Power reserve and additional outputs on SUSI? A tip for advanced model train DIYers…

Double objective

  • Add a power reserve with capacitors to improve your locomotive’s behaviour on dirty tracks
  • Use SUSI as 2 additional outputs via a small tweak.

Reminder about the SUSI interface in decoders

Most common N-scale decoders have up to 4 outputs: Light front, Light rear, AUX 1 & AUX 2 (the name of those last 2 may vary depending on the manufacturer). All these are “regular” outputs.

Some mini-decoders for the N-Scale also have a so-called “SUSI” connectors, with 4 soldering pads. For example:

  • Doehler & Haass DH05C & DH10C (see review here)
  • Zimo MX620 (old model) and MX622 (new model), but NOT the MX621.
  • Kuehn, for example N045.

The SUSI serial interface is actually designed to be connected to an external sound module. This is the default mode.
To see an example of how connecting sound modules work, refer to the post Model train sound for beginners.

SUSI interface on Doehler & Haass DH05C and DH10C decoders

SUSI interface on Doehler & Haass DH05C and DH10C decoders

SUSI as connector for power reserve

One of the 4 SUSI pads is the plus pole (aka “ZVS”/”V+”), another one is the negative pole (aka “GND”). Most decoder brands allow to connect small electronic capacitors to those pads. They will help when the track is dirty (unreliable power feeders). If you add enough capacitors (in N scale, 400uF or more), you should see greatly improved behaviour on points or at slower speeds.

Connecting capacitors to SUSI is usually possible. Check your manufacturer’s manual to see if that’s true (for some, the capacitors need to be connected elsewhere), or if some CV changes are required.
For Doehler & Haass for example, it is recommended to add “2” to the value of CV137 (bit 1 = on). Otherwise, your capacitors will have no visible effect.

SUSI alternatively used as 2 additional outputs

SUSI can be used for another purpose than connecting a sound decoder. You can deactivate the “serial interface” mode to get 2 extra logic outputs.

Beware that all brands may not allow this functionality, it needs to be configured in the decoder via CV configuration.

  • Zimo has allowed this for a while now.
  • Doehler & Haass has enabled this feature since January 2014, by firmware upgrade.
    You may need to check which firmware version you have: if you can’t read or write CV137, it probably means you are not up to date. If you can, then you should add “1” to the CV (bit 0 = on).
  • Kuehn allows this, at least for the N045 decoder.
    For Kuehn, CV56 bit 2 needs to be changed (refer to manual for details).

Also important: these 2 additional outputs are logic level outputs, also called “non amplified”. This means you cannot connect heavy loads to them, or you will destroy your decoder. Those logic outputs usually tolerate loads of 5V, with a max of 15/20mA.
You could use a transistor to amplify them…or just be cautious and respect the specifications of the outputs. If you only connect a single LED with a strong enough resistor (for example to light up the driver cab in a locomotive), you can connect those to a logic output directly.

In practice

In the recent post about converting a V100 locomotive, I made use of both those possibilities (power reserves + additional outputs) on a DH05C decoder. Head on to the article for more details. In the video below, the cab driver lighting is connected to the SUSI output:


Below is the schematic of the V100 locomotive above:

SUSI interface as additional outputs plus power reserve

Example: SUSI interface as additional outputs plus power reserve

Technical notes:

  • Resistor values are indicative. They will differ based on the LED models you choose and how many you use in a row.
  • For details and comments on the “power reserve” (in gray on the diagram), please refer to the other diagram on the KLV 53 vehicle here)


I hope this will help some of you, those little N decoders offer many possibilities to motivated DIYers!

Special thanks to Eduard from nscaleblog.wordpress.com who noted that Kuehn decoders also offer the same possibilities!

In any case, remember soldering directly on a small decoder is a difficult thing. You may destroy your decoder, and I am not accepting any liability for this!
The above works for sure for the mentionned Zimo, Doehler & Haass and Kuehn decoders. It may or may not work for other decoders, check your manuals before doing anything!