It is possible to programmatically disable the pull-down resistor

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uralpt
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It is possible to programmatically disable the pull-down resistor

Post by uralpt » Sat Mar 16, 2019 7:00 pm

Hi, friends.
I made two optocoupler boards, the first using 6N137, the second using PC817, both 16 channel. Pins KFlop as IN.
I want to use I/O pins JP4 and JP6 (LVTTL, 3.3V), but resistors 150 Ohm reduces the voltage on the input to a logical 0.
Can programmatically disable these pull-down resistors? I need only high-impedance input without a pull-down resistor on 16 channels.

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TomKerekes
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Re: It is possible to programmatically disable the pull-down resistor

Post by TomKerekes » Sat Mar 16, 2019 7:47 pm

Sorry no.
Regards,

Tom Kerekes
Dynomotion, Inc.

uralpt
Posts: 4
Joined: Sat Mar 16, 2019 6:34 pm
Location: Russia, Miass (very far)

Re: It is possible to programmatically disable the pull-down resistor

Post by uralpt » Sun Mar 17, 2019 8:06 am

Hi, Tom.
I know I'm wrong somewhere, but I don't know where.
Made the board 6N137, it works well with high impedance multimeter input, but does not work with KFlop board. Here is the schematic (111.jpg)
111.jpg
OK, I didn't know the board had pull-down resistors installed :))

I made the second board with PC817C (222.jpg), but I also get a high level voltage of about 1V on it.
222.jpg
What am I doing on the second board wrong?

ps.
sorry for the paint and bad camera on my phone :))
board_6N137.jpg
board_PC817.jpg
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TomKerekes
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Re: It is possible to programmatically disable the pull-down resistor

Post by TomKerekes » Sun Mar 17, 2019 4:39 pm

hi uralpt,

In circuit #2 I don't think you are driving the LED hard enough.

The 150ohms needs ~ 16ma to drive it to a solid high 16ma x 150ohms = 2.4V

Because the PC816C has a CTR of 50% the LED needs 32ma

If you assume the LED voltage is 1.4V then the current from the 820ohm resistor is:

(18V - 1.4V) / 820ohms = 20ma

But the 62ohm resistor steals all of it:

1.4V / 62ohms = 22 ma.

Maybe remove the 62 ohm and reduce the 820ohms to:

(18V - 1.4V) / 32ma = 520 ohms
Regards,

Tom Kerekes
Dynomotion, Inc.

uralpt
Posts: 4
Joined: Sat Mar 16, 2019 6:34 pm
Location: Russia, Miass (very far)

Re: It is possible to programmatically disable the pull-down resistor

Post by uralpt » Mon Mar 18, 2019 1:56 pm

Tom, thanks!
I`ll apply your advice soon. It takes a little time, which is never enough :))
I'll write soon.

uralpt
Posts: 4
Joined: Sat Mar 16, 2019 6:34 pm
Location: Russia, Miass (very far)

Re: It is possible to programmatically disable the pull-down resistor

Post by uralpt » Sat Mar 23, 2019 10:22 am

Hi, Tom.
Thanks for the support, all excellent!

My problem was that I did not supply power to the KFlop :)) and the output stage of the optocoupler passed all the current to KFlop.
I checked - the optocoupler works fine with a current of 20mA, since the CTR is> 50%.

But your circuit is better, there is no R1 (820) heater :))
Thanks!

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