Posts Tagged Memo to Self

Tek A6302 Current Probe Derating

Most currents around here come in tens-of-milliamps, maybe a few hundred, tops, but it’s worth noting some curves from the Tektronix AM503 current amplifier manual for A6302 Hall effect probes:

Tek A6302 Calibration Setup

Tek A6302 Calibration Setup

The maximum current drops from 20 A for frequencies above 20 kHz:

Tek A6302 Current Probe - Max Current Frequency Derating

Tek A6302 Current Probe – Max Current Frequency Derating

There’s a 100 A·μs pulse charge limit:

Tek A6302 Current Probe - Specified Operating Area

Tek A6302 Current Probe – Specified Operating Area

Because the probe is actually a pulse transformer, its internal termination imposes a (small) load on the input circuit:

Tek A6302 Current Probe - Insertion Impedance

Tek A6302 Current Probe – Insertion Impedance

The specs are 100 mΩ at 1 MHz and 500 mΩ at 50 MHz, which means the load is essentially zilch for the circuits and signals I deal with.

The Tektronix Probes for Current Measurement Systems has some useful descriptions.

Memo to Self: Should any of those limits matter, rethink what you’re doing.

An interesting story about the AM503 design from someone who lived through it.


Leave a comment

Ortlieb Backroller Pack Drop

Although the pair of Ortlieb Back-Roller packs on Mary’s bike make her look like a long-distance tourist, we’re actually on our way to her garden plot:



The left-side pack suddenly seemed unusually floppy:



One second later:



Another second and it’s visible under my right hand:



The view from her bike at about the same time:



I’m expecting to fall to my right, but it’d have been better if I hadn’t kicked the bag:



The pack went under the rear wheel and out the far side:



Where it came to rest in the middle of the trail:

Ortlib pack drop - aftermath

Ortlib pack drop – aftermath

Elapsed time from the first picture: just under 5 s.

Did you notice the other cyclist in the other pictures? She’s why I veered so hard to my right!

A pair of these latches hold the pack onto the rear rack:

Ortlieb pack drop - QL latch detail

Ortlieb pack drop – QL latch detail

When they’re properly engaged, they look like this:

Ortlieb pack drop - QL latch - secure

Ortlieb pack drop – QL latch – secure

When they’re not, they look like this:

Ortlieb pack drop - QL latch - whoopsie

Ortlieb pack drop – QL latch – whoopsie

Which is obvious in the picture and inconspicuous in real life.

The strap emerging from the top of the latch serves as both a carrying handle and latch release: pull upward to open the latches and release them from the bar, lift to remove the pack, and carry it away as you go. Installing the pack proceeds in reverse: lower the pack onto the rack bar, release the handle, and the latches engage.

Unless the pack is empty enough to not quite fully open the latches as you carry it, in which case the closed latches simply rest on the bar. We’ve both made that mistake and I generally give her packs a quick glance to ensure sure they’re latched. In this case, the plastic drawer atop the racks (carrying seedling pots on their way to the garden) completely concealed the pack latches.

Tree roots have been creasing the asphalt along that section of the rail trail: the bike finally bounced hard enough to lift the drawer and fall off the rack rod.

Memo to Self: In addition to the visual check, lift the packs using the strap across the middle holding the rolled-down top in place. Remember, don’t check by lifting the carrying handle, because it just releases the latches; another easy mistake to make.




Digital Tattoo Power Supply: Polarity Doesn’t Matter

When I rewired the guts of the digital tattoo power supply to eliminate the series foot switch, I kept the original wiring polarity, with the black wire to the sleeve and the red wire to the tip:

Tattoo Digital Power Supply - internal view

Tattoo Digital Power Supply – internal view

It’s the same color code I (strongly) recommend in the Squidwrench Electronics Workshops: use any color for the ground / common wire as long as it’s black, then, if you have a red wire, use it for the positive supply. You can use yellow for the higher supply voltage, but stop being clever.

I put suitably colored Powerpoles on the far end of the cable to replace the standard tattoo machine spring clip connector, so I can attach clip leads, battery test fixtures, and so forth and so on.

We wired the supply into a clip-leaded diode measurement setup with a current limiting resistor and a pair of multimeters to measure the diode current and forward voltage, whereupon we noticed all the meters displayed negative voltages and currents.

After a frenzy of wire-checking verified their setup was all good, I forced the simplest possible test, herein recreated on my bench:

Tattoo Digital Power Supply - polarity test

Tattoo Digital Power Supply – polarity test

Which produced this display:

Tattoo Digital Supply - reverse polarity

Tattoo Digital Supply – reverse polarity


After a brief exploration of “Trust, but verify” territory, we swapped the clip leads from the power supply and continued the mission.

Back on my bench, I pulled the supply apart and measured the voltage at the jack terminals:

Tattoo Digital Power Supply - jack wiring

Tattoo Digital Power Supply – jack wiring

Still negative. Huh.

The bottom of the power supply PCB shows exactly what you should expect by now:

Tattoo Digital Power Supply - reversed color code

Tattoo Digital Power Supply – reversed color code

The red wire near the top of the board is, indeed, soldered to the trace labeled GND and goes to the jack’s tip terminal; the adjacent black wire goes to the front-panel LED. Similarly, the black wire just below it, soldered to the same trace as the yellow wire, goes to the jack’s sleeve terminal; that trace also connects to a resistor leading to the trace labeled LED+ and the LED’s red wire.

Although tattoo machines run from DC supplies, their motors or vibrators don’t depend on any particular polarity and will run fine with a backwards supply.

Resoldering the red and black wires where they should go produces the expected sign at the jack:

Tattoo Digital Supply - meter leads

Tattoo Digital Supply – meter leads

Although measuring and plotting diode voltages and currents may seem tedious, actually wiring stuff together and taking data reveals how difficult the real world can be.

I trusted the supply’s internal color code and, although I’m certain I tested the Powerpoles, I obviously didn’t notice the meter’s sign.

Memo to self: Sheesh.

, ,


Tour Easy: Front Derailleur Cable Angle

Spotted while in the midst of replacing my Tour Easy’s rear grip shifter:

Tour Easy - front derailleur cable angle

Tour Easy – front derailleur cable angle

As you might expect, the cable saws through the side of its ferrule and the brazed-on frame fitting, because it’s been basically impossible (for me, anyhow) to find a replacement derailleur duplicating whatever the good folks at Easy Racers shipped back in 2001.

On the upside, this derailleur’s cable entry has a nicely rounded ramp eliminating the need for my brass cable pulley widget.

Memo to Self: Perhaps running the cable around a bearing anchored to the frame fitting would help?

I’ve obviously forgotten to fix this for several years, so putting it here may serve as a Round Tuit.



Subaru Forester Fuse Boxes

Speaking of automotive fuses, our 2015 Subaru Forester has a pair of fuse boxes, hereby documented in case of need.

One under the hood:

2015 Subaru Forester - engine compartment fuse box

2015 Subaru Forester – engine compartment fuse box

Note the white fuse puller near the top.

The layout chart doesn’t say what “SBF” might be, but we have a lot of whatever it is:

2015 Subaru Forester - engine compartment fuse ID

2015 Subaru Forester – engine compartment fuse ID

The spare fuses line up along the lower edge of the cover.

Another under the dashboard:

2015 Subaru Forester - dashboard fuse box

2015 Subaru Forester – dashboard fuse box

And their functions:

2015 Subaru Forester - dashboard fuse ID

2015 Subaru Forester – dashboard fuse ID

The string of fuses down the right side of the main block looks like a line of spares, but they’re not. What they might be isn’t documented anywhere, which seems to be very deliberate.

Memo to Self: Having never replaced an automotive fuse, I shouldn’t start worrying now.


Squidwrench Electronics Workshop: Session 2

Some ex post facto notes from the second SquidWrench Electronics Workshop. This turned out much more intense than the first session, with plenty of hands-on measurement and extemporized explanations.

Measure voltage across and current through 4.7 kΩ 5 W resistor from 0.5 V to 30 V. Note importance of writing down what you intend to measure, voltage values, units. Plot data, find slope, calculate 1/slope.

Introduce parallel resistors: 1/R = 1/R1 + 1/R2. Derive by adding branch currents, compute overall resistance, factor & reciprocal.

Review metric prefixes and units!

Introduce power equation (P = E I) and variations (P = I² R, P = E²/R)

Measure voltage across  and current through incandescent bulb (6 V flashlight) at 0.1 through 6 V, note difference between voltage at power supply and voltage across bulb. Plot data, find slopes at 1 V and 5 V, calculate 1/slopes.

Measure voltage across ammeter with bulb at 6 V, compute meter internal resistance, measure meter resistance. Note on ammeter resistance trimming.

Measure voltage across and current through hulking power diode from 50 mV – 850 mV. Note large difference between power supply voltage and diode voltage above 750-ish mV. Note power supply current limit at 3 A. Plot, find slopes at 100 mV and 800 mV, calculate 1/slopes. Compare diode resistance with ammeter resistance.

Review prefixes and units!

The final whiteboard:

Whiteboard - Session 2

Whiteboard – Session 2

Hand-measured data & crude plots FTW!


Squidwrench Electronics Workshop: Session 1

Some ex post facto notes from the first SquidWrench Electronics Workshop, in the expectation we’ll run the series from the start in a while. I should have taken pictures of my scribbles on the whiteboard.


  • Voltage – symbol E (Electromotive Force or some French phrase), unit V = volt
  • Current – symbol I (French “intensity” or some such), unit A = ampere
  • Resistance – symbol R (“resistance”), unit Ω (capital Greek Omega) = ohm

Introduce Ohm’s Law & permutations, postpone calculations.

Measure the actual voltage of assorted cells & batteries. Identify chemistry, internal wiring:

  • 1.2 = nickel-cadmium or nickel-metal-hydride
  • 1.5 = carbon-zinc or alkaline
  • 2 V = lead-acid
  • 3.0 = primary lithium
  • 3.6 – 3.7 = rechargeable lithium, several variations
  • 4.8 = 4 x 1.2 V
  • 7.2 = 6 x 1.2 V
  • 7.4 = 2 x 3.6 V
  • 9.6 = 8 x 1.2 V
  • 10.8 = 3 x 3.6 V
  • 12 = 6 x 2 V

Measure various resistors, favoring hulking finger-friendly sandstone blocks.

Introduce metric prefixes:

  • Engineering notation uses only multiple-of-three exponents
  • μ = micro = 10-6
  • m = milli = 10-3
  • k = kilo = 103
  • M = mega = 106

Discuss resistor power dissipation vs. size vs. location, postpone power formula.

Clip-lead various resistors to various batteries, measure voltage & current.

Introduce fixed & variable power supplies, repeat resistor measurements.

Now compute permutations of Ohm’s Law using actual data!

1 Comment