Peak-hold rectifier for HV pulse capture - Page 1
Forum section: electronics / projects

author: LukeW
author_rank: Frequent Contributor
timestamp: June 04, 2016, 01:02:36 am
content: Suppose we want to consistently, reproducibly measure the HT secondary voltage in an automotive ignition system. Building a HV resistive divider array is fine, but given the fast AC pulse, parasitic LC reactance is likely to be significant, more so than the DC resistance, so I suppose a capacitive divider approach is probably the best.You can get capacitive sensors for exactly this purpose, where you just kind of stick it in there near the spark plug, hold it there and read the HV. But they're fiddly, they fall off, they move around, and they are not held in a consistent geometry. Hence the capacitance changes and the reading is not consistent.I'd like to pull the coil out onto the bench, pulse the coil primary appropriately, and connect the secondary HV directly to a capacitor. Probably some sort of simple DIY parallel plate arrangement, set up to give say 1pF and capable of withstanding 1.21 jiggavolts without any breakdown obviously, insulated and with some shield/guard arrangement where the sense wire leaves the capacitor. Suppose the HV is say 50kV, and we want 5V out at 50kV. Let's make the low-side capacitor say 10nF, for a 10-4voltage divider ratio.Now we want a couple of components, maybe an opamp or two, for precision recification of the negative pulse and peak hold of the pulse, so we've got enough time for a microcontroller ADC to read it out and stick it on a display. The signal will enter the board at C2, the low-side divider capacitor, which is on the board.Does this sound sensible? Care to cast another set of eyes over the following circuit and tell me what you think?

author: joeqsmith
author_rank: Super Contributor
timestamp: June 04, 2016, 03:09:47 am
content: Not sure what your end goal is. The output will look different open circuited versus loaded. Spark plug breakdown will be different with pressure.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 04, 2016, 03:24:28 am
content: Why would the LC parameters be wrong for a divider? A 1:10 probe need only have a few pF, and 1:100 and greater ratios can have fractional pF equivalent at the tip. You can always put some damping resistance in series with that capacitance, too, which damps anything with L. The biggest downside, really, is adjusting it so it's compensated over a reasonable bandwidth (some MHz), and obtaining high voltage resistors and capacitors capable of reading it without breakdown.Can also inspect the primary side waveform, which includes leakage inductance unfortunately, but as I recall the spark voltage (or lack thereof, if it doesn't fire) is still visible. You'd have to test the coil to determine turns ratio of course.Tim

author: johansen
author_rank: Super Contributor
timestamp: June 04, 2016, 06:06:17 am
content: I think its the tektronix 6015 probe that is good to 40KV. I suspect this is plenty for automotive work, and with some work you could diy your own with an off the shelf 4 inch long, 100M ohm resistor.. some have filled the entire probe body with silicone, rather than filling it with a certain refrigerant that is no longer available.Though you may be able to fill it with r-152A which is found in canned-air. most any 6015 probe nowadays will be empty of refrigerant and only good to 20kv. i think its 75Mhz for the short cord version. i happened to find the replacement probe/resistor portion on ebay and had to build my own compensation box, which is not complicated.

author: Kleinstein
author_rank: Super Contributor
timestamp: June 04, 2016, 08:29:33 am
content: The peak hold circuit has several weak spots:The input needs a DC path to GND - could be in the 10 M range.The old capacitor should be much smaller - something like 100 pF is more suitable. A longer hold is better done digital.R5 is of no use - usually leave it out, or have is much larger (e.g. 100 M), just to compensate leakage of the OP.C5 is limiting the speed - depending on the OPs used, no need for this cap, or a much smaller value.If there is a scope available it might be simpler and give more information to just look at the waveform than just read the peak value.