Author: Ed

This type of switch is a nice alternative to the ordinary pin-header option jumpers: pull the white plunger up to open the switch, push it down to close. Nothing to lose or (worse) drop into the machinery.

Being that sort of bear, I test most components, particularly surface-mount parts, before soldering them onto the board. Switches, however… well, what could go wrong?

Unfortunately, both of these switches were defective.

The gutted switch at the top of the pictures stuck open after I soldered it in place: pushing the plunger down didn’t do anything at all. Leaning rather hard on it didn’t get its attention, so I unsoldered and tore it apart. The parts looked OK: no obvious corrosion or deformity.

I tested the second switch, found it worked perfectly, and soldered it in place, whereupon it failed just like the first: stuck open.

Perhaps the soldering iron’s heat (immeasurably) reshaped the plastic or (invisibly) oxidized the contact point? Maybe the design is close enough to not working that installing it pushes the tolerances over the edge? I’ll never know.

These were surplus parts, so there’s no recourse, but I’m pretty sure they’d misbehave the same way if I’d paid full retail for them. If you see any inside your widgets, this may be why you can’t select an option… or why the widget suddenly enters a mysterious new mode.

I tossed the rest of my supply in the trash.

After blowing up a MAX4372 high-side current amp, I thought a Schottky diode would do the trick as a voltage limiter for the delicate current-sensing inputs. The comments to that posting showed I might be close, but that I hadn’t figured it right.

The first step is finding out how the diode behaves.

Clip a multimeter (set to the 2 V range) across the diode, clip another multimeter (set to maybe 200 mA in series with the diode, then connect a bench power supply through a 100 kΩ (more or less) resistor to limit the diode current across everything.

Twiddle the power supply knob, record voltage and current pairs, type them into a spreadsheet (say, OpenOffice, but Excel is probably similar). That gives you a table & plot that looks like this:

mV	uA
20.1	0.4
40.1	1.4
60.0	3.5
80.6	8.2
100.2	18.0
125.4	48.9
150.5	131.3
175.0	343.0
183.8	481.0

Nothing surprising there: the current has an exponential relation to the forward voltage.

An exponential relation cries out for a semilog plot, so add a third column figuring the natural log (a.ka. ln or log-to-the-base-e) of the current values in the second column. The equation is just

=LN(B3)

copied down the column as needed.

That gives you another table & plot, thusly:

mV	uA	ln(current)
20.1	0.4	-0.92
40.1	1.4	0.34
60.0	3.5	1.25
80.6	8.2	2.10
100.2	18.0	2.89
125.4	48.9	3.89
150.5	131.3	4.88
175.0	343.0	5.84
183.8	481.0	6.18

The trick is to add a regression line to the data, which you do by selecting the data series, other-clicking, selecting “Add Regression Line”, selecting the regression line, other-clicking, selecting “Show Equation”, then futzing around until the equation shows enough decimal places. Also extend the X & Y axes so you can see the Y-axis intercept on the left and the current at the MAX4273’s Absolute Max rating of 300 mV on the right.

I threw out the first measurement point, as it didn’t quite fit the rest of the data. My measurement accuracy isn’t all that great below a microamp, sooo that seemed justified. Check the raw data and see for yourself.

The regression equation is, comfortingly, ln(current) = 0.040 * voltage – 1.187.

The slope of 0.040 = kT/q, which says the temperature of my basement laboratory is 464 K, a tad warmer than the actual 286 K. Feeding the actual temperature in, the slope should be 0.046.

What that really means is that the ideality factor n = 1.62. We usually forget about that little Fudge Factor, but here it is in action: 0.040 = nkT/q.

The Y-axis intercept is -1.187, which means:

saturation current = exp(-1.187) = 0.3 uA = 300 nA.

Not a number you’ll get from the datasheet, of course.

For more on all that, consult the Wikipedia diode entry.

It’s worth mentioning that the slope depends linearly on the temperature. The exponent causes the far end of that nice line to whip the current around something nasty.

Anyhow, with numbers in hand, it’s back to the schematic… and a bit of SPICE simulation that uses a canned diode model.

My initial thought was to stick a Schottky diode across the sense terminal inputs, but John Kasunich suggested that requires a much heftier diode and might not work anyway. He suggested sampling the current-sense voltage through high-value resistors, which will certainly affect the linearity & calibration of the sense voltage.

My sissy circuit has a peak fault current of maybe a few amps, which the diode should shrug off if I’m not stupid about it. But I like the resistor notion, as it dramatically reduces the diode current.

Maxim has a useful Application Note (AN-3888) describing the effect of common-mode filters on the amp’s calibration. A key suggestion: the two resistors should differ by a factor of two to match the input bias currents.

So here’s one approach that might work.

The schematic is a screen snapshot from Linear Technology’s LTSpice IV. The two current sources on the left model the MAX4372 sense amp inputs, with their max bias current values. R1 is the current-sense resistor: a whopping 0.5 ohm for my low-current application.

R2 and R3 isolate the diode from the sense resistor. High values introduce more error due to diode current, while also helping to protect the sense inputs from excessive voltage. Low values reduce those errors, while bypassing more load current through the diode. Ya can’t win.

Running the simulated load current up to 5 A shows that the diode clamps the input voltage to about 330 mV, which is likely good enough. Higher values for R2 and R3 reduce that; 10 and 5 ohms might suffice. The factor-of-two difference is really only important at very low currents for these very low resistors; at higher currents, the diode is all that matters.

What’s of more interest is the error induced by those resistors in normal operation. Here’s a screen snapshot of simulation up to a load current of 500 mA, well above my expected max of 300 mA. Pay attention to the middle trace in each group of three, which shows the results at 30 °C (the others are 20 and 40 °C).

The red traces angling down from the upper left represent the ratio of the diode voltage to the sense resistor voltage It starts a bit over 0.99 and gets down to 0.92 by 300 mA. So, basically this protection network introduces less than 10% error if you ignore temperature effects.

The board I’m building has a calibrated current sink, so (if I were doing this for a real project), I’d be sorely tempted to just build a lookup table on the fly. Then I could work backwards from the desired current setpoints to the PWM voltage outputs required to generate those values. But that’s a simple matter of software, right?

If you care a lot about accuracy, you’ll obviously want to measure the board temperature and tweak the table accordingly.

If you want to see how an actual diode behaves, you can measure it.