Need solution to level shift and amplify low frequency test signal - Page 1
Forum section: electronics / projects

author: rbm
author_rank: Regular Contributor
timestamp: February 16, 2019, 10:08:11 pm
content: I have built an motorcycle tachometer conditioning circuit and I need to functional testing on units that I build. I presently use a programmable arbitrary waveform signal generator to simulate the signal that would normally be present on the ignition coil primary. The picture below is of the actual ignition coil primary waveform on an operating engine:The signal frequency can vary anywhere between 20Hz and 300Hz. The signal generator I use (Feeltech FY3224) is limited to 20Vp-p and any waveform output by it is zero centred. The actual test waveform has to simulate the one above, swinging between 0V and 30V (max). I'd like to find a circuit that will level shift and amplify the waveform I'm able to produce on the FY3224 to be more like the actual signal. Does anyone have an idea as to how I can accomplish this?

author: donmr
author_rank: Regular Contributor
timestamp: February 16, 2019, 11:08:20 pm
content: Your best bet would probably be to set up an ignition coil, sparkplug, and a relay to simulate the distributor.

author: SilverSolder
author_rank: Super Contributor
timestamp: February 16, 2019, 11:21:27 pm
content: A simple solution might be to run the output from your signal generator through a transformer, to step up the voltage?You could easily add a DC offset to the secondary side.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: February 16, 2019, 11:25:38 pm
content: If you put a dremel to a toroidal ferrite core and then put a HALL sensor in the gap you can pick up the current in the wire to the ignition coil.This is probably the most reliable method.No need to interrupt or change the existing wiring in any way.No troubles with for example mosture or accidentally shorted wires.You do not want to get stranded in the middle of who-knows-where because your tacho meter addition is messing with your ignition's ability to generate proper sparks.Another way is to make a capacitive pickup.You can wrap a wire around the primary or secundary ignition coil wires a few times (no electrical contact) and fix it with tape or shrink wrap.The 80V or more spikes are mostly irrelevant, as long as your circuit can withstand them.Start with a 100k resistor in series with your wire and a few fast TVS diodes from your power supply.That should take care of clamping down those transients, especilally if combined with a small inductor in series with your resistor.

author: rbm
author_rank: Regular Contributor
timestamp: February 16, 2019, 11:32:34 pm
content: Thanks for the suggestions.@Doctorandus_P: I'm trying to make a bench test setup to do functional testing year 'round. I'm not intending to use the actual engine as a test device. Half the year, the motorcycle is winterized and unavailable anyway. As interest, there is already a manufacturer-provided connection to the coil primary available on the harness specifically for this tachometer purpose. The circuit I build is for people who modify the motorcycle into a cafe racer or bobber.The conditioning circuit works OK with the signal I can simulate but I'd really like to have the input as close to the actual conditions as possible.

author: langwadt
author_rank: Super Contributor
timestamp: February 16, 2019, 11:50:15 pm
content: Quote from: rbm on February 16, 2019, 11:32:34 pmI know a guy that makes ECUs he's occasionally used a regular automotive relay without the contacts driven by a regular low side mosfet output on his ECU, the flyback from the relay coil provide high enough voltage to triggerthe standard rev counter

author: David Hess
author_rank: Super Contributor
timestamp: February 17, 2019, 11:33:13 pm
content: Why not just use a x10 direct coupled linear amplifier with relatively wide bandwidth and a compliance of +/-100 volts on its output and rely on the signal generator for level shifting on the low level side?

author: SiliconWizard
author_rank: Super Contributor
timestamp: February 18, 2019, 12:08:06 am
content: Wouldn't that be an application for this one?https://www.eevblog.com/forum/projects/new-high-voltage-opamp-adhv4702-1/

author: David Hess
author_rank: Super Contributor
timestamp: February 18, 2019, 12:52:53 am
content: Quote from: SiliconWizard on February 18, 2019, 12:08:06 amHonestly, no. Once you design a circuit to get past the power dissipation limitations which will involve transistor cascodes, a higher performance lower voltage operational amplifier can be used.