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Grogster

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Joined: 31/12/2012
Location: New Zealand
Posts: 9063

Posted: 04:56pm 06 May 2015

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This is odd.

Standard 338 regulator circuit, 13.8v in, adjusted to 7v output.
That's fine, and voltage is 7v with 13.8v in.

Problem I am having, is that varying the input voltage results in a change in the output voltage, as the regulator is not even there.

If I crank up the voltage to 15, the output of the 338 goes up to about 9v, and if I wind the voltage down to 9v or so, the output drops to about 5v.

This is without my touching anything on the circuit.

With my other regulator circuits, changing the input voltage does not change the output voltage provided you don't go above or below the threshold voltages.

I have 220uF on the input, and 10uF on the output.

330R between output and adjust, 5k trimpot between adjust and ground - just as the datasheet says, really. I tried changing the 330R for a 100R, but other then increasing the adjust current, it had no effect on the changes of voltage at the output.

Datasheet says that the 338 needs a minimum load current of 3.5mA typical, 10mA max, so I have a small DC motor connected to the output, and it is drawing about 60mA, so the output load should be plenty to allow correct regulation.

Have tried three different 338's, and they all change output voltage in direct proportion to the input voltage, which is odd to me - all my other regulator circuits, such as the MCP series, you can vary the voltage up and down, and the output is always stable - I hooked one of those up and ran the same tests, and with a 3v3 regulator, the output was rock-solid at 3v3 regardless of where I set the input voltage between 5v and 15v.

Can anyone throw any light on what I am doing wrong?

Smoke makes things work. When the smoke gets out, it stops!

panky

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Joined: 02/10/2012
Location: Australia
Posts: 1094

Posted: 05:15pm 06 May 2015

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G,
With respect

, double check pinouts - almost seems like output and adjust are back the front?
Doug.

... almost all of the Maximites, the MicromMites, the MM Extremes, the ArmMites, the PicoMite and loving it!

Grogster

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Joined: 31/12/2012
Location: New Zealand
Posts: 9063

Posted: 05:25pm 06 May 2015

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Thanks for the pointer, but most definitely the correct pinout - checked that about a dozen times now!

EDIT: Chasing a possible line of enquiry - one moment folks.....

EDIT: Fixed it. Boy, sometimes I wonder about my own brain at times.

I had put the 330R resistor between INPUT and ADJUST, not OUTPUT and ADJUST.
Moved the resistor to where it is SUPPOSED to be

and funny enough, the regulator works fine, and now stays at 7v regardless of input voltage swings.

...dear oh dear.....

Edited by Grogster 2015-05-08

Smoke makes things work. When the smoke gets out, it stops!

BobDevries

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Joined: 08/06/2011
Location: Australia
Posts: 266

Posted: 05:55pm 06 May 2015

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Many is the time I have been caught by the different pinout of the 78xx series versus the 317/338.

L-R: Adjust, Out, In; and don't forget that the mounting tab is at Vout potential.

Regards,

Bob Devries
Dalby, QLD, Australia

Grogster

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Location: New Zealand
Posts: 9063

Posted: 07:13pm 06 May 2015

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I don't deal with the 338's THAT often, so as you say - easy to get caught out.
Most of the time, it is SOT-223 SMD regs or smaller, or biggest tend to be 78 and 79 series TO-220 regs - don't do much TO-3 stuff.

Yes, well aware of the body of the package being Vout - it is well isolated.

Wish I had spotted that "Ahem!" before posting the thread, but I could not see what I was doing wrong. Why does fate mock you like that?!(rhetorical!)

Smoke makes things work. When the smoke gets out, it stops!

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