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Variable Transformer

A variable AC transformer (Variac) is a very handy device and should be part of any electrical experimenters lab.

The Variac allows adjusting the line voltage anywhere from 0 to 130 VAC. The voltage output is usually provided through a single or dual 120 VAC outlet located on the Variac housing.

A Variac is available in many different VA output ratings.

Some uses for a Variac:

1.) Adjusting the speed of small AC power tools

2.) Creating a variable DC power supply by adjusting the primary voltage of a power transformer with the secondary output connected to a bridge rectifier and capacitor filter.

3.) Controlling the temperature of a heater or the speed of a fan motor.

4.) Re-forming power supply capacitors in old equipment by allowing the input voltage to the device to be increased slowly.

 

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Hot Wire Foam Cutter

Important Caution

The design of this foam cutter requires the use of an AC power transformer with a separate 120 VAC primary and a 24 VAC secondary winding in order to eliminate a potential shock hazard from the aluminum supports.

Construction Components

All measurements are approximate and non critical.

1.) The center support of the foam cutter is made from a scrap piece of 3in x 28in x 1in length of poplar wood.

2.) The side supports are made from a 18in x 1in x 0.125in length of aluminum angle.

3.) The rope used to tension the nichrome wire is a 60in length of nylon clothes line. A wood chopstick is used to apply tension to the cord. Note: The tension cord must be made of non-conductive material.

4.) The cutting wire is made from a 28in length of 27 awg nichrome wire.

5.) The black and white wire connections from the aluminum supports to the terminal block can be 16 or 14 awg TFFN stranded wire.

6.) Miscellaneous components include 16 awg #6 or #8 ring terminals, a two position terminal block, two small S hooks and some #6 and #8 machine screws and nuts.

7.) Power cable between transformer and foam cutter can be either separate 14 awg wires or a 14 awg two or three conductor cable.

8.) Power supply components consist of a 24 VAC transformer (Stancor P-8618) rated at 4 amps and a 0-130 VAC Variac transformer rated at 5A.

Note: Since most Variacs are NOT line isolated, a separate 24 VAC transformer is required to power the foam cutter.

Construction Notes

The aluminum support bars are spaced a nominal 24in apart and are attached to the poplar board with #8 machine screws.

Note: The aluminum bars also serve as electrical conductors to the nichrome wire.

The nichrome wire is looped several times through a nickel plated ring terminal and tightly crimped to the wire and attached to the aluminum bar with #6 machine screws and nuts.

Each aluminum bar is connected to the terminal block with a 16 awg wire using a 16 awg ring or spade terminal on each end.

The connection between the 24 VAC transformer and foam cutter is made with a length of 14 awg three conductor SJOOW power cable.

Finally, I added a two inch section of aluminum channel to the poplar wood support to allow me to clamp the foam cutter to my table saw in order to make long straight cuts using the table saw fence.

Using the table saw fence I have been able to cut strips of two inch styrofoam one tenth of an inch thick (0.1in).

Foam Cutter Voltage and Current Measurements
VARIAC OUTPUTXFRMR OUTPUTWIRE CURRENT
36 VAC7 VAC1.0 A
54 VAC11 VAC1.5 A
73 VAC15 VAC2.0 A
90 VAC18 VAC2.5 A
108 VAC22 VAC3.0 A

 

Construction Photographs

COMPLETED FOAM CUTTER.

 

TENSION CORD WITH S HOOK CONNECTION.

 

TENSION CORD WITH WOOD CHOPSTICK.

 

NICHROME WIRE CONNECTION TO ALUMINUM BAR.

 

VARIAC AND 24VAC POWER TRANSFORMER.

 

AIRFOIL SHAPE CUT FROM TWO INCH STYROFOAM.

 

SUPPORT CLEAT ON POPLAR SUPPORT.

 

REAR VIEW OF FOAM CUTTER CLAMPED TO TABLE SAW.

 

FRONT VIEW OF FOAM CUTTER CLAMPED TO TABLE SAW.

 

0.25 INCH AND 0.50 INCH FOAM STRIPS CUT WITH FOAM CUTTER.

 

Reference:

Variac and Cable Source

 

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Blade MCX2 RC Helicopter

Excellent beginners indoor helicopter. The coaxial main rotor design makes flying easier as it eliminates having to control a separate tail rotor while controlling the main rotor.

Ready to fly kit comes with preassembled helicopter, 4 channel 2.4 Ghz DSM2 transmitter, LiPo battery, LiPo charger, and 8 AA batteries for transmitter and charger. Recommended accessory: EFLC1005 6VDC (AC Input) power supply for charger. Eliminates having to use (4) AA batteries in the charger.

Comes with an excellent printed manual (also available online) with complete exploded view and replacement parts list.

Blade Helicopters

 

 

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DIY Optical Encoder

An optical encoder has some advantages over a mechanical encoder. One advantage is no bouncy or noisy electrical contacts that create false or extra pulses. Also, an optical encoder can operate at a higher rotation rate than a mechanical encoder.

I created a simple optical encoder from commonly available materials.

ENCODER HOUSING: I made the encoder housing from a 3/4 inch PVC pipe cap with a short length of 3/4 inch PVC pipe glued inside. The axle that supports the encoder disk was made from an inexpensive motor that I removed the magnets and the motor brushes from in order to allow the motor armature to spin freely.

The motor is then installed inside the PVC pipe. Since the fit was a little loose, I wrapped a couple of turns of tape around the body of the motor to make a tight fit. I also had to drill a small hole in the bottom end of the pipe cap to provide clearance for the rear motor bearing.

I sanded a flat spot on the outside of the pipe cap and drilled a small hole to accept a 4-40 flat head screw pointing out from the inside of the cap. This provides a threaded stud where the support for the encoder PCB is attached. The hole must be carefully located so that the opening in the photo-interrupters are at the correct level in relation to the encoder disk.

Finally I installed, with a press fit, a 1.5 inch diameter by 0.375 inch thick nylon disk to the motor shaft to which the encoder disk will be attached.

ENCODER CIRCUIT BOARD: I was able to fit the electronic components on a section of fiberglass circuit board that measured 1.375 inches by 1.625 inches. The photo-interrupters were located side by side and near the top edge of the PCB. The PCB also had a small hole drilled in it to allow mounting to the encoder support.

I installed two 0.1 inch spaced two position terminal blocks to facilitate the wire connections.

The encoder circuit uses two off-board 74HC14 Schmitt trigger inverters to clean up the pulses from the photo-interrupters and to drive two hi-efficiency blue indicator LEDs, one from each channel. This is not necessary for all applications.

The support for the encoder PCB was made from an approximately 1.25 inch long section of 0.375 diameter round nylon rod which was drilled and tapped at each end with a 4-40 thread.

ENCODER DISK: I designed the encoder disk by making a pattern using the spread sheet program Excel and then creating a pie chart with alternating black and white segments. The encoder pattern can be made with a variety of different diameters and with a different number of segments. It is important that the total number of segments used divide evenly into 360.

The artwork was printed out and then glued to a piece of thin cardboard with rubber cement. I used a compass to define the diameter of the disk and then cut it out with scissors. The notches at the edge of the disk were made with a 0.281 inch punch with the center of the hole located at the junction of the black and white segments.

This provides a total of 18 evenly spaced openings. The notches could also be cut out with a sharp x-acto knife instead of using a punch and cutting out the edge of either the white or black segment – but not both!

The finished disk was then glued to the nylon disk attached to the motor shaft.

The disk shown in the photos below had a diameter of 3.375 inches with a total of 36 segments – 18 black and 18 white.

More information on the encoder disk here: http://saroselectronics.com/create-encoder-pattern-using-excel/

 

PVC CAP WITH MOTOR AND MOUNTING STUD INSTALLED

 

ENCODER CIRCUIT BOARD

 

COMPLETED OPTICAL ENCODER WITHOUT DISK

 

COMPLETED OPTICAL ENCODER WITH DISK

 

ENCODER SCHEMATIC

 

CLOCKWISE PULSE OUTPUT FROM ENCODERS

 

COUNTER-CLOCKWISE PULSE OUTPUT FROM ENCODERS

 

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Adjustable DC Power Supply

This is a variable voltage analog power supply based on the LM317 adjustable voltage regulator. The LM317 includes on-chip current limiting, thermal overload protection, and safe operating area protection. All overload protection remains fully functional, even if the ADJUST terminal is disconnected.

The LM317 can output up to 2.2 amps, provided the difference between the input and output voltage is less than or equal to 15 VDC. The output current drops dramatically to 0.4 amps if the input-output difference is equal to 40 VDC. These values give an indication of the “safe operating area” protection.

The voltage is adjustable from 1.2 VDC to 30 VDC via a “VOLTS ADJ” potentiometer. There is also a “MAX ADJ” trim potentiometer that can be used to limit the maximum output voltage. The input is protected by a 1.8 Amp auto-reset fuse. Input voltage can be either 6-24 VAC or 8-35 VDC.

AC inputs are rectified by four 1N5402 (3 amp) diodes in a full bridge configuration and filtered by two 2200uF electrolytic capacitors. The main output regulator (TO-3 package) is fan cooled allowing continuous operation at high current levels and high ambient conditions. The regulator has built-in thermal protection and current limiting. The maximum output voltage is 30 VDC (with an input of 24 VAC or 35 VDC).

The two 6 inch diameter component mounting disks are made of 1/8 inch Delrin and the four 5/8 inch diameter support spacers are also made of Delrin.

The power transformer is a Triad F8-24 with a output of 24VAC at 4 Amps. A 1 amp fuse is installed on the transformer primary.

 

VERIFYING COMPONENT PLACEMENT

 

TOP AND BOTTOM COMPONENT TEMPLATES

 

TRANSFORMER AND TERMINAL BLOCK WIRING

 

COMPLETED POWER SUPPLY

 

1 AMP POWER SUPPLY PCB ARTWORK

 

 

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