New to EE... check my math pls? <3 - Page 1
Forum section: electronics / beginners

author: funbags
author_rank: Contributor
timestamp: January 30, 2018, 09:55:00 pm
content: Hey All,I'm new to EE. Been doing arduino stuff for a few years, but decided to learn analogue stuff two weeks ago.. (I'm an artist by trade, so maths are not my specialty.. (I actually had to relearn basic Algebra for this.))A bit of BG on my problem: I'm trying to get a pulse signal off my motorcycle's ignition. I've been told that the CDI signal voltage should be around 50V to 100V for the low voltage side, and 20KV on the high voltage side. Neither of which I'm eager to confirm with a smoking scope.I hooked a sensor wire up to the signal spade connector on the low voltage side of the spark coil. I have a DSO shell as a scope, but I'm afraid to hook it directly into spade connector because of voltage range I can potentially see.So I'm putting the signal through a 5.1V zener regulator (which I'm not convince is turning on) then in to a low-pass filter. The signal sample is taken after the low pass filter. For simplicity my schematic just shows the series resistors and not the 5.1V zener (grounded between R1 and R2), polarity protection diode (between R2 and R3) or RC 22uf cap (grounded after R3).My Problem: I'm trying to figure out the voltage of the pulses without actually reading them directly, but the voltage I came up with seems way too low...Givens:R1 = 320 ohmsR2 = 320 ohmsR3 = 100 ohmsVoltage sample after R3 (VR3) = 0.95VLine resistance of LV coil = 0.6 ohms (not in schematic)I get 7.03V as Vin... which seems way too low... So I must be doing something wrongMy Math for a simple series resistor circuit:Vin R1 R2 R3 Vout (the scope lead is technically Vout right now)---------------WWW----------WWW---------WWW----------------^||VR3Ohms Law - IR3 = VR3/R3 = 0.95V/100ohms = 9.5mAKirchoffs C Law - Itotal = I1 = I2 = I3 = 9.5mAOhms law - VR2 = IR2 x R2 = 9.5mA x 320ohms = 3.04VVR1 = VR2Kirchoffs V Law - Vin = VR1 + VR2 +VR3 = 3.04V + 3.04V + .95V = 7.03VThanks in advance!

author: danadak
author_rank: Super Contributor
timestamp: January 31, 2018, 12:14:37 am
content: You could always do a sniffer loop, a simple loop, start with 1 - 5 turns,loosely coupled close to HV lead. No direct contact, use back to back zenerprotection.https://www.eevblog.com/forum/beginners/inductive-probe-for-ignition-test/Capacitive coupling -https://www.picoauto.com/products/ignition-accessories/secondary-ignition-pickup-capacitive-with-bnchttps://www.amazon.com/Secondary-Ignition-Capacitive-Pickup-Probe/dp/B007I2IRG2http://dtec.net.au/Tech%20Articles/Ignition%20Coil%20Energy%20Testing%20V2.pdfRegards, Dana.

author: funbags
author_rank: Contributor
timestamp: January 31, 2018, 12:56:36 am
content: Awesome thanks Dana... I'm going to check out your links! Sweet.Thanks!

author: Brumby
author_rank: Unknown
timestamp: January 31, 2018, 03:17:38 am
content: Your caution indicates you have good instincts. You should do well.Understanding the potential for what could go wrong is a big part of keeping yourself and your equipment safe. Certainly there will be occasions when something does give up the magic smoke - but with care, this won't include anything too expensive ... or yourself.

author: funbags
author_rank: Contributor
timestamp: January 31, 2018, 02:59:02 pm
content: Thanks Brumby! I cant afford a mistake. haha Literally.I had some follow up questions to anyone about zener regulators before I abandon them for a much simpler capacitance sensor... I just don't like leaving things half understood. Plus I'm getting a very stable signal all the way up to 180Hz (my rev limiter).https://youtu.be/BdR9BRkh5isSo with the above schematic my calculated minimum voltage to activate the zener regulator is 8.986V which is greater than my calculated total 7.03V. (I actually have to check my reistors again. I may have misvalued the current limiting one... one of my earlier schematics has R1 at 100 ohms which would make Vzmin = 6.314V)Vzmin = Vz(R1+ (R2+R3)) = (5.1V x 740 ohms) / 420 ohms = 8.986V---------------------(R2+R3)If the Zener flips ON, would I see clamping on the scope instead? Or would the RC smooth that out too?Thanks again!

author: funbags
author_rank: Contributor
timestamp: February 04, 2018, 07:16:49 pm
content: I put it through a simple amp and it works.... I have many questions about why it works (because I had a lucky guess with the amp), but right now I'm just enjoying a delicious celebratory Centenial IPA... I'll collect my thoughts and draw up a schematic and possibly post a 2nd thread specifically to finding out why the amp circuit works. Super stable and accurate though... only red flag is that it isnt decoupled from the ignition. Thanks!