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I am building a 5-volt-to-24-volt DC converter using ULN2067B. According to the datasheet this device can support 1.5 A per darlington per collector.

Does that mean that all the darlingtons can draw 1.5 A at the same time? If no, how could a beginner in electronics have checked that while reading the datasheet?

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If you look at figure 17 of the datasheet, you can see that with no heatsink, at temperatures up to about 40 degrees celsius, the device can safely dissipate about 1.5 watts of power.

Per table 3, collector-emitter saturation voltage is at most 1.4 volts at 1.25 amps. 1.5 watts divided by 1.25 volts is 1.2 amps.

If your individual outputs are drawing lower power - say, 500mA, table 3 tells us the forward voltage will be at most 1.1 volts, in which case you can sustain a total of ~1.36 amps.

Adding a heatsink can increase the maximum dissipation to as high as 4 watts, depending on the size of the heatsink, which would allow total currents as high as about 3.5 amps.

tl;dr: Without any heatsinking, you can more or less handle one pin at 1.2A, or more pins at lower current. With heatsinking, this limit is increased substantially.

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  • \$\begingroup\$ You mean that power dissipation is related to the collector emitter voltage? \$\endgroup\$ Jul 22 '15 at 11:11
  • \$\begingroup\$ @Med.ali The power dissipation is the voltage dropped across the device multiplied by the current through it, so yes. \$\endgroup\$ Jul 22 '15 at 11:17
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Besides maximum current through one darlington transistor you also have to care about power dissipation. The datasheet tells you that the saturation voltage of one darlington is up to 1.5V at a collector current of 1.5A, leading to a power dissipation of 2.25W which is alread more than is allowed at 70°C Tamb. If you manage to cool all your pins down to 90°C a total power dissipation of 4.3W is allowed according the absolute maximum ratings. But this is already less that two times 1.5A over a single darlington. So in short: if you load only one transistor its collector current must stay below 1.5A while staying below the limiting power, if you load more, the total power dissipation must stay below both limiting values as well, no matter how you distribute the currents.

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