LinkPower
LPS1000 Adjustable Voltage PoE Switch
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Pre-standard PoE
These types of PoE devices deliver power over the spare pairs (pins 4, 5, 7 and 8) of a CAT3,
CAT5, CAT5E or CAT6 type Ethernet network cable. It delivers the voltage across the spare
pairs of an Ethernet cable. This pre-standard version of PoE is implemented in many different
manufactured products. These chipsets are lower cost than 802.3af compliant chipsets which
can result in lower product cost. Additionally, with pre-standard PoE, power may be easily
applied to a powered device using DC voltage from multiple sources such as automobile
batteries or solar solutions. Many PoE Vendors opt out of 802.3af due to the 13 watts
maximum power specification, which is too low for higher demanding powered IP applications.
Figure 2. Example of a PIP100 Passive PoE Mid-span Injectors
The most commonly used Ethernet cable, CAT5e, uses 24 AWG copper conductors, which can
safely carry 360 milliamp of current at 50 volts. The cable has eight conductors with only four
conductors being used to provide power. Therefore, the maximum direct current power
transmitted is 50 V × 0.360 A × 2 = 36 W. Considering the voltage drop of 3 volts of DC every
25 meters of cable length, at 100 meters, a powered device would only receive 31.6 watts of
usable power.
Power planning and cable length is an important consideration when using passive PoE
technology. Utilizing longer cables without accounting for the device’s power requirements may
cause intermittent operation and equipment malfunction.
