Mains Power to Scope Adapter - Page 1
Forum section: electronics / projects

author: dazz1
author_rank: Frequent Contributor
timestamp: March 07, 2025, 09:20:19 pm
content: HiI have a need for a mains power to scope adapter. The application requires the ability to display the mains current wave form on a scope.I am rebuilding a 1950's GEC Miniscope that was fitted with selenium diodes. I need to replace those with silicon versions.The selenium diodes have a forward voltage drop of around 80V compared to 0.5V for silicon. In addition, selenium diodes are leaky in the reverse.Another complication is that the Miniscope circuit uses the non-linear characteristics of the selenium diodes to regulate the HV voltage.If I just substitute selenium diodes with silicon versions the:HV output voltages will be higher,voltage regulation will be degraded and,peak currents from the transformer will be greater.The risk to the transformer is that it saturates and cooks out the transformer.So I need a means of measuring the 240VAC mains supply current waveform to identify transformer saturation. The most practical method I have, with what I have, is to measure the voltage across a 1ohm resistor on the mains supply side of the transformer. I can do that by measuring the voltage on both sides of a 1ohm 5W resistor, and subtracting the channels to display the current waveform on an oscilloscope. My Phillips scope has a 400Vpp limit, but the scale maxs to 10V/div. It is easy to wreck a scope by mistake.I decided to design/build a mains to scope adapter. Nothing too fancy. Just a pair of divide by 10 resistor voltage dividers with 50V zener diodes to limit/protect the scope inputs. No frequency compensation.The result is an adapter that is safe and easy to connect an oscilloscope to the mains. I can measure mains voltage and current wave forms.

author: Benta
author_rank: Super Contributor
timestamp: March 07, 2025, 10:42:47 pm
content: Are you using a differential probe?If yes, good.If no, your circuit is unsafe.

author: dazz1
author_rank: Frequent Contributor
timestamp: March 08, 2025, 01:35:54 am
content: Quote from: Benta on March 07, 2025, 10:42:47 pmElectrical distribution here used MEN, so earth and neutral are connected at the switchboard. Important because it means Neutral and Phase are not floating.My scopes are not isolated, so signals have to be referenced (connected via shield) to earth.All exposed metal (BNC Shields) is earthed.Max signal voltage at the BNC connectors is limited to 50V.The supply side of the adapter is fused.I use Ch A and Ch B subtract to get a differential voltage across the 1ohm current sensing resistor.What is unsafe?

author: Smokey
author_rank: Super Contributor
timestamp: March 08, 2025, 01:46:30 am
content: Just use an isolated current sensor? If you only need AC, that's super easy.https://www.digikey.com/en/products/filter/current-sensors/525Maybehttps://www.digikey.com/en/products/detail/olimex-ltd/SNS-CURRENT-CT013-100A/21662567

author: dazz1
author_rank: Frequent Contributor
timestamp: March 08, 2025, 02:57:07 am
content: Quote from: Smokey on March 08, 2025, 01:46:30 amEasy but also very expensive for me. If I don't order at least $66 parts, I pay $26 shipping. So a $2 part is then $28. I made the adapter with what I have at hand.The adapter protects the scope by ensuring the earthing is always correct. The adapter protects the user by limiting the signal voltage exposed at the BNC connectors. Have I missed something?

author: Smokey
author_rank: Super Contributor
timestamp: March 08, 2025, 03:06:20 am
content: Quote from: dazz1 on March 08, 2025, 02:57:07 amThese type of sensors are used all over the place. Every energy monitoring system out there has some version of these, usually just clamp on. Digikey is not the only source. Amazon ones would work just as well probably. Or alibaba, or whatever.All of the problems with grounding and floating things and differential and all that essentially go away. It would be worth $100 easy to have an isolated sensor and not have to worry about any of those issues.

author: bdunham7
author_rank: Super Contributor
timestamp: March 08, 2025, 03:07:34 am
content: You now have a pretty good passive differential probe from spare parts. What resistances did you use?A suggestion about your original selenium rectifier replacement project--one idea that may help is to use a power resistor on the primary side of the mains transformer. If you size it correctly and combine it with some other things on the secondary side, this can reduce saturation and improve conduction angles and power factor. Regulation by dissipation was the saying, the one thing you likely can't do is make it more efficient.What is the voltage and current on the secondary side?QuoteThe only thing I can think of is what might happen if your ground was interrupted. That's why I asked about the resistances used--if they're high enough this won't be a significant hazard.

author: dazz1
author_rank: Frequent Contributor
timestamp: March 08, 2025, 04:15:55 am
content: HiAttached is the circuit diagram. Only intended for my eyes, so a bit rough on presentation.The circuit includes two parallel divide x 10 voltage dividers. To work correctly, these require connection to a scope with 1Mohm inputs. The bottom end of the adapters has a 10k resistor.The resistance values then look weird, especially at the top half of the voltage dividers. To use standard range resistors, I ended up with two parallel strings of resistors. The key feature is the nominal 220R resistor. This is the trim resistor used to tune the resistor strings to the exact same resistance value. If the voltage dividers are not exactly matched, they will show a non-zero differential voltage across the 1R 5W current sensor resistor at zero current.Neutral is not shown in the diagram, but this goes straight through the adapter. Earth also goes straight through with a side connection. Also not shown is the 5A fuse that protects the 1R resistor and the power switch. I have not included any compensation capacitors. I don't need them for my application, but I could add them later if required.The back to back zener diodes limit the signal exposed on the BNC connectors to 50V max. Human and scope safe.This adapter has a single purpose. With this adapter, I can just plug things in and know that my scope and I are safe.For the Miniscope, the transformer has 4x secondary windings, so not much latitude for adding something on the primary side. There are two HT windings on the secondary. The original engineers would have designed for the inefficiencies of the selenium rectifiers. I need to add the right amount of loss to create a V/I curve that matches the requirements of the circuit.

author: dazz1
author_rank: Frequent Contributor
timestamp: March 09, 2025, 12:10:23 am
content: Quote from: Smokey on March 08, 2025, 01:46:30 amThese and other current sensors look like a good option. There would still be the requirement to connect power in, power out, and signal to the scope. So using one of these sensors would still need an enclosure with power and signal connectors.