Converting AC track power to 5 VDC

Maybe this?

http://www.ebay.com/itm/2-PCS-DC-BUCK-STEP-DOWN-7-24-VDC-TO-USB-5-VDC-3-AMPS-KIS3R33S-MODULE-/251275155513?pt=LH_DefaultDomain_0&hash=item3a812a9839

You would have to convert the track power to DC.

You get two so the first one is the guinea pig...

Did you use a heat sink on the regulator?

pa-- No, I didn't use a heat sink.  I'll add one and test again.

banjoflyer -- Thanks for the suggestion.  The board looks small enough to work.

If the power draw is 1 amp you will need a heat sink. Most likely the chip is getting too hot and shutting off. There are regulators in the TO92 package which can handle more than the 1 amp 7805. Fairchild MC78T05CT for example is a 3 amp one 5 volt fixed. LM350T is an adjustable one which can handle 3 amps. I keep these in my parts drawer to avoid stocking a lot of different ones. Circuit is similar except you need 2 resistors.  .

Dale H

We use some that are similar to this to light up LED in buildings. They handle five amps which is a lot of LEDs. You pretty much can't build one like this at this price.

http://www.ebay.com/itm/5-Amps...;hash=item5d334d0177

 Here's the basic schematic. The photo wouldn't display properly.

Pa.   I attached three alligator clips to the top of the 7805 regulator and tested again.  It ran longer (about 45 sec) but still failed.  I'm obviously driving the regulator too hard.  The alligator clips were very hot.

Thanks for everyone's input.  I got some good ideas and have ordered a higher rated power supply.  We'll see if that solves the problem.

A clip is not a very good heat sink. An aluminum strip or get one

from Radio Shack will be better

If using CC with 18v on the track the regulator is going from 25vdc

input to 5vdc output with a drop of 20v at 1amp is 20watts, that is a

lot of heat.

Reducing the capacitor size at the output of the bridge rectifier will reduce

the input voltage to the regulator which may help somewhat.

Lionel made a flat car with an LCD display that used a built in rechargeable battery.

The display worked for a long time without recharging the battery.

One amp at 5volts seems high for an LCD display. What is the size of the display

or is it an LCD TV?

The display Lionel used was 3.5 inch and cost about $35 at Walmart.

The directions that came with the car were about the worst that Lionel

ever wrote with many features that did not work and incorrect instructions.

Pa.  The display is 7 inches diagonally.  I used the alligator clips as a test to determine if the present problem is heat related; which it is.

I'm presently using a 470 uf capacitor on the output of the bridge rectifier.  What size electrolytic capacitor do you recommend?

The 470 cap is OK. What the actual current draw is hard to

tell. The 1 amp on the display is the same as the rating

on the plug in power souse but since the display has internal

batteries for power what current is going to charge the battery

and what current is for the display is hard to tell.

The problem is that you are going from a track voltage of 18vac

to 5vdc with a single regulator. If you use a 12v regulator 

feeding the 5v regulator first and heat sink both regulators you

still have the same heating effect but less stress on the 5v regulator.

Try powering the display from the plugin power source until the

battery is charged ( over night) then connect to track

power, use the 2 voltage regulators and heat sink both, it may

work.

The best power source would be one that will go from 18vac to 5vdc

and is what is called a switching power source, not a linear power

source, too inefficient.  

eBay has many nice little switchers that will take the 18V and give you 1A of 5V, they only cost a few bucks.

Banjoflyer and all others that offered suggestions for a power supply that will work off AC track voltage.  The USB 5 vdc power supply you referenced works very well.  It provides 5 volts to the photo frame and doesn't even get warm to the touch.  It's small enough that it may mount under the flatcar.

Thank you for your quick responses and great recommendations.

A question,

Where did you put the bridge rectifier and capacitors?  I'm wondering how small a capacitor you could get away with.

Pictures would be great!

I'm thinking about doing the same thing with a GPS.  The older Garmin I have does a slideshow from pictures on an SD card.  About 3.5".  Has a battery, but it doesn't hold a charge like it used to.  A long time back I posted some pictures of it on a building, where it's a lot easier to power.  Actually I have two of them and was thinking about mounting them back-to-back.

I've built some billboard cars too.  Watch out for the balance on the turns.  I added car tire weights under the flatcar to lower the center of gravity.

Ed

You can use a fairly small capacitor on most switching supplies.  Try something like a 470 uf cap, I suspect that will do the trick just fine.  If they had actually not hidden the make/model of that switching regulator, you could look up the spec sheet and know exactly what the requirements were.

For something fairly low power, you can just use a 5V three terminal regulator after the bridge and cap, and it's a lot cheaper.

You are welcome!

Mark

I too can use one of these AC-DC 5V regulators, but have a question. When they say it's rated at 1.2amps (for example) in the listing, is that for input or output? I'm hoping output because my feeder lines are carrying more than 1 amp.

If it is a linear regulator then input current is the

same as output current. If it is a switching regulator

power supply then the input current will change

with input voltage, the higher the input voltage

the lower the input current.

If it is rated for 1.2 amps, that is most likely the output current.

Take the output ratings with a grain of salt.  Many of them just parrot the specifications of the switching regulator they use, but they don't provide proper heatsinking to attain the rated output current.  You need to look closely at the module to see if it will actually supply the rated current as provided.

How does this one sound?

http://www.ebay.com/itm/281079...id=p3984.m1423.l2649

That one actually has a decent heatsink on the LM317.  Note that this is a linear supply, so the input current is the same as the output current.  If you're trying to get 5V from track power, you may be limited as to current, simply because you'll exceed the power dissipation of the LM317.

A trick you can use is insert a diode in series with the input, this cuts the input voltage in half, and since this supply accept AC or DC, it would also drop the power dissipation significantly.  A 3A diode is what I'd use.

Not sure I'm following that......the listing said there was a minimum voltage pressure (input to output) of 3V. ??

I'm up to about a 6 amp load on the lighting circuit of one of my KWs. Not sure I want to put a 3 amp load on this regulator. Maybe I'll be better off hooking the accessory this is for to my Miller Engineering terminal strip as was my original plan. 

The voltage pressure is a strange term. It means that

there must be a minimum voltage drop across the regulator of 3V.

If you are going to have a large voltage from input to output, it is better to

use a switching regulator.That regulator have have a large heat sink but

going from a track voltage of 18v to 5vdc at 1amp is still a power loss 

of 20 watts and all into wasted heat.

That listing for the 3amp adjustable regulator also has some

adjustable switching regulator boards for $1.20 each. You get 1Amp out but 

the input current will be under .2 amps.

There are 5v 1 amp cell phone chargers that take in 110 vac all in

a cube 1" on a side.

The only problem with a switcher is that some of

them make static on the radio.

For low current, that's perfect. Was looking at using that one to power my bluetooth TMCC receivers.

That should be plenty to power BT receivers.

Here's one that should work for many train applications.  You just need to add a rectifier.  There are lots more similar ones.

http://www.ebay.com/itm/Step-D...;hash=item2c5e1af420

They're saying that listing has been removed, John.

I just clicked on the link, that's not what they're saying to me.

Interesting.....I copied and pasted off the email version. Got that message. If I click it on the thread post, it works. Strange.  What kind of rectifier would this need?

Given that the max input voltage is 25 volts, I'd use a simple 3A diode and perhaps a 35V 470uf cap for some input filtering.

John,

I just found this in my parts box.....full wave.....Radio Shack 

http://www.radioshack.com/prod...sp?productId=2062581

Will that work? And will I need the cap for that one if it's all going to be used to connect to a micro computer fan to mod my Lionel Smokestack?

You'll probably peak at more than 25 volts with a full wave rectifier on 18 volts AC, so I'd stick with the single diode.

Truthfully, for a simple fan, I'd just use an LM340T-5 (LM7805T) three terminal 5V regulator, it's very low current and that's all that would be required.  You should be able to buy it for less than $1.  A diode, cap, and the regulator are all the parts needed.  A switching power supply for a small fan is overkill!

I also found a couple of 6amp diodes in the box. Is that overkill or will that be ok as well?

You only need one, but it will certainly work.  It'll have a lot of "reserve" capacity.

Ok, found one......

http://www.radioshack.com/prod...sp?productId=2062599

So, what is the order of connection.....AC lines to diode (I'm assuming I can use my 6amp diode) to cap to VR? Not sure about the 3 connectors on the VR.

John,

Since the diode and cap are polarized, do they go on the output side of the VR?

I realized after I typed the last entry, that that's probably incorrect. The diode is a rectifier, so I'm thinking AC to diode to cap to VR.   But those 3 terminals....something's not adding up as there are two AC lines going in (I assume one of the lines will connect in series to the diode then cap then to VR). 

The polarized cap is usually placed across the + and - of the bridge rectifier in proper polarity. This makes filtered DC from AC. A small NP one is sometimes placed on the regulated output. There are many ways to use a regulator.

Here are some circuits from the web

www.google.com/search?q=7805+c...biw=1695&bih=847

Dale H

And will I need a heat sink?

Dale,

Those circuit diagrams were very good.....explained the 3 terminals. I see now that one is a ground and both the AC input and DC output connect there.  

Is my thinking on this correct then.....AC line to diode (striped end) to VR, other AC line to ground on VR. Cap will connect to the diode and also to the ground on the VR. The cap is polarized, so will the neg side of it go to the ground on the VR?   Thanks