Jump-start: before & after

[Anonymous]

This is a comparison between two hypothetical pulse induction designs, one conventional resistance current limited, and one with jump-start. I have used my jumpstart topology as the basis, but the results would come out the same using JC's topology.
THE BASIS DESIGN
Coil circuit: 500 microhenries, 2 ohms, 500 pF parallel capacitance. Damping resistor presumed to be 680 ohms, although we'll ignore it in the calculations.
Flyback voltage: 100 volts.
Peak coil current just before flyback: 2 amperes.
Transmit on-time (including jumpstart): 90 microseconds.
Pulse rep rate: 2 kHz (500 microseconds)
Storage capacitor: for the purpose of analysis, it is presumed that this is a storage capacitor, not a half-cycle resonant capacitor. A half-cycle resonant capacitor would be 12 microfarads, so a storage cap would be much larger than that. Of course you'd want a fairly low ESR unit. Note: the design could be half-cycle resonant, but the numbers would come out a little different.
COMMENTS ON THE SPECTRUM: The flyback will have a lot of energy at 50 kHz and will go downhill beyond that. The period of current flow will have a lot of energy at 5 kHz, and the energy will extend down to the fundamental frequency of 2 kHz. The jumpstart version will have quite a bit more low frequency energy, improving its response on high-conductivity targets, but that's not my emphasis in this comparison.
BEFORE: CONVENTIONAL RESISTANCE CURRENT-LIMITED DESIGN
We'll do 2 time-constants. That's not a lot of current limiting, but it's enough that the waveform wouldn't look like a triangle on the 'scope.
The total resistance equals the inductance divided by the (45 us) time-constant, which comes to 11.1 ohms. We've already got 2 ohms in the coil, so our extra current-limiting resistance will be 9.1 ohms.
Let the power supply be +27 volts. Hey, we're generous here! Allow a loss of 1 volt in the transistor and rectifier, and we can deliver 26 volts to the coil circuit.
When the transistor is first turned on, the current is zero, and it rises at the rate of 52 milliamperes per microsecond. Without resistance, it'd hit 2.34 amperes in 45 us, but because that's one time-constant, it'll only hit 63% of that, or 1.47 amperes. During the second time-constant we'll add 37% of that, or .54 amperes. That totals 2.01 amps at the beginning of flyback.
The mean current is approx. 1.33 amperes over the 90 us transmit time. Averaged over the full rep interval of 500 microseconds, that's 239 milliamperes. Power consumption from a 27 volt power supply is 6.46 watts. Ouch.
AFTER: JUMP-STARTED DESIGN
Let the power supply be +5 volts, and assume a loss of 1 volt in the rectifier and transistor, as before. We can deliver 4 volts to the coil.
Let the jumpstart time be 10 microseconds. With 100 volts applied to the coil, the current rises to 2 amperes. 4 volts is being dropped across the resistance of the coil.
Now turn the main transmit transistor on. We continue to deliver 4 volts to the coil. The sustaining current is 2 amperes, dropped across the resistance of the coil, and no voltage is dropped across the inductance because the current isn't changing. This continues for another 80 microseconds.
The current drain during those 80 microseconds was 2 amperes the whole time. Averaged over the 500 us rep interval, that's 320 milliamperes. Power consumption from a 5 volt supply is 1.6 watts.
So did we cheat? only a little. The flyback is 10 microseconds, and the jumpstart is 10 microseconds, and they're both 100 volts, but... those are only approximations. There are resistive losses during flyback and jumpstart, although they're small in proportion to the losses during the transmit-on time. The easiest way to make up the difference is to have the jumpstart time slightly shorter than flyback, which means that jumpstart won't get it all the way up to 2 amps, maybe 1.6 amps. This leaves a voltage difference between the coil resistance and the driver transistor so the current can ramp up to 2 amps.
The extra circuitry in the jumpstart system will require power to switch it, so that's another loss that was ignored. However, one would expect this power to be much less than the power dissipated in the loop.
--A REALITY CHECK------
The jumpstart system is supposed to have efficiency somewhere in the same ballpark as an otherwise comparable VLF unit. 1.6 watts transmit power for a VLF unit is stompin' pretty loud. So, how do the numbers add up?
Of the energy being dissipated in the PI, most of it is in the ballpark of 5 kHz. We're hitting it with about 2 amperes at an 18% duty cycle. On an RMS basis, that's equivalent to about 800 milliamperes. Math a little sloppy, we're just doing a reality check.
The searchcoil has a reactance of 16 ohms at 5 kHz. If it were driven with a 32 volt P-P sinusoid, the RMS current would be 700 mA. Dissipated in 2 ohms resistance, that's an I-squared-R of 1 watt. Looks like we're not too far wrong here.
As far as I know, nobody in the industry drives VLF's this hard, not even close. At 5 kHz with a 500 uH 2 ohm loop, you might expect to see on the order of 14 volts P-P, dissipating 200 milliwatts in loop resistance, in a high end power hog.
-----CONCLUSIONS---------
In this example we got a 6.46:1.6 = 4.04:1 estimated reduction in power consumption. We cheated a little on the jumpstart system, but it has a better waveform, so for equal sensitivity to high conductivity targets the 4:1 improvement figure is reasonable.
Note that the way voltages scale is markedly different.

 

[Anonymous]

Dave,
"The jumpstart system is supposed to have efficiency somewhere in the same ballpark as an otherwise
comparable VLF unit. 1.6 watts transmit power for a VLF unit is stompin' pretty loud. So, how do the
numbers add up? "
The jumpstart having efficiency comparable to VLF unit are your words and not mine. Just wanted to make that clear. You've stated this before, and maybe it is true if you can short the coil, or whatever and save a bunch of power, but until then the PI unit is going drink some power. My main goal was never to save power, but to improve depth, though I know you have been on a course of saving power. And that is also a good goal, these lead acid gel cells are heavy. I can see where maybe 20% power can be saved with jumpstart by shortening the xmit pulse width and some power back from storage cap, the rest being the same.
VLFs definitely use less power than a PI which out performs them (detection depth on some objects). Now a PI which doesn't out perform them (I mean detection depth on same object of interest) really has no reason to exist. VLFs can discriminate.
Whooping lot of power in xmit, yes sir, you put this much power in a IB coil VLF detector and bring it near mineralized ground and the receiver is saturated, even if you did get it nulled that day. It ain't the FCC keeping the current down in VLF units. Its physics. Turn the receiver gain down to prevent saturation by the reactive signal and you got a Radio Shack special, low gain, lousy metal detector.
Whites Eagle SL 2, is one of the few vlf detectors (may be others) that allow you to turn down the receiver gain to operate in heavily mineralized ground, incase there are those out there who were wondering why you would ever want to turn down the gain, this is why.
Other vlf machines just scream their guts out.
Except the extra special gold only machines that can work in fairly bad ground, but not Aussieland.
JC

 

[Anonymous]

Before I get creamed on the statement that the Eagle SL 2 is one of the few detectors that allow you to turn down the gain.....
I misspoke, I should not have made the statement. There have been lots of vlfs made over the years and I meant the xmit power and not receiver gain.
Haven't messed with one in years either.
Sure there are many that the gain is knob adjustable or "sensitivity" is tied to receiver gain. My point is that turning down the gain or the transmit power leads to the same thing, poor depth performance. So can bad design. And that they don't handle mineralization well, something the PI are supposed to do better (and seem to).
VLFs seem to be stuck at a certain performance level for years, new LCD displays and digital gee wiz stuff, but same performance. And may never go much further. We will see. Like you said maybe with a lot of trouble get another inch. Oh boy!
Now I can over the same old ground again.
JC

 

[Anonymous]

Sorry, JC, didn't realize I'd worded things that badly.
Jumpstart doesn't necessarily have to be high-efficiency, it just offers the opportunity if one wants it. Since the more the efficiency is improved, the better the waveform becomes at the same time, it's not obvious to me why one wouldn't do it, unless one were trying to accomplish something that was proving to be unscalable from the standpoint of voltage or impedance.
The comparison with VLF was one of hypothetical transmitter power consumption, just as an order-of-magnitude reality check. Since a VLF transmitter which would be roughly equivalent to the described PI transmitter has power consumption far higher than anyone actually makes VLF's, the implication is that the described PI system has the potential to knock the socks off existing VLF's.
Another implication is that the PI could be rescaled to have both power consumption and sensitivity comparable to existing VLF's. Whether one would want to do that is a separate question.
--Dave J.

 

[Anonymous]

Hi Dave
I just don't want people to get the wrong idea about this jumpstart circuit thing.
It is a great idea to somehow short the coil and keep the current flowing without using any more power (or very little). I'm just not sure how to pull it off exactly, and don't want people to think the little jumpstart circuit is going to do this for them by itself.
You have some really good thoughts, keep it up.
JC

 

[Anonymous]

The jumpstart example I gave above, shorts out the coil to the +5 rail, the other end of the coil being grounded. Given that the otherwise similar non-jumpstart was running off a 27 volts supply, by comparison, +5 is almost but not quite a short to ground. The reason it isn't shorted to ground, is so we can maintain the current at 2 amperes as well as make up for losses during flyback and jumpstart.
If energy is supplied to the system by powering the +100 volt supply (presumably through an upswitcher), then we can literally short the coil out with a transistor during the "transmit pulse" (that word is really inappropriate, but we're apparently stuck with it). The current will drop slightly during this time, the decay time-constant being L/R = 250 microseconds. If we're going to have 2 amperes at flyback, we'll have to add a little more jumpstart time to stuff the extra horsepower into the inductor.
The result of doing this is, in effect, a single-ended CCPI system where the current alternates between +2 and zero rather than between +1 and -1. This has the advantage of allowing the transmitter coil to be used as a receiver.
A few days ago, I think it was you who made a post describing your jumpstart system as a system wherein the transmit/flyback circuit is, in effect, a "free" DC-DC upswitching power supply for powering the jumpstart. Continuing with that insight, we could conceive of the basic high-efficiency jumpstart system as being with the coil shorted out during the "transmit pulse", with the variants being answers to the question, "OK, now where do we inject power to overcome the losses?"
Two obvious possibilities are 1. furnish power to the +100 volt supply; and 2. during the "transmit pulse" short the coil out to a low voltage source rather than to ground.
--Dave J.

 

[Anonymous]

Hi Dave,
In your example of 500 uh 2 ohm coil if we can gracefully short it somehow with a 1 ohm device, then the total R is 3 ohms. Whatever the initial current is, it will drop by over 50% in 125 us and over 75% in 250 us. So the really low power idea of shorting the coil is ok but won't last forever. The good news is it is decaying in the right direction to match final turn off.
If you have the coil hooked to +5 volts and ground with the coil resistance of 2 ohms limiting the current then you are drawing 2.5 amps (of course) toward the last time constants of the pulse which is a still fair amount of power from a man portable battery. (Still need gel cel)
Yes, I did mention using the transmit coil as the inductor for a DC-DC boost/flyback power source for higher voltage, to get current moving faster, and absorbe some of the energy from the flyback (which it does).
JC

 

[Anonymous]

I can confirm that both approaches can be used to conserve power when driving inductive loads. We used them in the 70's for driving print head solenoids in dot matrix printers, and I imagine they were used long before that.
We had one design that used a 65 volt supply and a 12 volt supply. The 65 volts was used to ramp up the current quickly, then the 12 volts was used to keep it level, and then at turn off the current was dumped back into the 65 supply. We had to use an actual 65 volt supply because it had to be available the first time we fired the solenoids, we could not wait for it to get pumped up, but it did not have to provide much power because we recaptured energy at the end of the pulse.
We also had a chopper design that applied high voltage to ramp up the current then shorted out the coil to let the current circulate. The on time was long enough that we had to turn on the high voltage a couple more times whenever the current got too low.
Robert

 

[Anonymous]

JC:
Firstly thanks for you all for the informative discussion, - takes me most of my day to digest! With regards to Sealed Lead Acid Gell Cells, we are now importing direct from China 1.2Volt NiMh high powered rechargeable batterys rated at 9000mAh or 9Ah, - at a cost of US$ 3.60 each! These are only 175 grams each and will go a long way to alleviating the weight problem. Keep up the good work boys. John Kah Coiltek

 

[Anonymous]

Sounds good, do you know if they can be recharged rapidly, i.e. plugging into a truck cigarette lighter plug with the engine running about 14 voltages (10 batteries in series), without hurting them? This is what I do with the gel cels.
Not up to speed on the new batteries, I know some cell phones claim fast charge on this type, but is this just to sell em, or is it true?
JC

 

[Anonymous]

JC:
I havent had a look at that side of it, - if you look at the spec sheets they all recommend that you charge at .2C which is .2 x 9Ah or 1.8 aH constant then trickle back. When we get supply we will be making our own battery charger at about that rate, - unfortunatly there is not much available as stock well made chargers. NiMh batteries used in remote control car racing 'SUB -Cs' are designated with a different code 'RH' for rapid charging and discharging, - I think it would be best to stick to about .2C for the standard cells, - they are then guaranteed for >500 cycles. John