Primary switching power supplies or regulators of phase - with Oscilloscope - Page 1
Forum section: electronics / general

author: m.bolanos
author_rank: Newbie
timestamp: December 17, 2011, 10:57:52 pm
content: Hello,I have a question about Primary measurements in switching power supplies or regulators of phase with oscilloscope. which of the following ways is the best way to do both for the integrity of the device as well as the precision of measurement:-Isolate the scope ground.-Put the circuit to be analyzed under a 1:1 isolation transformer-Put the oscilloscope in a 1:1 isolation transformerThank you and congratulations on the BLOG.

author: Zero999
author_rank: Super Contributor
timestamp: December 17, 2011, 11:09:36 pm
content: Put the circuit to be analysed under a 1:1 isolation transformer is the only correct answer here.The other options are highly dangerous:Put the oscilloscope in a 1:1 isolation transformer. That won't do any good because the chassis is still connected to earth.Isolate the scope ground. The problem with this is the 'scope's chassis will become live when it's connected to the device under test and there will be no protection from a live to earth fault inside the 'scope.If the circuit under test is being powered from an isolation transformer it's safe because its power supply is floating so connecting it to the scope's earthed chassis won't cause a short circuit.

author: Psi
author_rank: Super Contributor
timestamp: December 17, 2011, 11:35:12 pm
content: Yeah, i've put a scope on an isolated transformer and remove the earth before.It's all fun and games until something touches the metal case of the scope.Then there's a loud bang and you spend the next two weeks fixing the scope.It's not fun.I'm seriously pondering getting one of those Nano scopes for when i need an isolated scope.

author: Unknown
author_rank: Unknown
timestamp: December 17, 2011, 11:53:07 pm
content: Agreed with the previous replies, don't mess with the scope's grounding, if it's a reputable manufacturer, there are likely instructions in the manual that explicitly recommend against ever using the scope without correct ground connection. I believe Tektronix also has an appnote about (not) floating scopes. Other solutions would be a high-voltage differential probe, or two probes in CH1-CH2 mode with the ground clip tied together if you can live with giving up one channel and the very limited CMRR. Even more expensive would be a scope with isolated inputs rated for mains voltage, like the Fluke ScopeMeters or Tektronix TPS2xxx series, with the correct probes rated for a mains voltage between the reference lead and ground. These scopes have floating inputs were the reference lead of the probe is isolated from the ground, and are properly designed for safe operation under these conditions. Don't expect the cheap handhelds to offer the same level of isolation.Quote from: Psi on December 17, 2011, 11:35:12 pmThat's the good outcome. The bad one involves funerals. I'm not aware of statistics about the number of fatalities due to floating scopes, but it's definitely non-zero.Quote from: Psi on December 17, 2011, 11:35:12 pmNot sure if that's much better, I wouldn't rely on there being any substantial insulation between the DSO Nano case/controls and the input, and would treat it as a low voltage only device. How much insulation would you expect between the headphone jack of an MP3 player and its case?

author: amspire
author_rank: Super Contributor
timestamp: December 18, 2011, 12:14:39 am
content: If I am using a mains powered scope, I have always preferred using a good differential amp in front of the scope - it puts much less capacitive loading on the circuit then the combination of an isolation transformer and an earthed scope probe. You often need to be able to look at the gate drive voltages, and the capacitance of an isolation transformer can mess things up pretty bad.Unfortunately, differential amplifiers are still quite expensive. The Cleverscope one is not bad at a bit over $300. Cleverscope will be coming out with two new scopes with 2.5KV isolation on each channel - one with 16 bit/ 250msps and the other with 5gbsps. They won't be cheap, but they sound like a power supply designers dream.The BWD Powerscopes used to be perfect - 4 channels of differential amplifiers. If you are working a lot in power circuits and you see one on ebay, it is not a bad thing to grab.Today, looking for some kind of small battery powered scope is not a bad option at all. If you can find one with something lie an optically coupled USB, then fantastic.Richard

author: NiHaoMike
author_rank: Super Contributor
timestamp: December 18, 2011, 04:30:50 pm
content: If you only need to operate the device under test for a short time, you can use a UPS as an isolated AC power supply. Just remember to unplug the UPS before connecting the scope. (A standalone inverter connected to a lead acid battery also works, but you might as well just use an actual isolation transformer unless you already have those items.)A long time ago, I know of someone who used a high voltage bench power supply for working on switching power supplies. It's a rather expensive way, but someone who designs power supplies can build one for cheap. And some CRT TVs and most CRT monitors have isolated 150V or so power supplies rated for the full wattage of the set.

author: Neilm
author_rank: Super Contributor
timestamp: December 18, 2011, 07:10:31 pm
content: Quote from: alm on December 17, 2011, 11:53:07 pmIt is high enough that the latest version of IEC61010 (that this the safety standard for test and measurement equipment) actually has some thing to say on removing the safety earth connection.Neil

author: m.bolanos
author_rank: Newbie
timestamp: December 19, 2011, 09:09:19 pm
content: Hi,Thanks for yours answers. I will use two transformer 690v-400v and the secundary connect, the primary are insolated. The operating voltage is 230vThank.