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DC to DC conversion, what a buzz.

A fairly small relay.

A fairly small relay.

Today I will take a step back into time, back before computers and even solid state electronics. The electrical properties have been there since the beginning of time.  We have learned newer and better ways but often all we have done is the same things that were done before, but in a better way.   In someways our improvements have actually made us more sloppy because it is relatively easy to change a program, but it used to take a major effort to rewire a panel.  I often stand in awe at some of the old technology, because those guys really had to know what they were doing.

The actual size of this relay with the cover still on the relay.

The actual size of this relay with the cover still on the relay.

In the first picture I show a small relay with the cover removed.   It is a coil around an iron post.  If you look closely at the picture you will see a metal frame on the left hand side of the coil.  This frame brings one pole of the magnetic field to the top of the relay.  When the coil is energized the movable metal piece, the armature, moves toward the metal post in the middle of the coil.  This causes the switches at the bottom, (labelled contacts) to change position.  The purpose of a relay is to use the current in the coil to switch other circuits.   The switch is called a “double throw” switch because as shown in the picture it is connected to one contact, called the normally closed, N.C., contact and when the coil is energized the movable contact moves over and connects to the other contact.   The other contact is called the normally open, N.O., contact. When the coil is deenergized, a spring pulls the armature back to the position shown in the picture.

The three sets of switches on this relay.

The three sets of switches on this relay.

This particular relay has three sets of these switches as is shown in the second picture looking at a different view of the relay.   The orange plastic device on this relay is a “flag” that can be seen through the cover of the relay and gives a visual indication if the relay is energized.  This contact arrangement is called 3PDT for three poles, (circuits) and double throw for each circuit.

Relays are used in many devices and are still in use today.   For example, the Arduino controller I played with back in a previous post, “Learning the Arduino …“, does not have the power to operate many devices so often a relay is used to control a high power device from a low power circuit.  Note: the Arduino could not directly power this relay.  Not all that long ago relays were used to perform logical decisions.  For example if the N.O. contacts of two relays were wired in series, then Both Relay 1 AND Relay 2 would have to be energized to have current flow.   It the two contacts were wired in parallel, then Relay 1 OR Relay 2 would have to be energized to have current flow.  I have worked on machines that had panel after panel full of relays doing these sorts of calculations.

Now imagine that we have wired the coil of this relay through the N.C. contacts on the same relay.   What we have done is create a buzzer, because as soon as we apply power the relay would be energized, but no sooner than the coil becomes magnetized the contacts would open and the coil would be deengerized.  Once the spring returned the contacts back to the original position the process would repeat.   The relay would not last very long because it is not really designed for this kind of abuse, but there are buzzers that are designed exactly for this purpose.  If the arm of the buzzer has a small hammer on it and there is a metal bell for the hammer to hit, this would be a bell.

If the “old days” (dang I can remember this!) car radios used to use electron tubes.  These are sometimes called vacuum tubes or just tubes in the U.S.  In places that speak the “Queens English” I believe they are called valves.  The problem with tubes is they require 160 to 250 Vdc to operate, but cars normally have either a 6V or 12V dc electrical system.  We cannot use a transformer directly to step up the voltage, because a transformer depends upon changing electrical fields to operate.  A transformer connected to DC is simply a wire and will draw a very large current.

The answer to this problem was to “chop” the car voltage before sending it to a transformer. This created a changing field and the transformer could then step up the voltage.   The device to do this chopping was called a vibrator and operated very similar to our relay wired as a buzzer.   The biggest difference was the vibrator used spring metal similar to the movable contact in our relay, but this was about two times longer.   At the top of the contact was a chunk of metal acting as a weight as well as being attracted to the magnet formed by the coil.  This weight on the end of the spring formed an upside down pendulum and had a natural frequency.  Some descriptions of this refer to this part as a reed.   The natural frequency made it possible to calculate the sizes of the capacitors and other components for the circuit.   Also this would feed a center tapped primary transformer so as the field was decreasing on one side of the transformer primary coil the other side was being energized.  An example of one type of this circuit can be found in this schematic picture.  The site containing that picture, Radio Remembered, also has a more complete description of the circuits.

Modern battery powered circuits still use this principle, but often the chopper is solid state devices. For example, the flash tubes on cameras require a capacitor charged at about 300 Vdc to create the arc across the gas in the tube to create the flash.   This is done with two to four AA or AAA batteries.  A chopper circuit feeding a coil builds up these charges.   I almost received a bad shock because I was working on a flash unit for a camera.  I assumed little bitty batteries nothing to worry about while working on it.  Luckily I turned on my head before I touched anything and discharged the capacitor by putting a jumper wire across the leads.  It made a nice blue arc and not one I would care to go across me.

We have several more circuits to talk about that use coils and capacitors before we move on.  Meanwhile I will be working on construction of the popsicle crane improvements and the thermistor cicuits.  To understand the next circuits I need to provide a little theory about some solid state devices.   I do not intend to go into much detail on those devices because the construction of them is beyond the means of any hobbyist. The description will not be very professional, but more toward having fun with it so you will be able to remember the important points as a user of these devices.

Hopefully you have found this post interesting and informative.
Gary

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