# Earth leakage from isolated power supply

I have recently put together a DIY bench power supply using a DPS5015 from Rui Deng and an old laptop power supply. Everything seems to be working fine except for a small amount of current leaking to earth even when the output is turned off.

I noticed this when I was testing after assembly. I was connecting an LED to the binding posts and noticed that if I was holding one leg of the LED it would light up when the other leg touched the binding post.

Now where I start to get confused is that the laptop power supply that I’ve repurposed has no earth connection and should therefore be floating. I believed this would mean leakage to earth was not possible as there was no return path for the current. I’ve confirmed the current is AC 50 Hz(I live in Australia with 50 Hz mains). Measuring the current straight to earth I get 148 uA. Current through my body is 133 uA. Current through my body with my feet insulated is 34 uA.

So my main questions are how can current leak to earth from a power supply that is not earth referenced and why does current still flow when not even connected to earth.

Am only a hobbyist just learning and would really love to understand what’s going on. Does this represent a safety risk? Are there any changes that can be made to reduce this effect/improve any safety factors?

• Concerning Class-Y capacitors in switching supplies read this answer electronics.stackexchange.com/questions/216959/… – replete Apr 12 '18 at 0:38
• Thanks for the link. So if I understand it correctly the Y capacitor in the supply should not be leaking more than 85 uA. If it is it may indicate a fault. I don’t think this was anywhere in the answer but I wonder if there is a way to test whether that is the cause. – Nick Dancer Apr 13 '18 at 5:47
• You first need to ensure that your measurement setup is appropriate for such small currents, that is, how reliable is your measurement of 3mA. If it is reliable, the power supply is not a very good one. The capacitively coupled current should certainly be under a milliamp. It's not uncommon to find overspecified Y-caps in cheaper supplies. – replete Apr 13 '18 at 5:52
• You should delete your recent comments and instead edit their content into your question. This will bump it and gain you more visibility. – replete Apr 13 '18 at 22:17
• @replete I’ve had a bit more time to accurately measure this and I’m not sure how I had the meter setup originally but am now only measuring uA so have updated the question. The accuracy of the meter I still can’t be certain of. It is an Aneng 8008 which is cheap but generally considered good value for money. I’m sure this means it has a margin of error but would have expected it to be within a few percentage points rather than an order of magnitude. – Nick Dancer Apr 14 '18 at 23:55