Electronic Circuits and Projects Forum

What's wrong with just using a 250 ohm resistor?.

Originally Posted by tahoka

Please can someone suggest me an alternative to IC 741 in the basic V to I converter and non inverting amplifier circuit as we require an opamp which uses only a single supply.
The input to V to I converter is 0 to 5 volts and output is 0 to 20 milliamperes.
the output of amplifier is 0 to 5 volts.

hi,
You can buy V2I ic's, this is one example, single supply

Originally Posted by Hero999

Because it isn't a normal standard value?

Nevertheless it can easily made by connecting 240Ω and 10Ω in series or just use 249Ω which is a 2% value and is only 0.4% out in this case.

However it seems like a pretty common thing to do so it wouldn't surprise me if you can buy 250Ω resistors specifically made for the purpose

You can buy HS 250R's

http://www.rhopointcomponents.com/pr...partno=H1L45AL

Originally Posted by Nigel Goodwin

What's wrong with just using a 250 ohm resistor?.

A resistor works well if the load is zero ohms. If he is trying to make a voltage-controlled current source, well, ....

Originally Posted by Roff

A resistor works well if the load is zero ohms. If he is trying to make a voltage-controlled current source, well, ....

The load can't be zero ohms, it couldn't possibly work no matter what you did if it was

Originally Posted by Nigel Goodwin

The load can't be zero ohms, it couldn't possibly work no matter what you did if it was

It could be virtual ground. But you just made my point. If the load is other than zero ohms, then the current won't be Vin/250.

Originally Posted by Willbe

http://www.electro-tech-online.com/c...09/LM324-1.pdf

Or LM358, if you just want an 8 pin DIP.

One problem with using a 250Ω resistor is that it drops 5V which might not be acceptable.

Just use a 10Ω resister and a non-inverting amplifier with a gain of 25 and it will drop just 200mV.


http://en.wikipedia.org/wiki/Operati...lifier_circuit
R2 = 240k
R1 = 10k