Measure voltages on an oscilliscope using a coil? - Page 1
Forum section: electronics / projects

author: DreadfulGranola
author_rank: Newbie
timestamp: January 29, 2022, 08:53:30 pm
content: I'm wanting to measure a waveform on an oscilloscope of an engine's ignition coil. However from what I understand the coil will generate voltages that exceed the capabilities of my scope (a HP 54504A).I was wondering if I were to wrap a coil around the wire if I would be able to calculate the voltage from what would be read on the scope since it would (in theory) be lower than what's actually flowing through the conductor.

author: bdunham7
author_rank: Super Contributor
timestamp: January 29, 2022, 09:00:25 pm
content: Typically that is done with a capacitive pickup, not inductive, although an inductive sort of pickup which reacts to current can be used to generate the trigger signal.So you would ground your oscilloscope normally and then fashion a divider probe that simply has the top end of the divider clamped to the wire. This 'connection' at the wire or coil is typically just a formed metal plate.

author: Benta
author_rank: Super Contributor
timestamp: January 29, 2022, 09:03:08 pm
content: No, that won't work.But you can snip off the "X2" connection and just connect "X1". Then you have a capacitive coupling to your probe. It'll say nothing about the voltage, but will show you the signal shape.

author: DreadfulGranola
author_rank: Newbie
timestamp: January 29, 2022, 09:14:56 pm
content: Quote from: bdunham7 on January 29, 2022, 09:00:25 pmQuote from: Benta on January 29, 2022, 09:03:08 pmOoop! I completely forgot about capacitive coupling. All this is still fairly new to me since this is my first scope and I'm still kinda a newbie to some of this. I'll try snipping the X2 connection off like Benta said since all I have is just a standard probe for my scope and while id like to see the voltage it's not the most important thing tbh.

author: Circlotron
author_rank: Super Contributor
timestamp: January 29, 2022, 09:34:52 pm
content: This is the sort of thing you want. Has an internal capacitive voltage divider.https://www.ebay.com.au/itm/154607051372?_trkparms=ispr%3D1&hash=item23ff4c626c:g:8ZoAAOSwcAJhPTSM&amdata=enc%3AAQAGAAACoPYe5NmHp%252B2JMhMi7yxGiTJkPrKr5t53CooMSQt2orsStEKTPzZMfQmny3knR97t0AB%252F1u8Dap5C83KYbNL1OO8uHS0ROXSF7u21DXzeKvuFjpWdA2mcrmFN8ERvWQLemcwt7WfJRoEZJatDr9Up2e5rbTCWgPa%252Fg5YHU1VOFjGFrpcXVWwsnoO1hUKczVJiStSsFTy1B0z74WiGGDPKAYzKMwXWX9lw2UtG0oCu9hVqjc2IpeA019YC8SoewjCPJkum2T5LP3uyLeE0qqjF%252FY%252Btgd8QHSeQ7cR6t97Z%252ByYYTwjGpmQBkb9IXPRmopyr9zLPa8hXuHowaVBpF7CIv3zmGWoC3RNDale7iRM4FAv%252BWaNg%252BhRjEaOJJnD1CySHALYBaZ%252BQ5e3Bt0%252FZfiddoyolg0S5CxLvSkq8i5tGWaBAZ3bwmZNonCenLL1gCWiyhjM9iihgiiMjVhaZ721oy5aHR1rWEZ9A71IzaGEDkjNyzuhldEP35UDf2ypfanMOvcJjD9ZkS9O6pyzmVz4YERWuMaz0kDWPT%252FnbpUk2WIIYI18Jk1wbB4FwGllTsGpRwXIwbei08ap%252FzQVSJTrDC4xq9E7TeIZrvebNX3p%252FWjI9YFYDm%252FVJZ0n54679vqpK%252F9s6LQ10RTQyuQmTwvQYx4iLeAL1U0aPO4%252BdC5hdQ%252FfQf4wSywRc3TSYlhoUyD1Wjyb1x1RwFgezs%252FLTLk%252BI%252Fl%252BMzo5kJ%252FjUJxzjR6u%252FzEdQhsX%252BU37HTbfUHRm3t3ifLbSwLyNeDEJtA2Q%252BcPZaM1HigBDeVFdagpBPT8rjlwOj%252BO4ZTcQ2%252B1NFaha%252Fy41FgkJ%252FX%252FUeUIdb%252BJxbRfePDPSiv1kR0sOO7ffXAxysKeBBtf9jfRQLwLNELPn0RwKsFA%253D%253D%7Cclp%3A2334524%7Ctkp%3ABFBM2Nnq-9RfSomething else that is quite instructive if you are bench testing Ignition coils is to look at the spark current with a scope. Run a 100 ohm 5 watt resistor to ground and put 33nF of cap across it. Send a spark to the ungrounded end of this combination and observe the voltage drop across the resistor. Then you get to see who’s telling the truth about ignition systems and who’s not.

author: CaptDon
author_rank: Super Contributor
timestamp: January 29, 2022, 11:57:01 pm
content: All of this sounds amazingly destructive to a scopes front end should there be insulation breakdown or punch through or just a f--kup in general. Even with a properly trimmed and adjusted 1000:1 H.V. divider you are still looking at up to 50kv / 50 volts to the scope open circuit and over 40 amps if you happen to be working on an MSD Magneto equipped top fuel funny car / dragster. Be careful!! I have a carefully constructed 1000:1 probe with trimming adjustment for accurate rise time. Built and calibrated specifically for one of my scopes. It lies like a rug when used with my other scopes of different input capacitance and absolute D.C. resistance!!!!