Power Supply, isolation? - Page 1
Forum section: electronics / beginners

author: coolyota
author_rank: Contributor
timestamp: November 03, 2018, 04:53:35 am
content: Hello first post on EEVBlog, I have recently taken up an interest in electronics and have bought myself a Mean Well PSU (https://www.meanwellaustralia.com.au/products/lrs-100). It didn't occur to me that I needed an isolation transformer PSU.. now that I am looking to buy my first oscilloscope. It's likely going to be a Rigol DS1504Z scope. I have watched Dave's YT video on danger of ground loop. Just a little concern that the MeanWell PSU is not isolated. I am not competent enough to read the spec sheet to determine if the MeanWell PSU is isolated.Am I correct to say the output on -Ve is connected to Earth? So, say if I connect the GND clip on the scope probe to a +ve rail on a circuit I'll have a ground loop?Thanks in advance.

author: ataradov
author_rank: Super Contributor
timestamp: November 03, 2018, 05:21:36 am
content: There is a capacitor there. This is an isolated design. You only need to worry about the DC isolation.Your isolation is achieved though a transformer in direct path and optocouplers in the feedback path.

author: coolyota
author_rank: Contributor
timestamp: November 03, 2018, 05:39:34 am
content: Quote from: ataradov on November 03, 2018, 05:21:36 amDoes it mean that it is safe for DUT to be connected to the PSU and probe with scope?

author: ataradov
author_rank: Super Contributor
timestamp: November 03, 2018, 05:40:58 am
content: Yes, absolutely. This is a typical design for the isolated PSU, they all are built like that.

author: coolyota
author_rank: Contributor
timestamp: November 03, 2018, 05:43:52 am
content: Fantastic. Thank you so much

author: tautech
author_rank: Super Contributor
timestamp: November 03, 2018, 06:22:51 am
content: Quote from: coolyota on November 03, 2018, 05:43:52 amWhenever you're worried about scope ground loops a DMM is a good thing to drag out and check for any voltage disparities between the probe reference point on the DUT and the reference (gnd) clip itself.

author: coolyota
author_rank: Contributor
timestamp: November 03, 2018, 06:49:06 am
content: Quote from: tautech on November 03, 2018, 06:22:51 amCan you please elaborate?Assuming I have a simple circuit consisting of one resistor and LED (2V @ 18mA) whereVin = 5VR = 166 OhmsConnect scope probe between Vin & RNot connecting the GND clip...Use DMM, switch to Voltage meter. Probe the Scope's probe and GND clip and see the reading...?

author: tautech
author_rank: Super Contributor
timestamp: November 03, 2018, 07:31:01 am
content: Quote from: coolyota on November 03, 2018, 06:49:06 amIf the supply is NOT mains ground referenced you can connect the probe reference most anywhere......within reason.With experience a glance at a schematic can tell where is safe to reference from and this can be confirmed with the DMM.Now remember this; the probe reference clip is where the waveform is referenced from BUT in most scopes the reference clip isalsoat mains ground potential so great care must be taken on where to reference from so to not create mains ground loops.Most everyone refers to the probe reference clip as the ground clip and while the scope has a mains ground connection this is correct; probe ground clip.Now most everyone also makes a mistake at some time and connect the probe clip to a point where they shouldn't and if we're lucky only a blown fuse or tripped MCB or RCD and a few sparks to wake us up and serve as a warning to be more observant.The potential disparity between the the DUT and probe reference clip (at mains ground potential) is what gets us into trouble so always know what it is !DMM is your friend here.

author: coolyota
author_rank: Contributor
timestamp: November 04, 2018, 07:47:27 am
content: So as rule of thumbs,alwaysconnect the GND clip to the GND terminal of power source. Correct?In the case of DUT powered from 2 prongs plug, then it is OK to connect the GND clip to any point of the circuit.

author: tautech
author_rank: Super Contributor
timestamp: November 04, 2018, 07:56:17 am
content: Quote from: coolyota on November 04, 2018, 07:47:27 amAlmost.GND of a circuit is not always mains GND.DMM use can indicate whether it's safe or not BUT you need be aware of phantom voltages and DMM's with a low Z range can help indicate true differences in potential.Play safe.

author: MichaelNL
author_rank: Newbie
timestamp: November 06, 2018, 11:26:13 am
content: Since we're talking scope probing now: please help me clear up some confusion in my mind.If I connect a DUT to an isolation transformer and use only one probe of my scope it's not possible to short something out via the scope, right?And if I were to use two probes in the same situation but remove the ground lead from the second probe (so that the ground of the scope is connected to only one point in the DUT) there's also no way to short something out. Is that assumption correct too?

author: Zero999
author_rank: Super Contributor
timestamp: November 06, 2018, 12:15:57 pm
content: Quote from: MichaelNL on November 06, 2018, 11:26:13 amIf only one ground clip is connected or they're both connected to the same point, then you should be safe.Beware that with the device connected to an isolation transformer and the secondary being earthed via the 'scope, the circuit is now referenced to mains earth and you lose the protection of any upstream RCD/GFCI.