Question about pot in LM317 circuit

[Cliff Matthews]

I read most everyone's posts, but I didn't see explanation for the OP's "off to full full bright" results. The 317 requires a minimum operating current of 10mA, so the OP needs to provide a suitable load.  I suspect once the LED's are biased on, he can get some dimming when turning back the pot?

[TerminalJack505]

Here are some simulations that show the DC transfer characteristics of your original circuit as well as some changes you can make to improve it.

Notice that the output voltage of the first circuit starts at 1.25V and reaches the maximum output voltage at about a half turn of the pot--50% of the pot isn't being utilized.

The second circuit introduces R2, which sets a minimum output voltage.  In this case, around 2.7V.  Most blue LED's V_f is about 3.2V.  The issue with the output reaching the maximum too quickly still hasn't been fixed.

The third circuit changes the pot's value to 1k.  The resistor in parallel effectively helps make it a little smaller than that, which helps stretch useful range of the pot a little.  The output voltage reaches its maximum at  94%, instead of 86%.

[edavid]

I read most everyone's posts, but I didn't see explanation for the OP's "off to full full bright" results.

That was not really what he said... it just wasn't using the full range of the adjustment pot.

The 317 requires a minimum operating current of 10mA, so the OP needs to provide a suitable load.

10mA is worst case... in the OP's circuit, it would be more like 1mA (or less).

[edavid]

One may be able to mitigate the effects of NE555 shoot-through with *enough* local decoupling, but its still undesirable.  Furthermore 555 circuits that take the feedback from the Output instead of the Discharge pin, without a pullup resistor for the timing, work far better with a CMOS 555, as its output is close to being symmetrical rail to rail but the original NE555 Output cant get very close to its +ve rail, which distorts the duty cycle of what should be a 50% duty cycle squarewave if no steering diodes are used.

Also good quality 220uF 25V caps usually cost more than you are likely to save (if any) by speccing a bipolar 555.  The economics may differ if using no-name far east parts, but I prefer not to go there.

I don't think the OP is going into production.  My point is that if you have a bipolar 555, it will work well enough... no need to hold up everything and buy a CMOS part.

[Cliff Matthews]

I read most everyone's posts, but I didn't see explanation for the OP's "off to full full bright" results.

That was not really what he said... it just wasn't using the full range of the adjustment pot.

The 317 requires a minimum operating current of 10mA, so the OP needs to provide a suitable load.

10mA is worst case... in the OP's circuit, it would be more like 1mA (or less).

Doesn't the 317 require passing 10ma just to establish the stability of its own internal circuits? (1.25Vref etc..)

[edavid]

The 317 requires a minimum operating current of 10mA, so the OP needs to provide a suitable load.

10mA is worst case... in the OP's circuit, it would be more like 1mA (or less).

Doesn't the 317 require passing 10ma just to establish the stability of its own internal circuits? (1.25Vref etc..)

Not under most conditions... here's the graph from the On Semi datasheet:

[Zero999]

The 317 requires a minimum operating current of 10mA, so the OP needs to provide a suitable load.

10mA is worst case... in the OP's circuit, it would be more like 1mA (or less).

Doesn't the 317 require passing 10ma just to establish the stability of its own internal circuits? (1.25Vref etc..)

Not under most conditions... here's the graph from the On Semi datasheet:

It's not recommended to rely on the LM317 to only need 10mA though.

[TerminalJack505]

Here are a couple more ideas.  These circuits show ways to get a nonlinear output.

The first one provides more sensitivity at the lower voltages/currents and the second gives more sensitivity at the higher voltages/currents.

[Audioguru]

The 317 requires a minimum operating current of 10mA, so the OP needs to provide a suitable load.

The LM317 needs to have a load of 10mA only when it has 40V across it and has maximum operating current.
The voltage-setting resistors usually load it. When the resistor from the output to the ADJ terminal is 120 ohms then it draws a current from the output of 1.25V/120 ohms= 10.4mA then the LM317 is loaded enough so its output voltage will not rise if it has no other load.

[edavid]

It's not recommended to rely on the LM317 to only need 10mA though.

1. Of course it is, 10mA is the datasheet maximum spec

2. OP is not going into production

3. If the load was not sufficient, the symptom would be the LEDs being on with the adjustment pot turned off, which is not happening, so this is a non-issue

[TerminalJack505]

Here's a more efficient version of circuit #5 that I posted above.  It requires some non-E12 series resistor values, however.  (The resistor values have basically been scaled-up to 2.5 times their previous values.)

The other circuit requires 18 mA of current to drive the ADJ pin.  This one requires about 7.5 mA.  (Which should also provide enough of a minimum load for the LM317.)

[OilsFan]

OP here. Thanks guys, really good discussion I'm learning a lot. As someone commented I'm not going into production but rather making this device as a way to learn more about circuits overall. I do hope it works in the end though. So it occurred to me that I could have a dimmer on those bright LED's but I just couldn't figure out how to implement it. Lots of good ideas and thank you to TerminalJack505 for all the simulations. Really helpful.

[Cliff Matthews]

I read most everyone's posts, but I didn't see explanation for the OP's "off to full full bright" results.

That was not really what he said... it just wasn't using the full range of the adjustment pot.

The 317 requires a minimum operating current of 10mA, so the OP needs to provide a suitable load.

10mA is worst case... in the OP's circuit, it would be more like 1mA (or less).

Ok my bad I guess I translated into that from a 317 experience I had where the pot acted like a switch. Still, some circuits act weird without loads.