Do R7 and R2 connect to each other? It looks like they don't. It seems like there is even a black line separating them, but doesn't that then mean that there is no path for electricity through those two resistors because the other side of them don't go anywhere? If that is so, then why do those two resistors even exist? I am just a little confused.

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Follow up question: Does D1 connect to M7? Its got that thick black border around it that makes me think that signifies it doesn't or maybe that is trying to signify the positive or negative side of the diode. I'm not sure.

  • \$\begingroup\$ Hey it's coffee mug heater. What is wrong with it? \$\endgroup\$
    – MiNiMe
    Commented Sep 18, 2023 at 0:25
  • \$\begingroup\$ @user2042006 - Hi, (a) You can't add new follow-up questions in comments - that is not an allowed use of comments (nor is answering in comments) - see here & here for the deliberately limited allowed uses of comments. (b) You also can't edit the question to add new parts or extend existing ones, once you have an answer. That makes existing answer(s) look wrong / incomplete, as they won't have answered any newly added parts. || These are some of the ways the Q&A format of Stack Exchange differs from typical discussion forums. \$\endgroup\$
    – SamGibson
    Commented Sep 18, 2023 at 0:44
  • \$\begingroup\$ user2042006 - Since the (currently only) answer was kindly updated by its author in response to your added question in a comment, I have moved your follow-up question into the actual question text, since now it does not invalidate any existing answer. Please be careful not to do this again, and do please read the two links I gave about the limited allowed uses for comments. Also, if you want to learn more about the problems of changing a question after it has received an answer, search for "chameleon question" on Meta Stack Exchange. Thanks. \$\endgroup\$
    – SamGibson
    Commented Sep 18, 2023 at 1:17

1 Answer 1


The black line is just a layer of ink on top of the circuit board (more commonly known as the silkscreen of a PCB). This layer can indicate anything while simultaneously being unable to actually influence the circuit in any way. That line between the resistors is just that: a line. A line of ink with no more significance than a line scribbled on top of the board with a sharpie.

If you were to carefully scrape off the ink (and the green solder mask below if you really wanted to), I am quite sure you would find a solid copper trace connecting those two resistors to each other in series.

As for where they go, they go across that big yellow mains-rated X2 safety capacitor, making those series connected resistors act like one 400kΩ resistor in parallel with the capacitor.

This for safety reasons - that capacitor could have 300V or more across its leads (depending on where you live and how high the wall outlet voltage is there) and with enough capacitance behind it to give a nasty little shock to any unweary fingers or other meat-based resistors that stray too close to that capacitor's leads. Once unplugged, a film capacitor like that will hold onto that charge for a while, half an hour or even hours or longer if its leakage is really low, which it probably is.

Those resistors (usually called bleeder resistors but sometimes discharge or leakage resistors as well) are there simply to ensure that when that circuit is unplugged, that potentially dangerous voltage is quickly discharged through those resistors in a matter of seconds instead of it sticking around for hours.

Essentially, those resistors are there to protect... you. Or anyone else who decides to open up the device, just in case it was plugged in recently.

EDIT: Just saw your comment about the diodes.

Does D1 connect to M7. Its got that think black border around it that makes me think that signifies it doesn't or maybe that is trying to signify the positive or negative side of the diode. I'm not sure.

Yes, D1 are connected to M7 as well as the trace near the 5.1V/1W label on the opposite side.

I have never seen nor heard of the silkscreen layer being used to indicate any information about the actual connects between components - you can see that already from the traces. I'm sure someone has done it at some point, but it would be very unique and unusual to do so.

Normally, the silkscreen layer is used almost exclusively to identify components with reference designators (R1, D1, M7, etc.), aid in correct placement of that component before it has been soldered down, and when applicable, denote things like polarity if the device is polarized or to point out the location of pin 1 for integrated circuits.

Other common uses for the silkscreen are to include human-readable descriptions of connectors or pins, give warnings or instructions, or simply for decoration. But again, almost never anything related to the actual circuit topology, except maybe to trace out important connections that are contained entirely in an inner layer of a 4+ layer circuit board (and such internal traces are obviously obscured by more circuit board in covering them up). Again, very rare to come across this however.

So yes, those thick black lines are being used to call out the cathode of the diodes.

  • \$\begingroup\$ metacollin - Hi, (a) The OP should not have asked a new question in a comment. That behavior has caused lots of trouble in the past, as answer writers (understandably) can feel aggrieved when their early answer is made to look wrong or incomplete compared to later answers, after new parts have been added. (b) However, since you've kindly updated your answer, I respectfully disagree. You said: "D1 are connected to M7" No, D1 is a diode of type M7, a common type similar to a surface mount 1N4007. See here for example \$\endgroup\$
    – SamGibson
    Commented Sep 18, 2023 at 0:58

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