An Inexpensive MERG Booster for DCC

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Peter Reynolds

Pictures by the author

The video that accompanies this article is available at MERG DCC Booster and also via the Gazette Extra tab on the Guild website.

AFTER BUILDING THE DCC Programmer, Gazette May 2022, the quest was on to make that item a bit more versatile for use on 7mm scale models, especially the current output. I looked at various solutions to this, including boosting the output of the LM556 dual timer used in the programmer. In the end I thought, “Why re- invent the wheel? It’s DCC, so use a booster”. The added benefit of this is that I could use the booster for my big layout – it is good practice to keep the traction separated from everything else, such as points, in case of faults. Now, I am competent when it comes to assembling electronic items (40+ years of experience), so I decided that I would build the MERG DCC Booster, rather than buy something ready-made. I duly parted with the money and placed an order from the MERG website (I am a member, and you need to be a member to purchase their kits; it is well worth it). It consisted of a plastic bag containing ten pages of instructions, an excellent screen-printed PCB, and various other bags and cards of components.

Everything you need is there, except a case (if you wish, but it must be metal), to make a 5A or a 10A booster. You will need to decide which version to make before you start – a few components differ between the two versions. Of course, you will also need a power supply to suit, and I bought a 15VDC 6A PSU meant for a Toshiba laptop – MERG will also sell you something similar, but if you wish to build the 10A version, you will need to source the appropriate power supply yourself.

A pluggable 15VDC 10A PSU will cost something approaching £80, unless you use a standard switch-mode unit which is about £35. The booster will work with a power supply of up to 24VDC at whatever current you choose. It should match the voltage of your base controller as near as possible. If you build the 10A version, it is also recommended that you replace LK1 with a thermal cut-out sensor (available from Rapid and other suppliers) which is attached to Q7 (this MOSFET controls the raw input power).

I spent a few days studying the instructions, just to get everything fixed in my mind before I started. I bought a case, some connectors, and two small 5VDC fans, so that the unit would look nice when finished; it is feasible to mount the PCB on your baseboard, but I wanted my unit to have an element of portability. If you build a 10A version, you will need to source some aluminium strip for the MOSFET heatsinks, but this is covered in the instructions

The instructions are very detailed in terms of construction order, although I did deviate from the enclosed plan slightly, due to my own construction preferences. I have always fitted any DIL IC sockets first. The components themselves are divided into passive and active, with a testing phase (just requiring a voltmeter) between finishing the passive items and starting to fit the active items. If you take your time, and work down the individual cards of components methodically, a professional looking unit is simple to complete.

Soldering techniques, when it comes to electronic items, are slightly different to what you may be used to for kit building, but provided you have the correct multicore solder, they are easily attained. MERG can also supply cheap, basic kits, that you can use to hone your skills first if you prefer. Check your work as you go along. Fix any soldering deficiencies as they arise.

As a result of adding the two cooling fans, I added a small heatsink (not included) to the 5V regulator. The inputs are on the left of the photograph. There are two purple wires for the DCC input; and one brown and one black wire which enable a remote shutdown via a switched 12V supply.

The outputs are on the right of the photograph. Taking into consideration my output current, I used 55/0.1 wire, which will handle 6 amps. The thinner wires are for an external moving coil meter, if you wish to fit one to register the output current.

You will need a digital voltmeter and a suitable DCC signal source, to test the final item, as well as your chosen power supply. It will not power up without a suitable source. This is shown in the Extra video that accompanies this article. It is just short of two hours long; I think I could have made the item in about 75 minutes, from start to finish, including testing, if I hadn’t been filming.

Overall, this is a well thought out kit, and certainly within the scope of anyone with reasonable electronics soldering skills. It is also relatively inexpensive; a basic 5A booster, with power supply, is less than £50.