Extending Range of a Precision Resistor Module

    Pickering Interfaces precision resistor modules offer very accurate resistance emulation. The modules do have to balance the conflicting needs for wide range and fine resolution with the need to provide an economical solution to the user requirement. Consequently the resistor modules offered as standard products may not cover all the requirement the user has.

    As an example a user may want a wide resistance range, say 100 Ohms to 10kOhms, with fine resistance resolution at the lower resistance end but coarser resolution at the higher resistance end. This page gives some suggestions on how the resistor modules can be combined to offer further capabilities when they are provided with internal short and open relays.

    Placing Modules in Series

    The 40-260 series and 40-297 resistor modules can be conveniently placed in series with each other.

    Diagram of resistors in series

    Suppose the user needs to cover 100 Ohms to 100kOhms with fine resolution at the lower resistances and coarser control as the resistance is increased.

    A convenient way of doing this is to chose a suitable fine control module such as 40-260-001 and a 40-297-003 to extend its range.

    The 40-260-001 covers the resistance range 90 Ohms to 8kOhms with a resolution of better than 10mOhm (0.01 Ohms).

    The 40-297-003 covers the range 3 Ohms to 1.5MOhms.

    Each of these modules has a short circuit setting to simulate a fault condition, in the case of the 40-297 the lowest resistance setting (3 Ohms for this model) is sufficiently low that the lowest resistance setting can be used rather than the short circuit setting. The resistance of the short circuit state for each channel can either be measured or retrieved from the calibration file if the lowest resistance setting of the 40-297 is not used.

    One channel form 40-260-001 and one channel from 40-297-003 can then be connected in series.

    For low resistance settings the resistance of the 40-297 is set to a minimum and the resistance of the series combination is the 40-260-001, plus the 40-297-003 plus the resistance of the interconnecting wire (which can be measured).
    When the required resistance exceeds 8kohm the 40-297 can be used to add resistance to the series combination.

    Users can therefore create a resistor channel that in this case varies from 93 Ohms to 1.5MOhms and has the resistance resolution of 10mOhms. For values above 8kOhm the user can either maintain the resolution of the 40-260-001 or can simplify control by reverting to the resolution of the 40-297-003.

    In either case control of the channel is simple since both modules respond to requests for a resistance value rather than relay on data bit patterns.

    The same principles can be applied to all the resistor modules. For example two 40-262 can be placed in series to increase the maximum resistance. For lower resistance values the short circuit setting can be used on one of the modules to take it out of the path, the resulting channel then needing to be corrected for the short circuit setting resistance and any wires used to configure them.

    Placing Modules in Parallel

    The resistor modules can also be placed in parallel. the effect then being to improve the power handling and resolution of the composite channel. Each of the channels can carry its rated current or power, so placing several channels in parallel with equal value resistance settings can improve the power handling.

    Diagram of resistors in parallel

    The same method can also be used to improve the setting resolution and extend the lowest value of resistance that can be achieved.

    Using 40-297-002 as an example the resistance range can be extended down from its normal 2 Ohms to 13.5k Ohms. If three channels are placed in parallel the lowest resistance that can be achieved can be reduced to 0.67 Ohms. If the channels are all set to the same value the resolution is changed from 0.25 Ohms to approximately 0.08 Ohms. However the resolution can be further improved by offsetting the channel values set by 0.25 Ohms. Consider this example:

    Set all three channels to 600 Ohms, the overall resistance is now 200 Ohms.
    Set two channels to 600 Ohms and one to 600.25 Ohms, the overall resistance is now 200.028 Ohms.
    Set one channel to 600 Ohms and two channels to 600.25 Ohms and the overall resistance is now 200.056 Ohms.

    The resolution of the composite channels has now improved to 28 mOhm as well the as the lowest resistance being extended.

    A slightly more complicated, but more efficient approach, is to use two channels in parallel but to use dissimilar settings. If for example a user wishes to get 100 Ohms and have fine setting resolution one channel could be set to say 105 Ohms and then the other channel set to nominally 2k1 Ohms. The basic accuracy will be substantially the same, being dictated by the accuracy of the lower resistance channel, in this case about 22 mOhms. Varying the resistance of the 2k1 Ohm channel in 0.25 Ohm steps will produce an overall fine control of less than 1 mOhm. It should be remembered that the accuracy is still limited and the current between the two channels is not shared equally.

    Precision modules such as 40-262 can also be used in parallel to lower the minimum resistance by using two in parallel, as the required resistance is increased then opening the internal series relay on one of the channels results in only one remaining in circuit and the maximum resistance of a single channel can be reached.

    Open and Short Relays

    For resistor modules with built in short and opening relays it can be seen these can offer very convenient means of expanding the range of the resistor modules, increasing their power or current handling or improving the ability to introduce fine resistance changes.

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