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Needed help for using ULN2803

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prathyks

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I needed help on using ULN2803. Like i wanted to drive some relays using uln2803. So how should i configure the uln.

The condition is i want to ouput 12 from ULN2803. I have provided ground to 9th pin and 12vcc to 10th pin. Now if i provide ground from microcontroller circuit. to input pins (i have shorted the ground from microcontroller circuit with 12v uln circuit). I am not able to get 12v output at output pins of ULN2803.

How to go about solving this problem.

Thanks in advance.
 

alexan_e

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ULN2803 has open collector outputs, you can only use them as low side switch to sink current (provide ground)
ULN2003_stage.jpeg

the ULN should be connected like this
ULN2003_relay.jpg

Alex
 

H.E.Tjelum

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Hi

The ULN2803 (and others in this family) are a very simple way to connect load to a MCU. The inputs should have 0V for OFF and 5V for on. The positive side of your loads shold be connected to your positive power supply (could be 12V) and the other end (negative) of your load should be connected to the output pin. This means that your output of the ULN is high when your input is low.

Friendly regards
H.E.Tjelum
 

betwixt

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Don't be confused by 'COM' on pin 9. It is NOT a power input to the '2803. In fact, the 2803 does not have a power input which is why it can't produce a voltage on its output. The IC is just a switch between the output pins and ground, it can only sink current to ground.

However, you SHOULD connect the COM pin to the relay power source, it connects to the internal clamp diodes and will help to protect the 2803 from voltage spikes as the relays are turned off. Never connect COM to a voltage lower than the relay supply as this will make the clamp diodes conduct all the time and could damage the IC and other devices connected to the COM pin.

Brian.
 

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