Using oscilloscope on SMPS - Page 1
Forum section: electronics / beginners

author: lordvader88
author_rank: Frequent Contributor
timestamp: July 12, 2018, 09:42:36 pm
content: So I don't have an isolation transformer (but I do have 2 microwave oven trans. so I need to get them from storage). Then I could use my bench scopes.So I'm going to use a pocket DSO138 on the hot side, with a 10x probe. And it's isolated.The bulk capacitor on most computer PSU's seems to be charged to 320V. What about the inductive spikes right on the flyback, how safe is it in general to probe the flyback ?

author: petemate
author_rank: Regular Contributor
timestamp: July 12, 2018, 09:48:06 pm
content: If you have an isolation transformer, its safe. But keep in mind that you'd still get a bang and perhaps a shock if you short something out. I'd be VERY concerned about working on the primary side of a PSU without adequate protection, so definitely get one. Another solution is to use real differential probes(not two channels subtracted by the scope).Most off-line flybacks use something like 650V mosfets. With derating, you should expect the inductive spike to be around 600V, depending on the load conditions and the snubber design.

author: Gyro
author_rank: Super Contributor
timestamp: July 12, 2018, 09:50:17 pm
content: https://www.eevblog.com/forum/beginners/back-to-back-microwave-oven-transformers-for-isolation/msg1409987/#msg1409987Oh, and I don't think the DSO138 has a very consistent input impedance, so doesn't play nicely with standard x10 probes. So the 50V input limit is about all you'll get.

author: bitseeker
author_rank: Super Contributor
timestamp: July 12, 2018, 10:45:14 pm
content: An isolation transformer provides some safety, but it is limited. Once you attach the ground clip from a grounded scope's probe, the power supply is once again ground referenced. In this arrangement, the isolation transformer makes the circuit safe for your scope's first probe, but not for you.Also note that any other probe that you attach has to have its ground clip attached to the same place as the first probe's ground clip.A differential probe is not cheap, but reduces the complexity of what is or isn't safe to probe. It makes probing with the scope as straightforward as probing with a DMM as far as isolation goes. The Micsig DP10013 probes are probably the most reasonably priced ones for the supported bandwidth. The EEVblog probes are quite a bit more expensive, but have a 10X mode which is great for general use.

author: wraper
author_rank: Unknown
timestamp: July 12, 2018, 10:58:07 pm
content: Get affordable high voltage differential probe like one from micsig, and use normal scope. It won't have safety problem and can safely work with flyback peak voltage. Bandwith of DSO138 is not sufficient to properly measure SMPS and I have doubts about it properly working with x10 probes as well which are not rated for such voltage to begin with.

author: BravoV
author_rank: Super Contributor
timestamp: July 13, 2018, 04:41:28 am
content: Quote from: wraper on July 12, 2018, 10:58:07 pm+1 , it will free you to probe "any" where you like at mains voltage points.Example with HV differential probes, the scope's probes were probing straight at the mains socket at 220V AC that powered the scope it self.