Odd Electronic References

Using a simple N.O. momentary switch, for a “toggle” (push-ON, push-OFF) switch:


Thermostat Hook up:


I just installed a Honeywell RTH221 thermostat, that I bought at Home Depot. The manual that came with the unit is cryptic, and assumes that the color code was adhered to, when the heater was installed. Of course, it was not, and I only have the heater hooked up. No AC, so I only have two wires.

Thermostats are basically simple devices. They have a few switches. One closes when the heater is to be ON. Another one closes (makes contact) when the AC is to be ON. Sometimes there is a third to close to turn ON a ventilation fan, for both moving heated or cooled air.

I worked them out myself, with an Ohmmeter. I put it in heat mode, and checked for continuity, across all terminal combinations, with the Heat ON and OFF. Did the same to find the cooling contacts, and the fan contacts. Then I found this diagram on the web, which confirmed my findings. I had to do this because both Honeywell and Home Depot were no help.

Here is the link to the wiring diagram: Heating/AC Wiring

Here is the link to the table of signal conventions: Signal Table

And here is the actual site: How-to-Wire-it

Also the thermostat switches 24VAC to various relay (aka contactor) coils, to engage the heat, AC, and ventilator fans. The 24V xfmr (transformer) that provides the power commonly resides inside the heater/furnace. One of those leads is common to all the “control relay” loads. The other connects to the red wire that goes to the thermostat. Here is a typical xfmr for the job:


Its rated for 40VA (or 40W, if the load is purely resistive). I’ve found AC coil relays drawing 1/2 amp (500mA) ! This to me seems to be a ridiculously high current. I commonly use electromechanical relays with coils that draw 150mA (1/6th amp), that have several 30A contacts. Very heavy duty relays. I don’t know what kind of neanderthal technology the use in the HVAC biz.

For non-HVAC use, this xfmr can also be used as an auto-transformer. You can stick 120VAC in, and get 208V or 240V out, on the extra primary taps. They are intended to be used when 208 or 240VAC are available, but can also be used as outputs, if isolation is not needed.

Photoformer – Arbitrary Waveform Generator from the Past :


Some EIA Capacitor Codes:


Tolerance codes are commonly known, but the voltage codes are less so, since they’re usually only embedded in the part number, which may not be printed on the actual part:


Here’s an example, for a old monolithic ceramic:


Electrolytic Capacitor life Calculator:



Electrolytic Capacitor Identification Markings:

Electrolytic Body IDs

Industry Color Code Convention for Power Transformers:

(click on Image to enlarge)


Common Radio Tubes:

(click on chart to enlarge)

Common Tubes

Improved SCR control:

Here is a simple circuit that can be used to improve the control of an SCR, for dimmers, night lights, and color organs:


A rising ramp is made using a 22K resistor and a 0.47uf cap, for a ~10mS time constant. The cap is discharge at every negative half cycle thru a 1N4004 rectifier. This ramp is fed to the emitter of a PNP transistor. The level for determining the brightness, is fed to the base of this same transistor. The more negative this signal, the brighter. That’s because the difference between the base and emitter, comes earlier in the cycle, the more negative the base potential. Any voltage can be fed to the base, including a Cadmium Sulfide (CdS) photocell divider, a potentiometer, or  the envelope of an audio signal.

Here is a video demonstrating the circuit, with a CdS photocell controlling a (C7) night light lamp. (Click here to watch).

A Word on TRIACs

TRIACs are the full wave cousins of the SCR. Both devices are used to control AC loads, though the SCR only passes half the wave, and rectifies the signal. The TRIAC passes the complete wave and therefore does NOT rectify the signal. A TRIAC can be switched ON, when the voltage at its output terminal MT2 is either positive or negative, and by a signal applied to its input, the gate, in either positive or negative polarity. These are commonly referred to a modes, or quadrants, as there are 4 possible operating modes. Below is a diagram, showing all four modes:


Quadrants I, II, III (also just 1, 2 or 3), have decent sensitivity, On most TRIACs Quadrant IV (4) is less sensitive than the other 3. On some, its so bad that these are know only as 3Q TRIACs. This is something to look at, in the datasheets, when deciding which device will suit your needs. Here are 3 common classes of driving a TRIAC:


Method (a) is basically the first way TRIACs were driven, when introduced in the mid 50’s. TRIACs are used to throttle power to a load, by only letting a small portion of the AC cycle pass to the load. Its a form of pulse wave modulation (PWM). A TRIAC can only be ON or OFF. In method (a), The AC wave is passed thru an RC network. Depending on the value ratio between the R and C, the resultant AC wave will be delayed. When the voltage of that wave exceeds the break-over point of the DIAC (or neon bulb, in earlier circuits), the DIAC will conduct and dump the C value into the TRIAC gate. Once ON the TRIAC will stay ON, until the AC cycle goes to zero, which it does twice per cycle. So even though the gate trigger current may be as high as 50mA, the brief period it takes to dump a small cap, lets say 0.01uf, will still be enough to turn ON the TRIAC. This method works in Quadrants I and III, where the polarity of the gate matches that of the output terminal (T2 or MT2), in both halves of the cycle.  This method is still used on inexpensive lamp dimmers.

Method (b) is used when the controlling circuitry needs to be electrically isolated from the AC. This method is usually hidden inside devices known as solid-state relays. There are special opto-isolators known as photoTRIACs. They are like most photo-couplers, except that the output device is a TRIAC. These TRIACs can’t pass to much current. Usually only about 100mA maximum, so the photoTRIAC drives a larger TRIAC, by routing the power TRIAC’s T2 to its gate. This will only cause a brief high current pulse, because once the main TRIAC turns ON, the T2 voltage will drop close to the T1 voltage. A small current limiting resistor, ~100 to 300 ohms is all that’s needed to protect the opto-device. This method also operates in Quadrants I & III.

Method (c) is used when the controlling circuit directly drives the TRIAC, and isolation is not needed. The driving circuit can be a logic gate, an op-amp, or voltage comparator stage, a microcontroller, or even a descrete transistor. Usually the control signal varies from zero to some positive voltage, relative to T1 (MT1). This circuit operates in quadrants I and IV (4), so it requires a special group of TRIACs known as Logic TRIACs. These TRIACs are more sensitive than ordinary TRIACs, with Quadrants I, II, & III, needing as little as 3mA, and Quadrant IV, as little as 5mA. These are also a subclass of 4-mode TRIACs. Its very important that the TRIACs used in such applications will work in Quadrant IV.

Here are some data points on a TRIAC driving a 6W and 60W incandescent lamps:

TRIACs and Incandescent lamps

Zero Crossing Detector, using only One Comparator Stage:


This circuit uses only one stage to form a window comparator. The window here is when the AC sine wave is near zero volts, or better known as a zero crossing detector.

Resistor Ladder Networks:



Threeneurons Kits – TO-92 Transistor Substitution Guide


Diode & Rectifier Orientation:

For hobby novices, or old timer who’s eyeball ain’t what they use to be (like me), here is a guide for properly orienting a diode, or rectifier, that come in the common DO-35 (small glass), or DO-41 (epoxy cylinder) packages:


A diode (or rectifier) has two leads (terminals): (1) an Anode, and a (2) Cathode. You can almost always see that the cathode side has a stripe painted on the body, near that lead. Sometime the printing equipment was not lined up properly, and the “stripe” is too close to the center. In this case, look for any text. Usually a part number. That text will be on the anode side, since that’s where most of the space is suppose to be, if it was aligned properly.

5-Way Binding Post

Ever wonder what are the 5 ways to hook up to a binding post


The Old 2N2222 Transistor

Parts can be used for other purposes than originally intended. Most think that the 2N2222 NPN transistor is a general purpose small signal NPN, that can be used for a lot of different applications. Well, it is. But, back in the 60’s it was originally designed for high speed switching applications. Here’s the top of an old datasheet:


Just because, that’s what it was designed for, originally, doesn’t mean you’re stuck  using that way. Think outside the box !

Japanese Transistor Number System:

Back when the US used the JEDEC “2N” numbering system for transistors, the Japanese had their own registering scheme. They stated with “2S”, but had a 3rd character, that represented the category of the particular device:


Air Core Inductor Calculations

Here are some equations for closely approximating the value of air core coils. I first encountered these in the ARRL Handbook, so they’ve been around for decades:


Krytron Cold Cathode Arc Trigger Tubes

Krytrons are specialized high current cold cathode gas trigger tubes that work in similar fashion to the 0A5 tube. This group of tubes claim to fame, is due to their use in detonating the chemical explosives, that initiate the nuclear explosion, of an implosion type atom bomb. These tubes were (or still are) not allowed to be exported. Several model numbers made by EG&G Corporation. Here are some photos (click on photos to enlarge):




Spark Gap: