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Summary:
This is a simple AC mains test indicator simulation. I have not fully tested it yet. I am trying to build a cheap AC current limiting device to add a layer of protection between an unproven circuit and mains power. I know I need to get an isolation transformer/variac but I'm too cash poor at the moment. The design must include a fuse and 3 indicators,
1) Mains Power Indicator
2) Blown Fuse Indicator
3) Load Current Indicator
I want all 3 indicators to light the red, green, or blue LED of a single common cathode RGB LED. Each indicator must cancel the operation of the others.
Parts:
Common Cathode RGB LED ×1
220k Resistors ×8 (1/4W)
MPSA42 NPN transistor ×4
MPSA92 PNP transistor ×1
1N400x Diode ×6
120V Light Bulb/Socket rated 25 watts ×4
Switches for how many bulbs are in series
Detailed Explanation:
The top left switch is representative of a fuse. When this "fuse"(switch) is open the red LED comes on. If this "fuse"(switch) is closed, but the second switch is open, the green led is on. This green LED is a mains power indicator letting the user know that the circuit has power, the fuse is good, but the device/circuit under test is not sinking current from the output. When a load is connected to the circuit the Blue LED is on. The 4 incandescent lights at the top are 25 watt bulbs. These 4 are placed in series with the live/hot/black AC mains, and creates a rudimentary AC current limiting device. Any device connected in series with these incandescent bulbs is unable to sink more current than the maximum power through these bulbs. This allows me to safely check for major faults in simple appliances and tools without relying on a domestic 15 amp circuit breaker. The output terminal of the circuit is the right side of these bulbs. The lower incandescent lamp is representative of a device or circuit under test that is trying to sink current. This lamp is setup as 120V RMS. It is limited by the 25W lamps on the series output. I have a Voltmeter set to measure the voltage between the 4 incandescent bulbs and the incandescent bulb that represents the device under test. The Voltmeter shows the relationship between the impedance of the 4 bulbs and the output voltage. Each of the 4 bulbs are individually switched in parallel with each other. A single 25w bulb is equivalent to a 1156 ohm impedance, two bulbs in parallel are equivalent to a 578 ohm impedance, three are 385 ohms, and four are 289 ohms. Each impedance will create a voltage drop according to the total impedance multiplied by the current the load is drawing. This is very important to calculate/understand this voltage/impedance/current relationship before testing any digital or active circuitry, as the voltage drop could cause problems or damage some circuits. This current limiting circuit is something I plan to use with small (less than 2 amp) step down isolation transformers, small appliances, and tools with a similar current draw.
The last switch below the test bulbs is there to fully demonstrates the load current indicator (Blue LED) functionality and it's relationship to the current drop across the current limiting bulbs.
As configured this circuit powers each LED with ~1 mA. The brightness of each color may vary slightly. I have also included the reverse current protection diodes for use with the Gallium Nitride LED chemistry used in RGB LEDs. These are the 3 diodes closest to the bottom of the circuit. The 3 additional diodes below the resistors on the upper rail are there to limit the current through the 220k resistors by cutting off 1/2 of the AC waveform. I'm not sure if I still need the "protection diodes" in the lower position, but I'm leaving them despite the possible redundancy unless someone informs me differently.
The circuit should work with MPSA42 NPN transistors as they are rated for 300V and 50mA maximum. The resistors, and diodes should be rated for peak mains voltage.
The LEDs have a common cathode (they all have one shared negative terminal//Don't PANIC = P_ositive A_node N_egative I_s C_athode ;)
Controlling each LED independently is done using transistors. The power indicator/green LED's anode/positive input, is connected to the collector of 2 different NPN transistors. Each base of these two transistors is connected to the Blown Fuse/Red LED, and Load Current/Blue LED indicators respectively through a 220k current limiting resistor. The Blown Fuse/Red LED circuit has the same transistor configuration connected to the Load Current/Blue LED indicator. These 3 transistor circuits are used to bypass the LEDs connected to their collector if the circuit connected to the base of the transistor has current flow.
There is a different 2 transistor configuration for the Load Current/Blue LED indicator. This indicator circuit block uses a PNP transistor that is turned on by the current drop across the current limiting incandescent lamp. Due to the voltage drop, currents involved, and gain of the transistor, the collector output of the PNP transistor is connected to the base of an NPN transistor that is used to switch the blue LED directly in series with the emitter of the NPN transistor. This transistor switch also provides the necessary current to bypass the other LEDs in the circuit ensuring only the blue LED is on when current is flowing on the output of the incandescent current limiting lamp. This load current LED is needed in order to indicate output currents that are lower than those needed to power the 25W current limiting incandescent lamp(s).
-Jake
I occasionally upload content on YouTube under my UpcycleElectronics account. It is not monotized. I will probably post a video on this project soon.
THIS IS A LIVE AC MAINS CIRCUIT. IF YOU DO NOT UNDERSTAND THE DEADLY POTENTIAL OF AC MAINS DO NOT TRY ANYTHING LIKE THIS CIRCUIT. IF YOU GET YOURSELF KILLED, IT'S 100% YOUR FAULT. I CAN'T FIX STUPID, MYSELF INCLUDED. I PROBABLY MADE SEVERAL MISTAKES HERE. IF YOU ARE NOT QUALIFIED TO UNDERSTAND THOSE MISTAKES AND CORRECT THEM I AM EXPLICITLY TELLING YOU NOT TO TRY THIS CIRCUIT. I AM NOT AN ELECTRICAL ENGINEER. I AM A DUMB HOBBYIST.
I welcome the opportunity to learn from anyone kind enough to help me improve my understanding and knowledge. Please feel free to take sarcasm and foul language elsewhere.
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