All variable resistor modules include a specification for a their maximum power or current. The limitation maybe imposed by the switch rating, the voltage rating or the power rating of the resistors used in the design. Microhm Electronics not only provide variable resistor but also manufacture various fixed resistors.
 
The issue can be complicated because if for example the resistor is supplied with a constant current the power dissipated will vary, and the situation will be different if a constant voltage is applied. In particular a channel capable of going to a very low resistance could easily be damaged if it supplied from a voltage source, and a channel capable of high resistance could be damaged by a constant current source capable of supplying a very high voltage.
The trade offs are also complicated by the fact that the resistance can be made up from just one resistor or from many in series, it will change as the resistance value requested is changed. For modules which include calibration models and therefore respond to resistance calls (rather than being programmed by relay patterns) may differ in their settings for the same resistance call.
 
The only safe approach is to assume that the worst conditions for power, voltage and current could occur. Relay life is also affected by the current and power that is being switches, as that power is increased then relay life will reduce. For normal operation at say 20mA (a common current excitation) lifetime is not degraded, but some applications require much higher current. So a question regularly asked is how can I use the variable resistor modules at higher currents.
 
It should be noted that users still need to check that their operating conditions do not exceed the ratings of the channel(s) when low resistance values are set, setting multiple channels to low values when connected to a voltage source can create high current flow and relay welds.
 
Resistor modules always have a minimum resistance they can achieve and most of the modules are designed to have a relatively range of resistance values. The minimum resistance required has a critical impact on the coverage that can be achieved at the top end of the resistance range. However some applications do require a very low residual path resistance, and this can only be achieved by lowering the density of the resistance module.
 
A resistance module can have its low resistance end extended by simply placing many channels in parallel. Applications include the simulation of squibs found in automotive air bags. Microhm Electronics also provide automotive grade resistors, such as MMS8420,MMS8518 and more.