I have a couple of HP ProCurve 2910al POE+ switches at my company that we are primarily using to power our VOIP phones that run on 48V DC. However, I have one wireless access point that I need to run off of POE, but it has to be 24V DC. I'm afraid to plug it into the POE ProCurve because I'm not sure if it will zap the device.
I'm wondering if there is a way to make sure to change the voltage on a specific port to 24V instead of the (seemingly) default value of 48V.
Thanks!
Joel
Best Answer
Most end point devoces should manage the voltage. Quote follows from this page PoE info The end point device maker should also have this info. Some early devices seeme dto weant special power injectors but have not seen that for some time
Assuming that the PD has been successfully detected and/or classified, the device should be ready to receive power. The PSE will place between 44 and 57 V on the line, which, due to resistive drops in the cable, will be between 36 and 57 V by the time it reaches the PD.
Since most PD applications are not run directly off of 48 V, the typical PD front end makes use of a DC/DC converter to generate more convenient lower voltages. In order to add more flexibility in choosing a DC/DC converter, most PD ICs have an on-board undervoltage lockout (UVLO) function provided and some form of a "power good" output. This allows a PD designer to now make use of a cheaper DC/DC converter that does not have a UVLO function.
Additionally, a DC/DC converter in the front end will usually make use of a bypass capacitor. This bypass capacitance looks like a short to the PSE trying to power the PD and will draw a large instantaneous current. However, according to the IEEE specification, the maximum a PD is allowed to draw is 450 mA.