I'm learning electronics and I have an old C64 power supply which was made at our local electrical shop back in the 80s. I'm taking a look at it to try and understand how it works.

What I don't understand is why the DC ground is connected to mains earth. From what I gather this is to give a ground reference to the DC side but why would you do that?

Also the power LED anode is connected to +5V output on the 7805 and the cathode to the metal chassis. I can see how this works but does this mean current albeit small is flowing through the chassis to get to the regulator's GND?

Finally I would like to probe this with an oscilloscope to measure the ripple at various stages of the circuit. My assumption is that as all grounds are connected there's no problem probing anywhere on the low voltage side. Would I just put the probe crocodile clip on the chassis ground point?

  • \$\begingroup\$ What you are proposing doing can be quite dangerous, and possibly even fatal if you don't know what you're doing or you touch the wrong thing. I will give you one hint. If your scope is powered from mains, the "crocodile clip" is ground, and you need to understand how that can create an additional current path. \$\endgroup\$
    – M D
    Commented Mar 5, 2015 at 17:38
  • 1
    \$\begingroup\$ @MD Yes, it is mains powered so if the clip is earthed and I connect it to the power supply earth there's no problem, right? I do understand that clipping it to, say, the 9VAC transformer output is going to short it to ground which is bad. \$\endgroup\$ Commented Mar 5, 2015 at 17:44

1 Answer 1


Why is the DC ground connected to mains earth? While I am not overly familiar with safety qualifications, I believe it is now mandatory, and has been "best practice" for quite some time, to have some potential in the system tied to earth, and typically to the chassis. As long as the DC output is not required to be floating, the most common connection is earth/chassis/DC ground all tied together. This output is generally the one of use to the human being utilizing the supply. This is different than say a lab/bench supply where you typically have a "-" prong that is the floating supply's return path and a GND/Earth prong that is tied to chassis and earth for safety reasons. This allows you to stack supplies which you can't do in the case of a earth-referenced output.

As for the LED, you are exactly correct. But, if chassis = DC GND = earth, this works. Whether or not that is allowable by modern-day safety standards is unknown to me. This isn't that much different than how an automobile is typically wired, and I'm sure that wasn't completely uncommon "back in the day". I believe that CRT TVs/monitors still had live chassis connections even into the 90s.

I don't feel very comfortable answering the last part of your question. If this is an isolated supply with earth connection on the DC/output side, the AC side has to be considered completely floating. Don't go sticking a probe in there with your ground clip on the DC side. Tektronix did a very good whitepaper on how to probe floating supplies several years back. I highly recommend you try to track that down. I believe it was called Fundamentals of Floating Supply Measurement. Be safe!

  • \$\begingroup\$ Thank you for your answers. I had completely forgotten that the -ve terminal of a car battery is connected to the chassis. I will make sure I read some more on how to probe power supplies. \$\endgroup\$ Commented Mar 5, 2015 at 20:43

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