Best way to clamp a signal, and some more questions - Page 1
Forum section: electronics / projects

author: senso
author_rank: Frequent Contributor
timestamp: March 24, 2013, 01:57:12 am
content: Good night, I'm in the process of designing some digital ignitions for my two stroke bikes, and I have some doubts on how to clamp the signal that is coming from the magnetic pick-up.My first idea was to use diodes to clamp it, but there is a small problem, when first starting the engine with the kick start, there is a delay until I have 5v to power up my micro, so it will be difficult to make it start with the kick starter.So I tough about using a zener diode, so I don't need to have the micro running, but I need to bypass the signal from the pick-up, instead of going to the micro, it must control directly the SCR, and I dont really know how to do that, maybe mosfets will solve my problem, I would like to avoid relays due to their size, and I think I would need to use a bi-stable one.The SCR will fire with 5v(or 4.7v from the clampling zener), and then the micro would start-up and take the SCR control.The part that I want from that signal is the positive rising edge.

author: lewis
author_rank: Frequent Contributor
timestamp: March 24, 2013, 02:05:33 am
content: Why would there be a delay before 5V appears at the micro?

author: senso
author_rank: Frequent Contributor
timestamp: March 24, 2013, 02:31:11 am
content: Because there I dont use(and don't have space) to use batteries, so the 5v are regulated from a 12v AC generated by the flywheel, and the pick signal also comes from that flywheel, using LiPo batteries or any other rechargeable battery is a bit of a burden, and commercial units like Zeeltronics, Ignitech, HPI, MVT, PVL and others dont need any sort of battery to start reliably every-time, so there must be a way do it right.

author: lewis
author_rank: Frequent Contributor
timestamp: March 24, 2013, 02:47:14 am
content: Aha, my bad, I assumed the bike would have electric start too.

author: gregariz
author_rank: Frequent Contributor
timestamp: March 24, 2013, 03:39:04 am
content: Clamp or Clipper?

author: senso
author_rank: Frequent Contributor
timestamp: March 24, 2013, 04:26:05 pm
content: I think that those are the same..

author: Harvs
author_rank: Super Contributor
timestamp: March 25, 2013, 12:05:10 am
content: Need to know more about how the original on the bike was setup. The bike ignition systems I've worked on had the trailing edge of the trigger "hump" (which is the second pulse,) represent the lowest amount of ignition advance that the engine uses.This is done because below about 2000rpm on a single or twin the engine speed can change too dramatically from one cycle to the next, hence giving an unstable ignition advance for a system based on delays from a fixed point. Therefore, at start up and anything below about 2000rpm, the ignition advance circuit does nothing and just uses the trailing pulse to directly trigger the firing circuit.Also of note, the bikes I've measured basically clamped the ignition trigger coil through some current sensing arrangement to make them more immune to noise and false triggering. Probably worth considering.

author: kxenos
author_rank: Frequent Contributor
timestamp: March 25, 2013, 12:32:20 am
content: You can add a PFET from ign. to scr with it's gate grounded through a resistor. Also gate will be connected to a pin of the uC. When the uC is off, the PFET is a closed switch and current flows. Now when the uC is on, it can pull high the pin and the PFET opens. Now you can inject whatever you want to the scr. Before the PFET you can place a bridge and a cap that will power a dc-dc that will generate 5Vdc for your circuit.Also you should protect your circuit for reverse voltages

author: senso
author_rank: Frequent Contributor
timestamp: March 25, 2013, 11:21:27 pm
content: I was thinking about using a PFET, so thanks for the suggestion, I will test it and see how it performs.Thanks to all the help provided by everyone as well.I know that the second pulse usually means the maximum advance, but this is from highly tuned engines that no longer use the stock stroke, so the only thing that I need is the first positive pulse and know how many degrees that pulse is from the top dead center, that is my static advance, from that the micro will follow my custom ignition curves.

author: Harvs
author_rank: Super Contributor
timestamp: March 26, 2013, 12:04:16 am
content: Quote from: senso on March 25, 2013, 11:21:27 pmOk, my experience with Ducati 2V engines is that this configuration gives a variation of up to about +/-6 degrees in ignition advance at idle due to changes in rotation velocity from one cycle to the next. This was unacceptable for me. YMMV depending on the specifics of your application.Personally, when getting an engine to idle well I'll take a fixed, dependable ignition advance over a variable yet erratic one any day.

author: senso
author_rank: Frequent Contributor
timestamp: March 26, 2013, 12:31:52 am
content: A low RPM I can just take the pick-up signal directly, so it should be stable, they are all single cylinder engines, with light weight crankshafts and rotors, so that shouldn't be a problem, but if it shows up I will come up with some solution.