The CMS ECAL Barrel HV system

The CMS electro-magnetic calorimeter comprises 75848 scintillating lead tungstate crystals. 61200 crystals are contained in the ECAL Barrel section and these are readout by avalanche photodiodes (APD) with internal gain. The APD gain strongly depends on the bias voltage that, for a gain G=50, is around 400 V. In order to match the requirements for gain stability, the power supply voltage must be stable to within 0.01\%. In this talk we describe our experience with the installed Barrel HV power supply system which has been used for data taking at the LHC since 2008.