Author Topic: Strange resistor in HCPL3700 example schematic...  (Read 720 times)

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Offline kirillandyTopic starter

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Strange resistor in HCPL3700 example schematic...
« on: February 22, 2020, 08:35:02 pm »
Hi all!
First time posting here :)
I've got a small question about a rogue resistor that appears on the example schematics of the HCPL3700 optocoupler, which I'd like to use as a switch.
I've managed to surmise (I think...) that the output of the IC is a Darlington/Sziklai transistor whose main function is to amplify the input current using the small current at the base from the photodiode. There's also a bypass capacitor installed (as you do), but what is the Rl resisitor between Vcc and Vout?
What's more, I've seen a version where it is connected to another capacitor (making it look like some low-pass filter, see Test circuits here: https://www.onsemi.com/pub/Collateral/HCPL3700-D.pdf) and a version where there is no additional capacitor which made me think it was some pull-up resistor (see attached schematics, I got them from some website)
The datasheets never even mention the Rl resistor and it's making me wonder whether it's some obvious important component which any self-respecting EE will immediately recognize... making me rue the fact that I'm not a self-respecting EE :P
What is it for? Is it necessary? And, more importantly, how will its resistance value affect the output of the circuit?

 

Offline wraper

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Re: Strange resistor in HCPL3700 example schematic...
« Reply #1 on: February 22, 2020, 08:57:44 pm »
Dunno why you find it strange. Optocouplers in general often are used with pull-up on the output. And for those with logic output like this one, it's given to be there. Optocoupler simply won't work without some sort of pull-up or some sort of small load from Vcc. Bypass capacitor is required too.
 
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Offline kirillandyTopic starter

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Re: Strange resistor in HCPL3700 example schematic...
« Reply #2 on: February 22, 2020, 09:11:13 pm »
Ah, OK, I was so distracted with figuring out how the transistors work that I forgot I was dealing with a simple logic circuit at the end of the day. So when the optocoupler is OFF (the led inside is OFF), the output is pulled up. And when the optocoupler is ON, the the transistor is open and current flows to ground? I always have trouble actually visualizing where current flows in different circuit states  :palm:
 

Offline wraper

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Re: Strange resistor in HCPL3700 example schematic...
« Reply #3 on: February 22, 2020, 09:16:45 pm »
So when the optocoupler is OFF (the led inside is OFF), the output is pulled up. And when the optocoupler is ON, the the transistor is open and current flows to ground?
Yes
 


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