Chris's noobie electronics questions

ChrisQ · Post by **ChrisQ** » Mon Mar 07, 2011 8:30 pm

I am finally beginning my adventures into lighting and I will use this thread to ask the questions that most kids learned in 7th grade shop class.

First up:

I intend to mod an ATX PSU (another one) so that I can use it to hook up the models for testing. This is because I will be using yet another 12v ATX PSU to power all the models when they are displayed. The various lighting kits call for 6 or 9 volts so I want to make sure I don't burn them up.

I believe that I have learned that the proper resistors will keep the LED's from frying. If so, it seems that when I wire up the first kit (a Deluxe 8) I should use a resistor calculator and replace the resistors if necessary. Is that correct? Do I need to consider anything else before wiring this up to 12v?

ChrisQ · Post by **ChrisQ** » Tue Mar 08, 2011 7:35 pm

A little more reading in the Madman Lighting Kits thread confirms that that kit will run on 12v right out of the box.

Still would like to have an idea of how to ensure that every model I build will handle 12v even if it was built with a 6v power supply in mind.

Ziz · Post by **Ziz** » Tue Mar 08, 2011 8:44 pm

So you want to be sure a 12v power supply can hook up to a model that was designed for 6v or 3v? That's easy.

Build an adapter that goes in between your power supply and the model that will cut the power down from 12v to 6v or 3v or whatever you need coming out the other side. I'm not sure of the math but you should be able to do it with a couple of resistors, maybe even make it small enough to fit inline with a fuse holder or something.

ChrisQ · Post by **ChrisQ** » Thu Mar 31, 2011 8:21 pm

After having spent the last few weeks learning more about electronics, I have discovered that I can wire up the 12v to positive and the 3.3v to the negative (ground) and this will give me a current of 8.7v, which is good 'nuff.

And I was already buying parts for my voltage regulator

Edit: NM it won't work for my purposes. The power supply will sense that something is wrong and shut down.

Back to the LVR...

Scott Hasty · Post by **Scott Hasty** » Fri Apr 01, 2011 7:14 am

I don't want to sound offensive, but why go to all the trouble (and potentially destroy something) when you can buy this?

Maybe I'm being an old curmudgeon, but rather than spend hours trying to figure something out, I can spend minutes ordering something that works out of the box.

Now, get off my lawn...

ChrisQ · Post by **ChrisQ** » Fri Apr 01, 2011 7:24 am

Hah I already have 12v coming out of my ATX power supply. I am trying to step it down to 9v. If I could find a 9v 1.5a wall wart I'd be in business!

And I am learning too. It's slowing down my modeling but I am having fun.

As an aside, the ATX power supplies also have a 5v rail that can evidently put out as much as 5.75 volts. It has been suggested that I just use that rail to power everything. I am just not sure that the blinker boards will still work properly and the LEDs will still be bright enough.

I'd appreciate any input on feeding these kits 5v instead of 6 to 9. The PSU is rated for 25A on the 5v line and I would not be using more than 1A.

Scott Hasty · Post by **Scott Hasty** » Fri Apr 01, 2011 8:58 am

Like this?

ChrisQ · Post by **ChrisQ** » Fri Apr 01, 2011 10:44 am

Wow I could have used your help in this thread.

Better late then never though, and a much cleaner solution. Thanks!

tetsujin · Post by **tetsujin** » Fri Apr 01, 2011 12:46 pm

ChrisQ wrote:After having spent the last few weeks learning more about electronics, I have discovered that I can wire up the 12v to positive and the 3.3v to the negative (ground) and this will give me a current of 8.7v, which is good 'nuff.

8.7 volts is not current... Not meaning to nitpick exactly but to me a good understanding of the basics is essential...

Anyway: to step down one voltage to another you pretty much need some kind of regulator circuit.

It is possible to divide voltage with resistors: the problem is that the voltage across those resistors varies directly with the current going through them. So if you had this:

12V source connected to nodes A and zero
5K Ohm resistor connected to nodes B and zero
7K Ohm resistor connected to nodes A and B

The voltage at node B with respect to zero would be 5V.

However, if you tried to use that 5V, the additional current draw through the 7K Ohm resistor would cause its voltage drop to increase: and thus the voltage across the 5K Ohm resistor would decrease. It's not a stable 5V, basically, it varies according to how much power you're drawing. Stabilizing that power output can be very important if you're running power to an IC of some kind. (Don't know about the Madman board: since it is, itself, a current regulator, the stability of its voltage input may not be as important...)

ChrisQ · Post by **ChrisQ** » Fri Apr 01, 2011 2:47 pm

Thanks for the terminology tip. I'm picking this stuff up as I go.

I already have the LM317 to build the regulator circuit and was about to go buy parts to build it until Scott posted a link for a 9v wall wart that will supply 1.5v.

That appears to be enough to run multiple lighting kits with no problem.

ChrisQ · Post by **ChrisQ** » Thu Apr 14, 2011 10:03 pm

Update: For my bench supply I did modify the power supply and built in the VR circuit. I made a box for the connectors that will look a bit nicer on the bench and ran the power switch next to the edge of the bench so I don't have to reach around the models. I will still just use a 1.5 or 2amp wall wart for the display but for building and testing purposes this will do just fine:

http://img.photobucket.com/albums/v260/ ... CI0049.jpg

Scott Hasty · Post by **Scott Hasty** » Fri Apr 15, 2011 7:20 am

Nice and clean, very nice!

Tchail · Post by **Tchail** » Fri Apr 15, 2011 11:47 am

Sparkfun has a DC to DC Converter breakout for about $22 that will take a DC 4.5 to 14V input voltage and reduce it to any voltage between 0.6 and 5.5V DC.

http://www.sparkfun.com/products/9370

Maybe this will help.

-Tchail

ChrisQ · Post by **ChrisQ** » Sat Apr 16, 2011 3:31 pm

Thanks, Tchail, I appreciate the effort. That product wouldn't help me regulate my 12v power supply to between 6 and 9 volts though. The VR circuit I built does a great job and was really simple to do as well.