# Need help building a transformer (240V to 18V 8A)

#### eddscott

Joined Jan 25, 2010
3
Hi, new to the forum and need help building a transformer.

For some background, I'm building a model railway and have come across a problem with the small control boards I'm using to control the track point motors.

The motors are solenoid type motors and require about 3 or 4 amps to throw them reliably. In some cases I require two motors to be thrown so 8 amps is required. The voltage can be from 15 to 20 but 18 seems about right with not too much heat generated.

These control boards have capacitors on them to aid the throwing of the solenoid motors. However, as its been explained to me, these capacitors cause a load to be present when power is first applied. Initially I used a laptop power supply rated at 18V 6A and this was fine until I used more than 3 of these control boards.

I then purchased a bench power supply but the circuit breaker kicked in with more than 4 of the control boards. If I added each board after the power was switched on it would work fine but when turned off and then back on the breaker would kick in again.

So, I want to build a transformer from 240V to 18V AC or DC and 8 amps. What I don't know is what other components are required other than switch, transformer, box, fuse etc.

If anyone could help I would be very grateful. Its become a big stumbling block with my layout.

Thanks

#### SgtWookie

Joined Jul 17, 2007
22,226
What happens when you add on another 5 control boards? You'll then need a 25A transformer!

The more control boards you add on, the greater the start-up load will be. However, this is a transient load; it will only be there when you first turn on the layout. Thereafter, you'll only be charging perhaps a couple of the control boards at a time.

There is an easy fix for your problem, and that is to use a power resistor to limit the maximum current draw from your power supply to your control boards.

The supply you are using now is 18v @ 6A.
When you first power up your system, all of the control boards are completely discharged, so they "look" like a dead short to the supply.
The problem is to limit the maximum current flow to 6A.

Since R=E/I, and you know that E=18V and I=6A, R=3 Ohms. You need a 3 Ohm resistor.

Let's calculate the resistor power rating requirement:
P=EI, so 18*6 = 108 Watts. Double for reliability = 216 Watts. Yeow, that's a big resistor!

However, you can use a number of automotive-type bulbs in parallel as resistors.
An 1156 is a single-filament brakelight bulb rated for 27W. You could try three or four of them wired in parallel as a current limiting resistor.

#### Bernard

Joined Aug 7, 2008
5,788

#### SgtWookie

Joined Jul 17, 2007
22,226
Thanks for the replies.

Sgt Wookie - So, if I put 3 or 4 automotive lights in parallel it might work?
If you go to your local breakers (auto junkyard) you could probably get a half-dozen tail lamps already in pigtails (lamp holders with sockets/wiring) for very little invested.

It's so inexpensive, it's worth a try or two.

#### Wendy

Joined Mar 24, 2008
22,800
How practical would it be to sequence them on in the space of a couple of seconds? Make that surge a bunch of small surges spread over time.

#### eddscott

Joined Jan 25, 2010
3
That would be OK. I'm not sure how many I can add at one time without the circuit breaker cutting in. Its happy with 3 or 4 when first turned on. Theres between 25 to 30 of these control boards. Thinking about it, if it would allow me, I could probably have 4 or 5 on a seperate switch so as the first few are turned on I could then switch on the rest in banks of 5.

Just in case it won't are there any diagrams online for a power supply?

Thanks guys.

#### Bernard

Joined Aug 7, 2008
5,788
Hoe much capacity [ capacitor ] on each board? Does pwr supply need to have good regulation? I needed a cheap & dirty power supply for motor testing, used surplus 18V 10A xfmr, 25A bridge rectifier, choke[ microwave filiment xfmr, 5000 uF cap to output, no regulation- and yes a fuse, slo-blow 7A.on primary.

#### SgtWookie

Joined Jul 17, 2007
22,226
eddscott,

You could probably do just fine with an unregulated supply like Bernard's talking about.

Get a transformer that has a secondary between 12.6v to 14v, and a full-wave bridge rectifier rated for twice to four times the current and twice the output voltage that the transformer is rated for. You'll lose about 1.2v-1.5v across the bridge, but you'll wind up with roughly 16v to 18v unregulated output after all the caps charge up. No complicated switching sequences or fiddling around with lamps.