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pull down resistor value calculation for digital output board

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animeshkhare

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I have a digital output which I am using to drive the relays through ULN2003. 82C55 is used in the digital output board with a pull up resistor of 10K Ohm. I am facing the problem that as soon as i turn on the digital board the output lines are pulled high and this in turn is driving my relays, so i am thinking of a solution to have a pull down resistor at the digital output pin of my board so that the digital output lines be pulled low on power up. Can I use the pull down resistor in this application and if yes what value i should use??
 

ericgibbs

Well-Known Member
Most Helpful Member
I have a digital output which I am using to drive the relays through ULN2003. 82C55 is used in the digital output board with a pull up resistor of 10K Ohm. I am facing the problem that as soon as i turn on the digital board the output lines are pulled high and this in turn is driving my relays, so i am thinking of a solution to have a pull down resistor at the digital output pin of my board so that the digital output lines be pulled low on power up. Can I use the pull down resistor in this application and if yes what value i should use??
hi,:)
Which version of the ULN2003 are you using.?

The 82C55 is capable of sink/source 2.5mA and ok to drive the CMOS version of the ULN without pullup/down reistors.
 
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animeshkhare

New Member
hi,:)
Which version of the ULN2003 are you using.?

The 82C55 is capable of sink/source 2.5mA and ok to drive the CMOS version of the ULN without pullup/down reistors.
thanks eric,
i am using ULN2003A for my application...
i know that 82C55 is capable of 2.5mA... but my problem is that the DAQ card(digital board) which i am using has an inbuilt 10K Ohm pull up resistor which is making the input to ULN to go high which inturn drives the relay... so i am thinking of using a pull down resistor at the output pin of the DAQ card (Digital board) which will pull the output low.
 

ericgibbs

Well-Known Member
Most Helpful Member
thanks eric,
i am using ULN2003A for my application...
i know that 82C55 is capable of 2.5mA... but my problem is that the DAQ card(digital board) which i am using has an inbuilt 10K Ohm pull up resistor which is making the input to ULN to go high which inturn drives the relay... so i am thinking of using a pull down resistor at the output pin of the DAQ card (Digital board) which will pull the output low.
hi,
The problem is you will have two resistors in series, connected between the +5V rail and 0v.
In order to pulldown the junction less than 2.5V [the turn on point for the ULN] the lower resistor will have to be at least 10K, say 4.7K to be sure of turn off.

Also its possible the DAQ card is 'actively' driving its outputs high during initialising, so the pull down will be ineffective.

I would try a 4.7Kpd on one pin and scope the signal during power up.
 

animeshkhare

New Member
hi,
The problem is you will have two resistors in series, connected between the +5V rail and 0v.
In order to pulldown the junction less than 2.5V [the turn on point for the ULN] the lower resistor will have to be at least 10K, say 4.7K to be sure of turn off.

Also its possible the DAQ card is 'actively' driving its outputs high during initialising, so the pull down will be ineffective.

I would try a 4.7Kpd on one pin and scope the signal during power up.
hi :)
thanks for the inputs. i will try using a 4.7K resistor and will see if the ULN gets turned on or remain off on power up.
 
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