Question about probing 12VDC to 200VDC boost converter safety  - Page 1
Forum section: electronics / projects

author: capf
author_rank: Newbie
timestamp: October 28, 2020, 04:58:18 pm
content: HelloI am designing a 12to 200V DC-DC boost converter to power some nixie tubes. I am powering it with a 220AC -> 12DC wall transformer, which does not have ground pin. The converter switching frequency is around 100kHzI read the specs for the Rigol DS1054z regarding the input voltages:- CAT I 300 Vrms, CAT II 100 Vrms, transient overvoltage 1000vpk- With RP2200 10:1 probe: CAT II 300 VrmsI have some doubts about the safety of the equipment. From my understandings, probing the converter classifies as a CATI measurement because the transformer provides isolation right ?Do the AC/DC coupling settings of the oscilloscope have an impact in terms of max input voltages ? I read that if I switch to AC coupling, the 10M input resistance is "bypassed" and the input capacitor of oscilloscope will see full voltageIn that case (or if I accidently switch to x1 probe settings) do I need to worry about anything, or am I in the safe side ?Thank you

author: Gyro
author_rank: Super Contributor
timestamp: October 28, 2020, 08:25:08 pm
content: Hi,Yes, you are right that once you are powering the converter from the output of a wall wart, you are safely in CAT I territory.It's generally a good idea to treat all scopes (apart from the specialist isolated input ones, or with proper HV differential probes) as CAT I instruments. CAT II implies that you are somewhere in the vicinity of mains, which is never a nice place to take them.You should also have no difficulty in referencing the output of a wall wart to scope ground. Yes, you should be using the probe on a x10 setting - this is good practice for pretty much all measurements anyway to minimise loading on the circuit that you're probing. Switchable X1/X10 are generally the spawn of the devil, it's worth looking out for some dedicated X10 ones, which should be just as cheap.Yes, In AC mode, the coupling capacitor inside the scope will see the full input DC voltage. For a small 200V converter like this, you ought to be able to stay in DC coupled mode unless you're particularly interested in viewing ripple voltage on the 200V rail - which shouldn't be critical for this application. Checking the output voltage is probably better left to a DMM anyway.I hope this helps.

author: capf
author_rank: Newbie
timestamp: October 28, 2020, 11:01:34 pm
content: Hi!Thank you very much for your help ☺️Regarding the output ripple, although not relevant for this application, if I want to measure it for "academic purposes" am I damaging my equipment by using AC coupling (using of course x10 probe settings)?Thank you

author: bdunham7
author_rank: Super Contributor
timestamp: October 28, 2020, 11:18:49 pm
content: Quote from: capf on October 28, 2020, 04:58:18 pmEven if you expose the scope to 200VDC, that is below its 300V rating, so it should be fine. However, the advice about AC coupling mooting the 10X voltage divider depends on the scope. On some, the AC blocking capacitor will eventually see the full DC bias voltage regardless of the resistance of the probe, for these scopes a 100X 10M probe that has its own voltage divider network will result in a much smaller DC voltage at the blocking capacitor. An example is the Probemaster 4910-2 at the bottom of this page:https://probemaster.com/4900-oscilloscope-probe-basic-kit-150-300-mhz/On other scopes, the AC coupling may be arranged so that the input is still shunted across the 1M resistance and the blocking capacitor only blocks the signal going to the preamp. You can test this by setting your scope to AC coupling and measuring the input resistance with a DMM. If it stays at 1M when switch from DC to AC coupling, you don't need to worry about overloading the AC blocking capacitor even at higher voltages.

author: capf
author_rank: Newbie
timestamp: October 29, 2020, 04:42:27 pm
content: Hi!Thank you very much for your help,For curiosity and peace of mind I will test it and then share the resultsThank you

author: capf
author_rank: Newbie
timestamp: October 30, 2020, 12:49:39 pm
content: I confirm that the input resistance is the same independently of the AC/DC coupling settings in a rigol ds1054z