Author Topic: Open collector driving a SSR  (Read 2899 times)

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Offline bootswithdeferTopic starter

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Open collector driving a SSR
« on: October 16, 2012, 11:41:13 pm »
Hello,

I am trying to design a circuit to control a series of SSRs via an Arduino using the I2C bus.  I am thinking of using something like the PCF8574 (datasheet) which has open collector outputs.  My SSR board is off the shelf (Sainsmart) and are driven by standard NPN transistors (not sure of exact part, it's SMD and labeled G1).  My power supply is a 5V wall adapter.

So my questions: how can I connect these two?  In my searching it seems maybe I could just use a pull up resister?  Is it that simple?  Is there a better choice than the PCF8574?  Any other suggestions?

Thanks,
Jesse
 

Offline Psi

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Re: Open collector driving a SSR
« Reply #1 on: October 16, 2012, 11:49:22 pm »
Here's how the ssr board input is wired, just so others don't have to go find the pdf.



But you are correct about needing a pullup resistor for each output of that I/O expander.
It can only pull high with ~100uA which isn't enough to switch on the NPN in the relay board. A 10K resistor to VCC would fix that.

Or you'd could find a I/O expander that can source current, then you waste very little energy and dont need the pullups.
« Last Edit: October 17, 2012, 12:25:44 am by Psi »
Greek letter 'Psi' (not Pounds per Square Inch)
 

Offline bootswithdeferTopic starter

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Re: Open collector driving a SSR
« Reply #2 on: October 20, 2012, 03:34:37 am »
Thanks.  After some searching I found the MCP23008 which will source current.
 


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