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.

Author: Ed

RAMBo Platform Heater MOSFET: VGS Specs

The improved platform for the M2 runs at 30 V, but the RAMBo board specs limit the max HBP voltage to 24 V, presumably because the 15 A ATO fuse won’t clear a high-voltage, high amperage DC short. While setting up the SSR that drives the new platform, I looked up the specs for the PSMN7R0-60YS MOSFET controlling the bed heater and … it doesn’t have a logic level gate.

The r_DS spec is an impressive 6.4 mΩ max, but that’s at V_GS = 10 V. The 1 mA threshold voltage V_GS(th) = 4 V max, which means there’s only 1 V of headroom to turn the transistor on enough to pass upwards of 10 A.

The typical I_D vs. V_GS curve (Fig 6) shows 20 A at maybe 4.2 V, but the typical R_DSon curve (Fig 8) shows the resistance skyrocketing for V_GS under maybe 4.8 V; sliding that curve a wee bit to the right would cause a Very Bad Thing to take place.

A 20 mΩ resistance dissipates 4.5 W at 15 A, which seems rather aggressive for the small PCB copper-pour heatsink on the RAMBo board. It’s a somewhat more bearable 2 W at 10 A, but I think that’s still too high. Of course, the typical dissipation ~~will~~ should be much lower…

A good engineering rule of thumb is to ignore the datasheet’s “Typical” column and design using the “Minimum” or “Maximum” columns, as appropriate. When you depend on typical specs, getting “the same part” from a different supplier can provide a real education in supply-chain management.

I suspect tolerance stacking works well enough that nearly all the MOSFETs on nearly all the RAMBo boards run cool enough to survive, but I’d rather see logic-level MOSFETs in Arduino circuits where the maximum gate voltage won’t ever get above 5 V.

2013-10-09
Tour Easy: Handlebar Wrap

After more than a few years, the handlebar grips on my Tour Easy are rather worn, so I recently wrapped them with cheerful red and yellow silicone tape.

Back in the day, you wrapped with cork tape and had to worry about the direction on each side. Silicone tape fuses into a solid mass and the orientation shouldn’t matter; that’s a Good Thing, because I’m not sure what direction would be correct in this situation.

The yellow section covers the SRAM twist grip, which means it has a moving joint at each end. I suspect the tape will pull back from the larger part of the grip and form an unsightly lump just behind it.

It’s certainly much grippier than I expected…

(The small pushbutton switch is the PTT for the amateur radio HT that does voice and APRS/GPS.)

2013-10-08
Humanoid Finger

Just for fun, I printed out Anthromod’s Kickstarter Hand finger:

Anthromod Finger – parts on M2 platform

If I were doing it, I’d add 1.75 mm alignment holes in each part, but clamping each phalange in both directions came out close enough:

Anthromod Finger – clamping

The tolerances were a bit tight and it required some trimming before all the joints flexed freely. I used short segments of 3 mm orange filament for the knuckle hinges and heat-staked the ends, rather than having to trim a trio of 3 mm screws:

Anthromod Finger – detail

After making three short rubber bands by tying and trimming loops from a longer band, the finger curled up just like yours:

Anthromod Finger – curled

The overall quality isn’t as good as I’d like: there’s a bit of uplift on the edges and corners. If I print another one, I want to try less than 0.2 infill and less cooling.

2013-10-07
Monster in the Mist

We biked to Saugerties for the Hudson Valley Garlic Festival and spotted this monster looming in the morning mist during the ride home:

Excavator on CSX gondola car – side

The end view shows it’s not an optical illusion:

Excavator on CSX gondola car – end

Some Google Maps fiddling reveals the plant, with the excavator atop the first car on the siding, down in the lower-left corner of the image:

Google Maps – Kings Highway at Tissal Rd

A zoomed view, rotated a quarter-turn CCW so it’s not quite so vertiginous:

Google Maps – Kings Highway at Tissal Rd – detail

My search-fu isn’t strong enough to uncover the plant’s name. They’ve obviously been doing something involving gravel and either asphalt or concrete for many years, so it’s not a prank…

2013-10-06
Getting More Clearance While Bicycling: Fluids Division

Based on recent experience, this “Baby, Think It Over” rig works even better than a propane tank:

Tour Easy BOB Yak – Gasoline can

I was going to take a picture with it posed next to the gas pump, but the whole affair isn’t all that stable: it’s tough to look cool when your fancy faired Tour Easy ‘bent flops over like dead possum…

2013-10-05
Sonicare Essence: Re-taping the Case

Much to my astonishment, the ordinary adhesive tape holding the Sonicare Essence power toothbrush together lasted for a bit over a year. As the tape splits along the gap in the case, the coil driving the brush head begins vibrating inside its nest, making a truly horrendous racket.

The new fix looks a bit odd, but works fine:

Sonicare Essence – red tape

The tape comes from Mad Phil’s stash and is, I think, splicing tape for reel-to-reel 1/4 inch recording tape: it has zero stretch, infinite strength, and adhesive that’s obviously lasted forever. The inside of the spool says “NOPI Made in Germany”, which doesn’t lead anywhere useful, although the NOPI name does seem to appear in a tape context.

After a year, the replacement NiMH cells are doing fine, still operating about once a day for three weeks from a 24 hour charge.

2013-10-04
Arduino Suicide Power Switch: First Light
This may not look like much, but it’s the first test of the p-MOSFET power switch that completely kills power to the Arduino Pro Mini board and the Hall Effect LED Blinky Light:

Power off – 30 mA load

The top trace is the base drive to the NPN transistor that holds the p-MOSFET on while the Arduino is running. When it’s time to shut off, the Arduino drops the base drive output, the MOSFET turns off, and the switched battery voltage in the bottom trace drops like a rock. The current is about 30 mA when the Arduino is running and immeasurably low when it’s off; the MOSFET spec says it’s less than 1 μA, which is fine with me.

I love it when reality matches the simulation.

That part of the schematic:

Hall Effect LED Blinky – Battery Switching

The PCB has those components clustered in the upper left corner, with the Arduino Pro Mini perched on header pins to the right:

Hall LED PCB – power switch test

The test code is a crudely hacked version of the canonical Blink sketch that waits 5 s after it starts running, then pulls the plug:
```
// Modified from Arduino Blink example
// Drives external p-MOSFET power switch
// Ed Nisley - KE4ZNU - Sep 2013

int led = 13;

// HIGH to enable power supply
int PowerOn = 4;

// HIGH to light Status LED
int Status = 10;

unsigned long MillisThen;

void setup() {

  pinMode(led, OUTPUT);

  pinMode(PowerOn,OUTPUT);
  digitalWrite(PowerOn,HIGH);
  pinMode(Status,OUTPUT);
  digitalWrite(Status,HIGH);

MillisThen = millis();
}

void loop() {
  digitalWrite(led, HIGH);
  delay(100);
  digitalWrite(led, LOW);
  delay(500);

  if (((millis() - MillisThen) > 5000ul)) {
      digitalWrite(Status,LOW);
      delay(50);
      digitalWrite(PowerOn,LOW);
      digitalWrite(Status,HIGH);
  }
}
```
It turns out that the Arduino runtime has a several-second delay after power comes up before the setup() routine starts running, so brief pulses from a vibration switch won’t last long enough to turn the thing on. That’s not a fatal flaw for now and, in fact, having to hold the power button in for a few seconds isn’t entirely a Bad Thing.

However, once the power turns on, a vibration switch could trigger an Arduino interrupt pin to reset a power-off timer. I’d be tempted to put the vibration switch in parallel with the button, with a pair of steering diodes that isolate the raw battery from the input pin.

This is, of course, a pure electronic implementation of a Useless Machine…
2013-10-03