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I was reading a bit from the excellent Ralim blog about the TS100 PCB design. There I met something I do not fully understand:

The output drive for the iron tip is run through a capacitor, then to a NPN transistor that switches the P MOSFET’s on/off to allow power to run through to the heating element directly from the DC IN. This is of note as it means that should the output be left HIGH or LOW the iron tip will be off, preventing any software errors from causing damage.

From: https://ralimtek.com/posts/2016/ts100/.

Can someone explain this to me? I am not sure what the circuit actually looks like from this description. Is this a common way to design a safe power controlling circuit? (It sounds like it based on the tone of the writing.)

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  • \$\begingroup\$ It's just using the capacitor in series with the heating element as an AC-pass, DC-block. So the power signal always has to be changing for any power to make it through and get to the heating element. \$\endgroup\$
    – DKNguyen
    Aug 29 at 7:57
  • \$\begingroup\$ But then you have to use AC as you say, and the NPN transistor can only be switched on half of the time if not using some form of rectifier, or am I missing something? \$\endgroup\$
    – Zorglub29
    Aug 29 at 8:07
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The circuit is designed such that the microcontroller GPIO controlling the heater circuit must be pulsing in order for the capacitor to couple a signal through to the NPN transistor in order for it to turn on the p-chan mosfet. This way if the microcontroller locks up for whatever reason, the GPIO won't pulse and the NPN transistor circuit will timeout and remove the power to the heater.

The circuit configuration is a 'one-shot' or monostable multivibrator.

It's a very common technique for fail-safe circuits. no pulse, no go.

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