Ed Nisley's Blog: Shop notes, electronics, firmware, machinery, 3D printing, laser cuttery, and curiosities. Contents: 100% human thinking, 0% AI slop.
The Victoreen 710-104 ionization chamber specs say it produces 1 pA in a 100 mR/h gamma environment, which suggests the actual current will be much, much lower in the Basement Laboratory. In fact, I’m hoping to spot individual gamma rays, rather than the overall radiation background current, which will involve counting groups of electrons as they march by.
The simplest possible electrometer amplifier, an MPSA14 NPN Darlington, produced 25 nA of current that, assuming a gain of 10 k, corresponds to an unrealistically high 2.5 pA of chamber current and is, realistically, entirely leakage current.
Adding an MPSA65 PNP Darlington boosts the overall gain to maybe (10 k)2 = 100 x 106:
Current Amp – Dual Darlington – Schematic
Granted, there’s not much to like about that circuit (“Any sufficiently sensitive instrument is indistinguishable from a thermometer”) and stuffing that much gain into a pair of inverters is basically crazy talk, but it looks like this:
Electrometer amp – circuitry
The blue trimpot in the foreground drives the base of a duplicate pair of transistors in a misguided attempt to make a differential amp that would balance out some thermal effects. Turned out to be not worth the effort, due to the adjustment’s fiddly nature, but also not worth unsoldering the parts.
The black lump covers two RG-174 coaxial cables that run off to the oscilloscope; they already had BNC connectors on the end and were small enough for the job.
Some DC measurements:
The output idles at 6.5 VDC → 550 μA of Q101 collector current → 6 pA of Q102 base current. Yeah, right.
Grounding the base of Q102 → 5.9 V output → 500 µA → 50 nA leakage into Q102’s collector. Maybe.
Shorting Q101’s base to its emitter produces 350 mV at the output → 30 µA of output current. Huh.
After restoring the status quo ante, the output idled at 10.2 V. See previous comment about thermometers, modulo soldering transistor leads.
So, given the predictably absurd temperature sensitivity of this whole lashup, it’s reasonable to say the entire DC output current comes from leakage, which also agrees with the fact that I’m not dying of gamma exposure. In point of fact, an ancient CDV-715 Radiological Survey Meter with a similar ionization chamber (which, at this late date, passes its “circuit test” function and seems to be perfectly happy) reports exactly zero background on its most sensitive 500 mR/h scale.
Lining the shield support box with copper foil tape turned out to be surprisingly easy:
Electrometer amp – shield – end view
The flat surface is two overlapping strips of 2 inch wide copper tape. I traced the exterior of the support box on the tape, cut neatly along the lines, slit the corners, bent the edges upward, peeled off the backing paper, stuck the tape into the box, pressed the edges into the corners, and didn’t cut myself once.
Applying 1 inch wide tape to the wall went just as smoothly, after I realized that I should cut it into strips just slightly longer than the hexagon’s sides.
The tape along the rim is adhesive copper mesh that’s springy enough to make contact all around the edge. I cut the 1 inch wide tape in half, which was just barely wide enough to reach::
Electrometer amp – shield – mesh soldering
Although you’re supposed to join the entire length of each seam for best RF-proofing, I tacked the corners and the middle of the long edge, then hoped for the best. The copper mesh seems to be plated on plastic threads that requires a fast hand to solder without melting, but I’m getting better at it. The adhesive is said to be conductive, but I loves me some good solder blob action.
The resistance from the flat bottom to the side panels and the fabric on the edge started out at a few ohms before soldering and dropped to 0.0 Ω after soldering, so I’ll call it a success. Didn’t even melt the outside of the PETG box, but I admit I didn’t take it apart to see what the copper-to-PETG surface looks like.
Covering the foil on the sides with 1 inch Kapton tape completed the decoration. I didn’t bother to cover the flat surface, because none of the circuitry should reach that far, and didn’t worry about covering the fabric tape for similar reasons. As madbodger pointed out, this violates the no-plastic-on-the-inside rule, but I’m still hoping for better results than having the entire plastic structure with all its charges on the inside.
A strip of horribly clashing orange plastic tape (which might be splicing tape for reel-to-reel recording tape) covers the outside edges of the fabric, prevents fraying, and gives the black electrical tape that holds the box down a solid grip:
Electrometer amp – shield – exterior
Yeah, like you’d notice mismatched colors around here.
Using black tape as an anchor seemed easier and better than messing with nesting pins & sockets. The copper fabric tape makes good contact with the rim of the PCB all the way around the perimeter and the black tape holds it firmly in place.
Early reports suggest the shield works pretty well…
Although I’d thought of a Mu-metal shield, copper foil tape should be easier and safer to shape into a simple shield. The general idea is to line the interior with copper tape, solder the joints together, cover with Kapton tape to reduce the likelihood of shorts, then stick it in place with some connector pin-and-socket combinations. Putting the tape on the outside would be much easier, but that would surround the circuitry with a layer of plastic that probably carries enough charge to throw things off.
Anyhow, the hexagonal circuit board model now sports a hexagonal cap to support the shield:
Victoreen 710-104 Ionization Chamber Fittings – Show with shield
The ad-hoc openings fit various switches, wires, & twiddlepots:
If white LED strips had existed in the early 1980s, the engineers responsible for the HP 7475A plotter would surely have done this:
HP 7475A Plotter – LED paper illumination
Not, that’s not stretched vertically: I bought a ream of B-size paper (11×17 inches) just for plotter demos.
Although the power supply does have a +12 V output, it comes from a TO220 transistor without a heatsink. The +5 V supply uses a robust TO3 transistor on a huge quad heatsink that can surely dissipate another watt or two without getting any sweatier.
I powered the LEDs from a dirt-cheap boost converter that provides a convenient brightness adjustment; it’s set to 10.5 V and that’s plenty bright enough. The converter attaches to pair of wires soldered across VR1, which is probably a crowbar that blows F3 (not shown) in the event the regulator fails hot:
HP 7475A – LED power tap – schematic
They don’t make power supplies like that any more.
The part locations (“O9” looks like a typo):
HP 7475A – LED power tap
The PCB has holes in exactly the right spot for a zip tie anchoring the wires exiting to the bottom:
HP 7475A Plotter – LED power tap – PCB top
This vertiginous view shows the inside of the case atop the chassis, with the boost converter affixed to the galvanized steel pan with foam tape and the LED wires stuck down with Gorilla Tape:
HP 7475A Plotter – LED strip and boost converter
Red silicone tape around a PCB-mount coax jack rounds out a true hack job.
Although I didn’t bring the plotter to the CNC Workshop, that venue’s dim light reminded me that you can never have enough light when you’re showing off your toys: the LED panels on the M2 and the LED light bars on the Model 158 sewing machine were the brightest spots to be seen.
I soldered up the simplest possible “electrometer amplifier” at Squidwrench, based on Charles Wenzel’s writeup:
Electrometer Amp – MPSA14 NPN Darlington
It’s an MPSA14 NPN Darlington transistor, with the base soldered directly to the Victoreen 710-104 ionization chamber collector pin. The flying leads connect to an ordinary digital voltmeter set to read voltage, rather than current, so that you see the voltage created by the transistor’s collector current through the meter’s input resistance.
The MPSA14 data sheet specifies DC current gain hFE > 10 k for low collector currents, with a graph suggesting it might be somewhat larger. Alas, all those are for “ordinary” currents, not the countably finite number of electrons coming from an ionization chamber, but let’s assume 10 k is close.
I used a Radio Shack 22-805 DMM, set to auto-ranging DC volts. The specs say the input “impedance” is 10 MΩ for all voltage ranges, so let’s run with that, too.
With 24 V (actually 24.6 V) applied to both the chamber (through the red wire) and the DMM (through the yellow wire), it read 250 mV: a mere 25 nA through the 10 MΩ meter resistance.
Assuming a transistor gain of 10 k, that’s a chamber current of 2.5 pA.
The ionization chamber specs say it produces 5 pA at 0.5 röentgen/hour → 100 mR/h produces 1 pA.
No, I do not believe the Squidwrench Operating Table is bathed in gamma radiation at 250 mR/h.
I should wipe down the transistor to see if that reduces the external leakage, then try a few others, but obviously the signal will remain lost in the noise.
We replaced the DMM with an oscilloscope and 10 MΩ probe, which conclusively demonstrated that unshielded high-impedance circuits make excellent 60 Hz receivers.
Just to have something to work with, I cut a hex from a sheet of double-sided PCB stock and bonded the edges with copper foil:
Victoreen 710-104 – Hex PCB – top
Slightly wider tape on three edges will clear the board supports:
Victoreen 710-104 – Hex PCB – bottom
For unknown reasons, the PCB has arrays of plated-through holes firmly bonding the top and bottom copper, so that’s pretty much solid copper with a glass-epoxy core. I think somebody (else) harvested it from a locally important company many, many decades ago, but it arrived with no provenance.
The first pass at the electrometer circuitry will be air-wired for low leakage, which is pretty much the only way I have to actually get low leakage; the holes should help glue the parts to that copper plane.
I’m not at all convinced the big hole in the middle is strictly necessary. The chamber has 10 pF from pin to can that should swamp any stray capacitance unless I do something really stupid.
Given my weak origami-fu and the need for hexagonality, I should print a 3D template.
It’s worth remembering that both the hex and the shield will be at the can’s +24 V potential, not “ground”. That makes no difference to the external circuitry, but will certainly cause me to blow a few junctions along the way.
The Smell of Molten Projects in the Morning 