{"id":1523,"date":"2025-04-11T06:39:44","date_gmt":"2025-04-11T06:39:44","guid":{"rendered":"https:\/\/webprojects.cloud\/wordpress\/splatco\/?post_type=spl_knowledgebase&p=1523"},"modified":"2025-06-10T06:51:24","modified_gmt":"2025-06-10T06:51:24","slug":"ms120-bidirectional-points-used-as-outputs","status":"publish","type":"spl_knowledgebase","link":"https:\/\/webprojects.cloud\/wordpress\/splatco\/knowledgebase\/product-documentation\/product-documentation-controllers\/ms120-product-documentation\/ms120-external-inputs-and-outputs\/ms120-bidirectional-points-used-as-outputs\/","title":{"rendered":"MS120: Bidirectional points used as outputs"},"content":{"rendered":"\n
\"\"<\/figure>\n\n\n\n

The adjacent picture illustrates the bidirectional I\/O points connected to some typical output loads. When an output is ON it connects the I\/O pin to 0V via a Darlington transistor.<\/p>\n\n\n\n

Each of the two connectors is wired to 4 bidirectional I\/O pins. Each connector also has a “convenience” pin connected to power supply positive. The output load should always be connected between the output pin and power supply positive. Use the +Supply convenience pin on the I\/O connector in preference to wiring directly to the power supply +.<\/p>\n\n\n\n

CAUTION: Take care never to short an output pin to the positive common pin. If the output is on at the time you will blow up the output chip instantly!<\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\n

The loads illustrated are:<\/p>\n\n\n\n

A: The input pins of a solid state relay. Note the polarity!<\/p>\n\n\n\n

B: A relay coil. Notice we have included a catch diode across the coil. While there are diodes on the board we recommend a diode across the coil, as close as possible to the coil, to minimize problems with switching noise. A suitable diode is a 1N4001 or similar 1A\/50V rectifier diode.<\/p>\n\n\n\n

C: An LED, with a series current limiting resistor. Resistance calculators<\/a><\/p>\n\n\n\n

The (digital) outputs are implemented using the ULN2803A, a chip that contains Darlington transistors. The outputs consist of transistor switches between the output pin and 0V. When the output transistor is off, a multimeter set to DC volts connected between the output and 0V will measure a voltage slightly less than the positive supply voltage.<\/p>\n\n\n\n

Take care never to short the output pin to the positive supply voltage. If the output turns on the output chip will be instantly destroyed. This includes never doing it with a multimeter set on a current range.<\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\n

There are catch diodes built into the chip. This means the board will not be damaged if you are driving relay coils, solenoid valves or other inductive loads (within the ratings of the output). We nevertheless recommend external catch diodes connected directly to the relay coil as close as possible to the relay, in order to reduce adverse effects of switching noise. The onboard diodes also mean that the load must not be connected to a supply voltage greater than the board’s positive supply voltage<\/em>.<\/p>\n\n\n\n

Maximum allowable output current<\/h6>\n\n\n\n

The maximum allowable output current is 400mA steady state, with brief (100mS) peaks of 500mA allowed. There is also a limit to how much total current one ULN2803A chip can handle. This relates to the internal heating of the chip.<\/p>\n\n\n\n

Let’s look at 2 extreme cases:<\/p>\n\n\n\n

    \n
  1. All 8 outputs on one chip conducting the same current<\/li>\n\n\n\n
  2. The allowable current concentrated in a few outputs.<\/li>\n<\/ol>\n\n\n\n
    Equal currents<\/h6>\n\n\n\n

    The following table lists allowable current versus ambient temperature, when all 8 outputs are conducting the same current at once:<\/p>\n\n\n\n

    Temperature<\/th>Allowable current per output<\/th><\/tr>
    25\u00b0C<\/th>180mA<\/td><\/tr>
    40\u00b0C<\/th>160mA<\/td><\/tr>
    50\u00b0C<\/th>145mA<\/td><\/tr>
    60\u00b0C<\/th>130mA<\/td><\/tr>
    70\u00b0C<\/th>115mA<\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\n
    Concentrated currents<\/h6>\n\n\n\n

    The following table shows the maximum allowable currents, when concentrated in a few outputs, versus temperature<\/p>\n\n\n\n

    Temp<\/th>1st O\/P<\/th>2nd O\/P<\/th>3rd O\/P<\/th>4th O\/P<\/th><\/tr>
    25\u00b0C<\/th>400mA<\/td>400mA<\/td>300mA<\/td>0<\/td><\/tr>
    40\u00b0C<\/th>400mA<\/td>400mA<\/td>150mA<\/td>0<\/td><\/tr>
    50\u00b0C<\/th>400mA<\/td>400mA<\/td>0<\/td>0<\/td><\/tr>
    60\u00b0C<\/th>400mA<\/td>325mA<\/td>0<\/td>0<\/td><\/tr>
    70\u00b0C<\/th>400mA<\/td>220mA<\/td>0<\/td>0<\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\n

    Naturally there are many other possible combinations, but the above should serve as a guide.<\/p>\n\n\n\n

    The above figures are intended as a guide only. In any event, the absolute maximum operating temperature for the ULN2803A chip is 70\u00b0C.<\/p>\n\n\n\n

    If you feel like doing the actual sums, here’s the procedure.<\/p>\n\n\n\n

      \n
    1. For each output, calculate its output voltage drop from the formula
      V(out) = 0.92 + 1.85 * I(out) [ I(out) in Amperes ]<\/li>\n\n\n\n
    2. For each output calculate the dissipation in watts from the formula
      P=V(out) * I(out)<\/li>\n\n\n\n
    3. Add up all the dissipation figures to give P(tot) in watts<\/li>\n\n\n\n
    4. Calculate the maximum allowable ambient temperature from the formula
      Ta(max) = 125 – P(tot) * 55<\/li>\n\n\n\n
    5. If you get an answer over 70\u00b0C then use 70\u00b0C<\/li>\n<\/ol>\n\n\n\n

      This calculation aims at a maximum internal (junction) temperature of 125\u00b0C, which is the maximum rating of the chip. There are 2 things to keep in mind with this:<\/p>\n\n\n\n