Ed Nisley's Blog: Shop notes, electronics, firmware, machinery, 3D printing, laser cuttery, and curiosities. Contents: 100% human thinking, 0% AI slop.
My V-750 dosimeter charger came with two (!) copies of the manual and the modification instructions (stamped JUN–1965) for adding the anti-kick capacitor.
The paperwork didn’t fare quite as well as the metal-cased charger, sporting far more mildew on the pages than I want on my shelves.
I cut the worst-looking copy right down the middle, scanned it with some attention to detail, and now there’s a nice version that looks just as bad but lacks the mildew.
If you’re really clever, you can figure out how to sequence the sheets and print them duplexed so they appear back-to-back, then bind them into a booklet just like the original. There’s a copy of a blank inside cover, too, so you can wrap your booklet in a nice Civil Defense Yellow cover.
The schematic shows what real engineers could do, back in the days when transistors came individually packaged with a ten-dollar price tag: 1.5 volts in, 200+ volts out, one transistor. Of course, they paid attention to their transformer lessons.
The charger had some corrosion on the cast aluminum (?) knobs, but seemed largely unscathed by four decades in its original box. The charging circuitry depends on a few electrical contacts and, as you might expect, those were badly intermittent.
A bit of background…
Charging contact pedestal
The charging pedestal has two parts visible from the outside: an outer sleeve that’s firmly secured to the case and an inner cylinder that slides within the sleeve, with springs inside the charger pressing it outward. Well, there’s a nut, toothed washer, and the bead-chain cap assembly, but those don’t count.
The inner cylinder has a transparent plastic insert crimped in place, with a metal rod protruding about 2 mm from the flat top of the plastic. That rod presses against the middle contact of the dosimeter and connects the charging voltage to the electrostatic fiber. The outer body of the dosimeter fits snugly over the cylinder to make the other electrical contact.
The directions tell you to press the dosimeter down gently to read it. A weak spring holds the cylinder outward with about 1.5 lb of force. After about 1 mm of travel an incandescent bulb (remember those?) turns on, transmits light through the plastic insert, and lights up the dosimeter scale and fiber.
To charge the dosimeter, you press down firmly and twiddle the adjusting knob to position the fiber. Pressing hard enough to force the dosimeter body down to the sleeve, another 3 mm of travel, compresses the dosimeter’s internal bellows (or plastic seal) enough to complete the circuit to the fiber; a sealed dry air gap normally isolates the fiber from the dosimeter’s external contact. A stout leaf spring holds the cylinder outward with (according to one instruction manual) 7.75 lb of force, so it takes more pressure than you’d expect to hold the dosimeter down.
Charging contact inside view
The internal parts of the charging pedestal makes all that stuff work without any formal switch contacts. That, unfortunately, causes the intermittent operation.
The gray “wire” inside the large 7-lb leaf spring is both the 1-lb spring and the high-voltage electrical contact. The purple wire soldered to the end of the wire spring carries the HV charging potential from the circuitry.
The black and red wires connect to the incandescent bulb, which fits into the holder near the top of the circuit board sticking up vertically just to the right of the pedestal base; I removed it to reveal the other parts. For what it’s worth, the bulb holder doesn’t do a good job of securing the bulb; I have some improvements in mind for that, too.
Note the spare bulb just beyond the center bulb contact near the top of the picture. The rubber grommet securing that has turned into black Gummi-bear substance; that sucker is in there forever.
The battery’s positive terminal connects to the case; this is a positive-ground circuit!
The leaf spring hitches over two shoulders on the circuit board and presses it firmly against the other side of the spring. The curved fork fingers pressing against the brown insulating washer are firmly mounted to the circuit board and act as one side of the switch contacts.
Pedestal removed from charger
When you push the dosimeter against the sleeve, the base of the cylinder slides through the ID of the fiber washer and contacts the fork fingers. Bingo, that completes the circuit, lights the lamp, and fires up the HV circuitry. The charging voltage doesn’t reach the dosimeter fiber because the leaf spring hasn’t started pressing the cylinder against the dosimeter’s innards: there’s no connection inside the dosimeter.
With that out of the way, here’s what’s needed to get the pedestal working reliably.
Get the whole pedestal assembly out of the charger, which requires a bit of wiggly jiggly action. This will be easier if you unsolder the three wires, which I didn’t do until I was sure it was absolutely necessary.
Grab the leaf spring on both sides of the bulb circuit board, pull up while pushing down on the spring’s base with some other fingers, and lift the tabs off the circuit board shoulders. This requires a surprising amount of force; don’t let the spring get you by the soft parts!
Leaf spring released
A small crimped metal connector mates the end of the wire spring to the center contact in the cylinder. Pay attention as you maneuver the pedestal out of the leaf spring: you don’t want to deform that connector too much. Or, much worse, lose it under your workbench.
There’s a rubber O-ring inside the outer sleeve that’s barely visible in the picture of the parts. The 1-lb wire spring had trouble forcing the cylinder back out through the O-ring, leaving the switch just barely closed even with the dosimeter removed. A touch of silicone gasket lube on the O-ring made it wonderfully slippery again.
The inner cylinder has wire snap ring in a groove that adds a bit of stability and maybe some contact friction inside the sleeve. You need not remove the snap ring; they’re not called Jesus clips for nothing. It’s outside the O-ring’s protection, exposed to the world.
Basically, clean everything without yielding to the Siren Call of sandpaper. What you want to do is get the oxidized metal off the base material without scarring it.
Pedestal contact components
I applied a tiny drop of Caig DeoxIT Red to the snap ring, worked it around & around, then wiped off the residue.
The actual switch “contacts” are the wide base of the inner cylinder (to the right in the picture) and the rounded end of the fork attached to the lamp base circuit board. The contact area is broad, smooth, plated-steel-on-steel, and utterly unsuited to the job. Wipe both of them clean, add DeoxIT, wipe them clean again.
I applied another minute drop of DeoxIT to the base of the cylinder after putting everything back together, rotated it against the fork, and wiped it off. Most likely that had only psychological benefit, but what the heck.
The parts go back together in the obvious way, again taking care not to let the leaf spring bite you. I routed the wires a bit differently, but I doubt it makes any difference.
Now the charger works perfectly again!
Memo to Self: replace that bulb with nice soldered-in-place LED
V-742 Dosimeter set to Zero
Update: It seems you can actually buy V-750 dosimeter chargers new from www.securityprousa.com/doch.html. However, eBay is significantly less expensive and you might get some quality shop time out of it. Your choice.
The default PWM frequency for PWM 3, 9, 10, & 11, at least for the Diecimila running at 16 MHz, is 488 Hz. That’s OK for dimming LEDs where you’re depending on persistence of vision, but it’s much too low when you must filter it down to DC.
The relevant file is hardware/cores/arduino/wiring.c, which is buried wherever your installation put it.
Turns out that the Arduino runtime setup configures the timer clock prescalers to 64, so the timers tick at 16 MHz / 64 = 250 kHz.
You can fix that by setting the Clock Select bits in the appropriate Timer Control Register B to 0x01, which gets you no prescaling and a 62.5 ns tick period:
If you’re finicky, you’ll bit-bash the values rather than do broadside loads. However, it probably doesn’t matter, because Timer 0 runs in Fast PWM mode and Timers 1 & 2 run in Phase-Correct PWM mode, so WGMx2 = 0 in all cases.
Fast PWM mode means Timer 0 produces PWM at 250 kHz / 256 = 976 Hz. However, the Arduino runtime runs the millis() function from the Timer 0 interrupt, so changing the Timer 0 prescaler poochesmillis(), delay(), and any routines that depend on them.
Phase-correct PWM mode means that Timers 1 & 2 count up to 0xff and down to 0x00 in each PWM cycle, so they run at 250 kHz / 512 = 488 Hz.
Adroit TCCRxB setting can prescale by 1, 8, 64, 256, or 1024. Or stop the Timer stone cold dead, if you’re not careful.
Before you fiddle with this stuff, you really should read the timer doc in the ATmega168 datasheet there.
In addition to building a Spice model for a transformer, it’s also important to know whether the core can support the flux generated by the primary winding. This is similar to the inductor problem I mentioned there.
Small HV transformer with test winding
Measure the core’s area and path length. One can reasonably expect all cores to have hard metric measurements these days: Yankees set those calipers to millimeters and get over it. Besides, you need metric units for everything that follows.
This transformer has two E-shaped core halves, so the center leg (the one with the windings on it) has twice the area of the outside legs, which are 7 mm thick and 5 mm wide. The central leg is twice that width: 10 mm.
Figure the stacking factor for a ferrite core is, oh, say, 0.9, making the effective core are:
Ac = 0.9 · 7 · 10 = 63 mm^2 = 0.63 cm^2
You need cm^2 here to get gauss later on.
The core is square, 30 mm on each side. Divide it in half, right down the middle of the center leg, then figure the mean path length around the middle of that rectangle:
MPL = 2 · (30 – 5) + 2 · (15 – 5) = 70 mm = 7 cm
Again, you need cm here to get oerstead down below.
Put a few turns Nt of fine wire around the core, outside all the other windings. This particular transformer has three small imperfections where the varnish / sealant didn’t quite bridge from the bobbin to the outer core legs, so I managed to sneak 20 turns of wire through the holes. Call this the test winding: Nt = 20.
Incidentally, that’s why you should always buy at least three units from surplus outlets: one to sacrifice, one to use, and one for a spare. I usually get five of anything.
Connect the transformer primary to a signal generator & oscilloscope Channel 1, connect the test winding to Channel 2, set the channels to maybe 100 mV/div. Set the signal generator for sine wave at maybe 1 kHz, crank on a few hundred millivolts, then read RMS voltages from both channels: Chan 1 = Vp, Chan 2 = Vt.
Knowing Vp and Vt and the number of turns Nt in the just-added extra winding, find the number of primary turns Np:
Vp / Vt = Np / Nt
136 / 40 = Np / 20
So Np = 68
Repeat that exercise, stuffing voltage into the transformer’s actual secondary winding (the HV winding):
4000 / 46 = Ns / 20
So Ns = 1739
Comfortingly, the turns ratio works out to what you’d expect from the voltage ratio measured while extracting those pi model parameters:
N = Np / Ns = 68 / 1739 = 0.039 = 1/25.6
(You may want the turns ratio as Ns/Np = 25.6. Either will work if you make the appropriate adjustments in the equations.)
Having measured the primary inductance as about 15 mH, the reactance at 60 Hz is:
5.6 Ω = 2 · π · 60 ·15e-3
So it’s reasonable to use a 100 mΩ current sensing resistor.
Plug a 6 VAC (not DC!) wall wart into the Variac and wire it to the primary through the resistor. Connect the oscilloscope X axis across the resistor, set the gain to maybe 10 mV/division.
Connect a 220 kΩ resistor in series with a 1 μF non-polarized capacitor, connect that to the normal HV secondary winding, connect the Y axis across the capacitor, set it for maybe 50 mV/div.
The capacitor voltage is the integral of the secondary voltage, scaled by 1/RC. The RC combination has a time constant of 220 ms, far longer than the 16.7 ms power-line period, so it’s a decent integrator.
Small HV transformer BH curve
Fire up the scope, set it for XY display, turn on the Variac, slowly crank up the voltage, and see something like this on the scope:
Tweak the offsets so the middle of the curve passes through the center of the graticule, maybe turn on the bandwidth limiting filters, adjust the gains as needed, then measure the point at the upper right at the end of the straightest section in the middle.
That point, as marked by the cursors, is more or less:
X = 6.5 mV
Y = 100 mV
Now plug all those numbers into the equations and turn the crank…
Finally, at that point where the cursors meet in the upper right part of the curve:
H = 122 · 6.5e-3 = 0.8 Oe
B = 14e3 · 0.1 = 1400 G
Assuming there’s a straight line from the origin to that point (which is close to the truth), the B/H ratio gives the slope of the line and, thus, the core’s permeability:
µ = B / H = 1400 / 0.8 = 1700
It’s allegedly a ferrite core, so that’s in the right ballpark given the rough-and-ready approximations in the measurements.
The answer to the key question comes right off the scope without any fancy math, though. Just beyond the upper-right point the BH curve becomes horizontal, which means the slope is zero, which means the core is saturated, which means the circuit stops working.
Sooo, the maximum value of the primary current is pretty nearly:
Imax = 6.5 mV ·10 A/V = 65 mA
My back of the envelope for the high-voltage DC supply is that a peak of 30 mA will pretty much do the trick, so I’m in good shape. Might be a bit higher during startup, but it’ll sort itself out in short order.
Whew!
Correction: I did a total arithmetic faceplant in the previous version. I think this is now correct, but you should always cross-check anything you find on the InterWeb, fer shure!
Every gadget comes with its own battery charger wall wart, every single one of which dissipates a watt or two even when it’s not charging. Add ’em up, multiply by $2 per watt per year (check your electric bill; that’s closer than you think!), and realize that you could afford some nice new tools just by unplugging the things between charges.
But that’s too much trouble and, really, AC outlets aren’t meant for that many mate/unmate cycles. I had one contact fall loose inside a power strip a while ago and the carnage was spectacular.
What to do?
Recharging Corner
Find an otherwise unoccupied flat spot (or build a shelf near an outlet), buy two or three Power Squid adapters (you don’t need surge suppression for this assignment, so get ’em on sale cheap), plug all your chargers into the Squids, and turn everything off with a single switch when you’re not charging anything.
Bonus: You certainly have some low duty cycle power tools that always have dead batteries when you need them. Plug ’em into the Squid you use most often for other batteries. That way, they’ll get a boost whenever you charge something else, which should keep ’em up to speed.
I set this tangle up before Power Squids existed, so I just plugged a bunch of Y-splitters into an ordinary power strip. It makes for a fearsome tangle of cords, but at least it’s out of the way atop the never-sufficiently-to-be-damned radon air exchanger in the basement.
Given a random transformer, create a decent Spice model… I have to do this rarely enough that I’d better write it down so it’s easy to find. There’s no magic here; it’s all described in ON Semi (nee Motorola) App Note AN-1679/D. See page 4 for the grisly details; I’ve reordered things a bit here.
Go to the basement lab and measure:
Primary & secondary voltages with a sine-wave input: Vp & Vs.
Primary inductance with secondary open: Lps(open)
Primary inductance with secondary shorted: Lps(short)
DC resistance of primary & secondary: Rp & Rs
Then return to the Comfy Chair and calculate:
Turns ratio N = Vp/Vs.
Coupling coefficent k = sqrt(1 – Lps(short)/Lps(open))
A quick trip to the basement lab produces these numbers for this small high-voltage transformer:
Small HV Transformer
Primary
Secondary
Voltage
1.08
27.32
DC resistance
2.03
349
L other open
15.5 mH
9.68 H
L other short
45.0 uH
31.3 mH
You don’t actually need the secondary inductances, but while you have the meter out, you may as well write those down, too. Maybe someday you’ll use the transformer backwards?
And a session with the calculator produces a Spice model:
N = 1.076 / 27.32 = 0.0394
k = sqrt(1 – 45.0 uH / 15.5 mH) = 0.998
LI1 = (1 – 0.998) ·15.5 mH = 22.5 uH
LI2 = (1 – 0.998) ·15.5 mH / 0.0394^2 = 20.0 mH
Lm = 0.998 ·15.5 mH = 15.48 mH
Note: the value of (1 – k) is the small difference of two nearly equal numbers, so you wind up with a bunch of significant figures that might not be all that significant. The values of LI1 and LI2 depend strongly on how many figures you carry in the calculations; if you don’t get the same numbers I did, that’s probably why.
The coupled inductors L1 & L2 form an ideal transformer with a primary inductance L1 chosen so that its reactance is large with respect to anything else. I picked L1 = 1 H here, which is probably excessive.
The coupling coefficient would be 1.0 if that were allowed in the Spice model, but it’s not, so use 0.9999. Notice that this is not the k you find from the real transformer: it’s as close to 1.0 as you can get. [Update: either I was mistaken about 1.0 not being allowed or something’s changed in a recent release; 1.0 works fine now.]
Spice transformer pi model
The primary inductance and turns ratio determine the secondary inductance according to:
Vp / Vs = N = sqrt(L1 / L2)
So:
L2 = L1 / (N^2) = 1 / 0.0394^2 = 644 H (!)
The models for LI1 and LI2 include the DC resistance, so that’s not visible in the schematic.
And now you can model a high-voltage DC supply…
Memo to Self: It’s G16821 from Electronic Goldmine
Primary on pins 2 & 10
HV secondary on pin 8 & flying wire
Electrostatic shield on pin 3
Note: You can compute the turns ratio either way, as long as you keep your wits about you. With any luck, I’ve done so… but always verify what you read!
Contrary to what I’d thought, the MAX4372 circuitry has a simple gain error: it’s about 10% low over the full-scale 300 mA current range.
A bench supply produces 5 V through an 8 Ω resistor, although the slope of the purple line is more like 7.3 Ω. Close enough.
The blue line is the current sense voltage, which is exactly the same as the setpoint voltage plus a little PWM noise contributing to the waviness. Unlike the previous solar-powered chart, the bench supply voltage doesn’t drop enough to saturate the current sink, so the result is a nice straight line.
The red line is the MAX4372 output, which is consistently 10% low right up to the end; I can fix that with simple software scaling. The curve doesn’t flatten out, either, because the common-mode voltage across the sense resistor stays well above the it-stops-working-well limit around 2 V.
MAX4372 Schottky Protection Hack
Conspicuous by its absence is any sign of nonlinearity due to the Schottky protection diode across the sense terminal inputs. The full-scale sense voltage is 300 mA x 0.5 Ω = 150 mV, which is sufficiently below the 1N5819 threshold of about 300 mV.
The picture shows the hack-job mod I applied to the circuit board; basically a cut-and-solder job with 10 Ω SMD resistors and a through-hole 1N5819. Yes, I stacked those two chips to get 5 Ω on the -Sense input; it’s a nice way to get good fixed ratios.
Despite what the stripes look like, both of those through-hole resistors are 1.0 Ω: brown-black-gold-gold.
The MAX4372T, the heart of this discussion, is the nearly invisible black rectangle just in front of the diode’s right-hand lead.
Although I should take a look at the high-value resistor / no diode protection circuitry, this one will suffice for now. It’s worth mentioning that I haven’t managed to burn this MAX4372 out, despite perpetrating much the same indignities on it as I did to the others, so the diode protection really is working.
The Smell of Molten Projects in the Morning 