Confirming Oscilloscope & Bench Power Supply Safety - Page 1
Forum section: electronics / beginners

author: crashless
author_rank: Newbie
timestamp: May 07, 2020, 06:13:35 pm
content: As with most people getting a new scope I found episode #279 on how not to blow up your oscilloscope.For me it had two effects, #1 a great feeling of new knowledge & #2 a great feeling of new fears.I mostly have a good feel for the concepts, but one place I wanted to confirm I had correct was the part around 9:25 when he tested a laptop power supply and checked resistance between the neutral on the DC & the ground on the AC plug.I purchased a 853D soldering station that came with a variable DC power supply and it only has two terminals. I used my multimeter to confirm there's no continuity from the negative post to the ground on the AC plug. This means I can safely use my oscilloscope on any device powered by this power supply...right? So long as any device I want to test satisfies this criteria, I can safely probe away (obviously so long as I stay on the DC side of the circuit that is). Do I have this right?Thanks so much for the help!

author: unitedatoms
author_rank: Frequent Contributor
timestamp: May 08, 2020, 11:16:19 am
content: Yes this is correct conclusion. However I heard about another fear of damaging your circuit with use of very old oscilloscopes. Tube based scopes on DC coupling input can have a bias voltage in range of several volts and kill low voltage low current circuits. Glad I could help someone with new fears.

author: crashless
author_rank: Newbie
timestamp: May 08, 2020, 03:57:11 pm
content: OK, great! Thank you for putting my mind at ease.Good to know about the old-school tube scopes - a lot are floating around for cheap and I have considered getting one for fun

author: Ian.M
author_rank: Super Contributor
timestamp: May 08, 2020, 05:36:17 pm
content: Better valve input scopes have compensation for the input grid bias voltage. However the compensation may go out of range if the input valve is failing due to low emission. The way to tell if there is a problem is to set the input to DC coupled and 1V/div and check the trace doesn't shift when you switch between input open circuit and input grounded (with no probe or an X1 probe). If it does, check the service manual if there is an adjustment. Check for open vs grounded shift again on the 100mV/div. As long as the input bias voltage is well under 100mV you are very unlikely to have a problem probing modern low voltage semiconductor circuits. N.B. allow at least 10 minutes warmup time before making any adjustment, and in use, after power-on wait for the trace to stop drifting vertically significantly before probing sensitive stuff as the input bias voltage is likely to be out of spec. during initial warmup.I have a Dynamco D7100 50Mhz chassis with 1Y2 dual input and 1X2 delay timebase modules (which has Nuvistor input valves, though most of the rest of it is transistorised), and I had to mod it to extend the compensation range.Where valve input scopes win is they generally have very robust input circuits, which can be important if you get into valve audio, Tesla coils or other high voltage stuff. You still need an appropriately rated HV probe, but if you make a mistake you are far less likely to damage one than a modern scope. However that's not a lot of consolation to your relatives if you've just fitted yourself for a pine box, so *PLEASE* be extremely careful when working on HV circuits, and don't do so unless you've had proper, supervised hands-on safety training!One thing to beware of that is far more likely to damage your circuit than a valve scope, is the ground leakage current of switched mode power supplies. To reduce EMI, they usually have a class Y capacitor between 0V on the secondary side and the negative terminal of the primary side main reservoir capacitor as a return path for high frequency switching noise capacitively coupled from the 'hot' end of the primary to the secondary. Unfortunately if the reservoir capacitor is (as is usual) fed from a bridge rectifier, its negative terminal 'bounces' between neutral, and a positive voltage equal to the mains peak voltage, at the mains frequency, and the class Y capacitor couples this to the secondary side, with a permitted current of up to 0.75mA (for non-grounded devices). It wont hurt most scopes, but its more than enough to blow the gate oxide of an unprotected MOSFET if you touch a grounded soldering iron bit to it while the circuit is still connected to the PSU, but is not otherwise grounded, even if its output is switched off!