Transformer puts out wrong volts

There could be an explanation or two as to why the voltage differances; 1) are you using a digital(LCD display) or analog meter(old swing needle type)? 2) the transformer has a bad circuit inside. Also are you checking for A.C. volts or D.C. volts with your meter?

Lee F.

Power a small 3-4 watt light bulb with the wall wart and check the voltage while it's under load.  That is quite a small wart, and the 16.5 v rating is at the stated 20VA load/output.

I agree. When I was slot car racing I was testing different transformers and got weird readings unless there was a load. Try adding something then check.

Rob

The specs say NON-REGULATED That's your answer. Test under load.

We have a huge HO - 1/64 slot collection and raceway.  If you use wall-warts for power(a lot of large tracks use deep cycle batteries), having matched independent power supplies for each lane is very important.  It's almost impossible to have a meaningful race when letting off your throttle sends the other car(s) into "hyperdrive" because you're sharing a small wall wart.

Thanks for all the posts. I left it at work by mistake. I won't get to test it till Monday. I will try it under load. I am using a digital meter. I had it set to read AC volts. if it tests ok under load is it then safe to use on the TIU? I don't want to damage it. The transformer was pretty cheap, like 5 bucks. Hate to damage the TIU with it.  

You can always call the manufacturer and ask them this question.

I would test it with a very light load, say one small bulb.  If it still reads high, I'd be leery of using it.

FWIW, I think 700ma is pretty marginal to power the TIU.  The specifications ask for twice that, but I think folks have run them on 1A at 18 volt Radio Shack supplies.  For a $5 investment, I'd go to a better transformer.

Edit: Maybe not...  I just powered up a TIU with nothing on the outputs, and it was only drawing 1/4 amp at 18 volts.

Thanks again...I decided it best to not use it. For some reason I interpreted the label to mean 16.9 VAC , 20 amp capacity. Now that I think about it how did I expect 20 amps out of that small thing?

Lots to learn. John, thanks for running test. I will purchase the recommended one from Radio Shack.  

A little knowledge is a dangerous thing...thanks to all for keeping me less dangerous. My trains and insurance company thank you also. 

Yep, VA and A mean two totally different things!

There is an inverse relation between amperage and voltage, ie no load= 0 amps gives a high open voltage.  The more the loading, higher amperage causes the voltage to drop.  All things being equal, when you put on a 700mA load the voltage should read 16V more or less. 

As a general rule, the more iron in the core and heavier copper windings, the voltage tends to remains somewhat stable as the load amperage varies.

an aside for my outside crossing gate, I have a really hefty iron transformer (12gauge secondary winding) feeding large rectifiers and capacitors.  At rest, the voltage is about 17V.  When gate rises the voltage drops to 15.8V which is close enough for government work.

A transformer has a figure of merit called "Regulation", which is defined as the ratio in percentage (no load voltage - full load voltage)/(full load voltage).  For very large power transformers the regulation is just a few percent.  As transformers get small, the regulation gets worse.  Wallwarts can be somewhere around 25%.

Small and cheap = poor regulation