Super. Thanks all... here's the latest "corrected" schematic. I'll post a celebratory note when I'm done. :-)

-c

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It provides a load. It sets the current that will flow through the adjustment resistor (which sets the voltage at the ADJ pin)

The minimum load current is specified as 3.5-5mA. With 240R (5mA), it's cutting it a little fine, but I would think it would work fine. Using 120R just adds safety margin. If you're worried, (and you have spare resistors) connect 2 x 240R in parallel for 120R. I personally wouldn't worry too much.

Yes.

 

Great, thanks, I won't worry about it, and I'll just use a 240 since that's what's coming (and will make 5.2mA, if I get the above math).

Just to confirm: the risk of using too high of an R1 value is that the voltage output can rise with too-low draws, and if I'm using this in some delicate little digital gadget, the potential for under-voltage when the gadget is connected is obviously potentially bad...? (E.g. I tune it to an expected X volts, but R1 is too high, then I connect the gadget, now the draw is enough to bring the voltage down under X volts). Can I use my multimeter to detect the voltage across the output while the gadget is hooked up? (Can't remember back to physics class well enough) Meaning, just to confirm that the voltage is the same in both the connected and unconnected states? Or I just need to design it right from the get go. :-)

Thanks,
-c

 

I think the value of your pots are too high.
1) Version B. R1= 240 ohms, R2= 10k ohms. When the pot is at max setting then the output will try to be 52.3V.
2) Version C. R1= 120 ohms, R2= 5k ohms. When the pot is at max setting then the output will try to be 53.3V.

What about heat? If the input is 40V and the output is set to 5V then with a load current of only 0.5A the IC will heat with 17.5W and even if the heatsink is huge the IC might shut-down.
The LM317 will reduce its max current to about 175ma to 500mA when it has 35V across it.
The LM150 will reduce its max current to 275mA to 1A when it has 35V across it.

__________________
Uncle $crooge

 

Yeah, I had come up with those numbers too, but since the data sheet said "The LM117 series of adjustable 3-terminal positive voltage regulators is capable of supplying in excess of 1.5A over a 1.2V to 37V output range" so I assumed there was some other limit coming in to play... But I see your point. Smaller pots would make more sense.

Duly noted. My intended application is coming from DC converters, so theoretically most are going to be 5-12V kinds of values... I just wanted a little flexibility for weird cases. But if I understand things, the greater the V-in V-out differential, the harder and less efficiently the unit will work?

Thanks,
-c

 

The greater is the Vin Vout differential then the IC gets hotter and its max current gets less.

__________________
Uncle $crooge

 

Ok, built the circuit in an old tape case. Works like a charm despite my terrible soldering and so on.

I order parts at my local hardware store and they get the orders wrong every time... but the mistakes seem to have been happy accidents: a 200 Ohm resistor instead of 240, which seems better given what we talked about above, and an audio-taper pot with a shaft (instead of flush-mounting screwdriver-adjusting linear -- the shaft more prone to accidental adjustment, you know...) The audio taper is actually a nice curve for this application, so that's cool.

I burned out the first voltage regulator by accidentally connecting an AC-output wall wart (read it wrong). That was a bummer :-) but that's why I always order at least two of everything.

One thing that i perhaps should have seen coming (and maybe just don't understand the math well enough) is that when I supplied 12v to it, it only went up to 11V. I supplied 5V, it only went up to 4. Maybe that's perfectly normal, I think I thought it would somehow increase the voltage above the supplied amount... is something wrong or is this normal?

I'll post pictures when I get the fake batteries made. :-)

-c

 

An ordinary voltage regulator needs to have an input voltage that is 2V to 3V higher than its output voltage.
"Low-dropout" voltage regulators are available that work fine when their input voltage is only 0.5V higher than their outpout voltage.

__________________
Uncle $crooge

 

Ah, that makes intuitive sense, thanks. I will plan accordingly.

-c

 

Well, it's all going swimmingly. My first use for this is a long-duration time-lapse with a digital camera and a computer and it's all coming together. Very exciting. :-)

I had a question about heat: when running, the little thing gets pretty warm -- on the edge of me being able to hold my fingers to it. I have attached a heat sink (see attached pic). The NTE970 that I'm using (the "equivalent" of the LM150) is spec'ed at:

Absolute Maximum Ratings:
...
Operating Junction Temperature Range, TJ . . . .. . 0° to +125°C
Storage Temperature Range, Tstg . . . . . . . . . . . –65° to +150°C
Thermal Resistance, Junction–to–Case, RthJC . . . . . . . . 2.5°C/W
...

...although I'm not 100% of the implications, 125°C seems well above the temperature it's reaching, so I assume I'm well in the clear? I just want to make sure I don't ruin another one of these. :-)

Thanks!
-c

P.s. the picture was taken by the camera being powered by the fake dowel-rod batteries from the unit. :-)

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The max allowed internal chip temperature is 125 degrees c. The case is probably much cooler.
The thermal resistance shows how hot its case is allowed to be at the power it is dissipating.

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Uncle $crooge

 

Hmmm... thanks, so, if I have this right:

I have 18.5 V into the thing, output to 3 V, so:
(18.5V - 3V) * 1A = 15.5W

(assuming 1A for the digital camera, which I'd guess is a fair overestimation)...

But I'm not sure how to interpret "2.5 deg C / W"?

I can also just use a lower-voltage power supply...

 

Many years ago, I purchased a wall-wart from Radio Shace that has an output voltage selector switch. You can select about 1/2 dozen output levels, starting from about 2.5V all the way up to 12V. I've never had a device since then that I couldn't power off this thing.

And it comes with a number of power connectors so it plugs right into most and DC connection jack.

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You don't need a quadraphonic Blaupunkt -- you need a curve
ball.

 

Make sure any audio circuits you are powering do not get any 100Hz or 120Hz hum.

 

You have the regulator dissipating 15.5W. Its chip to case heats with 2.5 degrees C above the ambient temperature for each Watt that it dissipates. So with an ambient of 30 degrees C, the minimum thermal resistance of a heatsink is (125 - 30)/(15.5 x 2.5)= 2.45 degrees per Watt. Select a heatsink with a thermal resistance of 1.5 degrees per Watt. It will be pretty big.

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Uncle $crooge