Expansion connector pin current limits



  • Hello, I was wondering how much current can one output pin on the expansion connector supply (or sink rather)? I'd like to drive 2 SSRs from one output. I'm aware that one is no problem, would just like to know if I can hook the second in parallel or if I should drive them through a small transistor.


  • administrators

    SSR control inputs draw very little current, so try connecting them both in parallel.

    The following processor pins can supply twice as much current as the others: PA[5–8], PA[12–13], PA[26–28], PA[30–31], PB[8–9], PB[14], PD[0–1], PD[3–17]. There is a link to the pin allocation table in the Developers page on the wiki.



  • Unfortunately none of the extra heaters are on those pins. The only numbers I found in the Atmel IC datasheet were the total limit of 150 mA and limits of 2 mA for the individual pins. The pins you mention are rated for 4 mA.

    @dc42:

    The following processor pins can supply twice as much current as the others: PA[5–8], PA[12–13], PA[26–28], PA[30–31], PB[8–9], PB[14], PD[0–1], PD[3–17]. There is a link to the pin allocation table in the Developers page on the wiki.

    A single SSR would draw around 10 mA to turn on fully. Since my heater worked fairly well off the pin without any apparent damage, I assume the IC can easily tolerate that… but I guess I'll err on the side of caution and drive it all from a transistor.


  • administrators

    The 2mA and 4mA are not ratings, they are maximum values for which the output is guaranteed to be no more than 0.4V from the appropriate supply rail. Microchip doesn't publish ratings for individual pins.

    I think most SSRs will draw much less than 10mA when driven from 3.3V. The 10mA figure probably refers to the draw when you use a voltage closer to the maximum input, which is typically 32V.



  • Ah, makes sense, thanks. 10 mA was for 12V I think.


 

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