I recently did a post regarding lighting up my layout and now have another question. As a result of input I received, I’ve broken up my layout into 6 districts, and plan to use a single power pack to light up each district…thus, a total of 6 power packs. Now I have another consideration. My layout room only has 12amps available for running the trains and lighting up the layout. In general, even though I have the track designed for 5 operators to be running trains, usually, I’ll be running a freight and passenger train on the mainlines, with the freight at slow speed and the passenger train, with 4 cars lit up, at a higher rate of speed. Guys dropping off cars and making up trains in the yard will obviously be running at very low speed, thus drawing very little amperage.
Based on a book I have about model railroad electricity (written by Andy Sparandeo), it looks like my passenger train will draw about 1.8 amps between the engine, its headlight and 4 lit up passenger cars. The freight train, running at prototypical speed, will probably draw less than 1.5 amp. So, I’m going to estimate that my 5 train power packs during a typical operating session in total will draw 5 amps of electricity or less.
Now for the lighting. Based on input I received before, I’m going to hook up my 6 power packs, with each one feeding power to a district on my layout with around 20-25 lights per district. I was told to use the DC output of each power pack so I could “dial the lighting down”, if necessary, to reduce brightness. I just installed one power pack for my first district. The power pack says it has 12VA output. My understanding is that in order to calculate the actual amps it draws, you divide the 12VA by 12…resulting in 1 amp. That’s the formula Sparandeo’s book gave me.
Until you connect a load to the pack there is no current draw. Setting the throttle at MAX does not create a load. The train it is running or the lamps it is powering are the load. The 12VA available is likely for the entire pack, AC and DC. If you are not using the AC terminals you have %80 of the 12VA available on the DC terminals for your lamps.
You have to add up your individual current ratings for EACH bulb, that will give you your load. Remember not to go over %80 of max load for a pack.
The part you are missing is that when you step down the voltage, you increase the current. Think about a computer power supply - they can put out 40 ore more amps at 12 volts - obviously they don’t draw 40 amps from the wall socket yor you’d need a dedicate circuit for a single basic computer.
It’s not 1 for 1, ie reduce the voltage by 10 (from 120 to 12) does not change the amps by a factor of 10, there are internal losses in the transformer that eat some of that up. But a transformer producing 1 amp at 12 volts will not draw anywhere near 1 amp from the 120 AC input side. Probably even less than .2 of an amp - so you need not fear plugging in 5 or 6 1 amp power supplies with a single 15 amp household circuit.
Now here’s a couple of questions regarding Jsperan’s input:
You said that …"You have to add up your individual current ratings for EACH bulb, that will give you your load. Remember not to go over %80 of max load for a pack. "
Unfortunately, I cannot add up the current ratings for each bulb. For example, the Walther’s lights that I bought (lamposts and gree-shaded loading dock lights don’t even mention on their website or the box they came in what the current rating is. Many of my other bulbs, like the Life-Like ones to light up the inside of a building, only say that they are 12 or 16 volts.
You say I should not go over 80% of the max load for the pack. Without knowing the load and only knowing the pack is 12VA, I’m not sure where I go from here.
I can tell you that when I hooked up the 25 lights I have connected to district 1 on my layout to the DC output from the pack, with the throttle set to maximum, the lights seemed just about right…not too bright and not too dim. When I turned down the throttle a little, the lights were okay but honestly, they looked best at full throttle. If I’m overloading the pack, will the internal breaker kick in?
By the way, all my lights are hooked up in “parallel”.
You said “But a transformer producing 1 amp at 12 volts will not draw anywhere near 1 amp from the 120 AC input side. Probably even less than .2 of an amp - so you need not fear plugging in 5 or 6 1 amp power supplies with a single 15 amp household circuit”.
Getting back to what I said above, I’m not talking about only five or six 1-amp power supplies. I’m talking about 5 power packs that are hooked up to my Aristo-Craft walk-around throttles for running trains and another 6 for lighting up the 6 “districts” on my layout. Why is it that you’re saying that each of the lighting power packs will only draw about .2 amps? Is it really possible that I’m powering those 25 lights in my first district with only .2 amps?
Do you agree with what Andy Sporandeo says in his book, when he’s discussing what size power pack to buy, that a passenger train, with engine, its headlight and its lit-up cars could pull 1.8 amps?
I’d appreciate you commenting on what you would guess my 5 trains might be pulling for amps, again realizing that during an operating session, trains are not barreling around at full throttle.
I had a “duh” moment. I took out my volt meter and turned the dial to where it said “DC/ma”. When I touched the red/black testers to the wires going to my district 1 lights, it read 0.10…Does anybody know what that means? Does it mean 100 milliamps which would actually be 1/10th of an amp? Or, should I be setting the dial on my meter to something else, like “DC10A” to get a reading?
If you connected your meter in amp mode to a voltage source, it may mean you fried your meter like I did, so the .1 is meaningless. Better check the meters other functions.
The current draw of your power packs is going to be enough to not matter, in the scheme of things. Assuming 5 trains, 2A each, which is probably high by at least a factor of 2, at 15 Volts (also a high number), they are using about 30VA total. The 12A you have available on your circuit, can provide 1440 VA. At the very most, your trains will be drawing about 2% of your available power.
(OK, it could be a little more to allow for some inefficiencies, but it is barely a blip on the radar.)
Going a bit farther, 5 12VA supplies would be outputting 60VA, they will be drawing more than that, but even if it is double it’s only 10% of your available power.
To read current with any meter (not counting clamp on ones), it needs to be connected in series to the part of the circuit being tested. In other words, you need to disconnect one wire from your power pack, connect one lead from the meter to the power pack terminal and the other lead to the wire just disconnected. Then energize the circuit (turn it on). ALWAYS start with the highest amp setting on the meter and scale down for a reading to protect your meter.
The current draw of a power pack should be listed on its ratings. Look for the “Input” ratings. For a simple transformer, the formula is volts times amps in the primary equals volts times amps in the secondary. Of course this is not an exact reading since transformers are not 100% efficient.
Yes, unfortunately many manufacturers do not provide the information they should and sometimes we have to guess. If you were to assume that your 12-16 Volt bulbs draw 50ma .050 X 25 = 1.25 Amps
If your pack had a 1 Amp load capacity that would have been overloading it, so likely the bulbs are more in the 30ma range, .030 X 25 = .75 Amps. When you added 7 more bulbs, .030 X 32 =.96 Amps this load could very well cause a pack rated for 1 Amp to overload.
Unfortunately, if you do not know the loads of the devices you are connecting the only way to find out the load is to connect them with an Ammeter in series, as Tom described. I’m not sure if there is a clamp-on ammeter for inductive load testing for loads this small. I have an Amprobe clamp-on meter but it does not range that low.
If the pack you are using has overload protection, (most do I think) it will kick out if you overload it. Beware though, that the “inexpensive” packs may be a one time affair. Overload it once and it’s done for good. I think most MRR power packs have some sort of auto resetting overload protection but some may not.
I hope some of this is starting to make sense. It’s not the easiest part of the hobby to get a good grasp on. Keep asking questions and we’ll keep trying to help you out.
Thanks again to everybody who responded to my questions!
Jeff,
You said:
“The current draw of your power packs is going to be enough to not matter, in the scheme of things. Assuming 5 trains, 2A each, which is probably high by at least a factor of 2, at 15 Volts (also a high number), they are using about 30VA total. The 12A you have available on your circuit, can provide 1440 VA. At the very most, your trains will be drawing about 2% of your available power.”
How did you calculate the 30VA total for the 5 trains? I don’t understand the relationship between the 2A for each train, at 15 volts, resulting in 30VA total. It looks like you’re multiplying the 2A times 15 volts, getting 30VA, but isn’t that for one train…not all five? If each train is 2A, don’t you need to multiply that by 5 for the 5 trains? I guess I need you to spell it out with an equation.
I thought that if a power pack says “12VA”, that it meant it would put out 1 amp…because Andy Sporandeo’s book says to determine the amp capacity of a power pack, you divide the 12VA by 12, resulting in 1 AMP. And when he says that a train draws 1.5 amps between the engine and headlight, then I thought I needed a power pack rated for at least 18VA’s because 18VA divided by 12 = 1.5 amps. Based on what you’re telling me, I can see where either Andy is wrong or I misunderstood what he is saying. Actually, I think he forgot a step in between the 2 calculations. I think you can see where my confusion is. Andy has a graphic where he says a passenger train with lighting will require 1.8AMPS. It really is disappointing to think that in his chapter explaining “Choosing a Power Pack”, he shows you how to calculate a power pack’s amp capacity (the VA’s divided by 12) and then tells you how much a train draws, but
I think the biggest flaw in the calculation you’ve been doing is that a train will draw 1.8A. That’s a lot. A modern loco draws well under .5A. Looking at the Feb. MRR review of the Bachmann DDA40X, which has two motors, the most it can draw when stalled is 1A. The BLI SW7 draws 0.18A when stalled! (Says something about the difference between B’mann and BLI, perhaps.) Remember, if the train is moving, the motor isn’t stalled, and will be drawing significantly less current. As far as car lighting goes, if it is newer cars with LED lighting the current will be very small. Any lighting isn’t going to draw a lot, or you’d have a melted shell on your hand.
The importatnt thing to remember in your musings is that 1A of 12V is the same power as 0.1A of 12V. So even with the inefficiencies in the power pack taken into account, the power to run your trains is barely a concern with respect to your house wiring’s ability to supply it.
Layout lighting can be another issue, if you don’t watch it.
I did what you suggested and hooked up my meter in series to the power pack on the DC circuit with the 25 lights. I got a reading of 1.3amps. Now what I don’t understand is that my power pack says it is rated at max. DC 12VA. Again, if I divide the 12VA by 12, like Andy Sporandeo suggested, I would get 1amp. If my meter reads 1.3 amp draw on the circuit, how come the pack didn’t get overloaded if it can only produce a max. of 1 amp?
In parallel circuits (like house wiring or train track) the current is additive. If you have 20 lamps each drawing 50ma then the total current of all the lamps would be be 1amp (as pointed out above by Jeff). The same is true of locomotive is they are all running on the same track.
The current is equal to the voltage divided by the resistance. If you had a 12 volt source and a lamp with a 12 ohm resistance the current draw would be 1 amp. If you reduced the voltage to 6 volts the current would be 1/2 amp. So dropping the voltage drops the current flow if the resistance stays the same.
While LEDs tend to use less power, many of the common types draw 30 to 40 ma, so one has to pay attention to them as well for calculating power needs.
Maybe I can help without confusing the issues any more.
First, drawing 1.3 amps from a power pack rated for only 1 amp. Electrical and electronic ratings usually are based on the sustained heat load, and are usually on the conservative side. Your 12VA power pack has diodes and a transformer sized to provide 1 amp at 12 volts without overheating. Can they do more, especially for a few minutes? Almost always yes, unless the circuit breaker prevents it. That’s probably the scariest part (but not all that unusual) - that your circuit breaker is not kicking out at 1.3 amps. The circuit breaker is not protecting the rest of the power pack from overheating the way that it should. The case of the power pack is probably getting warm supplying 1.3 amps. Another confirmation of the power pack struggling to provide above its rating is that the lights got dimmer as you added more - an indication that the voltage is dropping noticeably under full load - which is a natural characteristic of a highly loaded transformer.
Electrical and electronic components have very high variability between individual components of the same model and rating. If you want less variability, you pay a lot more per component. Standard resistors vary in actual resistance by +/- 20% from their rated value. Transistor gains have similar variability. This variability is why
In a word, YES. Again, you are still confusing the current draw on the low voltage size with the current draw on your AC wall outlets. Yes, a given train as in you exampel could draw 1.8 amps, depending on the make and how many lights are in each lightes cars - many model models draw a half amp or less, even with sound. The old rule of thumbd for HO was 1 amp per locomotive. But that train drawing 1.8 amps at 12 volts on one of tyour Aristo power supplies IS NOT drawing 1.8 amps from your wall outlet. Without testing, my guess would be about .3 of an amp, allowing for power supply inefficiency plus overhead of the controller. Voltage and current are inversely proportional
I think I’m finally getting this issue understood. I hope you realize my confusion was caused by what I reported reading in Sporandeo’s book.
Now that I’m convinced that my 5 power packs running trains, along with 6 or more providing power to my lights, will in no way come close to using the 12amps left in my train room’s house circuit, I will concentrate on getting the right amount of lights on each of the power packs, based on the power pack’s individual ratings. Some are 12VA and others are 17VA. I’m going to break up my first “district” to include only 20 lights and then see what I get for a reading on my meter, regarding ma draw. I had my meter set to “DC10A”, when I got the reading of 1.3amps. Per the instruction manual, this setting on the meter allows me to determine anywhere from 0 to 10amps…I happened to get 1.3amps.
Thanks again for you guys taking the time to respond to my questions.
At least now, I can rest assured that I have enough power in the room.
If you’re talking about basic train set type power packs, you need to read the wording on the rating. VA is “volt-amps” or basically watts. If only one numebr is given, it is the total you can draw from botht he variable DC and the AC terminals combined. 12VA would be about 1 amp at 12 volts, although if the AC terminals are a more common 16V then it’s more like 3/4 amp max.
Now you see why a train set power pack works fine with the single loco and 3 cars that come with the set but fall flat with double headed trains with 4 or 5 lighted cars. They just don’t put out that much power.
I’ve been looking at the prices of second-hand MRC power packs on Ebay and it looks like I’m going to spend quite a bit of money, if I buy 6 power packs to run the lighting on my layout.
I’m thinking of buying the “Pyramid” 5-amp power supply in this link:
For $29.99, with free shipping, this power supply could probably run the same number of lights as 5 MRC power packs, with each rated at 12VA.
And I guess if I want to absolutely sure I can run 100-200 lights, I could buy this “Pyramid” 10-AMP power supply…anybody have any opinions on this one?
In one of my previous postings, “Fred” suggested that he’s not a fan of 5amp power supplies, unless 18 gauge wire or bigger is used for the “bus” feeding all the lights. Is there anything else I should be concerned about if I go to this larger single power supply?