jump-started transmitter

[Anonymous]

Reference: JC's posts "Another Idea" 2 Jan 02
JC describes a system for using energy recovered from the flyback pulse to "jump-start" the transmit pulse, allowing improved efficiency and increasing sensitivity to high-conductivity and iron objects.
Here is a different topology for accomplishing the same thing. Sorry, I don't have the ability to publish a drawn schematic, so this will have to be done awkwardly with words. Voltages, component types, etc. are given for the purpose of illustration, and it will be evident to those skilled in the art that the invention comprises a particular operating principle and is not constrained to the details given here for convenience in explanation.
This implementation uses three diodes.
BASIC TRANSMIT DRIVE ARCHITECTURE
The power supply is +12 volts. This voltage is switched with a PNP transistor to the coil, the other end of which is grounded. Therefore, the transmit voltage is positive and the flyback is negative.
For reasons which will become apparent later, a first diode is wired in series with the collector of the transistor, the cathode of the diode being connected to the coil.
BASIC FLYBACK ARCHITECTURE
The hot side of the coil goes to the cathode of a catch diode (the second diode), which stores the energy on the negative end of a capacitor. The positive end of the capacitor is wired in series with a third diode the anode of which goes to the +12 supply. The purpose for the third diode will be explained later.
For the sake of illustration, we'll say that the voltage on the negative end of the capacitor is -100 volts. The actual voltage will depend on a variety of other variables, particularly the electrical Q of the coil, and the ratio of transmit jump-start time to total transmit on-time.
BASIC CHARGE PUMP ARCHITECTURE
A PNP transistor is wired with the emitter to +12 volts, and the collector to the negative end of the capacitor.
When it is time to begin the transmit pulse, this transistor is turned on. The negative end of the capacitor goes to +12 volts and the positive end goes to +112 volts. The third diode is now reverse biased, which leaves the positive end of the capacitor floating.
THE JUMP-START CIRCUIT
A PNP transistor is connected with its emitter going to the positive end of the capacitor and its collector to the hot side of the coil. If base current is furnished, the transistor will be switched on, pulling the hot end of the coil up to +112 volts. The first diode prevents the main transmit transistor from being subjected to reverse voltage.
One way to turn the jump-start transistor on is to wire a biasing resistor from the +12 supply to its base. When the emitter goes up to +112 volts, the bias circuit will turn on. If this proves to be too slow, a capacitor can be wired in parallel with the resistor.
TERMINATION OF THE JUMP-START PULSE
To terminate the jump-start pulse and begin the main transmit-on period, first the main transmit transistor is turned on (timing not critical), and then the charge pump transistor is turned off.
When the charge pump transistor which had been shorting out the negative end of the capacitor to the +12 rail is turned off, the negative end of the capacitor is left floating. Current can no longer flow through the capacitor, turning off the jump-start transistor. Without the jump-start transistor being turned on, the voltage on the coil collapses until the first diode is forward biased, clamping the coil voltage at approximately +11 volts.
The main transmit drive transistor is kept turned on until it is turned off in order to initiate flyback.
FLYBACK VOLTAGE
In a given design, the flyback voltage can be regulated by varying the on-time of the charge pump transistor. Long on-time will give low voltage; short on-time will give high voltage.
Alternatively it could be regulated with an avalanche diode wired across the capacitor; however, this sacrifices some of the energy efficiency benefits of the design.
COMPARISON WITH JC'S METHOD
The circuit complexity is about the same either way, when you include the base/gate drive need to time the jump-start transistor in JC's design. Efficiency is also about the same.
The choice between one or the other probably comes down to the following factors:
1. Compatibility with other aspects of the design, for instance receiver circuitry, or constant-current regulation circuitry.
2. Whether or not the designer likes charge pumps.
3. Whether or not the designer dislikes high-side transistor base/gate drive circuits.
4. As fully implemented, comparing two otherwise equivalent designs, one may be more economical than the other.
5. As fully implemented, comparing two otherwise generally similar designs, one may be faster than the other.
6. It may be that one design is more compatible with MOSFETS than the other, and that the designer prefers MOSFETS. (or vice versa, and bipolars.)
--Dave J.

 

[Anonymous]

Hi Dave
I like the way you get the voltage on the cap turned around and fed back in.
The circuit I envisioned went more like this. Standard PI drive with mosfet grounded and coil to +12 volts. Now the fly back pulse at the junction of the fet and coil is positive, this works better for my simple mind.
Between the +12 supply and the coil put in a first diode, anode to +12 and cathode to coil. Also put in a second diode with the anode at the junction of the fet and coil and the cathode to a capacitor. Put a PNP transistor with the emitter at the cap/diode junction and the collector at the first diode/coil junction.
This PNP transistor needs to be turned on at transmitter turn on (not too critical on timing) and should be turned off sometime during the transmitter on time after the capacitor has dumped some energy into the coil and the +12 (minus a diode drop now) has taken over, but before flyback (timing not too critical). The transistor remains off during entire receive period, turned back on in rough conjunction with the fet.
The transistor cannot be a mosfet in this circuit because of the parasitic diode in fets will short around the coil, like a flyback suppression diode. Also the reason the transistor can't be another diode, which would be really simple.
This scheme may save some power, but there will still be a healthly amount of power wasted in a strong PI. The main thing was to get a faster rise time on the current in the coil and get the current steady state quicker, by having a higher voltage availible for awhile. Of course still have to wait for the target time constant to die down.
So the world still isn't perfect.
JC

 

[Anonymous]

The flyback storage capacitor in the above circuit is grounded on the negative side. I would recommend some kind of film capacitor (polypropalene or even cheaper polyester) rather than electrolytic. Dissapation in a electrolytic is probably going to cause it to spew its guts all over your workbench at these frequencies and power.
PNP transistor should probably be turned off with a resistor between the base and emitter, because the emitter on this transistor is going to see fairly high rates of change of voltage which is going to try to turn it on, as the capacitor is charged during flyback.
The pnp transistor can be turned on with a resistor and npn transistor to ground.
No companies better try to swipe my design, cause I gots witnesses now. <IMG SRC="/forums/images/smile.gif" BORDER=0 ALT=""> Besides it would cost 3 dollars in parts, raising the final cost of the detector 600 dollars due to advertising costs increases buying long ad space to show nuggets detected at ten times the coil diameter.
Everybody else enjoy!
JC

 

[Anonymous]

JC:
We're all familiar with "fold-outs" in magazines.
You've just invented the "fold-down"! You open the mag to that page, and the advertisement de-accordions and drops to the floor quicker than you can unroll toilet paper.
In your description of the ad, you forgot to mention 17 lines going from the loop down to the target with little arrows. It'll make people with amblyopia dizzy and could even trigger an attack of migraine. I hear the Association of American Opthamologists has petitioned the Federal Trade Commission to ban advertisements showing 28 parallel vertical lines or more, on the grounds it could cause permanent eye damage.
--DJ

 

[Anonymous]

If you've seen certain metal detector ads you know what I'm talking about, already at 5 times coil diameter.
What about my circuit, think it will work?
JC

 

[Anonymous]

The circuit you just detailed, is what I deduced from your original post 'way back when, and yes I believe it will work. From an analytical point of view, yours is a lot more straightforward than mine. If either has a catastrophic surprise lurking, more likely mine than yours.
Speaking of advertising, a careful study of men's magazines reveals that nobody wants another foot, but everybody wants another inch. Now, getting another inch is a difficult and painful process, but it happens more often in our industry than in theirs. And, in our industry, you can actually buy a thing called a "Bigfoot"! We are sooo, soooo far ahead of the rest of the world, ain't we?
--Dave J.