Grounding a DC Circuit

Higgy

I have a quick silly question for a first-timer at assembling circuits. I have a circuit with a photodiode input and a BNC/coaxial output to a scope. The circuit diagram is below.

In assembling this circuit, I'm not sure where to ground to. I've put it in a metal enclosure. One book says that "grounding" usually means connecting it to the metal enclosure. Should I do that? Another source seems to suggest that coaxial cables provide the ground via the scope or whatever they are connected to. Which would mean that I don't need the "ground" in the circuit diagram.

I've drawn up a quick visual to clarify the question (notice the question mark). Thanks!

Attached Thumbnails

   

MATLABdude

(I can't see your schematic)

Remember that ground is only a reference point. All you need is a completed loop.

You probably should case ground your circuit, if they recommend it. However, there is no way to take measurements single-ended (not your standard voltage measurements with your standard oscilloscopes, at any rate). You need to hook up the ground of the oscilloscope probe to the ground of your circuit (or another reference point) in order to take measurements, otherwise, you'll just get tiny little spikes when voltage transitions happen (due to Lenz's law). If you can see them at all.

The problem sometimes occurs when grounds don't line up. If your oscilloscope is hooked up to one ground, but the circuits ground is 50 or 100 V above / below the other ground, bad things can occur. This happens all the time when you have mains voltages on different circuits, routing different paths through the building. Or grounding stuck in physically different locations.

Higgy

Thanks MATLABdude - I've attached the images differently now, so they should be visible.

So I should consider whatever it is I'm connecting my circuit to in deciding where to ground (scope, dmm, etc)? That makes sense. Unfortunately, I don't know where the scope's ground is.

Would it be a safe assumption that the scope is grounded through its power cable (which goes to the wall power)? Assuming that's the case, should I ground the circuit to the case, and then ground the case to .... the ground (presumably a cable with a clamp I could attach to a workbench, etc)? Or is there anything magical about the coaxial cable between the circuit and scope that gives them an equivalent ground.

Sorry for all the stupid questions - said "sources" just breeze over this stuff. I'm not working from any particular instructions here - really just on my own, that's why I came here - and the "book" and "source" I mentioned were just books lying around the lab.

Thanks again!

MATLABdude

Do you come from an electrical background? I'd grab the old hand in the lab and have them give you a tutorial. And if you have no background / experience (or it's been a while), and nobody around the lab who's got that experience, I'd talk to your supervisor / manager / departmental electronics tech. Unless you are any of the above!

In order to make a measurement, any measurement, you need a reference. For a DMM, that means hooking up the black terminal to the ground in your circuit. For an oscilloscope, that usually means hooking up the little clippy thing dangling off your main hook / probe up to your ground circuit (and yes, a ground connection is made via the cable's outer jacket):
http://en.wikipedia.org/wiki/Test_probe

Like many people doing this for the first (few) times, you've still got a notion that ground is a special point to which all electrons flow. However, ground is also the place from which all electrons (in your circuit) flow. In short, it's just a node with current_in equal to current_out!

Higgy

...And I thought I did a thorough search of Wiki - didn't come across that article! I'll give it a read, think about what you said, and come back if I'm still confused.

(I think I'm getting confused because the last time I did any electronics was ~3-4 years ago in a simple "Electronics for Physicists" course, where everything was done on huge clunky breadboards with a few gigantic variable resistors nailed to pieces of wood. In there, we would make a common ground the way you mentioned. But I've been using circuits (pre-built) in here that I just connect to the scope and go - one cable, one job, no extra thoughts. Particularly, when I've used pre-mounted photodiodes before, it's just a coaxial (BNC) connection to the scope. So when the circuit diagram seemed to suggest a second cable going somewhere... I wasn't sure if that was needed.

And no, there isn't anybody around I can turn to at the moment. It's the summer, and people tend to be scarce!)

Higgy

(please excuse the double post)

Hmmm... I'm not using an "oscilloscope probe" as discussed in that article - I'm literally taking a BNC cable, plugging one end into the "output" of my photodiode circuit (as in the diagram above), and plugging the other end into "Input 1" of the scope. So no clippy thing. But given the last bit you said (which is not as easy to learn searching the internet as you might think!), - since I've wired together the "-" end of my 9V battery and the "ground" side of the capacitor in the diagram above to the outer sheath of the BNC connector that goes to the scope, I'm guessing that my circuit is indeed grounded through the scope, i.e., it shares a common ground with the scope through the coaxial cable, and since I know for a fact that the scope is indeed grounded (and not floating), all is well with the world, and there's no reason to physically solder a connection between the "ground" point in the circuit diagram to the physical metal case it sits in.

*phew*

Thanks again.

MATLABdude

Of course it isn't easy to find; that's the sort of thing that's usually taken for granted and passed on from person to person in about 5 minutes. You shouldn't be afraid to ask questions in your lab for fear of looking dumb; we've all been there, and you're playing around with stuff that cost thousands of dollars. Even though it may be summer, there should be *somebody* where you work (physics department?) that's still around that knows something about instrumentation.

Now take a look at your BNC connector, as if you were about to plug it into your eye socket (but don't do it--you don't have the right impedance!) The pin right at the centre is where the signal goes, and this is connected to the centre conductor of your coax cable. The concentric metal shell around it (just inside of the metal shell that holds the connector in place) is where the negative reference goes, and this is connected to the jacket of your coax cable.

Redbelly98

I wouldn't think case-grounding is necessary here, as long as the battery voltage is at a safe level. I.e., if it's safe to touch the battery's terminals, then there are no real safety issues that necessitate case-grounding.

However, I do have a question about the circuit as drawn. The photodiode essentially acts as a current source. It is trying to move that current through the oscilloscope. Will the oscilloscope impedance be set to a nominally high value (typicall 1 MΩ), or is there a low-impedance (typically 50 Ω) setting that will be used? Also, what is the purpose of the 1 kΩ resistor?

Higgy

The oscilloscope impedance is 1 M[itex]\Omega[/itex].

Reverse biasing a photodiode gives it a faster response. The response time depends on the size of the capacitor and load resistor. I'd also guess that you could swap out the 9V battery for larger voltage sources, in which case the resistor would protect the photodiode from risk of damage, since the photodiode has only a very small impedance.

Redbelly98

Okay.

Note that the load resistance is actually

R_L = 1MΩ + 1kΩ ≈ 1MΩ

The 1kΩ is not having any appreciable effect on the circuit as presently drawn. In practice the 1kΩ should be placed in parallel with the scope's 1MΩ.