# transformer VA needed?



## Don Trinko (Nov 9, 2012)

I am putting a small (7x11) layout outside in a shed. i am wondering what the typical locomotive requires in current draw. I know it varies but at the moment I have not yet purchased the locomotive.
The transformers i have seen vary from 5 VA to 60 VA. ( yes I know this is not current)
My present transformer is rated at 1 amp. It handles every thing i have at the moment but I do plan to buy a fancier locomotive in the future. Any specific suggestions for a reasonably priced transformer, possibly 2 train transformer? Thanks; Don


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## Greg Elmassian (Jan 3, 2008)

I bought a 60 VA transformer and it could not even put out 2 amps at max voltage. 

VA is a very misleading rating... you would think that 60 is the rating, and if the supply went to 30 volts, then you would get 2 amps... NOT TRUE... 

60 VA means that at some arbitrary combination of volts and amps, the product is 60... so you really do not have any guarantee of amps at a selected voltage and vice versa. 

There's very few 5 or 6 amps supplies there which is what you are basically asking, need 5 or 6 amps at 24 volts to be safe (some G scale actually wants 27 volts to get a prototype speed or operation) 

I fall back on my standard recommendation, buy the MRC Power G, 10 amps at over 24 volts. Search the web and you will find it for a reasonable price. 

(figure worst case 2.5 to 3 amps per loco, and don't forget lighted cars, that is often overlooked) 

Greg


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## Don Trinko (Nov 9, 2012)

Thanks; as a retired tech I was aware of the "VA" rating. I would rather see an amp or watt ratting. What voltage does ot take for a reasonable speed? ( I have a 12 volt/100 amp hour battery) I'm guessing 12 volts (actual 13 to 14 with my charger on) is not enough. Thanks; Don


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## Greg Elmassian (Jan 3, 2008)

Voltage varies a lot, due to different motors and different gear ratios. Some locomotives are going way over 100 at 20 volts some others barely hit 65. 

Like I said above... 5 to 6 amps, and 24 volts, unless you are never going over 35 scale miles per hour on NG locos... 

Greg


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## Dan Pierce (Jan 2, 2008)

Some manufacturers use a 12 volt motor and these are usually found in slow running locos. Accucraft did this in the 90's. 

Now, locos have decoders or other electronics and only reach reasonable speeds if you have 18 volts. 

And then there is the fast passenger trains that need 24 volts or more. 

As Greg put it, go big, it will be there when you need it!!


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## ezaircon4jc (Mar 31, 2012)

Can I upgrade the transformer later and use the same throttle controller? 


I hope so as I purchased this today with the idea of buying a higher power transformer later.


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## TheRoundHouseRnR (Jul 15, 2012)

No, the variable throttle needs to be able to cope with the higher amp draw of a bigger transformer. It needs bigger heat sinks and upgraded electronics. Im afraid there will be know upgrading using this throttle. The Roundhouse RnR


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## Greg Elmassian (Jan 3, 2008)

You can use any transformer you want, but the ouput is limited by the capability of the "controller"... so if you buy a 5 amp controller, it's maximum output will be 5 amps, no matter what the input supply is capable of. 

Be aware that a train only draws what it "wants", it is not "forced" to use all the amperage that is available. 

So if you use a 5 amp transformer and a 5 amp controller, and run a small loco, maybe it will only draw 2 amps or less. 

Also, if you buy a 10 amp transformer and a 5 amp controller the controller will only be capable of 5 amps. You could put two 5 amp controllers on the 10 amp supply and run 2 separate tracks. 

Greg


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## ralphbrades (Jan 3, 2008)

G1MRA specifies 24 Volts as its running "line voltage". Given this, it is quite common in the UK and parts of the EU (when no-one is looking!) to use AT and ATX computer PSUs in series. These are then coupled to off the shelf "panels" from people like Gauge Master or PCBs from people like MERG. However one of the common problems is that despite the high current rating of the PSUs used they still blow up... An old friend of mine is currently(!) using twin 750Watt ATX server PSUs on his G1 layout, he has yet to blow one up but should he do so he will simply get another one out of the server farm... 

The VA rating of a transformer should be used as a guide and not a be all and end all. I have had to design some very strange PSUs in my time and as guide I have always used the rough rule that my Amperes multiplied by my constant Voltage multiplied by 1.76 (the cube root of 3) gives me the minimum VA rating for an E+I laminated transformer. Yes I know this is a 3-phase rule but if you apply it to single phase systems then you would end up with a slightly too big transformer that runs a lot cooler... 

I used this formula to size the transformer that I required for my Amplifier -it worked out at 700 Volts at 201mA. It is a Hammond 373BX and the PSU is rated at *63.99 JOULES...* This is a Class "A" amplifier thus the load never varies. 

regards 

ralph


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## Greg Elmassian (Jan 3, 2008)

One thing that may not be apparent to a newcomer in this thread, is that there are differences between power supplies that are merely the rectified and filtered output of an AC transformer, and power supplies that have actual circuitry to maintain a constant output voltage. 

In the example "Ralph" gives, the "ATX" power supplies are very closely regulated switching power supplies. The output voltage will be constant regardless of the load unless there is an overload. 

Sorry to add another wrinkle, but not all are the same "type". 

So, my recommendation is usually a meanwell 24 volt power supply (regulated power supply, switcher) and a controller... if you are NOT using any DCC decoders. 
If you are using locos that have DCC decoders (running in DC mode) I recommend the unregulated MRC Power G, which has a power supply and controller in one box, and has a 10 amp capability. 

Greg


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## TheRoundHouseRnR (Jul 15, 2012)

Im sorry greg , i was not as specific as i should have been. I just meant a 1 amp variable throttle will only take the load of one amp. You can use a bigger transformer with the 1 amp throttle but it will still only put out 1 amp. 
The Roundhouse RnR


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