still backwards. Its output is its collector but you have the reset pins connected to its base. The reset pins are supposed to be connected to its collector.

MrDEB · Feb 10, 2009

Boy you are sharp!!!

I must of looked at that 20 times and didn't catch the wrong connection.
I held the drawing from the web next to my drawing and BAM!! I see what your talking about.
I think I need to change my snap to grid from .025 to 1mm
might help in making straight lines.

Hero999 · Feb 10, 2009

Roff said:
I said 78L12. Notice the "L". It's in a TO92 package.
7812 requires 10mA minimum load. 78L12 only requires 1mA.

That's not true, both will work with no load connected. I think you were referring to the quiescent current requirement.

Roff · Feb 10, 2009

Hero999 said:
That's not true, both will work with no load connected. I think you were referring to the quiescent current requirement.

That's not what the datasheets say.

audioguru · Feb 10, 2009

The 78xx has its quiescent current through its ground pin. It works perfectly without a load.
It spec's its regulation when its load is from 5mA to 1A.
Its typical max outoput current is over 2A so they could have spec'd its regulation with a load current from zero to 1.5A but the numbers would not be as nice.

The LM317 adjustable regulator has its quiescent current through its output pin so that the current in its ADJ pin is as low as is possible (typically only 50uA). It needs to have a minimum load of up to 10mA which is usually provided by its voltage-setting resistors.

Hero999 · Feb 10, 2009

Roff said:
That's not what the datasheets say.

Where does it say they require a minimum load current?

The dropout graph even specifies a load of 0A.

Roff · Feb 10, 2009

Hero999 said:
Where does it say they require a minimum load current?

The dropout graph even specifies a load of 0A.

I was looking at the load regulation specs. You and Audioguru set me straight.

buffered zener power supply

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