I have an isolated switching power supply which takes in 30V and outputs 55V 1.5A. It's a power source for a PoE+ powered device. When the device which is supposed to be powered is not connected (via RJ45), the only load present on the 55V rail is an LED which draws 6mA and the output voltage goes up to somewhere between 56V and 57V. When the powered device is connected (it draws ~400mA) I have a nice and stable 55V output.

Now, the transformer requires a minimum load of 20mA. So the question is, how can I add a temporary load on the output that would automatically disconnect when I connect my powered device? As mentioned above there is only a 6mA load, so I need to add a temporary 14mA load.

As the voltage increase when there is no load, I was thinking about adding a zener with a breakdown voltage of 55V in series with an LED and a resistor. So if the voltage was greater than 55V the zener would start conducting and I could adjust the resistor value accordingly to draw at least 14mA. However I couldn't find a zener with a Zv of 55V.

Any other ideas? I have limited space on the pcb so the component count/footprint is important.

• Do you mind having an extra constant 20 mA draw? If not, just add a resistor. Commented Mar 23, 2018 at 10:34
• I wouldn't like to waste those 14mA, that's why I need it to be temporary. Commented Mar 23, 2018 at 10:42
• @A Silva - ok! Suggest you add that to your question, but Andy's answer will get you started :) Commented Mar 23, 2018 at 10:45
• Is the actual goal to assure that the output is not above 55V, even with no external device connected? If so, there are potentially other ways of doing that instead of loading the supply so that it is regulating to 55V. Commented Mar 23, 2018 at 13:13
• @crj11 No, it's ok if the output is above 55V. The actual goal is to draw at least 20mA with no external device connected. Then disconnect this temporary load when the external device is connected. Commented Mar 23, 2018 at 13:40