How to match a DC power supply to a DC accessory?

Power is Watts and its how much voltage (E) and current (I) the power supply can supply. P=I*E.

I would purchase a supply that is close to the voltage requirement and its power requirement is equal to or greater than the accessory requirement. The current draw will take care of itself. Higher wattage supply would be able to supply more current, but the current draw will be dictated by the accessory resistance. Current equals power divided by voltage.

Hope this helps.

Thanks for the info.

Rich, you gave me a formula, P= I*E.  You said that "I" is the current.  But, you didn't tell me how to find out what that number is so that I can run the formula.   How can I tell what the current number is, when I only see Watts and Volts on the box of the accessory.  I know the current is a DC current, but that is all.  :-)

Thanks for your help.

Mannyrock

For DC, divide volts into watts and you'll get current in amps.  Assuming you have a 30W accessory that takes 12 VDC, it will have a 2.5 amp requirement.

Just divide power by the voltage, that will give you current. For example 24 watts divided by 12 volts results in 2 amps current.

Look for power supplies that have Y volts DC (amazon, ebay perhaps?). 

Then see if it has a wattage greater than x watts - that's the one to buy.

If the power supply you're interested in is rated in volts and amps, Divide the watts you need for the accessory (x watts) and divide it by the volts you need (Y volts).  That will tell you the amps of the power supply.  Get one with and amp rating greater than the amps you need.  

So if you see at power supply rated at 12V  and 36 watts, that the same as 12 volts and 3 amps.

12V x 3 amps = 36 watts

36 watts divided by 12V = 3 amps

ps. exactly what Rich said above, in a slightly different format!

pps.  (I type slower than the guys above!)  

Wow.  Great info from all.

I'm printing them all out and putting them in my notebook.

Eddie has the easiest answer, for dummies like me:  

                                   "Look for power supplies that have Y volts DC (amazon, ebay perhaps?). "

                                    "Then see if it has a wattage greater than x watts - that's the one to buy."

Thanks to all,  Mannyrock

@Mannyrock posted:

...

Assume I have a large accessory, which needs DC power.   And, the box says that the accessory is X watts, and Y volts.

 

Just curious if this is a manufactured accessory with a nicely labeled box...or if it's a one-off custom device that just happened to be in a random cardboard box with someone's hand-scribbled musings on the power requirements?  It's just that in the train world, manufacturers either include, sell, or at minimum recommend compatible power supplies.

Not clear if you're talking 10 Watts, 100 Watts, or more but for fixed-voltage DC adapters, expect to pay between 5-cents and 50-cents per Watt.  Here's a recycled photo from a previous OGR thread - in this case it is a 12V x 2A = 24 Watt adapter so just under 10-cents per Watt.  Many adapters use barrel/coax plug connectors for which there are handy screw-terminal adapters so you don't have to splice into cable. 

Always supply more amps than the item uses normally. (think of trying to start your car with a low amp 12v latern battery vs 12v car battery. Voltage is the same, huge amp difference. But both light the little interior bulbs fine though)

Voltage is a bit more fudgable; depending on the item.  e.g. an LED (alone) won't tolerate overvoltage but works at a lower than labeled voltage. Motors may run too fast, to slow if not exact volts etc..  Bulbs may last longer but be dimmer at low volts or brighter with shorter life at a high volt.  (ie. 12v bulb run at 24v will burn twice as bright for half as long(a guess-timate formula with a large varience, but usable rule of thumb most of the time if you pay attention to the parts and issue at hand..and bulb heat outside normal voltage)

Remember adding a diode or two can dial down voltage if you are close but over and can't be.

I see you are leaning at wallwarts. Fuse the wires off them, some have zero protection inside. Nearly every one in my house is also on a fused power strip after a few meltdowns (including need to repaint a few inches of fire soot) and a yet unexplained spark/ plasma streak looping at the ac plug on a sm. pw phone charger.   I just don't trust the plastic cases anymore)  (the failing ones were not in train service, they were used with the items they came with and not even in use, just plugged in)

Well, since someone asked, . . .. 

Google   "Holo 360 Train Horn Electric."

It is a 12 volt horn, with three train sounds.  Meant for attaching as a novelty item under the hood of your car.  Only about 25 bucks.  Many reviewers say it is just not loud enough for a car, but put it on their rider mowers or golf carts.  So, probably pretty low watts.

I'm thinking of mounting it in the ceiling of my big basement, and blowing it when I run the trains.  If nothing else, it will at least annoy the wife.  :-)

Unfortunately, there are no specs  given on the Amazon page for this horn, other than 12 volts.

(I warned you guys about my McGiver solutions.)

Thanks,

Mannyrock

Ooops.  I misspelled it.  It is the Wolo 360 train horn.

Anytime you have two of three factors, amps, watts, voltage; you can figure the missing number using the other two numbers and the relationship formula. The formula breakdown never changes and can be found in some good  dictionaries to Wikipedia if you don't want to memorize it or want to double check yourself.

The three amp fuse shown is another tell tale. It certainly doesn't use all 3a or the fuse would burn. If not listed at 1a, I would assume 2.5a or more like likely 2a was needed.  (it may have a peak draw slightly over 1a... or 3a fuses might have been a better mfg deal than 2a). I fuse a little closer than most folk, but that low an amp rate isn't much to worry about most of time anyhow, after rtping....the fuse at 3a is likely more for the wire itself in case it shorts than the boards.)

OK, now we're getting somewhere.

We got  a Wolo horn with 12 volts DC, 1 amp, and 12 watts.   (Plus we'll need a 1.5 amp fuse or so.)

Are there any  model railroad power packs that would fit the bill?   Or is 12 volts just off the scale for DC railroad stuff?

Thanks,

Mannyrock

Well Heck Fire Uncle Jed, I jest found me one on Ebay, for $6.49!

Looks like a phone charger.

Specs say:   It is 12 volt and 12 watts output.

Description:

Output Voltage: DC 12V +- 5%  

Input Voltage:AC100- 240 V, 50/60Hz 

Input Current:<0.3 A

AC inrush current:<20 A

Leakage current:<1 mA

Over load protection:>2.5 A

Over-voltage protection:15V

Start:100VAC<3S

Hold Time:240VAC:>20mS

Withstand voltage:3KVac 5mA 1MIN

Working Temperature:0degree  to + 40degree

Humidity:20% to 90%RH

Safety standards:GB4943-2001, IEC60950

EMC:En55022

Efficiency:>= 80%

Power:12W

Size: 2.84*1.66*1.19 inch (L*H*W),Plug cable: 110cm

Made in China.

Mannyrock.

You'll have many options on eBay, Amazon, etc.  Here's a 12V 2-Amp version WITH the screw-terminal adapter for even less...and it's a U.S. seller.  Same seller has a 1-Amp version for less...also with the screw-terminal adapter as a "free" gift!

Thanks Stan,

But, what will a screw-terminal adapter do for me?

I was thinking I would have to cut the wire, and hopefully find two DC wires to work with, and then do some soldering.

Thx,

Mannyrock

The. Screw terminal adapter allows you to not cut the DC output plug off of the power supply. Use the adapter and connect the two accessory wires to it, no soldering required.