Five relay modules, three distinguishing features: number of channels, current per channel, and whether switching is single-pole or double-pole. Additionally, three of them have a dimming function. Answering these four questions in order will inevitably lead to the right module.
The five modules at a glance
- CMR 411 S — 4 channels, 10 A, single-pole, 4 PWM dimmer channels.
- CMR 412 S — 4 channels, 10 A, double-pole, 4 PWM dimmer channels.
- CMR 811 S — 8 channels, 10 A, single-pole, 4 PWM dimmer channels.
- CMR 431 S — 4 channels, 30 A, single-pole, no dimming function.
- CMR 232 S — 2 channels, 30 A, double-pole, no dimming function.
The system behind this is simple: if you need 30 A per channel, you forgo the dimming function; if you want to dim, you stick with 10 A. And the double-pole variants each have half the number of channels of their single-pole counterparts at the same size – double-pole switching takes up space.
Single-pole or double-pole – the fundamental question
Single-pole means: only the positive line is switched, the ground remains continuously connected. This is the normal case and correct for most consumers.
Double-pole means: positive and negative are disconnected simultaneously, and the consumer is then potential-free. This is necessary where, for safety or installation reasons, no connection to the vehicle electrical system should exist — and useful where leakage currents via the ground can lead to corrosion.
This question should be at the beginning of the selection, because it cannot be changed afterwards: a single-pole module does not become double-pole by reconfiguration.
Ten or thirty Amps
Again, the decisive factor is not solely the continuous current of the consumer, but its inrush current. Lighting, electronics, and small pumps usually remain well below 10 A. Compressor cooler boxes, larger pumps, and winches exceed this upon startup — here, the path leads to the 30-A modules.
The price for this is the absence of the dimming function. In practice, this is not a loss, as consumers that draw 30 A are not dimmed anyway.
The PWM dimmer channels
Three modules come with four PWM control channels, which allow dimmable LED lights or LED power drivers with a suitable control input to be regulated directly from the system monitor.
Two points are important here. First, there are four dimmer channels — even with the CMR 811 S with its eight switching channels. If you plan eight dimmable circuits, you need a second module. Second, the luminaire must be approved for this control; a luminaire not intended for this purpose will flicker, hum, or have a shorter lifespan.
NoteWhich circuits are to be dimmed should be planned before purchasing the luminaires — not configured after installation.
Emergency operation: the underestimated feature
All five modules have a manual emergency operation. By repositioning the fuse or circuit breaker, each output channel can be bypassed — the circuit remains operable, and the protection is maintained.
This is the most valuable point when deciding on a digital distribution. The obvious concern — what happens if the control fails — is thus answered: the light will still turn on, it just won't be via the monitor anymore.
All modules are available either with ATO fuse or with circuit breaker. The circuit breaker is reusable, the ATO fuse is cheaper and available everywhere as a replacement.
Main/Secondary and Feedback
Two functions come from the bus connection in all modules. The Main/Secondary function allows a switching command to control several outputs simultaneously — for example, to switch lights together at several locations.
Feedback is more important in everyday life: switching status and defective fuses appear directly on the monitor. A blown fuse is thus reported instead of searched for — and that is the difference between five minutes and an afternoon.
Typical errors in selection
- Selected by number of channels, forgot polarity. Single-pole cannot be made double-pole retrospectively.
- Planned eight dimmable circuits on the CMR 811. There are four dimmer channels, not eight.
- Designed for continuous current. Inrush current determines whether 10 A is sufficient.
- Planned non-dimmable luminaires. Approval can be found in the luminaire's datasheet.
- Emergency operation not documented. It is only useful if someone knows it exists in an emergency.
