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I own some animatronic robots that use solenoid valves to operate their mechanical movements. enter image description here Recently I created a pcb that uses three 8 channel Darlington Arrays ULN2803A to control the individual solenoids and make the robot move. Each Solenoid is 6.7 watts (or about 300milliamps) and 24 volts. The power supply I use is 4.16 amps and 24v. I use an arduino to control the darlington arrays. When I first tested the darlington arrays, the robot moved and it's solenoids were triggered by the darlington arrays, but upon inspection of the board, the darlington arrays began to smoke. I replaced them, and tested them again. I tested them multiple times having similar results, only twice did they work without failing. When they didn't fail, I noticed the chips get extremely hot. This has concerned me, because I know someone who also uses them and has had no heating or failure issues with them. I did not use diodes for the outputs as it says they have built in output clamps diodes. I don't understand how I could possibly be overloading them because they can handle up to 50 volts and 500 milliamps. Is there any output protection I should have used? perhaps addition of some resistors? Thanks

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  • \$\begingroup\$ Could you share a wiring diagram (or better yet, a schematic) of exactly what was connected and to where? \$\endgroup\$ – rdtsc Apr 18 '17 at 22:20
  • \$\begingroup\$ Added a schematic view \$\endgroup\$ – Jonathan Kramer Apr 18 '17 at 22:30
  • \$\begingroup\$ Darlingtons are the wrong arrangement, as they don't saturate and they will dissipate horribly -- as indicated by The Photon already in an answer. Each relay is only about \$\frac{6.7\:\textrm{W}}{24\:\textrm{V}}=280\:\textrm{mA}\$. So this can be done with any BJT rated for about half an amp. Since you can realistically expect to achieve perhaps \$V_{CE}\le 300\:\textrm{mV}\$, this is under \$100\:\textrm{mW}\$. So even a TO-92 could do it. But you'd need 24 of them. You'd still need to work out how to drive them. Or get some NFETs (dual SOT23-6 exist.) \$\endgroup\$ – jonk Apr 18 '17 at 23:10
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    \$\begingroup\$ you need larger heatsinks on your transistors. or any. to get anywhere near absmax specs... \$\endgroup\$ – dandavis Apr 19 '17 at 2:37
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With Vce-sat of maybe 1.6 V (at 500 mA), each darlington will be heated by ~0.8 W when run at maximum current.

If all 8 darlingtons are active at once that's 6.4 W.

Junction-to-ambient thermal resistance is 66 C/W (for some assumed copper pattern on the board).

So with with all 8 darlingtons active at 500 mA, you can expect a temperature rise of 422 degrees.

What this means in practice is: Don't operate more than one or two of the darlingtons at full current at the same time.

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