resistor across analog output?

panthripu

Member
P
Join Date
Mar 2004
Posts
228
hi everybody
i have a small question, i am using siemens step s7-400.One of my analog output i am using for ref. to proportion valve with +/-10V.
I found one resistor of 500Ohms across output wires at plc .What is this for?
 
Then the cards been configured for -20 to +20mA.


**edit**
Panthripu,
Not wishing to appear rude but you really should know simple stuff like this - it's Electrical 101 - Ohms Law.

I apologise in advance if you are still a student/trainee.

.
.
 
Last edited:
I don't know the S7-400 analogue I/O but the S7-200 has a minumum resistance for the device that you are sending a voltage to. For the S7-200 the minumum is 5k0 (5000 ohms) so having a 500 ohm resistor would overload the output.

Bryan
 
thanx friends
first of all let me clear to my friend that nothing in this world is simple as you see it, it may seem simple but in actual not so simple.
Second i am not a trainee , rather i am working for last 15 years.infact s7 is my new field.
i can very well understand and seen it that if we have output of 4-20mA and our application need 0-10V then we can put a resistance across the output terminal and can convert in voltage.
but when i saw that the output from plc itself is +/- 10 V and a resistor of 500Ohm across it.
Sometime in some application we need some minimum load all the time ,in such condition we can use it but i dont know about s7-400.
 
panthripu my friend,

I believed that your S7-400 analog output module had been wired and configured to produce an output current signal of -20 to +20mA.
Bridging that output with a 500Ώ resistor would convert it to -10 to +10V

But if you say that it's been configured and wired for -10 to +10V then I've no idea why anyone would fit a 500Ώ resistor.



 
Last edited:
Was the resistor directly across the terminals?


That is, in parallel with the load (IP valve).

This maybe a hardwired way of dividing the potential to limit the maximum voltage to the ip valve....... but pretty wacky idea.

Cheers
 
Is it possible that some trainee put it there thinking that the output is -20/+20mA?
 
Last edited:
resistor across o/p

i agree with cjd...
it seems like that but let me check the whole system throughly and i will let you know.
 
Putting a 500 ohm resistor in parallel with the load will only limit the voltage to a maximum of 9.63 V (assuming a typical output resistance of 19 ohms)
 
I misread the ops original post as 5000 not 500 Ohmsdue to the missing space.
 

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