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I'm trying to build a timed fan, but I seem to be drawing more current from the battery than expected.

enter image description here

Not shown: 3.7V Li-Ion battery connected to a 5V boost converter (which is how I'm getting the 5V).

When D0 is disconnected from the BJTs (and resistors), I draw around 100mA from the battery and the Attiny begins to heat up to around 33-35 degrees. I've checked that there are no shorts anywhere, and when I completely disconnect the Attiny (ie pulled it out of its socket), the current draw drops to zero, so it seems like the Attiny is pulling 100mA for no apparent reason.

The boost converter is able to handle the load, as the output voltage remains at 5V so it's most likely not the boost converter.

Any potential reasons for this?

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    \$\begingroup\$ Latch-up due to ESD? Overshoots due to complete lack of bypass capacitors? \$\endgroup\$
    – Justme
    Commented Jan 5, 2023 at 20:41
  • \$\begingroup\$ Some stuff to check: A) Check your program for any sort of loops or logic that would have any effect on unused pins that could be shorted to ground or VCC. B) Cody Phipps suggestion is certainly a possibility. C) show us a complete schematics with all connections ( or no connection ) to the other pins not shown. D) reprogram the ATtiny with a do nothing short program just to confirm it is not the problem source. E) Check if your program send a fast PWM output at OFF state. ( what is the D0 measured output voltage ? ) \$\endgroup\$ Commented Jan 5, 2023 at 20:49

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If you are confident that the tiny is connected correctly, make sure it wasn't shorted out to begin with (sometimes if there is a fault within the Atmel chips to begin with, GND and VCC get shorted together).

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    \$\begingroup\$ Yep that was it. Resistance between VCC and GND was 70 ohms, whereas a different Attiny had around 3.1 megaohms. Thanks! \$\endgroup\$ Commented Jan 5, 2023 at 20:57
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    \$\begingroup\$ So Atmel sells faulty chips? \$\endgroup\$ Commented Jan 5, 2023 at 21:01
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    \$\begingroup\$ Exposure to ESD can cause them to short. If you have them laying around on a desk or out of the packaging or static bag, chances are it will get exposed to something. @user253751 \$\endgroup\$
    – PhippsTech
    Commented Jan 6, 2023 at 4:30

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