# Help, I want to build this circuit.

#### Chris333

Joined Nov 20, 2008
8
Hello.
I am a complete idiot about electronics. I need to build this circuit:

It needs to be driven with 3 volts AC.
I know how to read the diagram and can make it if I had the parts, but I don't know what parts to buy. There are many different 10nF capacitors or instance.

Is there a single place I can order these components and could some one please list what the part numbers would be for me.

#### beenthere

Joined Apr 20, 2004
15,808
All you have to worry about is the voltage rating, which is not stated. It's probably under 50 volts, as that is the max for the 1N4001's.

What is the output voltage and current? - also critical for the selection of the SCR.

Mouser will have all this stuff in its catalog, same for Digi-key.

#### Chris333

Joined Nov 20, 2008
8
Thank you.

The output should be around 3 V DC. It is to power a very tiny model train with a pager motor in it. The diagram was sent to me by the man in this video. He is using the circuit in the second half of the video:

Any clue to where I can get a 3 Volt AC power supply?

#### Chris333

Joined Nov 20, 2008
8
I just searched for R2 (1k resistor) and got 11,186 results at Mouser. How do I know what to buy?

This is my problem...

#### beenthere

Joined Apr 20, 2004
15,808
See if you can find a transformer. Using half the secondary of a 6.3 volt center tapped transformer will work. Any SCR in TO-92 will handle the current.

#### SgtWookie

Joined Jul 17, 2007
22,201
This transformer could be made to work:
http://www.mpja.com/prodinfo.asp?number=7838+TR
It's listed as 12VAC CT (6v-0v-6v), but it has primary windings for both 110 and 220.
If you connected the 220 primary to 110VAC (a normal outlet) then you'd have 6VAC CD secondary (3v-0v-3v)

#### Chris333

Joined Nov 20, 2008
8
How do I read the circuit to find out what type of components will work? For instance when I search for a 1k resistor I get:
How do I know what I need?

Also I found a 10nF cap, but could not find a 1uF cap. Any ideas?

Thanks

#### SgtWookie

Joined Jul 17, 2007
22,201
How do I read the circuit to find out what type of components will work? For instance when I search for a 1k resistor I get:

How do I know what I need?
Carbon composition, carbon film or metal film will work just fine. Of the three, metal film are the most stable, least noisy and most expensive.
Also I found a 10nF cap, but could not find a 1uF cap. Any ideas?
You should be able to find a 1uF cap no problem.
Look in the aluminum electrolytic section.

#### Wendy

Joined Mar 24, 2008
21,840
Radio Shack even has these particular parts.

#### SgtWookie

Joined Jul 17, 2007
22,201
Holy cow! You paid WAY too much for those capacitors!

You really have to watch it buying stuff on E-bay. Do some shopping on places like Digikey.com Mouser.com and Jameco.com - all reputable suppliers.

Sometimes you can find deals on Electronic Goldmine:
http://www.goldmine-elec.com
But they have a $10 minimum order. In their GoldPak section, they have a bag of 50 100nF caps for$2.49. http://www.goldmine-elec-products.com/prodinfo.asp?number=GP59
They're mixed, of course - and not HV like the Wimas you bought on the auction site. But for low voltage stuff, they're just fine.

Digikey is your friend. They will ship small orders via US 1st Class Mail, which will save you a lot in shipping.

The caps you ordered will work fine for bypass caps on IC's. You could use one of them on the output, but you really need more like 1uF to 10uF.
100nF = 0.1uF

#### beenthere

Joined Apr 20, 2004
15,808
Caps as small as that are not going to have a polarity.

#### Wendy

Joined Mar 24, 2008
21,840
Even Radio Shack would have been cheaper in this case, much as hate to say it. I am not really a fan of RS, but they are a standard every one can agree on, in more than one country (I think). They even have the transformer you were needing.

#### Chris333

Joined Nov 20, 2008
8
In the last year or two Radio Shacks around here have pulled all their electronic components off the shelves. And you can't get out of the store without hearing an offer for a cell phone.

#### Chris333

Joined Nov 20, 2008
8
OK I now have this circuit built, but need to wire the power to it.

How do I connect the 110v to this transformer to get 3 volts out of it?

Do I connect both blacks and both reds together?
Or do I connect the red and black in the middle to each other and connect power to the other two wires?

Thanks
Chris

#### beenthere

Joined Apr 20, 2004
15,808
Connect the center black and red together, with 120 VAC onto the other black and red. That puts the primary windings in series. Use the yellow as the ground, and put diodes on the two blues for a full wave rectifier.

With that transformer, that means about 4.3 volts on the filter cap. You will have to use a low drop out regulator to reduce it to 3.

#### Chris333

Joined Nov 20, 2008
8
OK Thanks

I wired the 120VAC as you said and I get 2.9v at the yellow and one blue end.

But when hooked to the circuit I get nothing out the other end.

So can you explain more about the diodes and drop out regulator?

After spending around \$50 on this I'm a bit disappointed it doesn't work : (