Hallo,
I am using SN7407D hex buffer with open collector output.
For calculating pull-up resistors, I tried to look on internet but there are couple of different formulas which confuses me a bit. Can somebody suggest me the correct formula for calculating pull-up register.

Thank you in advance

 

hi vd,
It partly depends upon the load the pulled up o/c output is driving into, what is the load.?
E

 

24V is load

 

24V is load

24V is your voltage source. That does not describe the load.
What is the circuit or device that will be connected to the SN7407 output?

Circuit calculations will provide two answers, i.e. a minimum and maximum value of the pull-up resistor. This will establish a range of values. After that, you choose a suitable value within that range.

Here is how to calculate the minimum pull-up resistor:

Supply voltage = 24V
Sink current = 16mA

R = V/I = 24/.016 = 1500Ω

Now we need information on the load to determine the maximum pull-up resistor.

 

You reported only on the voltage supply. It carries information, but it's not enough. It will be easier if you say that it is connected to a resistor. The load capacity and the switching speed are also important.

 

MrChips, You calculated the minimum value of the resistor, but this is not always necessary!
Presistor~24V*16mA= 384mW hex output 6.144W !

 

MrChips, You calculated the minimum value of the resistor, but this is not always necessary!
Presistor~24V*16mA= 384mW hex output 6.144W !

Sorry, I get 2.3W for six resistors.

Each resistor can be 1.5kΩ ½W. I don't see that this is a problem.
When we know more about the load requirements we may be able to go to a higher resistance.

 

The 7407 output transistor is rated for +30 V and 40 mA. This sets the absolute minimum value of the 24 V load impedance to 600 ohms.

ak

 

The 7407 output transistor is rated for +30 V and 40 mA. This sets the absolute minimum value of the 24 V load impedance to 600 ohms.

ak

Thanks for that. I know I should have done my due diligence and checked the datasheet. I was being a bit lax and used the standard TTL IOL value off the top of my head.

At 600Ω, the power dissipated is 1W. This would call for a 600Ω 2W pull-up resistor.

(We still need the load requirements from the TS.)

 

For load device:
Input voltage = 24V
Input current = 7mA

 

hi,
So the load resistance is nominally 3k3,, 24v/7mA = 3428R.

If the supply to the pull up resistor is 24v then you will not get exactly 24v output from 7407 when the output is High.
What is the lowest voltage the load will accept.????

Assume the load will accept 20v, then the pullup would have to drop 24v - 20v = 4v at 7mA.
That is a resistance of 4v/7mA = 560R nominal.
When the 7407 output is Low the 560R will have a current of 42mA, approx 1Watt, so a 3Watt rated resistor would be my choice.

E

 

okay, Thanks

 

If a load device of 24 V / 7 mA is attached between a +24 V source and the 7407 collector, why do you need a pull up resistor? When the 7407 output is high, the load will be off. When the 7407 output is low, the load will have almost the full 24 V across it and the 7407 easily can sink 7 mA. If this is not what you want, then what are you trying to achieve?

Please post a schematic of your circuit.

ak