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Old 15th June 2013, 11:56 AM   #1
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Default Is my Home Ohm free ??

Here's one that is baffling me.

The original problem was that when I tried to build a multiplexed LED display the LEDs were too dim to be visible under normal lighting.

Here is a little test circuit that I'm playing with around an IL-1 opto-isolator.

The circuit is fed with a 5V 333Hz square wave 1mS (ON) 2mS (OFF).

Now Ic(max) of the IL-1 is 50mA (cont) and 400mA (t(on) < 1mS).

So ignoring Vce of the IL-1 I tried R = 220R and measured the voltage across R with a 'scope. V=1.5V so I=7mA.

A lot lower than I expected.

OK I thought lets try R = 100R. V=0.7V so I=7mA.

Weird.

OK I thought lets try R = 47R. V=0.34V so I=7mA.

Weird.

I'm missing a trick here somewhere but I can't figure out where.....
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Old 15th June 2013, 12:02 PM   #2
Mooly is offline Mooly  United Kingdom
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Not an unexpected result I think. Try an opto with a higher "current transfer ratio".
Current Transfer Ratio (CTR) and Response Time of Photocouplers / Optocouplers | Renesas Electronics Europe

Are you driving the opto hard enough ?
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Old 15th June 2013, 12:03 PM   #3
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That's a thought, I'm only driving the opto at 10mA, the LED is rated to 60mA but I took that as a maximum.
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Old 15th June 2013, 12:20 PM   #4
FoMoCo is offline FoMoCo  United States
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CTR of an opto can be anywhere from 20% to 600%. Most are between 50% and 150%. So... Nothing funny with your circuit.

Why on earth use an opto for a multiplexed display?
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Old 15th June 2013, 12:21 PM   #5
Mooly is offline Mooly  United Kingdom
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I think tbh you really need a high transfer ratio device. They are really darlingtons internally and ratios up to 15000% (maybe more nowadays) are available.

Buy Optocouplers Optocoupler Transistor O/P, 300V, SMD4 Lite-On LTV-852S-TA1 online from RS for next day delivery.
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Old 15th June 2013, 12:24 PM   #6
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The problem is I'm playing with PICs. The maximum output current of the PIC that I'm using is only 25mA per pin. The LED displays need about 160mA with the low duty cycle to achieve full brilliance.

I've now increased the opto LED drive to the maximum of the PIC (ie 25mA) and with R = 100R I'm now seeing ~ 13mA through R.

I'm guessing that the IL-1 I'm playing with is a bit outdated.

The LEDs are Common Cathode so I've had to add an amplifying element between Vcc and the Anodes to increase the available current. Using PNP transistors isn't viable because of the 5V logic level of the PIC. My solution was to try opto-isolators. I'm only playing at the moment and thinking with my fingers and bits box.

Last edited by KatieandDad; 15th June 2013 at 12:30 PM.
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Old 15th June 2013, 12:27 PM   #7
FoMoCo is offline FoMoCo  United States
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Not outdated, just not designed for that purpose. Why not a simple 2n4401 or 2n2222(A) level shifter? Or you could just add a 4401 to your existing opto as an emitter follower.
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Old 15th June 2013, 12:28 PM   #8
Mooly is offline Mooly  United Kingdom
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Not so much outdated, rather just a typical characteristic of a "single transistor" opto.

Something like a 2N7000 FET would be ideal for a PIC. You wouldn't need any gate resistors and the 2N7000 makes a great power switch... unless you really need the isolation of course.
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Old 15th June 2013, 12:38 PM   #9
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My mistake was that I opted to drive the displays with an 18V battery..

I've got to maintain 0V integrity because of other logic elements in the circuit.

I suppose I could drop Vcc (LED) down to 5V.

Originally I used 2N3906 (PNP) transistors to supply the additional current but then I lost 0V integrity because +18V(LED) had to be tied to +5V(PIC) so that the transistors would operate. That it what took me down the opto route.
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Old 15th June 2013, 12:48 PM   #10
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Is the LED ground shared with the PIC ? If it is shared a 2N7000 is still perfect even if the LED's run on 18 volts.
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