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Registered Member #54278
Joined: Sat Jan 17 2015, 04:42AM
Location: Amite, La.
Posts: 367
I have heard favorable mention of current monitoring devices on 4HV concerning the "Pearsons" monitor which appears to be a VERY expensive item. Is this device designed for continuous AC input or just single current pulses or both...and what about DC monitoring?? I ask since I am about ready to make such current measurements on coigun and pulsed laser flashlamp circuits in attempt to tune the coil to critical damping. I originally wrote this message in the "Tesla coils" forum, but thought it really should be placed here...printed in the "Tesla Coil" forum rules is:
"This is the dedicated Tesla coil board. If it has to do with Tesla coils, post it here. Air-cored resonant transformers only, please!"
So moved it here. I have several standard current shunts with "DC ratings". I don't understand why -any- low frequency current waveform or pulse, should work (which is what I need). HOWEVER, I dug out a vintage induction probe I got at an auction years ago (you thread the wire through it as usual) that seems to only work on AC. I first tried this last night. I now assume nothing is wrong with this coil, as of yesterday, as it gives correct answers for AC. It occurred to me to try AC when I got a significant, short, pulse when I switched the wires during DC measurement attempts. Thereafter, it worked perfectly!
It is made by a company called "GAUSS CONTROL" and rated at 0.1 volt/amp (I verified this as accurate with AC, but get nothing with DC-even with hundreds of amps supplied with a supercapacitor!). I posted two photos of it on the "Tesla Coils" forum in the "Oscilloscope Choice" thread.
Since I heard several members are "watching" for a low ebay price to appear on a Pearsons coil, I will, IF one of the other shunts work for me, consider selling this vintage GAUSS CONTROL unit at a lower price (compared to a similar Pearsons unit on ebay) only to a 4HV member.
Any information as to why a DC-only monitor is manufactured will be gratefully appreciated.
Registered Member #162
Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140
A d.c. current shunt is usually a low value resistor inserted in series with the circuit being monitored, and the voltage across the resistor is proportional to the current through it. all resistors have some inductance, very high current shunts are often just a straight piece of metal, low current shunts may be wirewound, having more inductance. The ratio of resistance/inductance determines the maximum operating frequency. V = (I x R) + (L x dI/dt) so a d.c. shunt will also accurately measure current, up to a certain frequency. pulses or impulses have large dI/dt so false readings will be obtained (too high)
a.c. current monitors are commonly simple transformers, typically a toroidal core but not necessarily. e.g. the current monitor that you posted could be a toroidal core with a 1000 turns winding, giving a current of 1mA per amp.turn passed through the core. With a 100 Ohm resistor across the coil you would have 0.1 V per amp.turn.
at very low frequencies the reactance of the winding will be too small for an accurate measurement at very high frequencies the self-capacitance of the coil will cause errors, or possibly the core may overheat.
For 'tuning' a flash or coilgun discharge you don't need an accurate/absolute measurement, it's only the waveform that is important, so if you clip your 'scope 'earth' to the 'earthy' side of a wire that leads to the tube/coil and probe tip to the end of the 'earthy' wire near the tube/coil you will get an accurate current waveform, usable for 'tuning'
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Anyone familiar with "GAUSS CONTROL" current monitors?
