First, before I get to the meat of the matter, this circuit has nothing whatsoever to do with automotive modifications, even though it contains high voltage ignition circuitry. It's also not designed, or used to shock people.

The circuit itself is used to ignite waste gas from natural gas drilling operations, and quite simply, delivers a high voltage spark at the top of a flare stack every 20 seconds, or so. If the drill hits a gas pocket, the gas is routed to the flare stack, where it can safely be burned off.

The problem I'm seeing is that the circuit is prone to failure, and out of nine igniters, I have four down for the same problem. The problem manifests itself with high voltages present throughout the circuit, which generally scrambles a pair of 555 timers. The device that's failing is a Fairchild [SIZE=-1]ISL9V5036P3, which states in it's datasheet that external components for protection are not required.

The collector of the device is connected to the primary of the ignition coil through a relay, without the use of any external protection components at all. The 'problem' first manifests itself by loss of consistent spark delay, duration and 120hz drive signal from the two 555 timers that control those functions. The chip also becomes very warm...too warm to even touch, by which point, the chip is fried.

My solution is to connect a suitably sized MOV from collector, to ground, which can be done easily by soldering the MOV on to the back of the circuit board. Which is about all I can do without removing the [/SIZE][SIZE=-1][SIZE=-1][SIZE=-1][SIZE=-1]ISL9V5036P3[/SIZE][/SIZE] and placing it on it's own daughter board.


Question: Since the MOV is going to be conducting on a regular basis in order to protect the ISL9V5036P3, can I expect premature failure of the MOV? The MOV I'm looking at is rated at 200 volts, 10 amps.


Thanks in advance, everyone! [/SIZE]

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A few questions:
- Does the [SIZE=-1]ISL9V5036P3 run cooler with the MOV in place?
- How hot does the MOV get after running for a while?
- I assume the [/SIZE][SIZE=-1]ISL9V5036P3 drives a transformer. Have you tried connecting the MOV across the primary?
- Have you tried a spark gap device instead of an MOV?
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- Have you tried a spark gap device instead of an MOV?

Spark gaps would have a much higher breakdown voltage than the transistor or MOV so I don't see how that would help.

 

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Question: Since the MOV is going to be conducting on a regular basis in order to protect the ISL9V5036P3, can I expect premature failure of the MOV? The MOV I'm looking at is rated at 200 volts, 10 amps.

That likely should be adequate. Monitoring its temperature after a period of operational time should tell you if it's being overstressed.

 

I have attached a Littelfuse data sheet for their gas filled spark gaps. Their break bown voltages run from 145 volts to 1100 volts. They cost about $3 each.

Interesting. I obviously didn't realize they go that low in voltage.

P.S. That attachment doesn't seem to work.

 

I withdraw my spark gap suggestion. The data sheet says they are only rated for 400 surges (500 amp). I guess I had used a different spark gap in the past that had a much greater lifetime.

 

Why not just use a grill igniter and a spark plug? My grill lighter runs on a single AA battery and generates about 5 sparks every second. Internally it's an oscillator and a big step-up coil.

 

[SIZE=-1]The collector of the device is connected to the primary of the ignition coil through a relay, without the use of any external protection components at all. The 'problem' first manifests itself by loss of consistent spark delay, duration and 120hz drive signal from the two 555 timers that control those functions. The chip also becomes very warm...too warm to even touch, by which point, the chip is fried.[/SIZE][SIZE=-1][/SIZE]

What chip gets warm?

So the MOSFET drives a relay and the relay drives the ignition coil? There should be a diode across the relay coil. You may not see this as that diode may well be inside the relay itself. It doesn't hurt anything to add an external diode anyway right at the relay coil; 1N4000 thru 1N4007 should work OK.

However, I'm guessing without a schematic, or even a good clean picture of the unit itself.

(A grill ignighter may only work in the short term. That is what they seem to do on my grill even when subjected to only the occasional use.)

 

(A grill ignighter may only work in the short term. That is what they seem to do on my grill even when subjected to only the occasional use.)

Mine also failed, but that was due to corrosion of the PCB because of environmental exposure (they seem to assume we'll store our grills indoors ). My replacement igniter has been in there for years. Admittedly, service is very intermittent. It would make sense to slow it down a lot for this application. Or maybe just turn it on and off every 20 seconds.