LM317 Adjustable Regulator Question

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Why screw around with the 317? Hook the LED between two outputs of the 595. Output "10" lights one color and "01 the other. "00" or "11" is no light. Don't forget the R in series with the LED. Art

Thanks Art but that will ultimately double the number of 595's I'd need. Also, this is a totally discrete serial application (no microcontroller) which will ultimately have 60-70 bicolor LED's.

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Ah! This changes things. Yes, the 317 will not sink current and is the problem. You need something which both sinks and sources current. Like an opamp. Art

I once had an engineer who had a saying, "a circuit should be as simple as possible, but no simpler." I think yours is simpler. Gotta have a pulldown that can supply ALL the led current. Stick in a resistor that can sink 70x+ the led current. And heat sink the lm317. I'd also be concerned whether you can reliably turn on both sides with only 5V. If you're building one that has to sorta work, you're close. If you want something reliable and repeatable, a new topology would be in order.

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(View in fixed font, such as Courier)

Fig. 1 LED -----. .--||--+--. HC595 | | | | | |\ | | '--| >o--R1--' | |/

hi / lo / tri-state =3D red / green / off

Fig. 2

_______ L1 clk |\ .-.-.-. ---| >o---' ' ' '--+---> 2.5v |/ | --- C1 U1 --- | =3D=3D=3D

The +2.5v supply (Fig. 2) doesn't leave a lot of headroom for controlling current, if that matters.

-- Cheers, James Arthur

Neat!

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Yeah not much current sinking. If you don't care about wasting power you might add a 'pull down' resistor to the output of the LM317.

George H.

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doesn't national make a 'ground reference' chip for splitting a power supplly? that thing can sink/source large currents.

(A) You are correct that the 317 only sources current; it does not sink current. A simple voltage divider backed up by an op-amp of sufficient current capability is what you need.

(B) You _are_ aware that about 3% of the population (6% of men) can't distinguish between the red and the green of a bicolor LED, so your display will just look like a bunch of amber lights, vaguely twinkling?

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It's been a while since I played with the 317 so I took a look at the datasheet and am surprised it worked at all for you. It has a min Vin-Vout of 5V and you're attempting 2.5V.

For lighting LEDs you really don't need a tightly regulated 2.5V rail.

How about using a couple of 2.6V zeners connected in series from +5 to gnd? There's not enough V to turn both of them on but a load from the center to gnd gets 2.4V as does a load from the center to +5.

A chain of 8 diodes would work similarly but the load would see less voltage. Replacing the center 2 diodes with bipolar Q's would give a better regulation if needed as the load current varies.

Multiple instances of any of the above would spread the power dissipation over a wider area and may make routing the PCB easier.

Just some random thoughts. Art

All great ideas and I hope to try some of them soon limited only by what I have in the 30+ year stockpile I have.

Art, in your last post you mentioned Vin-Vout of 5V. I actually have 12 volts into each regulator.

I had one other idea in addition to those you all provided. I'm thinking about getting rid of the 2.5 volt regulator. I'd keep one 317 set at 2.5 for Vcc of the 595. I'll add a 337 negative regulator set at -2.5 to supply Gnd on the 595. So the 595 is now running off a split supply but essentially sees 5 volts. I'll run my other gates off this same supply. The outputs from the 595 are to one side of the bicolor leds as before but now the other side of the LED's goes to the center tap of the power supply transformer. This should provide good regulation for both polarities and give consistent brightness no matter how many greens or reds are lit.

Sorry if this is carrying on too long. I really enjoy all the expertise here though.

So, splain me how it works to turn off the leds? What's in that little triangle between Q and R1?

The upside is that you can probably add a cap and have tristate = yellow.

Well, the symbol is for an inverter. So, if it's an inverter, please 'splain the voltages on the input and output when the led is off.

You mean, like, splitting the load and using more than one????

how about using one of those Virtual voltage IC's? you can get then in many forms.. but, the most common one is a 2.5V type. it has a op-amp output so it can source and sink. THis is all in one package with no components needed.

Jamie