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.
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.
Increasing the Ratings
- Increasing Current.The simplest way of increasing the current rating is add resistor channels in parallel and instruct them to the same resistance or relay pattern.
- Increasing Power.This can be achieved by connecting the channels in series or parallel
In practice using a combination of these methods may provide the best solution. Choosing the best route needs to take account the way the channels are being driven (voltage or current) and the resistance range required.
PXI modules such as the 40-297 and 40-260 series include both short circuit and open circuit settings and these can be used to help configure the resistance range of the channel without external switching.
A variable resistor is required to make a voltage source (fixed) appear to be a variable current source to a low load resistance. The resistance range required is up to 25kohm, and at the lowest resistance setting the power dissipation is 1W. Two channels of the 40-297-002 can be used to achieve this connected in series since each is capable of being set to up to 13.5kohm.
A variable resistor is required to operate from a 150mA source and the maximum power is calculated to be 1 W. Although 40-297 can handle the current two channels are needed to handle the power. These could be in series or parallel, but placing in parallel halves their current. So for best life it maybe best to use a parallel arrangement.
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.
Lowering the minimum resistance
Resistor modules always have a minimum resistance they can achieve and most of Pickering Interfaces 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.Example. A resistance range up to 25 ohms is required with a low minimum resistance. This can be achieved by using 40-297-001, having a range of 1 to 230 ohms (18 channels) and placing 8 channels in parallel to provide a minimum resistance of 0.125 ohms, resolution of 0.125 ohms and a maximum resistance of greater than 25 ohms.