"tweaking" a fixed 3-pin regulator

You need a negative supply to operate a 7805 at less than 5V, or LM317 at less than 1.25, etc. I don't recall if there are any three-terminal regulators that go to zero on a single supply.

You may be better off using a TL431, op-amp and power transistor.

Tim

-- Deep Friar: a very philosophical monk. Website:

if you look at the datasheet you'll see that the output voltage isn't very accurate, 4%/2% so your 4.9 looks to be within with in spec.

you could tweeak the output voltage to be higher with a resistor in the ground pin, if you keep the input voltage and th output current resonably constant, because the current in the ground pin changes with output current and input voltage.

first datasheet I found says 5mA so try 20R

and all the specs are for >5mA Io so I think you'd want to load it at least that

-Lasse

Hi ! According to...

.. you can take the control input to a voltage divider between the output and ground so making it adjustable above the nominal 5 V setting.

--------------------------------------- Posted through

you need to generate a -5 Volt signal to connect to terminal 2.. This signal of course can be scaled via a pot. So, you would place the wiper on terminal 2 of the 7805 and low side to common, high side to the output of your -5 supply..

If you don't have a -5 supply available and, are using a single/bipolar supply? you can then employ a very basic circuit via a 555 timer to generate a -5 volts into a 5.1 V zener shunted reg source to drive the top side of the pot...

Working with 555's are great because they're cheap, plentiful and quick to get you out of trouble fast.. Just the other day I had a fixed (-) supply via a 555 to bias a pot for +/- control to drive a small low end AC drive that didn't have a (-) reference, nor could it offset it in software..

Have a good day.

Sure. Just lift the "ground" pin up above ground, and the output will track that. A series resistor, or better yet a voltage divider from its own output, will work. To go lower, you'll have to pull the regulator gnd pin negative.

I like to stack regulator gnd or adj pins on top of the outputs of other regs. You can make 1.25 and 2.5, for an FPGA, by stacking two LM1117s, no resistors required.

John

when xilinx first came out with 1.2V fpgas, AFAIR the 'normalt' 1.25V ref regulator with tolerances was above max spec voltage.

-Lasse

Move voltage just makes them faster!

John

John Ferrell W8CCW

Yeah as others have said you can tweak it up in voltage, down is a bit more difficult. If you are just using this for a reference (very little current) then a real 5 volt reference might be better. REF02, or even one of the zener references LM4040. You then don't get the voltage variation that a voltage regulator gives you.

George H.

Did you have a resistor across the output and common terminals?

--
Politicians should only get paid if the budget is balanced, and there is
enough left over to pay them.

If you need more accuracy (the uA7805 only promises 4.75 to 5.25V) there are tighter-specified regulators. Consider LT1083, for instance (it's an adjustable 3-terminal regulator with about +/- 1% accuracy).

If you need REAL accuracy, three-terminal regulators are not ideal (four-terminal ones are better, they allow point-of-load sensing).

If you just need to cover the 5.00 volt setting on your divider, a 7806 regulator would guarantee that...

Did you pour extra current down the reference lead to swamp the regulator's quiescent current? Like running a LM317 with 5V ref. instead of 1.25V?

Opamps would do even better.

Grant.