I made a card controlling 24 electromagnets with an Arduino board via 3 darlington ULN2803A

My power supply for the electromagnets was 20 V and 300 mA for each electromagnet.

Due to the physical power of the electromagnets being too weak, I replaced my supply of 20 V by one of 36 V (the electromagnets support 40 V).

When I tried the new 36 V supply, it worked for a minute and then my ULN2803A burned!

However, the ULN2803A documentation indicates that they support 500 mA at 50 V!

Is there a Darlington that supports 36 V and 400 mA per channel, that I could use in place of the ULN2803A?

Or is there another way to control my electromagnets under 36 V?

Thanks for your help. Did I understand right ? This five time to  order 24 solenoïds

Sunnyskyguy EE75, You warn me that the Solenoids will dissipate Pd=V²/DCR=36²/67=19W.

Half of my solenoid are under tension about Half a second every 2 seconds. Do you think it will be OK ?

I wonder if to double the SLA5085 is a solution ? enter image description here

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    \$\begingroup\$ Just checking: did you observe the limit for the total chip current? It's not 500 mA per channel, all channels on. From memory it's 1 A total max. \$\endgroup\$ – Transistor Apr 7 at 17:10
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    \$\begingroup\$ The datasheet also states that there is a maximum amount of power that the package can dissipate; you exceeded that with the increased voltage. Read section \$\endgroup\$ – Dave Tweed Apr 7 at 17:13
  • \$\begingroup\$ Here's a few overkill transistors with much higher ratings than you need. \$\endgroup\$ – Hearth Apr 7 at 17:16
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    \$\begingroup\$ And don't forget the clamping diodes, to absorb inductive energy. \$\endgroup\$ – analogsystemsrf Apr 7 at 17:30
  • \$\begingroup\$ If possible, use a MOSFET driver instead. These kind of transistor arrays tend to be crap for industrial use, even with proper flyback diodes on the outputs. \$\endgroup\$ – Lundin Apr 9 at 8:37

If solenoid is 300mA @ 20V or DCR=20/0.3 = 67 Ω, then @ 36V Iout= 34.5V / 67Ω = 515mA

From table in 6.5
If Iin= 0.5mA Ic = 350 mA 1.6Vmax = 0.56W / driver *8= 4.48W > * \$R_{θJA}=66.4°C/W\$ = 297°C rise above ambient worst case. ( Fail )
- then 36V? Melt-down.

To keep overall heat rise of 36V * 0.5A= 18W per driver with a DCR=67Ω to a cool 20°C rise the driver resistance needs to dissipate about 2/3W each with an electrically-insulated shared-heatsink or use a MOSFET with low RdsOn such that \$R_{θJA}*Ids^2*DCR <= 20°C\$

One solution that satisfies this is five Mosfet Array 5 N-Channel, (< $4 @ 1k) Common Source 60V 10A 5W Through Hole 12-SIP w/fin (this is the only suitable array of this type) and I see it has coincidentally been suggested.

  • \$\begingroup\$ Thanks for your help. Did I understand right ? See my diagram link \$\endgroup\$ – Olivier Masson Apr 9 at 12:02
  • \$\begingroup\$ That's better. You know the Solenoids will dissipate Pd=V²/DCR=36²/67=19W so they will heat up fast and if you need to hold longer than it takes to heat up to burn your fingers, the current must be reduced with the result of less holding force but cooler or use forced air cooling with thermal conduction. But that needs a timing, force and temp rise spec from you to optimize the design. \$\endgroup\$ – Sunnyskyguy EE75 Apr 9 at 15:04
  • \$\begingroup\$ Did you see my suggestion to double the SLA5085 ? \$\endgroup\$ – Olivier Masson Apr 11 at 8:51
  • \$\begingroup\$ You can always parallel these FET drivers safely since the Ron rise with temp. But with 0.22 Ω V GS =4V, The power dissipation is 0.22Ω * 0.5A^2= Pd= 60mW max per driver so it is not necessary. \$\endgroup\$ – Sunnyskyguy EE75 Apr 11 at 14:03

Personally, I'd just use discrete MOSFETs plus flyback diodes (eg. IRLZ44N + 1N4004), but you can get 5 of them in one package (SLA5085 from Sanken) with logic-level (4V) drive:

enter image description here

Don't forget the diodes across the solenoids, whichever way you choose to go.

Darlingtons are not great for switching high current at low voltage as they drop a lot of voltage causing a lot of power dissipation, which is part of the reason why they're very seldom seen these days in such applications. If you insist on Darlingtons there's always the TIP131 single power darlington from 40 years ago. Again, you need the diode across the load.

  • \$\begingroup\$ Thanks for your help. Did I understand right ? \$\endgroup\$ – Olivier Masson Apr 9 at 11:48
  • \$\begingroup\$ Yes, you understand correctly. \$\endgroup\$ – Spehro Pefhany Apr 9 at 15:03

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