The Smell of Molten Projects in the Morning

Ed Nisley's Blog: Shop notes, electronics, firmware, machinery, 3D printing, laser cuttery, and curiosities. Contents: 100% human thinking, 0% AI slop.

Category: Electronics Workbench

Electrical & Electronic gadgets

Improved M2 Heated Build Platform: First Light

Although the M2’s heated build platform works well enough, somebody who knows what he’s doing (you know who you are: thanks!) sent me an improved version. It’s a PCB heater, laid out to compensate for the usual edge cooling, firmly attached to a tempered glass plate with genuine 3M thermally conductive tape:

Improved M2 HBP – test setup

They designed the heater around the 30 VDC power supply used in their other equipment. Although I had ~~high~~ moderate hopes that a boost power supply would convert the 24 V supply I already had for the stepper driver bricks into the 30 V for the heater, it was not to be. So there’s a 36 V 9.7 A 350 W supply arcing around the planet that (I think) should work better: adjust the voltage down as far as it’ll go, soak up another few volts in the solid-state relay, and Things Should Be Close Enough to 30 V. One can buy a genuine 30 V supply, but it costs surprisingly more than either 24 V or 36 V supplies on the surplus / eBay market and won’t really provide the proper voltage without upward tweaking anyway.

I replaced their standard 0.156 inch square terminals with Anderson Powerpoles, soldered a length of shielded cable to the 100 kΩ thermistor pads, and gimmicked up a connection to the 24 V supply; it delivered 23.7 V at the PCB terminals. The thermistor is 100 kΩ at 25 °C and 11.4 kΩ at 77 °C. The PCB heater is 5.9 Ω at 25 °C and 7.3 Ω at 77 °C; it dissipates 77 W at 77 °C (no, that’s not a typo).

The ultimate temperature looks to be about 90 °C with a 24 V supply, which isn’t quite enough for ABS (which I’m not using in the M2 right now, but probably will eventually). The time constant, assuming the 1-e^-1 point is 66 °C, works out to about 9 minutes; it’ll be up to final temperature in half an hour. Those numbers aren’t quite as accurate as one might wish, because the heater power drops as the temperature rises and the copper resistance increases.

A 30 V supply would dissipate 120 W at 77 °C and rumor has it that the ultimate temperature is around 125 °C, which would be fine for ABS. Goosing the power a bit would produce more heat, but I’v been running the Thing-O-Matic at 110 °C and that’s good enough. More power, of course, gets it to the temperature setpoint faster, which is probably a Very Good Thing.

Obviously, you need PWM to control the temperature; given a 9 minute time constant, a bang-bang controller will work perfectly well.

The original data, including the thermistor resistance after I got my act together, plus a cute little temperature-vs-time graph:

Improved M2 HBP – 24 V supply

The colored flyspecks are part of the paper; I salvaged a stack of fancy menu cards from a trash can and padded them up as geek scratch paper.

2013-09-05
Baofeng UV-5RE: BL-5 Battery Overview

After I mentioned I was thinking of repurposing the nearly unused lithium-ion batteries from the Wouxun KG-UV3D radios for a blinky light, Dragorn of Kismet introduced me to his Baofeng UV-5 radio. The radio itself seems to be the worst amateur radio you’d be willing to use, but when seen as a standardized battery and drop-in charger with a free radio and antenna tossed into the deal, it’s not all that bad:

Baofeng UV-5RE radio – overview

The Wouxun and Baofeng 7.4 V batteries allegedly have similar capacities: 1700 vs 1800 mA·h. The Baofeng also has a 3800 (or 3600) mA·h pack that extends well below the base of the radio (not all large packs seem to be compatible with the UV-5RE radios I got); that would be roughly equivalent to the larger packs that power the Wouxun / APRS / voice gadgetry on the bike.

The Baofeng battery pack is smaller and has features that seem less likely to misbehave on a bike.

It has a latching tab with a ramp and a positive notch, with ridges around the edge that engage the radio shell:

Baofeng UV-5RE radio – battery latch tab

The radio body (which is what I must duplicate) has a movable latch tab above the battery contact pins, so the latch holds the battery into the compartment. The spring-loaded pin pairs are wired in parallel, presumably for redundant contact with each battery terminal:

Baofeng UV-5RE radio – battery compartment latch and contacts

The battery terminal pads are reasonably well protected by the tab:

Baofeng UV-5RE radio – battery contact pads

The battery slides into the radio compartment and latches with a snap. Two holes on the battery base engage a pair of pegs on the radio case:

Baofeng UV-5RE radio – battery base detail

The holes are rounded rectangles and the pegs have one corner sliced off. The pegs seem entirely too fragile and not well suited for 3D printing, so some metalwork may be in order. The pegs must resist only pulling forces perpendicular to the case back, not sliding forces, and the case constrains side-to-side motion.

The two square posts (with two others not shown) form the “feet” that support the radio when it’s standing on the desk or in the charger.

Now, to doodle up the dimensions and measure the actual capacity.

Speaking of capacity, BL-5 batteries on eBay range from $23 for “genuine Baofeng” that may or may not actually have that name on the label, all the way down to $8 for the usual no-name equivalent.

2013-09-04
Cheap Boost Power Supply Evaluation

For reasons that will become apparent in a while, I got a pair (*) of boost power supplies from the usual eBay source, allegedly capable of boosting a 10-to-32 VDC input to a 12-to-35 VDC output, up to 10 A and 150 W:

Boost power supply

After establishing that it would not produce 30 V into a 5.9 Ω load (5.1 A, but 152 W), I got systematic.

A 100 Ω resistor drew 1/4 A while I set the output to 28 V. Doubling up the resistors showed that it worked OK at half an amp:

Boost supply – 50 ohm load

Four 6 Ω resistors in series draw 1.2 A, then (channeling the true spirit of DIY 3D printing) two in series drew 2.3 A:

Boost supply – 12 ohm load

That’s 32 W each and, yes, they did get toasty, but, no, I didn’t leave them turned on all that long.

But a 6 Ω resistance still didn’t work, so the supply can’t provide 4.7 A at 130 W. In case you were wondering, that’s two 6 Ω resistors in series and a pair of those strings in parallel, so each resistor still sees 32 W.

In terms of driving the actual load, these supplies aren’t going to light it up.

Ah, well, whaddaya want for five buck from halfway around the planet?

(*) Davy’s Aphorism: Never buy only one of any surplus item, because you’ll never find another. Get at least two, maybe three if it’s something you might actually use.

2013-09-03
Soldering Project: Astable Multivibrator

We’ve been kicking around ideas for introductory soldering / electronics projects at Squidwrench, which prompted me to come up with a classic astable multivibrator circuit:

Astable Multivibrator – simulation

Everything is totally non-critical: it should oscillate as long as the base resistors are small enough to lightly saturate the transistors. The LEDs let you know it’s actually working.

You’d probably want a few decades of caps and a decade’s worth of resistors on pin headers, so as to cover the range from visible blinkiness to nasty audio squeal.

The pulse generator and cap at the bottom apply a jolt at T=0 to knock the circuit off balance. Otherwise, the simulation will just sit there and do nothing; in the real world, of course, nothing stays balanced for very long.

2013-08-27
P-channel MOSFET Power Switch

This is a simulation showing that a p-channel power MOSFET should work fine as a battery cutoff / switch for the bike taillight (clicky for many more dots):

P-MOSFET power switch

The general idea is to have a pushbutton or vibration sensor turn on the power, whereupon the Arduino wakes up and activates an output pin that holds the power on. When it’s time to shut down, the Arduino turns that output pin off, the power goes away, and everybody’s happy.

The MOSFET must be not only p-channel, but also have a logic-level gate, which is a rare and precious combination among cheap surplus MOSFETs. I’m hoping those FDS6675 MOSFETs work better than their package looks.

The capacitor and resistor over on the right simulate a reasonable load.

The voltage-controlled switch in the middle represents the vibration sensor, which is either shorted or open as determined by the voltage source at the bottom. There doesn’t seem to be any other Spice-ish way to do that.

The Arduino output, simulated by another voltage source drives the NPN transistor, which isolates the output pin from the 7.4 V (up to maybe 8.5 V when fully charged) Li-ion battery. It also isolates it from the switch, which would otherwise yank the output pin to ground if you pushed the button when the power was already on.

You’d want a few more pullup and pulldown resistors to ensure things stay where they’re put while the lights are out. I’d want to measure an actual vibration sensor; it may require a pulse stretcher to ensure the Arduino has enough time to wake up and smell the electrons.

The overall concept seems workable.

2013-08-23
Arduino Pro Mini Pin Coordinates
Arduino Pro Mini – top

Measured from the Official PCB Layout, with the board origin at the lower-left corner of the PCB, down there by the D9 pin, in mils (0.001 inch):
- D9 = (50,50)
- D10 = (650,50)
- A5 = (535,805)
- A4 = (535,705)
- A7 = (535,393)
- A6 = (535,293)
- FTDI header = (100,1250) to (600,1250)
- Reset button = (350,105)
- D13 LED = (540,100)
- PWR LED = (350,850)
- Upper fiducial = (160,1140)
- Lower fiducial = (490,100)
Subtract 50 mils from each of those coordinates to put the origin at the middle of the D9 pin, which may be more useful. Doing that in inches produces:
- D9 = (0.000,0.000)
- D10 = (0.600,0.000)
- A5 = (0.485,0.755)
- A4 = (0.485,0.655)
- A7 = (0.485,0.343)
- A6 = (0.485,0.243)
- FTDI header = (0.050,1.200) to (0.550,1.200)
- Reset button = (0.300,0.055)
- D13 LED = (0.490,0.050)
- PWR LED = (0.300,0.800)
- Upper fiducial = (0.110,1.090)
- Lower fiducial = (0.440,0.050)
Trust, but verify…

Yes, this is a knockoff PCB from the usual eBay vendor, not from Sparkfun. Contents may settle during shipment. Enlarged to show texture. Your mileage may vary. No warranty, either express or implied, no lie. Do not eat.
2013-08-22
Semiconductors From eBay: A Tinge Of Doubt Crosses My Mind

So I picked up a lot of 20 p-channel MOSFETs from the usual eBay supplier in China, which arrived in good order. As is often the case, the SOIC chips are in snippets of tape-and-reel carrier, but this tape looked decidedly odd:

eBay FDS6675 Tape Cover Contamination

Peeling back the tape shows that the crud is just on (or perhaps inside) the tape, not on the ICs or inside the carrier pockets:

img_3456 – eBay FDS6675 Tape Cover Contamination – interior

Some of those specks are dirt, some seem to be bubbles, other are just, well, I don’t know what they might be. Maybe they were having a bad day in the tape factory?

One might reasonably conclude the chips aren’t in their original carrier…

I must gimmick up a quick test to verify that the chips behave like p-channel MOSFETs, instead of, oh, solid plastic; that Fairchild logo looks a bit grotty, doesn’t it?

2013-08-21