Ed Nisley's Blog: Shop notes, electronics, firmware, machinery, 3D printing, laser cuttery, and curiosities. Contents: 100% human thinking, 0% AI slop.
I needed a rail-to-rail op amp in a DIP package to breadboard the amplifier for those toroids and came up dry, so I sawed out a protoboard square, glued a MAX4330 in SOT23-5 atop it in classic dead-bug style, rearranged the leads into the standard DIP pinout, and moved on:
MAX4330 dead-bug style on DIP adapter
The trick to getting the header pins aligned is to stick ’em in a pile of perfboards, which instantly makes them stand up straight and parallel:
Then wound them with grossly excessive amounts of wire (the up-armored core on the right appeared earlier):
Slit Ferrite Toroid current sensors
The smaller toroid is an FT37-43 that barely covers the active area of an SS49-style Hall effect sensor, but experience with the FT50 toroid suggests that’ll be entirely enough:
slit FT37 toroid trial fit to SS48-style Hall effect sensor
Data on the uncut toroids:
Property
FT50-61
FT37-43
Outer diameter (OD) – inch
0.50
0.375
Inner diameter (ID) – inch
0.281
0.187
Length – inch
0.188
0.125
Cross section area – cm2
0.133
0.133
Mean path length (MPL) – cm
3.02
2.15
Volume – cm3
0.401
0.163
Relative Permeability (μr )
125
850
Saturation flux G @ 10 Oe
2350
2750
Inductance factor (AL) – nH/turn2
68.0
420
Those overstuffed windings improved the sensitivity, but increased the winding resistance far beyond what’s reasonable.
Data on the slit toroids:
Toroid ID
FT50-61
FT37-43
FT50-61
Measured air gap – cm
0.15
0.15
0.17
Winding data
Turns
120
80
25
Wire gauge – AWG
28
32
26
Winding resistance – mΩ
530
920
100
Predicted B field – G/A
872
660
163
Hall effect sensor @ 1.9 mV/G
Predicted output – mV/mA
1.7
1.3
0.31
Actual output – mV/mA
1.9
1.9
0.37
Actual/predicted ratio – %
+12
+46
+19
The last few lines in that table show the transimpedance (transresistance, really, but …) based on the winding current to Hall sensor output voltage ratio (in either mV/mA or V/A, both dimensionally equivalent to ohms), which is why the toroid’s internal magnetic flux doesn’t matter as long as it’s well below saturation.
Gnawing the 80 turn winding off the FT37-43 toroid and rewinding it with 15 turns of 24 AWG wire dropped the winding resistance to 23 mΩ and the transimpedance to 0.36 mV/mA:
FT37-43 with 15 turns 24 AWG – Hall sensor
However, applying a voltage gain of about 28 (after removing the sensor’s VCC/2 bias) will produce a 0-to-5 V output from 500 mA input, which seems reasonable.
The first Wouxun (evidently pronounced “ocean”) KG-UV3D HT spent a month or two in my bike, lashed to a kludged version of the APRS+voice interface box and powered by its own lithium-ion pack. After I got the circuit worked out and built a duplicate, I picked up a second HT for Mary’s bike; as a result, that battery pack never got much use.
A pair of discharge tests shows the difference:
Wouxun 7.4 V Packs
The 2011-03 battery has almost exactly the rated 1.7 A·h capacity, at least if you’re willing to run it down to 6 V, and the 2012-06 pack delivers 1.9 A·h. Electronic gadgets measure state-of-charge using the battery voltage, so the older pack “looks” like it has much less capacity: it runs about 100 mV lower than the newer pack out to 1.2 A·h, then falls off the cliff. Looks to me like one of the two cells inside is fading faster than the other; so it goes.
I’m still thinking of using these to power some LED taillights, because they have a nice form factor and built-in latches:
If you happen to own an MFJ-260B dummy load and it’s giving you weird SWR values, take the cover off and roll the power resistor in its mounting clips:
MFJ-260B HF Dummy Load – power resistor
My buddy Aitch discovered that oxide / corrosion / dirt buildup between the resistor and the clips can produce absolutely baffling results, even while passing enough current to warm up the element, far more power than you’d think would burn away any crud.
A box of surplus Vexta NEMA 23 stepper motors arrived:
Vexta C6925-9212K stepper motors
The data plate sayeth:
Model C6925-9212K
2 phase
1.8°/step
2.3 V
3 A
According to Dan, who happened into the deal, that Vexta model number applies to their custom motors, which accounts for the fact that there’s no further data available anywhere.
Dividing 2.3 V by 3 A = 0.77 Ω windings. Multiplying 2.3 V by 2 A suggests a 7 W maximum dissipation.
Poking around with a meter identifies the windings:
Blue – White – Red
Green – Yellow – Black
Given those colors, the Y G B W R K color sequence on the connector doesn’t make any sense to me. Most likely, there’s a standard I’m unaware of.
The resistance from the center taps outward measures 1.0 Ω, which is close enough to 0.8 Ω for me. Measuring across the whole winding gives 1.8 Ω.
The inductance is 1.0 mH from the center tap and 4.0 mH across the whole winding. Remember that inductance varies as the square of the number of turns.
The time constant for a complete winding = 2.5 ms = 4 mH / 1.6 Ω.
Adding a strip of white LEDs under the X stage helps shed some light on events atop the M2’s build platform; this was very nearly the first improvement after getting the printer, but somehow I’ve never written down where that nice white glow comes from.
This view shows the strip from below, looking up from the -Y direction in front of the stage:
White LED strip under X axis frame
I originally screwed the wires into the terminals from the hulking 12 V Dell laptop brick for the platform heater, but then I had to unscrew the wires whenever I moved the M2 and I didn’t like sharing the connectors with those huge conductors. Now the LEDs are in parallel with the extruder fan (which runs continuously), sharing the FAN1 screw terminals inside the electronics case.
The M2 firmware uses PWM to cut the 19.5 V supply from a much smaller laptop brick down to roughly 12 V RMS for the fans, but that isn’t such a Good Thing for LEDs. The strip has 120 Ω resistors that drop about 2.4 V at 20 mA from a 12 V supply, leaving 9.6 V for the LEDs (at about 3.2 V each). Running from 19.5 V means the resistors will see about 9.5 V and pass nearly 80 mA, four times the nominal rating, during each PWM pulse.
Based on those measurements, the light output doesn’t go up by nearly a factor of four during each pulse.
I plan to add a 12 V supply to the LinuxCNC box, probably by recycling the 12 V brick from the M2, which will get the LED current back down to a reasonable level. With any luck, they’ll survive this mistreatment and not carry a grudge.
You could, of course, just power the LEDs from a separate 12 V wall wart, but that adds Yet Another Thing when I carry the M2 to demos.
A simple test of additional insulation below the Makergear M2’s heated build platform, measuring the time required to heat the platform from 30 °C to 80 °C:
As-shipped without insulation: 8:20
Cardboard + cotton cloth: 8:30
Cardboard + aluminum foil + cotton: 8:00
That’s with a resolution of about 10 seconds and 1 °C. Ambient temperature was 25 °C; I preheated the platform to 30 °C for a repeatable starting point. The heater was full-on for the entire time and I tried to record the time until it first turned off at the setpoint temperature.
So my initial insulation didn’t make any difference; ten seconds (in the wrong direction!) seems down in the noise.
Adding aluminum improved the situation, but not by much.
The platform wasn’t moving, so there’s no air circulation on either surface. I think it will be possible to record / plot the platform heater duty cycle during printing using LinuxCNC’s HAL components, so some useful data should emerge from that.
I think the bottom line is that there’s so much heat transfer up through the glass plate and away that reducing the heat flow from the bottom by a little bit doesn’t matter…
The Smell of Molten Projects in the Morning 