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I'm looking at converting my positive output HV supply (capacitor charging) to a negative output supply. These capacitors discharge through flash lamps that require the anode side of the lamps to be positive relative to the cathode side of the lamps. If the anodes are connected to ground (through the HV switch) and the cathodes become the HV (-) supply, am I cool? The trigger circuit will have to be inverted as well, but that's a simple rewiring. Am I missing something?
No thoughts on this?
can you draw a schematic?
Actually...I don't want to. I'm not being stubborn, I just think it would take me a long time to sketch something in Paint and post it. Ultimately, I suppose I could, but it is the concept that I am most curious about. In general, flash lamp charging circuits are drawn and built to drive these lamps with a positive anode voltage relative to ground. I am happily cranking out kW into these lamps with this arrangement. I want to replace my big, bulky (and inefficient) linear power supply with a pulsed, constant current switching supply, but the output in this case is negative relative ground.
Maybe I am being naive, but I would think that as long as the anode side of the lamp remains electron deficient relative to the cathode, there would be no problem. The cathode would then no longer be common with ground and the anode would become common to ground. Is this weird or not?
The concept seems sound. The lamp won't know the difference. My concerns would be the input of the trigger circuit is isolated from the HV circuit. And what ever device is triggering the lamp is now on the correct side of the lamp.
Mixedgas has more experience on these things. Hopefully he chimes in.
The trigger is a capacitively coupled plate(reflector), (+)50kV through the xenon, grounding through the cathode to the capacitor bank @ around +/- zero volts (now) and grounding to the new (-)4 to (-)6kV cathode, yet to be.
Steve?
Pulse is done with a trigger coil?
How many volts does the tube operate at? If its like 400-600 or I guess less than maybe 2kv you could upgrade/add something like an IGBT + simmer ignition
Each of the four tubes has a hold off up to approximately 2200V and then would spontaneously and unpredictably discharge. When triggered, all four of these tubes will conduct a precisely timed pulse (jitter at < 1 usec) from the main capacitors that can be as little as 800 V. The common,adjustable, over voltage spark gap that is in line with the cathode side of the tubes allows the operating range of 800V to 2200V to be elevated to as high as I want it, up to the maximum energy capacity of the lamps. The trigger is an adjustable, 50-60kV, 8J, 4usec pulse from a coil that is conducted to the aluminum housing that supports the chamber reflectors. The original trigger wires absorbed some pump light and were removed. The chamber is 9cm in diameter and 30 cm long. Main pulse energies are up to 4,000J, 30usec long and 20kA.
pics? unless this is somewhere cameras are not allowed ;)
reflector electrically floating? air or watercooled? one end of the trigger coil secondary goes to the reflector, where does the other go?
the lamp anode has to stay positive if its designed as such since the electrodes would be different, ie one pointy the other round or something like that and/or different compositions (tungsten with something radioactive added)
Take a look at these:
http://www.photonlexicon.com/forums/...3636-New-Laser
This was a couple of months ago and the trigger wires are gone, but the reflector is very much as it is shown here. The reflector is floating and air cooled (for this reason). The ground of the trigger is now directly connected to the zero volt, cathode ground and would then be connected to the zero volt anode "ground". The main pulse would still be positive on the anode end of the lamps relative to the cathode.
Unless some portion of the PSU sensing and control system is grounded on the output side, generally there is no issue reversing the polarity. Only folks who would know are the folks that engineered and configured the cap charger. Some times that is mentioned in the manual, some times it is not.
I like dedicated cap chargers as opposed to HV psus. Cap Chargers generally have a circuit that allows faster, more optimal, charging with far less wasted energy by changing the switching frequency in the PSU as the cap charges.
I would switch to series injection triggering if I were you. External field triggering is so out of date. Some times it is needed because of lamp cooling issues etc. Series injection allows two series lamps to run off a given cap bank, as well.
Without pictures and seeing the system, its difficult to comment outside of generalities.
Steve
Panasonic. Reassigned microwave oven, inverter power supplies. The current is controlled by the duty cycle of the PWM %. I think this will be much better than the big linear system I am using now. I just have to get a 220hz variable PW control circuit running.Quote:
Unless some portion of the PSU sensing and control system is grounded on the output side, generally there is no issue reversing the polarity. Only folks who would know are the folks that engineered and configured the cap charger. Some times that is mentioned in the manual, some times it is not.
I actually moved away from series injection to what seems an anachronistic triggering system. I elected to use the external triggering so as to make the lamps themselves the HV, HC switch. This reduces the rather significant inductance that the secondary coil of a series transformer introduces. I also decided to operate each lamp with its own capacitor bank to avoid the increased impedance associated with operating the lamps in series. This impedance could be countered by going to very high discharge voltages and therefore low capacitance (maintaining the LC product), but we're talking 10's of kV's @ kw's for the power supply. Also, the interesting thing about these lamps is that a single lamp will spontaneously break down at about 2,200V, but two lamps in series will break down at about 2,800V. So, with multiple lamps in series, the switch would have to be a large, triggered SG or Thyratron that could hold back nearly the full discharge voltage and survive well over 10 kA. That gets expensive.Quote:
I would switch to series injection triggering if I were you. External field triggering is so out of date. Some times it is needed because of lamp cooling issues etc. Series injection allows two series lamps to run off a given cap bank, as well.
After reviewing the literature, where much of the criticism of external triggering seems to come from is jitter and poor reliability. However, I think I've discovered an interesting phenomenon. Pulse rise time and duration is the result of the increasing density of ionized charge carriers vs the decreasing drive voltage of the capacitor bank as the pulse progresses. As the external trigger pulse becomes more and more vigorous, the main pulse rise and duration decreases. This makes sense because the trigger itself introduces charge carriers and the more of these that are present before the capacitor voltage drops, the higher the average current. This is a lot like the pre-pulse technique without the simmer drive. Eight joules at 55kV is a REALLY HIGH trigger energy and I am considering increasing it further. The jitter is low enough that the light output from one lamp has the same duration as from four lamps and I have yet to see a dropped pulse. I have a laser threshold at about 120J and above 1200J I'm now struggling with damage to the cavity optics. Suca pRoblem? It's like being too thin or too rich. In the words of Jeremy Clarkson "more powwwwwwer" I miss him, but I digress.
I'm happy with the discharge circuit with the exception that the power supply needs to be more efficient to allow a higher rep rate.
One end of the Uwave suppy is probably grounded. Do you have a model number, some of the Magnetron Driver schematics are on line.
Commercial cap chargers tend to be very efficient and faster then a raw voltage source, just something to think about.
Steve
I don't have a line on any cap chargers that might be close to the need. 5,000V @3-6 kW. The power supplies I'm looking at are the Panasonic #220943086526.Quote:
Commercial cap chargers tend to be very efficient and faster then a raw voltage source, just something to think about.
is there a spec sheet for the tubes? how are they wired? 2 parallel? do you need them to all go off exactly at once?
These were custom made. Four tubes are wired in parallel. Each tube is attached to its own 10kV, 50uF capacitor. They have to discharge within a usec or two of each other otherwise the envelope of the light pulse will be widened.