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Ignition coil arc generator

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iJim

Sat Mar 19 2016, 08:27PM

Registered Member #2662 Joined: Fri Jan 29 2010, 10:14AM
Location:
Posts: 36

Hi all,

I wonder if someone could help me out... I've built a mosfet half-bridge using a self-oscillating IRS2153D. With no load, the circuit works exactly as expected. When I attach an ignition coil I can generate a high voltage with the secondary open circuited. An issue arises when I try to draw an arc to ground. As soon as I bring a grounded rod to the HV terminal the forming arc causes my benchtop DC power supply to reset (it's a sensitive smps variety). I've tried all sorts of interim fixes, adding additional capacitance to the output of the bench supply had no effect, adding some capacitive impedance on the HV output to reduce the current didn't work (<50 pF @ 20khz). Any ideas? I've built the half bridge circuit several times and never had a problem with it before, but I've never tried to draw arcs before!

Cheers!

Antonio

Sat Mar 19 2016, 10:46PM

Registered Member #834 Joined: Tue Jun 12 2007, 10:57PM
Location: Brazil
Posts: 644

Look at your ground connections. The return current from the arc may be interfering with the control of the power supply. Or just the power supply can't supply enough current for the bridge with an arc at the output.

iJim

Sat Mar 19 2016, 11:00PM

Registered Member #2662 Joined: Fri Jan 29 2010, 10:14AM
Location:
Posts: 36

Thanks Antonio! The PSU can provide 60V @ 4A, the arc probably strikes at 25 V ~1 A, so I'm quite a long way off the current limit. The way in which the power supply shuts down suggests interference (basically it beeps, the lcd goes blank and the output is disabled; it requires a mains reset to get working again). The grounding isn't great, as there's some pretty long wires from the supply to the PCB, I did try some shorter ones but they had limited effect. I'm arcing to the earth terminal on a different mains socket, not sure if this is the source of the problem? Additionally, I assume the HV winding on the ignition coil is connected to one of the primary terminals, it isn't at all obvious where else it could be. I can see this as being a problem, but I don't see an easy way to fix it.

Cheers,

Jim

hen918

Sun Mar 20 2016, 10:17AM

Registered Member #11591 Joined: Wed Mar 20 2013, 08:20PM
Location: UK
Posts: 556

Try putting a low pass LC filter on the output of the PSU, on both the negative and positive rail.
i.e. extra capacitance in parallel, an inductance in series (an inductor on the negative and one on the positive rail) and more capacitance in parallel. You shouldn't need / want too much inductance, maybe 100uH total

If that doesn't fix it, the interference is being transmitted over the air... Physical separation? Or you could use a different power supply

Proud Mary

Sun Mar 20 2016, 02:22PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

Field collapse around laminated iron core make plenty big back EMF. Back EMF pulse sees wires going to power supply and think "Ah there's a handy transmission line. Let's see where it goes." Maybe PSU go FUTT next time reflected pulse go looking for end of transmission line.

Patrick

Sun Mar 20 2016, 04:48PM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

the back EMF pretty much causes a crowbar, or similar protection circuit to latch, thus needing you to unplug it. really you should use a linear PS or car battery.

woodchuck

Sun Mar 20 2016, 05:24PM

Registered Member #39190 Joined: Sat Oct 26 2013, 09:15AM
Location: Boise National Forest
Posts: 65

Yup. Despite the wonderful efficiency, compactness, and cost-effectiveness of switching power supplies, there are times when a Variac, rectifier, and a few large electrolytic capacitors are golden. Also, the continuous control they provide is an invaluable aid to what you're doing.

iJim

Sun Mar 20 2016, 06:55PM

Registered Member #2662 Joined: Fri Jan 29 2010, 10:14AM
Location:
Posts: 36

It's mighty annoying... I do have an old linear supply sitting around, I haven't tried it but I'm pretty sure it'll be fine in this application. Rather than digging it out and admitting defeat I'll add some
Inductance between the psu and half bridge with some capacitance either side.

Cheers.

Proud Mary

Mon Mar 21 2016, 04:04PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

As Patrick points out, batteries are an excellent investment if you do lots of HV stuff.

I have two programmable power supplies and I would never ever conect them to a HV experiment because I've wrecked PSUs in the past which is frustrating and expensive.

I don't use car batteries because I am always short of space, so I find the Yaesu series of compact lead acid accumulators work very well for me.

I also use the very robust Cyclon 2V 8AH single cell lead acid batteries if I need a power supply floating with respect to Earth for X-ray cathode heaters. By series-ing with single 4AH Nickel cadmium cells, I can lash-up the voltage range 1.2V, 2V, 3.2V, 4V, 5.2V and 6.4V, which completely covers me for floating cathode heater supplies across a range of diffraction and spectroscopic tubes.

Lead acid cells also offer very high instanteous currents, which is handy where something starts off with a good kick.

Where equipment is only required to work for relatively short periods of time, storage batteries offer a very simple alternative to building a more vulnerable and more complex PSU.

Patrick

Mon Mar 21 2016, 08:12PM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

just watch out for feeding back high voltage into the battery and/or evolving Hydrogen.

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