making a model of a power plant...

[Leejames Merlas]

I'm making a model of a power plant that supplies electricity to different buildings and houses. It's for physics.

I have a 220v - 12 v transformer (2A). I want to light about 20 LED lights. My teacher told me to use parallel wiring and have a 1/2 watt 1Kohm resistor for each LED. Do i still need to use a diode for the transformer (i bought two 6A diodes. did i buy the right thing??) What part of the transformer should i solder my plug so that the transformer can connect to a wall socket? How do i solder the switch so that i can turn my whole project on and off? I attached the materials i'll be using. Please help :grin:

 

[betwixt]

Re: Help me pleeeeaaaaseee

Simple - and you do have the right parts! In fact you could have used smaller diodes but big ones will work just as well.

The transformer has two sides, a 220V primary side for connection to the wall socket and a 12-0-12V side for the LEDs. I can't download the full size pictures on my slow internet connection but from the thumbnails it looks like you have a power cord with a "figure of 8" (two round connections molded side by side like a number 8) plug on it. Cut the plug off so you can connect to the wires inside the cable. One wire goes to the "0" pin on the primary side of the transformer, the other wire goes to one side of the switch. Then link the other side of the switch to the transformer pin marked "220V". You should have a loop, cord->wire->transformer '0'->switch->wire_>cord. PLEASE be very careful to make sure all those connections are insulated so nobody can touch any of the joints or pins, they are all potentially 'live' and therefore dangerous.

The secondary side of the transformer is a little more complicated but not difficult. You will need some insulated wire to connect to the LEDs, I would suggest you use red for positive and black for negative if possible as this is an industry standard. The actual colors make no difference to the electronics but make it clearer to identify when wiring it up. Connect the negative (black) wire to the transformer pin marked '0' on the secondary side, make it long enough to reach the LEDs but don't connect the other end for now. Next connect the two diodes, the anode ends (end WITHOUT the paint band around it) go one to each of the transformer secondary pins marked '12V' and the cathode ends (the end WITH the paint band) are joined together. Connect the positive (red) wire to the point where the two diode cathodes join. You now have a 12V DC supply!

The final step is to connect the LEDs, you have to be careful here because they can be damaged if you connect them the wrong way around. Before going further, look closely at them and you should see a small flat side on their body, it is on the wider rim around the flange where the pins enter the plastic body. You might also have LEDs where one leg is longer than the other, unfortunately not all do so use the flat to double check the polarity. The pin next to the flat is the LED cathode, the other is the anode. It would be easiest to connect the LEDs and resistors first so join one resistor leg to each LED anode side. The circuit should work even with only one LED connected so you can try it out before adding more. The cathode (flat) side of the LED goes to the negative (black) wire and the other leg of the resistor goes to the positive (red) wire. If you power it up the LED should light up. You can add as many LEDs as you need across the red and black wires in the same way, just add a resistor in series with each one and make sure you have them the right way around.

CAUTION: The transformer has two pins marked '0', make sure you connect the one on the primary (0 - 110V - 220V) side to the switch and cord and the one on the secondary side (12V - 0 - 12V) to the diodes and LEDs. DO NOT link them together!

You might notice a slight flicker in the LEDs, particularly if you move them around. This is quite normal, it's because they are actually flashing on and off. If you want to try an experiment after getting it working, buy a capacitor of 1,000uF / 50V and connect it between the red and black wires. It will be polarized, make sure you connect the capacitor negative side to the black wire and the other side to the red wire. The standard for capacitors is to mark the negative side with a stripe if it doesn't clearly have a minus symbol on it. Be careful not to wire the capacitor the wrong way around as they usually explode if you do (danger and a bad smell!).

Brian.

 

[Leejames Merlas]

Re: Help me pleeeeaaaaseee

Thanks a lot brian. One question: When i wire the switch, can i wire it to any of the two terminals of the switch?

- - - Updated - - -

Thanks a lot brian. One question: When i wire the switch, can i wire it to any of the two terminals of the switch?

 

[betwixt]

Yes, a switch is basically a conductive metal bridge across two contacts. You might think of it as a swing bridge that opens or closes to let vehicles across but in this case carrying electrical currents. It doesn't matter which way around you connect it.

Brian.