Darlington Array - Switching 3V3 with 4.2V.

Thread Starter

Pidgeydoyle

Joined Dec 4, 2012
17
Hi Everyone,
I was wondering if anyone could help me with a small problem I've been having.

I'm trying to use a ULN2003 darlington array IC to individually control a series of relays.
I am using the 4.2v signal voltage from my microcontroller to switch the transistors and a regulated 3.3V supply for the IC and relay.
Unfortunately when I try to turn the relay on using the microcontroller, all of the darlington pairs on the IC turn on and all my relays turn on as a consequence.
If I remove the 3v3 regulator and power everything from Vbatt, the problem goes away however I would prefer to power the IC and relays from the regulator for power saving reasons as I can turn it off when not required.

Does anyone know why my initial setup does work?

Thanks!!

upload_2019-5-30_17-7-40.png
 

dl324

Joined Mar 30, 2015
9,111
I'm trying to use a ULN2003 darlington array IC to individually control a series of relays.
I am using the 4.2v signal voltage from my microcontroller to switch the transistors and a regulated 3.3V supply for the IC and relay.
You're using the wrong part. ULN2003 are designed to be driven from 5V logic:
upload_2019-5-30_15-12-48.png

What is the pickup voltage and coil current for the relays you're using? Those darlingtons can have a saturation voltage that's almost half of your 3.3V.

Unfortunately when I try to turn the relay on using the microcontroller, all of the darlington pairs on the IC turn on and all my relays turn on as a consequence.
Doesn't make sense. The darlingtons are independent. Post more complete information (like a schematic).
 

Alec_t

Joined Sep 17, 2013
10,429
Depending on the wiring of the MCU, power supply and IC your problem may be due to 'ground bounce', where the comparatively heavy current drawn by a relay is raising the ground rail potential to a point where the MCU resets or the regulator drops out of regulation. Are you using a 'star ground' arrangement?
 

bertus

Joined Apr 5, 2008
20,092
Hello,

Did you think of the mosfet alternative of the ULN2003, the TBD62003?
The ULN will have quite a voltage drop.
There is also an 8 fold version the TBD62083.

Bertus
 

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Thread Starter

Pidgeydoyle

Joined Dec 4, 2012
17
Hello,

Did you think of the mosfet alternative of the ULN2003, the TBD62003?
The ULN will have quite a voltage drop.
There is also an 8 fold version the TBD62083.

Bertus
Hi! I tried the TBD62003 but unfortunately it resulted in the same outcome.
 

Thread Starter

Pidgeydoyle

Joined Dec 4, 2012
17
You're using the wrong part. ULN2003 are designed to be driven from 5V logic:
View attachment 178585

What is the pickup voltage and coil current for the relays you're using? Those darlingtons can have a saturation voltage that's almost half of your 3.3V.

Doesn't make sense. The darlingtons are independent. Post more complete information (like a schematic).
so the ULN2003 is connected to a MUX..

upload_2019-6-4_15-47-21.png
 
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