How do I verify if a DC power supply is grounder or not? - Page 1
Forum section: electronics / beginners

author: Rene
author_rank: Regular Contributor
timestamp: October 15, 2014, 12:34:44 am
content: Hello,I recently purchased a new power supply (Velleman PS1503SBU). This power supply features a negative and a positive terminals in front of the unit but it does not have a ground terminal (red and black banana jacks but no green ground banana jack). The power supply cord plug (the one that plugs to the wall) does have a ground pin.Can someone tell if there is a way to determine if the negative terminal in front of the unit (black banana jack) is connected to the ground terminal of the power supply plug? I did a conductivity test (a continuity voltmeter test) between the negative terminal (black banana plug) and the ground ping of the power cord plug and I did not get a beep. Does that mean the negative terminal is not connected to ground?Thanks.

author: RobertHolcombe
author_rank: Frequent Contributor
timestamp: October 15, 2014, 12:42:14 am
content: You tested correctly, it has a floating outputThe chassis will be mains earthed

author: ice595
author_rank: Regular Contributor
timestamp: October 15, 2014, 12:56:49 am
content: Test for continuity at the positive output terminal and the ground pin on the plug as well.There could be the possibility that the power supply is a negative voltage regulator.meaning that the positive terminal is grounded and the negative terminal has a negative voltage with reference to ground.believe it or not, I have deep fried projects before because I've used a negative voltage supply in reverse to give me a positive voltage.

author: Rene
author_rank: Regular Contributor
timestamp: October 15, 2014, 01:22:25 am
content: Thank you. I tested both negative and positive terminals and I did not get a beep on either one so I guess I am floating for sure.In case anyone is curious, the reason I am asking this is because I just ordered a brand new oscilloscope and I was made aware that you can blow it up if you are not careful. Specifically, I was made aware that if your power supply is grounded and you connect the ground clip of the oscilloscope in the wrong place your oscilloscope can go booom!This video made me aware of the issue:http://www.eevblog.com/2012/05/18/eevblog-279-how-not-to-blow-up-your-oscilloscope/By the way, what if the negative terminal was grounded and I used one of those “Cheater plugs (http://en.wikipedia.org/wiki/Cheater_plug)”? Would that do the trick of making the power supply a floating ground power supply?Thanks.

author: ice595
author_rank: Regular Contributor
timestamp: October 15, 2014, 01:26:34 am
content: instead of plugging the non-floating power supply directly into the wall, you can plug it in an isolating transformer.OR plug your scope in the isolation transformer so the scope ground will float

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 15, 2014, 03:11:51 am
content: Also check if there's much if any capacitance. Not usually a problem but can be interesting.

author: RobertHolcombe
author_rank: Frequent Contributor
timestamp: October 15, 2014, 10:02:12 am
content: Quote from: ice595 on October 15, 2014, 01:26:34 amThat's not safe practice, seehttp://www.cbtricks.com/miscellaneous/tech_publications/scope/floating.pdf

author: Seekonk
author_rank: Super Contributor
timestamp: October 15, 2014, 07:29:12 pm
content: Speaking looooooosly about scope isolation...... I remember a long time ago HP or Tektronix had a tech note about inserting a 25A bridge rectifier into the ground wire. That would keep the scope grounded as far as electrical shock but allow it to "float" and to prevent ground loops when attached to another device that was grounded. Anyone remember this?