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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets. |
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9th Sep 2019, 1:00 pm | #41 | |
Dekatron
Join Date: Dec 2007
Location: Haarlem, Netherlands
Posts: 4,203
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Re: Defective/ fake LM317T
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9th Sep 2019, 1:40 pm | #42 | |
Dekatron
Join Date: Jun 2016
Location: London, UK.
Posts: 3,687
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Re: Defective/ fake LM317T
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A neat trick to making calculations easier is to use a 120 ohm resistor there. This does two things: Firstly it makes the other resistor value easy to calculate in your head as the reference adj is 1.25 volts. So for 12 volts you just take 1.2 volts off and multiply by 1000 then round down to the nearest E12 so 1K. This pops out around 11.7 volts, slightly under intentionally, so you can then slide a 100 ohm trimmer on the end to give 11.7-12.7 volts for precise trimming or use a precision E24 in there instead. Secondly this divider also sinks 10mA which is the minimum required DC load current for regulation. That means it'll be quite happy with whatever is or isn't connected to the output. With a 240 there it's only about 5mA which is borderline unstable with about a 10% chance of doing something weird. There are some real cock ups with the LM317 out there. QRP labs sell a few ... |
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9th Sep 2019, 3:36 pm | #43 | ||||
Nonode
Join Date: May 2004
Location: Halifax, West Yorkshire, UK.
Posts: 2,587
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Re: Defective/ fake LM317T
This is one of those threads that has generated so much noise that it's hard to see the wood for the trees. To my (simple) mind the following chronological extracts and the attached image crop may hold the clues needed to solve the mystery:
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1. Remove the flying pot with its leads and put back the original. 2. Reinstate the supplied LM317 assuming it too hasn't been fried. 3. Make sure that any circuit board damage is made good and that there are no shorts or discontinuities. 4. Try again. Alan |
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9th Sep 2019, 4:24 pm | #44 |
Octode
Join Date: Feb 2018
Location: Southampton, Hampshire, UK.
Posts: 1,063
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Re: Defective/ fake LM317T
The aim of mounting the pot the way I did was to make it panel mountable and make the entire unit fit in a case. I would not have been able to do that with the supplied blue pot with a tiny screw for voltage adjustment. In relation to the wiring of the pot, the PCB already came with a tracing linking the wiper to one end of the track.
I selected a 22K panel mountable pot because I have no more 10K ones, at least for now. I also suspect my 10K pot may have been burnt by the IC, because it was reading in the megaohm range when I tested it after desoldering. |
9th Sep 2019, 4:35 pm | #45 |
Octode
Join Date: Feb 2018
Location: Southampton, Hampshire, UK.
Posts: 1,063
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Re: Defective/ fake LM317T
@ajgriff : here is what I am hoping to achieve in the end. The picture is of a unit I built last year from a preassembled regulator module but it is too noisy for radio circuits, hence the new LM317T circuit
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9th Sep 2019, 5:47 pm | #46 |
Dekatron
Join Date: Apr 2012
Location: Wiltshire, UK.
Posts: 14,007
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Re: Defective/ fake LM317T
As others have noted, you need a certain minimum current through the resistor-network; I've generally used a 5K variable, *and* a separate load-resistor from the +ve output to ground to place a consistent load on the chip.
The diodes and decoupling caps are highly recommended too! [I've used loads of the TO3-cased LM117/LM317 3A-rated "Steel" regulators in the past and they behave well if treated well; I've also built quite a few little power-supplies using a Vellemann kit http://www.velleman.co.uk/contents/e...198_k1823.html based on the TO220 LM317 and they work just fine for powering Roberts radios like the 707 and 606]. |
9th Sep 2019, 6:36 pm | #47 | |
Nonode
Join Date: May 2004
Location: Halifax, West Yorkshire, UK.
Posts: 2,587
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Re: Defective/ fake LM317T
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Alan |
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25th Sep 2019, 12:31 am | #48 |
Octode
Join Date: Feb 2018
Location: Southampton, Hampshire, UK.
Posts: 1,063
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Re: Defective/ fake LM317T
Well, here's finally an LM317 that's not fake. I built a fully functioning voltage regulator from it and some other parts, including a scrap VCR transformer referred to in another thread, a heatsink from an old Woolworths wall wart and a special 220 ohm resistor (reading 250 ohms) from BVWTM ! It works fine with both a 6V radio circuit and a 24V motor.
Many thanks to everyone who helped me build this regulator properly from the scratch and without a kit !! The kit that failed earlier on has been repurposed into a fixed 6V regulator. It works fine with a 'flying pot'. I selected a 5K one, a better wirewound Bourns one is on its way. Last edited by Jolly 7; 25th Sep 2019 at 12:38 am. |