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LED's for spark monitoring

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LAN_403

Tesladownunder

Sun Sept 24 2006, 04:17AM

Registered Member #10 Joined: Thu Feb 02 2006, 09:45AM
Location: Bunbury, Australia
Posts: 1424

This follows on from investigations into a method to monitor the polarity of individual sparks and streamers in the High speed Tesla spark photography thread.
I found that one could make a very effective and simple monitor with two LED's in anti-parallel and two 10 ohm resistors. This seems to be a rather bizarre combination of values which should make the LED's blow from overvoltage and over current but the red LED's haven't blown yet with solid 2 foot sparks from my 4 MOT Tesla coil.
They are quite bright considering that they are only responding to bursts of microsecond pulses that make up a TC streamer ring up, strike and ring down.
Note that you can't run the LED's off DC without putting about 0.4A through the 10 ohm resistor (=4W) and burning it up rapidly.

Pics show
1 circuit diagram,
2 actual low inductance wiring and
3 in use with a Tesla coil. The TC shot actually has them at both ends but the toroid end was mis-wired

.
4 shows a high speed streak camera view of the negative strike and alternating polarity ringdown of sparks (which are not very visible to the camera)

Next post I will show ignition coil sparks and spark arresters.

Peter

1159071472 10 FT0 Hvpolarityledschematic

1159071472 10 FT0 Hvrotmirror24inmirroroff

1159071473 10 FT0 Hvrotmirrorpolaritydetailnegstrikelong

Steve Conner

Sun Sept 24 2006, 08:15PM

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

Excellent stuff!

I'd never have thought LEDs would survive that kind of abuse either, but 4hv wrote the book on calculated component abuse

Tesladownunder

Mon Sept 25 2006, 01:19AM

Registered Member #10 Joined: Thu Feb 02 2006, 09:45AM
Location: Bunbury, Australia
Posts: 1424

Well, it gets weirder.

Photo 1 Main resistor 10 ohms, no shunt resistor. Still OK.
Photo 2 No resistors at all

You don't need the resistors at all. Well at least for ignition coil stuff.
Photo 3 Actually I have known about this for 3 years. Here is my flyback inverter running of a 2N3055 (remember them?) and two antiparallel LED's taking the spark output. These were square non high intensity types but no dropping resistors. The dust confirms how long it has been since I have used them.
I have a recollection of trying this with a TC but they didn't last long but they did for a while. I still haven't blown a red LED so maybe I haven't reached the limit.
Photo 4 I incorrectly wired two LEDs in parallel instead of anti-parallel in the 10 + 10 ohm arrangement with 2 foot TC sparks. Amazingly they didn't blow even though massively overvolted in reverse. The incorrect LED's are at the right and are faint (not aimed well at the camera. I think one of these LED's had reduced output later. Presumably they avalanche very well.
Photo 5 A neon across an 82 ohm resistor just fires. Assuming inductive reactance is not the major factor, and that a neon fires at about 70V this suggests that 10 ohms will have a voltage drop of about 12 volts in this ignition coil setup. It also gives an estimated current of about 800mA peak.
Photo 6 A photoflash Xenon firing across a 1000 ohm resistor. I haven't tested the flash over voltage but perhaps 1000V might be reasonable (although the E fields may trigger it at lower voltages. So similarly about 1 amp seems to be the current.
I now need to test this in a TC circuit and also repeat the readings with the neon /xenon shielded. I will also try smaller resistors which should have a lower inductance to see if inductance is playing a part. I have a few other thoughts as well.

Peter

1159147147 10 FT16359 Hvpolarityled10ohmoneonly

1159147147 10 FT16359 Hvpolaritylednoresistors

1159147147 10 FT16359 Hvpolarityledflyback

1159147147 10 FT16359 Hvpolarityneon82ohm

1159147147 10 FT16359 Hvpolarityxenon1000ohm

LarsE

Mon Sept 25 2006, 01:54AM

Registered Member #153 Joined: Sun Feb 12 2006, 03:44PM
Location: Sweden
Posts: 31

I tried to put red LEDÂ´s on the top load of my small SST and they survived with no problem.
]1159149222_153_FT16359_highvoltageled.wmv[/file]

EDY19

Mon Sept 25 2006, 02:15AM

Registered Member #105 Joined: Thu Feb 09 2006, 08:54PM
Location:
Posts: 408

Thats pretty cool LarsE- Not many people have the problem of the leads melting before the LED's die

Nik

Mon Sept 25 2006, 05:55AM

Registered Member #53 Joined: Thu Feb 09 2006, 04:31AM
Location: Ontario, Canada
Posts: 638

It has to do with current. As long as you dotn run too much current though the LED they really don't seem to care what voltage goes across them. I know this is diffrent but I have run a regular 60w incandecent lightbulb off of a mot (ballasted to ~1000w) and the bulb hapily ran without protest because it was passing the 0.5amps it was rated for. Now what is a little perplexing to me is even though it was running at the correct current it was disipating 1000w insted of 60w, shouldn't that cause the filliment to burn up? Or did I jsut find the least efficint way to light my workspace

Marko

Mon Sept 25 2006, 10:44AM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

Voltage across something truly IS just a product of it's resistance and current, leds etc. have a constant voltage drop and with few mA they will shine nicely no matter is the current sourced by flyback or 3V battery.

Only care must be taken not to overvolt the LED in reverse with HV.

Your mot bulb didn't 'care' for anything because voltage drop over it's resistnace was probably around it's working voltage adn it shined happily dropping say 200V and dissipating it's 60W. (very poor impedance amtch).

Remaining kolowatt and half remained circling as reactive power being fed back to source.

With TC's it's a bit different: LED's seem to suffer high peak currents but aren't bothered for much, since durations are very short.

Tesladownunder

Mon Sept 25 2006, 01:54PM

Registered Member #10 Joined: Thu Feb 02 2006, 09:45AM
Location: Bunbury, Australia
Posts: 1424

Firkragg wrote ...

..Only care must be taken not to overvolt the LED in reverse with HV...

Not quite true given the previous video where unprotected single diodes have their leads melted on a TC.

I have now developed an current meter for HV.
Photo 1 Single unprotected LED, avalanching away happily in reverse.
Photo 2 The schematic of the meter showing one reverse LED and 6 forward ones. THe theoretical currents to run the LED's at 2 V are also shown.
Photo 3 The current meter in action.
Photo 4 I have recorded a death of a LED which was a gradual affair with decreasing brightness over minutes of sparks until it stopped. Here it is fading (leftmost LED)

Peter

1159192464 10 FT16359 Hvpolarityledsingle

1159192464 10 FT16359 Hvpolarityvu1schematic

1159192464 10 FT16359 Hvpolarityvuleddying1

Steve Conner

Mon Sept 25 2006, 02:17PM

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

Nik, you ballasted it to 1000VA, not 1000W. The resistance of the bulb is so badly matched to what a MOT is designed to drive, that it runs at a very poor power factor.

TDU: Could you connect two of those current meters so one shows negative current and the other positive, and then put them side by side in your streak photos? It should make an oscilloscope-like display, similar to what Terry is trying to do with the LM3914.

Tesladownunder

Mon Sept 25 2006, 07:08PM

Registered Member #10 Joined: Thu Feb 02 2006, 09:45AM
Location: Bunbury, Australia
Posts: 1424

Steve Conner wrote ...

TDU: Could you connect two of those current meters so one shows negative current and the other positive, and then put them side by side in your streak photos? It should make an oscilloscope-like display, similar to what Terry is trying to do with the LM3914.

I want to have positive and negative represented on both ground and toroid ends and I need a wider dynamic range as well with perhaps 8 to 10 orders of magnitude.
I also want to be able to short the LED's progressively to avoid exposing the most sensitive ones to high current. I have in mind much higher currents than TC's.
Of course, this still needs to be tested on a TC

It would be good to be able to replace the LED's easily for failures.
I think the metal or carbon film resistors are small enough to have low inductance. Perhaps the 1/4 W ones will have less than the 1W ones. Really I don't know how accurate the divider will be. Still any information is better than none but some calibration would be good.
I could check linearity by putting this setup within a 1000:1 divider and ensuring that it does go down by 3 orders of magnitude. Also should be designed for 1 rather than 2 amps and multiples thereof.
A potential problem is that LED brightness will depend on the time that the current is above the threshhold for that LED. For example the 20A LED still lights dimly, suggesting that the current exceeds 20A for a short time. The first three LED's do seem to be saturated though and there is a drop after that.
It also means that there is no real set endpoint but relative brightness will be easy to see, even without a streak camera. THe streak camera should give really good shots, but the pictures are much dimmer.
Lots to think about but sadly, I have run out of red LED's at the moment and I don't want to pay $2.90 each locally so will have to wait for an eBay order.

Pic shows the reverse polarity LED has died. RIP

Peter

1159211294 10 FT16359 Hvpolarityvuleddying2

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