Zalman CPU & VGA Power Consumption Meter

Fans|Controls
Product
CPU & VGA Power Consumption Meter
ZM-PCM1, ZM-VPM1
Manufacturer
Zalman
Market Price
ZM-PCM1: ~$50, ZM-VPM1: $40

Power measurements have been part and parcel of SPCR's various testing methodologies for many years. In many cases, instrumentation to measure the power of specific components either don't exist or costs the heavens. As a result, we've custom-made our own power measurement devices or customized existing tools for the job. Now, finally, a PC components and accessories company is offering a couple of power consumption meters that seem to offer serious CPU and VGA power measurements.


Fairly plain little boxes for Zalman's new CPU and VGA Power Meters.

These are not the first power meters to bear the Zalman name. The multi-fan controller Zalman ZM-MFC2 (replaced by the ZM-MFC3) also featured a module which allows it to monitor the AC power consumption of a PC in real time. The difference is that these new devices are power meters only, with no function other than to report the voltage, current and power of a CPU or a video card. They are two parts of a modular series; the VGA meter appears to be an add-on to the CPU meter.

ZM-PCM1

The CPU power meter consists of two main parts: A front panel display and control section, getting data via a thin cable from a black box that goes between the AUX12V 4- or 8-pin connector on the motherboard, and the equivalent output plug from the PSU. The front panel portion fits into a standard optical drive bay.


CPU power meter: Front panel display, breaker box, and power cable.


Shown here ready to go. Note that there are blank faceplates on either side that can be removed for other disply modules.

Most power meters of this sort use a simple shunt resistor of extremely low impedance to measure current. DC electrical circuits are consistent in that the same amount of current (Amperes) passes through every component in the circuit, so when the voltage drop across the known low resistance is measured, this information can be used to calculate the current in the circuit. Multiply the current by the voltage and you get power, in Watts. We at SPCR use inline 0.01 Ohm resistors and a highly accurate multimeter costing several hundred dollars for this purpose. It's clear that the little black breaker box of the ZM-PCM1 contains some kind of shunt resistor, and perhaps a portion of the circuitry which measures the voltage drop across it, and calculates the current and power being delivered through the AUX12V connector.

(Note: More sophisticated devices might use some kind of Hall-effect system, which measures the magnetic field created by the current to derive the actual current, thus avoiding any voltage drop, which is important when dealing with very low voltages. But with 12V here, and the modest price, it's probably a shunt resistor.)


SPCR's AUX12V power meter on our CPU heatsink test platform: A 0.01 Ohm Shunt resistor and expensive high accuracy multimeter.

It was a simple matter to get the ZM-PCM1 installed on the Intel i7 CPU testing platform shown above. Within a couple of minutes, we had the display showing us the voltage across the AUX12V connector, the current through the line, or the total power dissipated through it.


The display is showing the voltage across the AUX12V connector.

How accurate was it? Well, it basically read within about 1% of our own shunt resistor and multimeter combo. For example, on loads where we saw 86~88W, the ZM-PCM1 displayed 87~89W. So it can be said to be accurate enough, we'd say within 1-2% of absolute values (given that our own DIY setup cannot be perfectly accurate either).

This device is perfectly useful for load power measurements of almost all CPU types in the market today, as all but one type draws all its current through the AUX12V line. The single exception is the Intel i7-1366. We know that up to 30W of the power for this CPU family can come through the main 24-pin ATX power cable.

The low limit of the meter is 10W, which is a bit too high to compare many modern CPUs at idle. This is a serious limitation for anyone who wants to examine the minimum operating power of various CPUs.

ZM-VPM1

The VGA power meter has almost exactly the same components. What is missing is the front panel bracket; the square bracket is meant to be screwed into one of the spost on either side of the main display of the CPU power meter display.


VGA power meter: Front panel display, breaker box, and power cable.


Here is a closeup of the VGA power display.

There is an obvious limitation to the VGA power meter. It has only one 6- or 8-pin PCI-Express Graphics (PEG) 12V power connector, which performs the same function as the AUX12V 4 or 8 pin plug for the CPU. Many of the more powerful video cards require two PEGs; not having a tap on the other power input line means missing out on some of the current. So you can get an accurate AUX12V power reading into the VGA card only if it uses just one PEG. The other complication is that this is clearly not the only path for power into a VGA card. The PCIe 16x slot itself delivers current on both 3.3V and 12V lines into a video card, up to 75W in total. Usually the power through the PCIe slot is much less than through the PEG, but this means that even with a sinle 6- or 8-pin PEG video card, the Zalman power meter will not provide an accurate measure of the power consumed by the video card. It will always read too low. The product seems to exist because it can, never mind how accurate or useful it is.


VGA display showing voltage on left, CPU power in Watts in center.

SUMMARY

The Zalman ZM-PCM1 CPU power meter does exactly what it is intended to do: Measure the power delivered to the AUX 12V socket on any motherboard. It gives the metrics-happy geek an accurate picture of real CPU power demand (except for the aforementioned i7-1366 models which draw additional power from the motherboard via the main ATX12V lines).

The add-on ZM-VPM1 VGA power meter is another story. Even though the module is virtually identical to the CPU meter, because of the complex way in which PCIe 16x VGA cards are powered, the meter can only provide a partial and incorrect measurement of power consumed by the card. Unless you have a need to get at least a rough idea of power draw froim a single PEG connector VGA card, the ZM-VPM1 is not recommended.

Keep in mind that the ZM-PCM1 CPU power meter readings for power and current includes losses in conversion through the voltage regulation module of the motherboard (where the 12VDC is converted to the much lower voltage the CPU needs, usually well under 2VDC). The shown power is actually higher than what the CPU pulls from its socket, by at least 10%. Because there are real differences in VRM efficiency, the same CPU can pull more power (or less) from the AUX12V socket in different motherboards, while doing exactly the same tasks. To measure the power delivered to the CPU itself, the power meter would have to tap into the CPU socket terminals, past the VRM circuitry. This is beyond the realm of any consumer-level hardware monitor.

Except for the high 10W minimum measurement limit, the Zalman ZM-PCM1 is about as good as can be expected for a consumer product, and the ~$50 asking price is not unreasonable, given that not everyone has the skill and patience to make shunt resistor breakout device, or want to invest in an expensive multimeter for a power meter like SPCR's.

Articles of Related Interest:
Zalman ZM-MFC2: 4 x Fan Controller + Power Meter

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