Advanced Study of Accurate Voltage Measurements - Basics of VCC Measuring
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Basics of VCC Measuring
Now it's time for us to measure the VCC (CPU core voltage). As I mentioned a while ago, your target VCC is generated within your VRM circuitry. Remember? That's the location that's next to your CPU socket with MOSFETs, inductors, and a few electrolytic capacitors. The measuring technique is the same one we used for the rails.
You should set your DMM on the appropriate voltage range and insert your black probe into a GND. Technically it can be any ground, that is, you could pick a black connector from a 4-pin Molex or AUX (or any of the motherboard holes where standoffs are placed); however, that's not very accurate because of resistance. We know that even a piece of wire has resistance. It would be best to find a VGND or a ground that's closer.
What do we do with the red probe? Obviously, you should reverse engineer the motherboard's VRM circuitry to find the most suitable spot for accurate VCC measurements. The design of this VRM sub-circuit is different on each motherboard, so there's no default spot.
You need to understand the main concept of VRM circuitry. The VCC regulator gets its feedback from a point of the plane, and according to these received values it tries to maintain or generate the appropriate voltage as seen from this feedback point.
Furthermore, you could use the output leg of an inductor (find them next to the MOSFET stages and capacitors) as a measuring point. Some motherboards might have copper planes behind the processor socket on the back of your board; this usually happens with CPU sockets that have an open center. If that's so, it is possible that there are several ceramic capacitors connected in parallel. Their position is between the VCC and ground plane, thus you measure the voltage across these capacitors.
A while ago it was advocated pretty enthusiastically to measure VCC prior to the inductor(s). Popular spots were the legs of a MOSFET or one of the inductor legs. Thankfully, this technique was proven wrong by Johnny Bravo (kudos!), one of the most knowledgeable hardware enthusiasts.
He explains that these spots are incorrect because the voltage fluctuates (being turned off and on) several hundred times each second. Therefore, depending on the quality of your DMM, you should be measuring an average of this fluctuation. No doubt you're roughly measuring your VCC, but you want accuracy not an average.
The solution is to measure VCC after the inductor. On older boards you can do this by measuring it from one of those legs of an inductor that point toward your CPU socket. He explains that this won't work for the newer cube-like inductors. The workaround is to find either an empty socket for a smoothing capacitor and use the positive terminal, or using the positive side of an SMD electrolytic capacitor.
Stopping for a moment, in the aforementioned paragraph I mentioned two types of inductors: older and newer. Older inductors are de facto standard common inductors and they look plain and simple. You can clearly see the coil of wire on the component. Check out the attached image below. Notice those capacitors and that classic inductor.
The above shot is part of the VRM circuitry on an older board (circa 2004). But nowadays almost all of the high-end motherboards are sporting new, sort of cube-like power inductors. They're shielded. They can take many forms from square cuboids up to regular hexagons and whatnot. The below VRM image helps you recognize them.
Pay close attention to those cube-shaped TRIO R47 power inductors. That's the new design we're talking about. You cannot see any coil of wires. The rest of red-striped gray components are the aluminum electrolytic capacitors. You might also notice those MOSFETs right next to those inductors. Their color is also black and they have three legs.
This zoomed in picture might help you recognize the aforementioned components. Oh, and to dispel any confusion, I've rotated them counter clockwise.
If you cannot approach either of these places on the top side, then try the rear of the board. This way you will accurately measure the VCC. Regardless, always be conscious of your techniques and do not attempt to do any of these if you aren't comfortable yet. Also, be careful not to short out any of those tiny SMDs (surface mounted device) by touching (connecting them) more than once with the same probe.
Sometimes you might have access to the capacitor leads of those caps, which follow that one inductor I was talking about. This most often is on the rear of the motherboard. Measuring here is a bit tricky but by using needles to the probes of DMM, the process can be done fairly decently. You can link this technique with the aforementioned and try both of them out, since you'll work on the rear of the board. Compare results!
Next: Cutting Edge VCC Measuring >>
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