Nuts and Volts - September 2017

Is It Dead or Not?

Great editorial by Bryan Bergeron in the July 2017 issue on a grab-and-go kit, but I'm flummoxed regarding the inclusion and use of a clamp-on ammeter to "verify that the circuit is, indeed dead." I'm sure you must have meant something else.

As I'm sure Bryan must know, the ammeter will only indicate the presence of current; it won't tell you if the circuit is 'dead' or if any potential is present. That's where a DVM would be used.

Put simply, if you're replacing an electrical outlet, a reading of zero from your clamp-on ammeter around any or all of the wires is no guarantee at all that the breaker is off.

Ira Wexler

You are, of course, correct Ira.

Thanks for pointing out the error.

Readers like you help make it a better magazine for all.

Bryan Bergeron

Values MIA

I was reading the June Q/A column and realized Figure 1 has no component values except for transistor types. Do you have an updated schematic? Also, tying the emitters of Q3 and Q4 together looks very odd! Should the collector and emitter connections of Q4 be swapped? Dan NE7JN

My bad Dan! Below is an updated schematic that has the values I used when I built it. They must have been buried in the metadata somewhere when I pulled the schematic out from the board layout.

As for Q3 and Q4, that’s quite intentional. It makes it into a comparator. Only when Q3’s emitter is below Q4’s by ~1.2V does the circuit begin to do anything. In other cases, they are reverse-biased and the fan doesn’t turn on at all because there is no current flowing in R4.

With no current flowing in R4, Q2 doesn’t begin to be a current source. Q3’s emitter is lowered when Q1 becomes warm, shrinking it’s Vbe, which is then multiplied by the gain around the potentiometer connected to its collector.

You’ll notice that I’m pretty boring about the component values I choose. I like to make things easy on myself. Unless something needs a precise amount of current, I usually don’t worry too much about more than the order of magnitude.

In the same way, D1’s voltage could be different, as long as there’s about 20 mA of current available. Even that current is a guess. It’s enough current to keep the zener diode stable and far more than the attached transistors will need.

One other thing of note is that Q1 is a pretty hefty NPN only because those TO-220 packages are easy to mount as a temperature sensor. Mine is extended by a shielded three-conductor cable to reach the radio. It’s shielded to lessen RF incursion.

I think this is a fun circuit since it’s a bit of a puzzler. It uses transistors in both conventional (class-A amplifier, current source) and unconventional ways (Vbe multiplier, comparator, thermal feedback loop). Try it out on your friends the next time you’re at a cocktail party!

Kristen McIntyre Q&A Editor READER FEEDBACK

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