benchtop power supply

[Steve GarnettParticipant @stevegarnett62550]

Well, if you take pin 2 of the LM317/338 to ground (-ve side of C1) and you still get the full output, that would indicate that you’ve got a dead short from input to output of the regulator – internally. Also, if the resistor is getting fried, it implies that pin 2 (which should only take a maximum of 100uA) is drawing current. Can’t see any explanation other than this, I’m afraid – one fried regulator.

Edited By Steve Garnett on 29/09/2011 20:38:48

[ian weeks 1Participant @ianweeks1]

Thanks steve,

Will try what tou say but wont be till next weekend now as am off to snowdonia for the week

Regards Ian

[Sub MandrelParticipant @submandrel]

I know this is a return to the beginning of thios thread, but this afternoon I found a scan of the 7805 datasheet on my PC. Hand typed, by the look of it. Apparently you DON’T need any capacitors with a 7805 as long as (a) the supply reservoir capacitor is big enough and (b) you aren’t too picky about the transient response. So the 0.1-1.0uF cap on the output is only needed if you have a ‘spiky’ load. I think I’ll keep using a 47uF on input and 0.1uF on output anyway.

 

Neil

[ian weeks 1Participant @ianweeks1]

Dear All ,

Once again thank you for your erudite postings in the recent past,they were a great help. Could I trouble you again for some information.

last night at last found some spare time to ‘play’ in the shed . decided to build as much of the the ht circuit as I could [ really want to spend cold winters nights with the valve radio on the kitchen table!] Have done this apart from last 4 220 microfarad capacitors-finding hard to source 100v ones.

While ‘on a roll’ Decided to re look at power supply and started on the variable side of the power supply as detailed in Mew and you were right Steve putting pin 2 to ground indicated dead short in lm 338. .Checked wiring and replaced 338 Same again 37v unvariable at out put. Spent rest of night poring over schematic [building using terminal block and my attempts at wiring. Cannot find discrepency . If you have time can you tell me if there is a problem with the published schematic? I,m beginning to become obessed with getting this to work!! On a lighter note while on the walking holiday in wales spotted an copy of electronics servicing vol2 for 50p in charity shop window- serendipity

Thanks in anticipation

Ian

[Les Jones 1Participant @lesjones1]

Hi Ian,

I have sent you a message about the problem. Look in “My messages” under your account.

Les.

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