EEVblog Electronics Community Forum
Electronics => Projects, Designs, and Technical Stuff => Topic started by: HighZ on December 30, 2018, 12:50:10 am
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Hi Guys,
I have been working on a circuit for a white now and ive just managed to get the chance to assemble it to test and im having some weird issues.
Description of Circuit:
The circuit is basically a regurgitation of a SiliconChip Magazine design "High-Energy multi Spark For performance Cars" from their December 2014 issue, i will not link the whole article here or pics directly from their magazine in order to avoid any copyright issues here for the message board, i will however post a picture of the circuit i have drawn up (or at least the relevant sections to this topic). The circuit is basically a High voltage generator using a TL494 to drive some fets that drive a coil to produce +300V DC. This 300V then gets fed to a L6571 High Voltage Half Bridge Driver with oscillator to charge a capacitor. The L6571 works like a SPDT switch, charging the 1uF cap in one direction and discharging it in another. upon Trigger, the 1uF cap is discharged through Q4 collapsing the magnetic field in the primary winding of the coil and you get your spark(s). So, sounds easy enough right.
(https://i.ibb.co/DYPGp2b/circuit.jpg) (https://ibb.co/2S08N46)
Description of Problem:
i have the trigger to this circuit being controlled by a microcontroller, driving a 5V digial signal to a BC817 (NPN) (T2 in schematic), i have this trigger signal running at 14Hz. the issue is when i start the program to output the 14MHz signal to T2, the system constantly gets reset, as if the microcontroller is being either browned out, or a high voltage spike is causing it to reset. This kind of spurious output continues for a while until the microcontroller gives up and just resets to the start of its program (where no 14Hz output is given to T2).
Testing and Limitations:
The 300V HV circuit works fine i have tested the voltage with a DMM, it looks like the High voltage half bridge driver works when i signal a trigger to T2 however.. this is where the problem begins.
Below you will see two logic captures (they are only at 4Mhz, i have a crappy logic analyser) :
The first is what the signal "should" look like, the channel on the bottom is the output trigger to the CDI circuit. this is the only one that really means anything here.. everything from chanel 0-6 is just diagnostics for my own peace of mind. Channel 7 shows a 1ms pulse to drive T2 which then trigger the Half Bridge Driver (U7), this is WITHOUT the R36 in place.. so nothing is getting driven.. this is what we should see.
(https://i.ibb.co/HzsBMXm/good-logic.jpg) (https://ibb.co/DCyz6L3)
The second logic capture is what i get when R36 is installed and the half bridge driver (U7) is being triggered. This is all in between the microcontroller being reset and picting back up the Trigger signal (channel 0) and sending the "CDIout p79" signal (Channel 07) to the half bridge driver.
(https://i.ibb.co/Px2bwDt/bad-logic.jpg) (https://ibb.co/wynbMcY)
What the problem could be?
1. there is some high voltage transients moving onto the GND plane after the 1uF cap gets discharged causing a host of problems for the power regulation circuits of the microcontroller.
2. Once the spark plug is extinguished, the collapsing field of the ignition coil develops a reverse current flow and this sucks out all the juice from the circuit causing resets?
3. There is not enough supply current and the circuit is being brown'd out when the cap is gettig charged by the HV generator (there is at least 10A of 12V supply availabel to this circuit, so i doubt it?)
limitation --> i dont have a decent enough scope that will record the GND plane voltage when the 1uF cap is being discharged.
Idea of Possible Solution:
Add some filter caps and diodes to the circuit to remove any HV spikes.. i need your help for this.. where and what exactly.
Gain some more information about the functional state of the circuit when the problem happens to help get a better idea of whats going on (data gathering)
Other info and Thoughts:
Throughout the SiliconChip article they mention trigger signals coming from automotive devices. like Hall Effect, Points, Reluctor pickup, Piranha optical picups... never a digital signal. i suppose that these traditional methods of triggering in engine are more "rugged" and might not be so susceptible to spikes. however.. when we are dealing with a circuit involving 3.3V and 5V logic these digital signals need to be isolated from high voltage spikes for sure.
any help with this would be much appreciated!
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HighZ, I've got a few questions:
1.) do you have the capacitor across the coil primary all the time? If memory serves, a typical CDI does not have one. In a CDI the coil is used as an autotransformer. In an inductive discharge system, the capacitor forms a tuned circuit with the coil and shapes the current waveform to deliver a higher secondary voltage & current plus reducing primary voltage when the points open.
2.) have you tried connecting the coils' HV lead directly back to the coil negative or GND connection? This eliminates the spark & the radiated wide spectrum EMI is reduced. AFAIK, this should be OK but maybe put a 10K resistor or a supression style ignition lead in series to limit current.
3.) do you have a sketch showing all the subassemblies & their relative distances and connections, particularly the ground or common connections? In most electronically controlled ignitions, the coil or coils is or are mounted directly on the engine to provide a solid return connection for the HV currents. You've got a few high current, fast risetime currents interacting with looped connections which are dandy antennas.
4.) the diagram seems a bit fuzzy & it's hard to make out the text.
Cheers,
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Hi I am having similar problems with my silicon chip CDI.
It had worked OK for 100 miles.
Have not solved it yet but I have notice that the supply on the L6571 chip is very close if not crossing its power down threshold.
Going to reduce the value of the two 33K Ohm resistors to 27K tomorrow.
I have noticed a similar problem with IC1 TL494, when the coil is fired the voltage at the transformer primary supply will fall, this will cause current to flow from the 100uF capacitor in parallel with ZD1 back to the transformer. This lowers the voltage at pin 2 of the TL494 and can cause a power down. Solution place diode in series to the 10 Ohm and 8.2K Ohm to prevent the out rush current from the 100uF capacitor.
Will progress if time tomorrow.
Regards
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CD ignition systems produce a lot of radio frequency noise since dumping the charge of a capacitor directly into an inductive load - the ignition coil primary - forms a resonant tank circuit. I did some recent work on just this thing and the noise was so bad it caused two Fluke meters and a very expensive isolated, battery-powered scope to malfunction - stuff which really shouldn't misbehave like that.
The best cure is to use proper shielding and board layout techniques to minimized the ability of external RFI to cause problems with your circuit, but that takes lots of experience, or lots of failed tries (which leads to experience... if you pay attention, anyway). A quicker solution is to insert a small resistance in between the capacitor and primary to dampen the oscillation; start with 1 ohm and work your way up to a maximum of 2x the characteristic impedance of the LC network (which you can find by using the frequency of oscillation and the capacitance to find the inductance, then using the inductance and capacitance to find the characteristic Z).
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A half bridge for a capacitive discharge ignition seems like an overcomplicated design - a simple SCR circuit is a more usual solution.
CD ignition systems produce a lot of radio frequency noise since dumping the charge of a capacitor directly into an inductive load - the ignition coil primary - forms a resonant tank circuit. I did some recent work on just this thing and the noise was so bad it caused two Fluke meters and a very expensive isolated, battery-powered scope to malfunction - stuff which really shouldn't misbehave like that.
I'd imagine the spark plug itself would also act as a spark gap transmitter, only more pronounced with the higher spark energy. Joe Smith built a rig out of actual ignition parts specifically to test automotive multimeters for EMI immunity.
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A half bridge for a capacitive discharge ignition seems like an overcomplicated design - a simple SCR circuit is a more usual solution.
One advantage to the half-bridge approach is that it disconnects the HV supply from the capacitor during the firing of the spark plug, so a modest savings in energy use. That said, I otherwise agree with you that it isn't worth the bother.
CD ignition systems produce a lot of radio frequency noise since dumping the charge of a capacitor directly into an inductive load - the ignition coil primary - forms a resonant tank circuit. I did some recent work on just this thing and the noise was so bad it caused two Fluke meters and a very expensive isolated, battery-powered scope to malfunction - stuff which really shouldn't misbehave like that.
I'd imagine the spark plug itself would also act as a spark gap transmitter, only more pronounced with the higher spark energy. Joe Smith built a rig out of actual ignition parts specifically to test automotive multimeters for EMI immunity.
The spark plug does emit quite a bit of E- and H-field noise, but during my testing of different ignition systems (with the end goal of somehow getting 20J into each spark discharge, vs. the 2mJ that is more typical), only the CDI type (both an off-the-shelf unit from MSD and one I designed) wreaked havoc with my test gear; two different small engine magneto systems (Honda and Briggs) and a much higher energy aftermarket "coil on plug" system intended for late model cars had no effect.
Interesting side note that relates to this issue: the ignition transformer operates in flyback mode in magneto, Kettering (aka "points-style") and even the modern coil-on-plug system I bought, but it operates in forward mode (ie - as a true transformer) in the CDI system. However, the peak current is so high in the CDI system (>100A) that it is guaranteed to saturate the primary up until the plug fires (at which point normal flux cancellation from secondary load current can occur).
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Sorry did not get back to the bench to test. Changed the two 33k resistors feeding the output drive IC power to 27k and 24k. Intermittent false triggering of output stopped. Confirmed that the output IC supply was dipping two low and causing the IC to power down triggering the output FET's.
If you want to stop the EMI from the ignition system go fully FFR military or do Diesel.
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When Q4 turns on, C24 pulls the BOOT pin on down toward zero. D13 will turn on and pull Vs and all associated circuits down to zero, also.
I think what you need is a small resistor in series with D13 to limit current. Also, C24 is vastly too large, I've never used more than 1 uF for bootstrap capacitors.
Also, R44 and R45 seem awfully large to keep all that circuitry powered, they can only deliver about 3 mA. Also, I see nothing to limit voltage on C20.
R44-R45 is delivering 3 mA from a 225 V source. If the circuit ever draws less than 3 mA, the voltage on C20 and U7-Vs will go above 16 V.
Jon
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The operational frequency of this circuit is low, therefore the boot strap capacitor is larger.
Not using the oscillator at high frequency, used at RPM + multiple spark produced by the additional components around the oscillator pins.
Secondly U7 has a shunt regulator which can limit the voltage to~14.4 to 16.6Volts @5mA, 15mA max.
Turn on is 8.3 to 9.7 volts and turn off 7.3 to 8.7 Volts, so voltage must be above 8.7 volts under all events.
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One last comment, check the bottom FET for gate breakdown. Gate drive not reaching ~15Volts.