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Question re: Calculating V drop across spark gaps, components

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thrival

Fri Oct 19 2007, 09:06PM

Registered Member #1019 Joined: Sat Sept 22 2007, 02:39AM
Location:
Posts: 29

Assuming the typical ignition coil circuit where spark jumps a gap,
how would a person calculate:

1) the V drop across the gap?

2) the V drop across any component in series with the gap, when breakdown occurs?

I have 4 loaner books on HV engineering, go into great detail re: the physics of
spark discharges, but not how value of components are affected. In days of early
radio, was well-known that spark gaps converted HV, low current, to low V, high current.
Can anyone refer to me to sources where and how the latter values are quantified?
Thank you

...

Fri Oct 19 2007, 10:10PM

Registered Member #56 Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445

The voltage drop across anything connected to the secondary in a circuit like that will be negligible. The voltage drop across D1 will be about 10v (roughly 2v times the number of diodes in series), and the voltage drop across the spark gap will depend in the current in the gap. If you have a nice high current running, it should be well under 100v.

thrival

Fri Oct 19 2007, 11:30PM

Registered Member #1019 Joined: Sat Sept 22 2007, 02:39AM
Location:
Posts: 29

Dear *** :

Thanks for your reply, however I'm still a bit confused. You said:

The voltage drop across D1 will be about 10v (roughly 2v times the number of diodes in series),

I only intend to use one diode. I know diodes typically have V drops from .7 to maybe 10V
depending upon type. Do I need a HV diode for this if most all the V drop occurs across the
spark gap, or could a lower rated diode serve? How do you come by your figures of 100V
across the spark gap, et al ? Actually current out the ignition coil in this case will be pretty
low, <200 watts. Thanks again!

Simon

Fri Oct 19 2007, 11:40PM

Registered Member #32 Joined: Sat Feb 04 2006, 08:58AM
Location: Australia
Posts: 549

A spark or arc happens when you ionise air (free electrons from nuclei) and make a conducting path. This can happen two ways: heat or electric fields.

When you first apply a high voltage to air, the air ionises and you get a spark. This takes the Paschen voltage to happen. As soon as current flows through the air, this heats it up a lot. Heating the air reduces the voltage needed to ionise the air.

Calculating this stuff is difficult.

...

Sat Oct 20 2007, 12:21AM

Registered Member #56 Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445

There is no such thing as a single diode that can handle >10kv, hv diodes are made internally as a stack of smaller diodes inside a single package.

Steve Conner

Sat Oct 20 2007, 12:46PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

The voltage rating of the diode is for the reverse direction anyway, when it's not conducting.

As for the spark gap voltage, look at this:

thrival

Sat Oct 20 2007, 12:59PM

Registered Member #1019 Joined: Sat Sept 22 2007, 02:39AM
Location:
Posts: 29

Dear *** :

Well I believe you, have no reason to doubt that, but do I need a diode stack?
Again, the diode is ONLY forward biased and will never experience a reverse
voltage. It's a DC circuit and the diode is redundant from that perspective, but I still
need it for other reasons, hence the need to calculate the V drop across it during forward
bias, when the arc has formed and is conducting. Arcs lower the V in a circuit by acting
like a big resistance. That's why what comes out after is lower V than what went in.

I know voltage across gaps is determined by distance and breakdown of the air, and that
there's a resistance drop across the gap when it occurs. But now that an arc has formed,
how does one calculate the V drop across other components in that series circuit, while
the arc exists? If you say I need a 10kV diode stack, OK; I'm just trying to understand
how you came by that figure

likewhat

Sat Oct 20 2007, 03:24PM

Account deactivated by user request on 6/11/2009.
Registered Member #1071 Joined: Fri Oct 19 2007, 02:13AM
Location:
Posts: 44

It seems like it should just be an ohms law type thing to me. The voltage drop across a component should just be related to its resistance like in any other circuit. At some point when things are small and the voltage is high you might just be arcing across stuff so a "high voltage" one of them is needed. I dont think the presence of an arc should matter if you are worrying about something else in the series circuit. You should be able to model everything else in the circuit as a black box feeding some voltage and current through the component in question.

...

Sat Oct 20 2007, 04:45PM

Registered Member #56 Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445

I suppose I misunderstoodd you, if you are using a a single low voltage diode you can assume about 2v drop across it.

thrival

Sat Oct 20 2007, 05:43PM

Registered Member #1019 Joined: Sat Sept 22 2007, 02:39AM
Location:
Posts: 29

Thanks guys, appreciate the input, but still need to calculate.

Imagine for example, we have a source putting out 35kV, but the spark
gap is moved close enough to break down at 10kV. Where does the other
25kV go? Across the diode? If the diode is the only other R in the circuit,
then I'm afraid it might be toast.

If someone can tell me they know for certain, that forward-biased, low voltage
diodes can be placed in HV DC circuits without worry (within rated amps), then
I'll be very happy.

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