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Step 4AC Control Unit
Caution: You are dealing with 115V AC! This is capable of giving you a lethal shock. You will be controlling an appliance that could burn your house down. You’ve been warned! Proceed at your own risk and with all due caution. Follow the directions carefully. Check your work twice! Don’t take shortcuts! If you're not comfortable doing house wiring (115AC) get someone to help you.
Note that this unit works only for purely resistive loads. This means only standard (not high intensity) incandescent (not fluorescent) lights and heating units without fans (like hot plates or electric frying pans). Do not try to control inductive or capacitive loads like motors, relays, or fluorescent lights. There is no snubbing in the circuit and the SCRs could be destroyed if used on loads that are not purely resistive.
Download the Applications Note that contains the circuit schematic from the Fairchild site. The schematic we'll use is Figure 14 in that apps note, except that we'll use an MOC3042. Note that the 180 Ohm resistor is R3 in our parts list and SCR1 is closest to D1, R1 and the MOC, while SCR2 is closest to D2 and R2. I'll use these numbers consistently in drawings and figures to follow.
The MOC is an optocoupler that isolates our Pulse Width Regulation Unit from the AC line while allowing the pulses to control the SCRs. Another function of the MOC3042 is to ensure that the SCRs are switched on only when the AC voltage is at 0. This is known as zero-crossing switching and minimizes EMI and any disturbances to the AC power lines. Read more about it in the Fairchild apps note.
Gather the parts you need. In addition to the parts you ordered in Step 1 and shown in Picture 2, you’ll also want to visit a hardware store and buy a sturdy plastic, double outlet box, a 15A rated receptacle, a 15A rated wall plug, a single outlet or switch box (that you’ll cut up), a terminal block rated for 115AC (Radio Shack, or hardware store), and some 14 gauge, solid copper wire. Use 14 gauge copper wire for main conductors, 18 or 20 gauge for smaller conductors. These items are shown in Picture 7. I used 14 gauge, 3 wire (2 conductors plus ground) electrical cable to connect to the wall plug. You may want a wall plug with stranded (more flexible) wire. That's fine, just be sure it's rated for 15A service. Use plain perf board with no plating or plated holes on it to build the circuit. This makes sure that conductor spacing can be maintained. Our goal is to maintain 0.1” to 0.2” spacing between all lines carrying 115VA. If you follow my pictures, you'll be fine. Note that the SCRs specified have an isolated heat sink tab. That is, the heat sink is not electrically connected. This means that if you accidentally touch it, you won't have any risk of a shock. This is a good thing.
I've provided a number of pictures and diagrams to guide you. Picture 8 shows the completed circuit board with the heat sinks in place. Picture 9 is the top view of the circuit board without heat sinks and with the parts labeled. Picture 10 is the bottom view of the circuit board showing construction details. Picture 11 is the bottom view with parts and pins labeled. The AC Board Diagram is a schematic showing parts placement and wire routing. It is a bottom view corresponding to Picture 11; parts that go on top of the board are shown in pink. If you study these a bit, you should have no trouble building the unit following the directions below.
Start by cutting out the perf board to fit in one half of the double outlet box. The dimensions I used are 2.9” by 1.6”. (If you're in doubt about part locations, just count the holes – they're 0.1” apart.)
Make the holes for the heat sinks in the locations shown (count the holes). They will be mounted by bolting them to the SCRs and soldering the SCRs in place.
Bend the middle SCR lead as shown in Picture 12. The bent leg should extend 0.2 inches in front of the other two legs. Bending the middle lead like this helps maintain the conductor spacing. Bolt the SCRs to the heat sinks and test fit them in place. You may want to tape the assemblies in place temporarily until you solder the leads. Once the SCRs are soldered in you can remove the heat sinks for more room to work.
Cut about 16 inches of 14 gauge wire and strip about 6 inches of it so you’ll have some bare copper wire to work with. Cut two lengths and bend them to make the drain to source (K to A) connections to the SCRs. Look at the pictures!
One end of each length of wire should come out of the perf board to connect to the terminal block. We'll bend that later to fit during the mechanical assembly.
Bend the leads of the SCRs over the wires you’ve just fitted and solder them in place. Make these joints very sound mechanically and solder them carefully. This ain’t 5 volts you’re dealing with here! Picture 10 shows this clearly.
Mount the MOC3042 in place. Again, use some tape to hold it until it’s soldered. Use an insulated piece of 18 gauge wire to connect from pin 6 of the MOC to the gate of SCR1. Picture 11 and the AC Board Diagram show this.
Route the leads for the resistors and diodes. Make solid mechanical connections and do a good job soldering. Picture 11 shows where I connected each component while Picture 9 shows the top view. These should be a useful guides.
Once all the soldering is done, dismount the heatsinks from the SCRs and put a thin layer of heat sink grease (Radio Shack) on the back of the SCRs and remount the heatsinks. Tighten the screws firmly, but don't overdue it. You don't want to distort the tabs on the SCRs. Your circuit board should look like Picture 8. The resistor shown as “R5 (uProc Gnd)” is R5 on the Pulse Width Regulation Unit Schematic. Mount this where it is most convenient. I put it on the AC Control Unit since it's a convenient place to connect ground from the Pulse Width Regulation Unit. The control signal from PB4 on U1 (the uProcessor) connects to the wire to MOC pin 1.
Note that this unit works only for purely resistive loads. This means only standard (not high intensity) incandescent (not fluorescent) lights and heating units without fans (like hot plates or electric frying pans). Do not try to control inductive or capacitive loads like motors, relays, or fluorescent lights. There is no snubbing in the circuit and the SCRs could be destroyed if used on loads that are not purely resistive.
Download the Applications Note that contains the circuit schematic from the Fairchild site. The schematic we'll use is Figure 14 in that apps note, except that we'll use an MOC3042. Note that the 180 Ohm resistor is R3 in our parts list and SCR1 is closest to D1, R1 and the MOC, while SCR2 is closest to D2 and R2. I'll use these numbers consistently in drawings and figures to follow.
The MOC is an optocoupler that isolates our Pulse Width Regulation Unit from the AC line while allowing the pulses to control the SCRs. Another function of the MOC3042 is to ensure that the SCRs are switched on only when the AC voltage is at 0. This is known as zero-crossing switching and minimizes EMI and any disturbances to the AC power lines. Read more about it in the Fairchild apps note.
Gather the parts you need. In addition to the parts you ordered in Step 1 and shown in Picture 2, you’ll also want to visit a hardware store and buy a sturdy plastic, double outlet box, a 15A rated receptacle, a 15A rated wall plug, a single outlet or switch box (that you’ll cut up), a terminal block rated for 115AC (Radio Shack, or hardware store), and some 14 gauge, solid copper wire. Use 14 gauge copper wire for main conductors, 18 or 20 gauge for smaller conductors. These items are shown in Picture 7. I used 14 gauge, 3 wire (2 conductors plus ground) electrical cable to connect to the wall plug. You may want a wall plug with stranded (more flexible) wire. That's fine, just be sure it's rated for 15A service. Use plain perf board with no plating or plated holes on it to build the circuit. This makes sure that conductor spacing can be maintained. Our goal is to maintain 0.1” to 0.2” spacing between all lines carrying 115VA. If you follow my pictures, you'll be fine. Note that the SCRs specified have an isolated heat sink tab. That is, the heat sink is not electrically connected. This means that if you accidentally touch it, you won't have any risk of a shock. This is a good thing.
I've provided a number of pictures and diagrams to guide you. Picture 8 shows the completed circuit board with the heat sinks in place. Picture 9 is the top view of the circuit board without heat sinks and with the parts labeled. Picture 10 is the bottom view of the circuit board showing construction details. Picture 11 is the bottom view with parts and pins labeled. The AC Board Diagram is a schematic showing parts placement and wire routing. It is a bottom view corresponding to Picture 11; parts that go on top of the board are shown in pink. If you study these a bit, you should have no trouble building the unit following the directions below.
Start by cutting out the perf board to fit in one half of the double outlet box. The dimensions I used are 2.9” by 1.6”. (If you're in doubt about part locations, just count the holes – they're 0.1” apart.)
Make the holes for the heat sinks in the locations shown (count the holes). They will be mounted by bolting them to the SCRs and soldering the SCRs in place.
Bend the middle SCR lead as shown in Picture 12. The bent leg should extend 0.2 inches in front of the other two legs. Bending the middle lead like this helps maintain the conductor spacing. Bolt the SCRs to the heat sinks and test fit them in place. You may want to tape the assemblies in place temporarily until you solder the leads. Once the SCRs are soldered in you can remove the heat sinks for more room to work.
Cut about 16 inches of 14 gauge wire and strip about 6 inches of it so you’ll have some bare copper wire to work with. Cut two lengths and bend them to make the drain to source (K to A) connections to the SCRs. Look at the pictures!
One end of each length of wire should come out of the perf board to connect to the terminal block. We'll bend that later to fit during the mechanical assembly.
Bend the leads of the SCRs over the wires you’ve just fitted and solder them in place. Make these joints very sound mechanically and solder them carefully. This ain’t 5 volts you’re dealing with here! Picture 10 shows this clearly.
Mount the MOC3042 in place. Again, use some tape to hold it until it’s soldered. Use an insulated piece of 18 gauge wire to connect from pin 6 of the MOC to the gate of SCR1. Picture 11 and the AC Board Diagram show this.
Route the leads for the resistors and diodes. Make solid mechanical connections and do a good job soldering. Picture 11 shows where I connected each component while Picture 9 shows the top view. These should be a useful guides.
Once all the soldering is done, dismount the heatsinks from the SCRs and put a thin layer of heat sink grease (Radio Shack) on the back of the SCRs and remount the heatsinks. Tighten the screws firmly, but don't overdue it. You don't want to distort the tabs on the SCRs. Your circuit board should look like Picture 8. The resistor shown as “R5 (uProc Gnd)” is R5 on the Pulse Width Regulation Unit Schematic. Mount this where it is most convenient. I put it on the AC Control Unit since it's a convenient place to connect ground from the Pulse Width Regulation Unit. The control signal from PB4 on U1 (the uProcessor) connects to the wire to MOC pin 1.
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Feb 14, 2011. 10:44 PMsalmansheikh
says:
May I use 220 ohm resistors instead of the 180ohm?
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