Author Topic: What am i doing wrong?  (Read 4694 times)

Offline shipto

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What am i doing wrong?
« on: April 05, 2015, 05:40:09 AM »
Working on a project that requires a dual voltage power supply 24v and 5v my electronics skills are limited to following other peoples circuits and this one is me combining 2 seperate circuits.
Please excuse the crude drawing but I have included it to show the way I am attempting to accomplish this.
The lm338k is a 5 amp power regulator which is supplying the 24v line and this is working fine however the lm317t which is intended to take that 24v down to 5v is the problem no matter what i do I cant seem to get it to work I am getting 24v on the 5v line.
I have searched and cant seem to find the answer but I am sure it should work and can only assume I am missing something.
Thanks for any help
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Offline John Rudd

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Re: What am i doing wrong?
« Reply #1 on: April 05, 2015, 05:53:45 AM »
Firstly have you got the 317 connected the right way round?
What value resistor have you got from the o/p to the adj pin?

Your cct should look like this:


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Offline andreas

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Re: What am i doing wrong?
« Reply #2 on: April 05, 2015, 05:57:40 AM »
You can check for bad potentiometer on the adjust pin or in the worst situation replace the lm317 as a test.
Check this also for more info. http://www.ti.com/lit/ds/symlink/lm317.pdf

Offline DMIOM

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Re: What am i doing wrong?
« Reply #3 on: April 05, 2015, 06:00:19 AM »
First quick thoughts, and please forgive if these are things you've already checked/considered:

1/ Pinout - are you absolutely sure the device is wired right? (I know I've blown, or at least shut-down, regulators before when prototyping)

2/ Is the LM317T getting hot - that's a lot of voltage for a linear device to be dropping? - it may be in thermal shutdown, and since it doesn't have a ground terminal, I'm not sure what'll you see if it can't cope.

3/ What values have you got for the output-to-adjust feedback resistor and the pot? and is it a lin or log pot?

4/ Is there any real reason to use an adjustable regulator? - a linear 7805 variant reg would be dissipating vast heat, but  I make extensive use now of switching regulators such as Recom R-78C5.0 - they can run off up to about 40v input, but because they're switching devices, their efficiency is much much higher & they don't heat-up anything like so much

Dave

Edit: oops - too sloooooow a typist! - cross-posted same time as John & Andreas

Offline shipto

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Re: What am i doing wrong?
« Reply #4 on: April 05, 2015, 06:09:30 AM »
Actually I just noticed the potentiometer on the 317 is wrong I actually have two resisters r1= 270ohm r2=820ohm following the below chart which should result in 5.05v. I am very sure I have it all wired the right way around but I will try it all again just in case. As no one has (yet) said theres no way it will work I am guessing it has to be a wiring problem, which I wont rule out.
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Offline John Rudd

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Re: What am i doing wrong?
« Reply #5 on: April 05, 2015, 06:15:11 AM »
From a design perspective, as per the manu's recommendations, R1 should really be 240, make R2 a 1K preset to trim the volts....
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Offline shipto

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Re: What am i doing wrong?
« Reply #6 on: April 05, 2015, 07:03:50 AM »
thank you all I have a successful circuit, at least on the breadboard lets hope I can transfer it to the verostrip with the same results
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Offline John Rudd

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Re: What am i doing wrong?
« Reply #7 on: April 05, 2015, 07:07:13 AM »
lets hope I can transfer it to the verostrip with the same results

Couple of tips for using Veroboard, use a small drill to create breaks in the tracks, run a thin blade down all tracks after you have finished soldering....ensures you gave no adjacent shorts.... :zap:
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Offline shipto

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Re: What am i doing wrong?
« Reply #8 on: April 05, 2015, 07:11:56 AM »
just found the 317 on the board is knackered probably my fault but good to know reason.

Andreas that pdf is teaching me many things, as i stated above my experience is very limited and I have never fully understood a lot of what i do for example I didnt put protection diodes on the lm317 because the schematic didnt show them. I now know they are there to protect from discharge from the capacitors and their inclusion would have stopped me blowing 317's
« Last Edit: April 05, 2015, 08:04:33 AM by shipto »
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Offline John Rudd

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Re: What am i doing wrong?
« Reply #9 on: April 05, 2015, 07:12:55 AM »
Oh dear  :zap: Fortunately they don't cost a lot.... :lol: :lol:
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Offline Bluechip

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Re: What am i doing wrong?
« Reply #10 on: April 05, 2015, 10:05:53 AM »
Some LM317T chips need a MINIMUM load of 10 - 15mA to regulate correctly. ( Been there !! )

Also,

First Diag: The diode above the LM338 is wrong. It don't seem to do anything ??

See att.
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Offline shipto

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Re: What am i doing wrong?
« Reply #11 on: April 05, 2015, 10:27:33 AM »
I think during normal operation it doesn't I believe its there in case the cap somehow manages to discharge through that line.
dont ask me how I am guessing.

Because I am still struggling the circuit seems absolutely fine until I try to put a cap on the voltage out line then even with diodes it makes the 317 go pop, lucky I have so many but I dont really want to blow any more if i can help it.
Heres the rub I tried to remove caps from the 5v line and thats fine except the infrared driver goes haywire without any  :bang:
although I currently have the driver working from a seperate power source maybe it will be fine if I connect it as it needs to be?
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Offline Bluechip

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Re: What am i doing wrong?
« Reply #12 on: April 05, 2015, 10:50:58 AM »
If the capacitor on the output is charged up ( would be anyway ) and say the input got disconnected. The output would then be +ve of the input. This is back to front. Most regulators will not like this and promptly perish. If it happens, D1 will conduct and connect the input to the output, ( less some 0.6V, the volt drop across D1 ), this is relatively harmless and the chip will survive.

There does not seem to be any DC path on the cct. you first posted from the output, through R1, adj. terminal, and R2 to deck as shown in the diag. I got from the LM338 data sheet. It's not marked on your diag. but it could be the attempt at D2 on the one I posted ???

It looks to be the wrong way round ???

Check the differences between the two LM338 diagrams ??

Where did you get yours from ??
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Offline shipto

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Re: What am i doing wrong?
« Reply #13 on: April 05, 2015, 11:22:27 AM »
from google images some have c2 some dont but I think i see what your saying c2 may need to be there.
I am going to a friends his daughter in law has qualifications maybe she can help me.
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Offline shipto

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Re: What am i doing wrong?
« Reply #14 on: April 05, 2015, 02:39:53 PM »
Went back to drawing board and I think (no fingers crossed emoticon?) I have it, rather than trying to run the 5V of the 24V line I thought what if I ran them both from the feed line. So I hooked a 12V supply to the breadboard along with 2 lm317's and got the result in the picture.
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Offline shipto

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Re: What am i doing wrong?
« Reply #15 on: April 24, 2015, 03:37:56 PM »

The lm338k is a 5 amp power regulator which is supplying the 24v line and this is working fine
:lol: :lol: :lol: :bang: :bang: :bang: :bang: :lol: :lol: :lol:
The resisters must have been the only voltage regulation that was going on here because the lm338k's where fake.
I have solved the problem I was having another way but took another one of these to make another power supply for something else and while checking on the net for the diagram I found <a href="http://www.youtube.com/watch?v=SNClz-QeXxw" target="_blank">http://www.youtube.com/watch?v=SNClz-QeXxw</a> so I opened up a few of mine and see pic
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Offline rowbare

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Re: What am i doing wrong?
« Reply #16 on: April 27, 2015, 12:47:52 PM »
What did the guy in the video expect to find in there? A pony? There is no way that one could tell using a gram kitchen scale whether or not a chip is real.

There isn't a whole lot in any IC. See the little square "nose" under the eyes made by the leads in the third picture? That, in all likelyhood is the circuit.

I am not saying that your chip was not counterfeit but that test is worthless.

bob

Offline shipto

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Re: What am i doing wrong?
« Reply #17 on: April 27, 2015, 05:36:16 PM »
I know that part is the chip but that is approx 2.5mm square and the wires going to it where 0.06mm thick I would guess that its a chip thats meant to carry a few milliamps just so that you get the correct voltage. The lm338k is a 5 amp regulator I would say the wire would need to be a bit thicker than 0.06mm I am sure someone with more knowledge would be able to work out what thickness would be required to carry 32v at 5 amps but judging by car fuses I would guess around 0.8mm maybe?
There are other clues too the lettering is easy erased by just rubbing with fingers, I believe the origional manufacturers use a more robust printing process also it is not quite square and none of the measurements on the spec sheet are correct all are close but I suspect the manufacturer would be pretty precise about these details.
All of which is no help to me I have solved the problem another way and ordered more chips from rs components now, the cheaper lm338t rather than the lm338k. I am just thankful that I didnt pay the price the proper lm338k's are worth.
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Offline John Swift

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Re: What am i doing wrong?
« Reply #18 on: April 27, 2015, 06:04:50 PM »
 more info on fake semiconductors

http://www.hq.nasa.gov/office/codeq/trismac/apr08/day2/hughitt_NASA_HQ.pdf

http://www.cti-us.com/pdf/CCAP-101InspectExamplesA6.pdf


on an IC data sheet I have
it noted the IC package can be dissolved by - Chlorine-based solvents: Trichloroethylene, Trichloroethane, etc.
Aromatic hydrogen compounds: Benzene, Toluene, Xylene, etc.
Keton and Acetone group solvents

I may have to try it  some time  :D

   John

PS
 for a bit of fun ,  if you  was  interested in photos of the silicon dies   http://micro.magnet.fsu.edu/creatures/pages/enterprise.html
http://micro.magnet.fsu.edu/creatures/index.html

Offline Baczlwding

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Re: What am i doing wrong?
« Reply #19 on: June 28, 2015, 12:26:21 PM »
Working on a project that requires a dual voltage power supply 24v and 5v my electronics skills are limited to following other peoples circuits and this one is me combining 2 seperate circuits.
Please excuse the crude drawing but I have included it to show the way I am attempting to accomplish this.
The lm338k is a 5 amp power regulator which is supplying the 24v line and this is working fine however the lm317t which is intended to take that 24v down to 5v is the problem no matter what i do I cant seem to get it to work I am getting 24v on the 5v line.
I have searched and cant seem to find the answer but I am sure it should work and can only assume I am missing something.
Thanks for any help

Offline picclock

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Re: What am i doing wrong?
« Reply #20 on: July 02, 2015, 05:22:11 PM »
Just as an observation - I think if you want to add a 5v power supply the easiest way is to get a scrap phone charger and connect the 0v phone charger output to the 0v of the 24 volt supply. You then have a 5v and 24 volt supply. Also no heat issues and very little to do in the way of connections .. .  :med:

Best Regards

picclock

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