I am using a ULN2803 with a RPi PICO. The PICO's outputs are 3.3V logic. The relays I will be controlling have 24v coils. Is this an acceptable application for this device? At first glance, it seems okay, but the more I (overthink) things, the more I am questioning this. Can someone help me out?

Thanks much!
Chris

 

Hello,

You might want to have a look at the TBD62083.


Bertus

 

Hi Chris,
What current is required by the relay coil.?
E

 

The coil is 24v @ 320 Ohms R, 75mA. There will be other devices I want to drive with the ULN2803 as well and hoping to find a "universal" go do buffer for a lot of other projects, thus the ULN2803.

Chris

 

hu Chris,
The d/s suggests that a 3.3V input level to the ULN2803 will be more than enough for those relays.
And up to 250mA load.
E

 

3.3V is enough to drive the Darlington into saturation. And if you only need 75mA of output current you're fine; it can do much more than that.

 

You might also consider one or two MIC4467 quad drivers.. They are fully characterised for TTL-level input (2v) and therefore are able to sink or source up to 1.2A if need be (5v <Vs <=18v per output, total package usage limited)

 

Hello,

You might also consider one or two MIC4467 quad drivers.

The TS stated he wants to drive 24 Volts coils. As the MIC4467 is only capable of 18 Volts, the TS can not use it.

Bertus

 

Hello,

The TS stated he wants to drive 24 Volts coils. As the MIC4467 is only capable of 18 Volts, the TS can not use it.

Bertus

Ah, good point, I missed that.

 

Is there any concern about joining the grounds together of the logic and power side? The 24V power side will have 0 and 24V connection to the ULN2803. However, I am only seeing a +3.3V input to the device and therefore am concerned about there being a difference in potential between the two power supplies. My assumption is to join the two zero lines from each power supply which hopefully brings them to the same potential. Is this thinking correct?

Thanks for the help on this guys, it has been a very long time since I tinkered and not so sure on my memory anymore.

Chris

 

Yes, the grounds must be connected together.

 

Be sure the ULN2803 snubber/flyback protection diodes COMmon is connected correctly to the relay + voltage.

 

Also, watch out for total current output on all 8 pins. You cannot drive all 8 at maximum current. The output currents are derated as more ports are on at the same time. Check the datasheets (may have to look for older ULN2803 sheets), there is a graph showing 100% duty cycle and number of ports on vs current per port. It is all related to maximum power dissipation.
At 75mA, you are ok, not an issue. If all 8 ports are on, maximum current per port is just above 150mA per port. With 4 ports on continuous, about 300mA per port is allowed. (all at 50 deg. C, DIP package)

 

Awesome guys, thank you all very much for the help and clarification. I am VERY glad I found this resource!
Cheers!
Chris