# Resistors

Discussion in 'The Projects Forum' started by rigers, Jul 13, 2012.

1. ### rigers Thread Starter New Member

Jun 14, 2011
20
0
Hello,

I am trying to built a circuit that will have very low current in it something in the micro-amp, even nano-amp if possible. I know the voltage across the resistors so by using ohms law I can get current.
I wanted to know if I should be looking for any specific resistor. I know about the tolerance, power rating and thermal coefficient (power and heat shouldn't be a problem at such low currents, right?) I was wondering if there is any other specification that I should look at or if there are any specific resistors designed for this. The largest resistor that I want to have is 1GΩ, and go down a decade until 1kΩ.

Thanks!

2. ### Markd77 Senior Member

Sep 7, 2009
2,803
596
Without knowing the reason, it might be hard to get good answers.
Diodes and other semiconductors have a reverse leakage current which is low but probably hard to predict.
1N914 reverse leakage current from datasheet is maximum 25nA at 20V and 25 celcius.

3. ### rigers Thread Starter New Member

Jun 14, 2011
20
0
The voltage range will 0v to 5v. I want to send a constant current/voltage to a battery, and because the battery will be charging and discharging I want to measure the current through the resistor. Depending on the measured current I'll change the input voltage so the current stays constant.

DAC->resistor->battery->GND

The dac only provides 0-5v but I have 12v going through the board.
However, I know there are circuits with op-amps and stuff but this system seems to be working really well. Also I wanted something that goes really close to GND. Also, I can do negative currents by flipping the order to

DAC->battery->resistor->GND

Any suggestions would be welcome, but the resistors as mentioned in the initial post are more of a concern right now.

4. ### WBahn Moderator

Mar 31, 2012
22,755
6,779
You description leaves me scratching my head.

What is the basic, fundamental piece of information you are trying to determine? In other words, if you could ask one question about the state of the system and couldn't do any computation on the answer at all, what would the question be?

How is the battery being charged? I assume it is NOT through this large valued resistor.

With large resistors, at some point your measurement will become dominated by thermal noise.

Are your two descriptions intended to show a series connection between the four components in each case? A simply diagram (just something thrown together in Paint) would be quite helpful.

5. ### wayneh Expert

Sep 9, 2010
14,616
5,161
Designed for what? I can't quite tell what you hope to accomplish with your resistor. If you need an accurate resistance so that you can convert voltage to current accurately, then choose a low tolerance range and a low temperature sensitivity.

And read the wiki on resistors - you'll see that they're made of different materials that might impact your system. Carbon film resistors, for instance, are known to be "noisy". EVERY real device has capacitance and inductance in addition to the DC resistance, and everything responds to ambient conditions such as temperature. These effects are usually so small that they're of little consequence most of the time, unless you're designing for high frequency.

6. ### rigers Thread Starter New Member

Jun 14, 2011
20
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Sorry for taking long to reply back. I added a picture so maybe you guys can get a better idea.
Also think of the batteries (V5,V4) as DACs. The opamp can act as a switch because it has an on/off pin and I'm using it to provide higher current. When off it has high impedance. I don't have negative voltage that's why I have the opamps like that and the DACs like that.

Again sorry for taking this long to reply back!

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7. ### DerStrom8 Well-Known Member

Feb 20, 2011
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That schematic is very confusing. Not to mention it breaks about a dozen standard conventions for drawing electronic schematic diagrams

It still doesn't answer what exactly you're trying to do.