My answers are below. If there are any ambiguity just post again :) My unde= rstanding needs to=20 be perfect.....=20 Thanks Sean, John ----- Original Message ----- From: Sean Breheny To: Microcontroller discussion list - Public. Cc:=20 Sent: Monday, December 5, 2011 11:57 AM Subject: Re: [EE] Question of bench PSU safety. A few more questions: 1) what do you mean by "transformer leads to the banana jack"? I checked the resistance between transformer and the -ve terminal on the PS= U . It was low in resistance under 10 ohms.... The tranformer is earth grounded.There was impedance between the transforme= r and earth ground. Not sure of the exact value. In series there was a 22 ohms resistor and a caps.... Can't rmb the caps va= lue. Normally the transformer is NOT connected directly to the output jacks 2) what were you trying to power with the power supply? Were you connecting the NEGATIVE output of the power supply to something other than the ground of the device you were trying to power? I connected the -ve output to a point that I thought was DC ground that I w= as powering.In reality that point was 145 AC volts against earth ground. I know this has to be a V= ERY silly mistake. But this circuit created DC voltage from AC without a transformer. Something like th= is: http://www.daycounter.com/Circuits/Transformerless-Power-Supplies/Transform= erless-Power-Supplies.phtml The point is at D1 to Live. 3) When you say "145 volts on the tested circuit", can you specify which two points you measured this between? Based on the previous diagram, figure 1: At D1 to Live and referenced to ea= rth ground............ As others have said, it seems like you have a bigger problem here than just a non-isolated power supply output. Be careful trying to use that power supply if some of the insulation inside has melted - it may be on the verge of shorting out internally. I may need to call the co. again since I don have the exact transformer to = replace the one that got burnt...... On Sun, Dec 4, 2011 at 11:41 AM, John Chung wrote: > Hi guys, > > Had an eventful night...... Tried to power a circuit "which" I thought wa= s a DC circuit ONLY to be greeted > by a power trip and my bench power supply was smoking...... > > Needless to say I opened up the bench power supply to find that the trans= former leads to the > banana jack had it's enamel coating melted... I found out later that the = DC ground was > actually tied to the AC ground....... 145volts on the tested circuit and = the bench PSU banana jack was hooked to AC ground too....... > There was some reactance between the bench banana jack to AC ground which= KIND of save the PSU life...... The > bench PSU still powers up and works*despite the transformer's enamel bein= g melted.* > > To NOT repeat history here are some questions: > > 1) Do I need to check ALL DC circuit to determine if it is tied to AC gro= und? > 2) When the DC circuit*tied to AC ground* is tied to ground hooking up to= the PSU which is also tied to ground would ruin it.... But IF the > =A0=A0 PSU is NOT tied to ground, would it be safe to use. Since it canno= t be grounded since the PSU has not connection to AC ground. > 3) Case scenario. The notebook DC ground has resistance of 1MOhm to AC gr= ound which is okay to hook up a scope probe BUT it was a reactance of 1Kohm > =A0=A0=A0 it would be hazardous? > > > > Thanks, > John > > -- > http://www.piclist.com PIC/SX FAQ & list archive > View/change your membership options at > http://mailman.mit.edu/mailman/listinfo/piclist --=20 http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist --=20 http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .