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 Forum index » DIY Hardware and Software » Lunettas - circuits inspired by Stanley Lunetta
Constant Current LED Indication
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piedwagtail



Joined: Apr 15, 2006
Posts: 274
Location: shoreditch
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PostPosted: Fri Mar 30, 2018 6:49 am    Post subject: Constant Current LED Indication
Subject description: an Easter empirical from the Wagtail notebook
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The distress of switching noise has bothered me enough that I researched ways of eradicating it for Easter. Wink

Graham Hinton alluded to using J112s and J175s over at the other spot but they're becoming hard to obtain. So I pulled out my trusty IRF610 and tested the following circuit. The current doesn't dump into ground, in fact it only changes on power up. After that it pulls a steady current between the 15V Rails. There's a play off between biasing the FET and the current you need. FETs are notoriously approximate; so a test jig may be needed to assess a particular FET's Rx. Rx has a range which needs to take into account the voltage needed for the LM317 to regulate the current.
The IRF610 inverts - shorts the LED off when logic goes high.
The second diagram uses the complementary IRF9610 which produces a non-inverting action- logic high LED on.
With modern LEDs 2.5mA is bright.
TO-220 are hilariously big components but they are robust electrically; go blow a J112 and keep smiling.
............
4066 and 405x in series with the LED and LM317 could be used but the +/-rails cannot extend to 15V either side.
DG418 can suffice, but the price point is at least 2.5X the Mosfet whilst it must be the right version not a low voltage type; the advantage is the possibility of a SOIC package.
.................

3/4/2018 update
Ordered BSS84 SOT23-3 package for feasibility study along with 100mA LM317s
..............
8/4/2018 update
The above combination is now projected to be the goto selection for boards.
Works as second picture 470/470k/8k2
Tests 2.7mA
Stay with adaptions of LM317/IRF9610s for installation situations


InvertingCCLed.jpg
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Inverting
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