# design a 500ma current source

#### rijinca

Joined Feb 23, 2010
5
can anyone help me to design an constant current source with high ouput impedence in 50M ohm range

#### Ron H

Joined Apr 14, 2005
7,014
Use a field-effect current regulator diode! Google for the right type of device.

Alberto
I don't think those are available at 500mA.

Rijinca, how precise does it have to be?
How stable does it have to be as a function of temperature?
What supply voltage are you planning to use, or is that a choice that depends on the final design?

#### rijinca

Joined Feb 23, 2010
5
my project was to construct a insulator surface resistivity meter using 4 point probe methord,
the load ranges around 5mega ohms

let me give a brief idea of the process:

current is channeled though two probe into the insulator layer and the resulting voltage ismeasured
resistivity is calculaated as r=k*v/i
voltage source can be design dependent

#### beenthere

Joined Apr 20, 2004
15,819
You might want to scale back your current - 500 milliamps through 5 million ohms means a driving voltage of 2,500,000. That is difficult to realize.

#### DickCappels

Joined Aug 21, 2008
6,532
You might be able to acheive 50 Megohms by using a MOSFET and a MOSFET input op amp.

You choice of 500 ma to test the surface resistance of an insulator sounds like it might not be optimum.

What is the range of resistances that you expect the curent source to be able to drive? That would tell you the required compliance voltage, which is a key design parameter.

#### rijinca

Joined Feb 23, 2010
5
the load would be in mega ohms range

#### Ron H

Joined Apr 14, 2005
7,014
the load would be in mega ohms range
How much current do you need? Keep in mind that a microamp times a megohm is a volt.

#### russ_hensel

Joined Jan 11, 2009
825
High current current sources are often built using voltage regulators. Not sure what the output impedance is. Look at the application notes ( maybe even the spec sheets ). Maximum voltage is voltage in - voltage of the regulator ( i think )

#### SgtWookie

Joined Jul 17, 2007
22,210
I'm wondering what load could survive 1.25megawatts power dissipation?

500mA current through 5MEG Ohms is 1.25megawatts.

#### Ron H

Joined Apr 14, 2005
7,014
I'm wondering what load could survive 1.25megawatts power dissipation?

500mA current through 5MEG Ohms is 1.25megawatts.
He's measuring surface resistivity. Perhaps it's the Earth's surface.

#### SgtWookie

Joined Jul 17, 2007
22,210
He's measuring surface resistivity. Perhaps it's the Earth's surface.
There are probably easier ways to bore a hole through the planet.

#### russ_hensel

Joined Jan 11, 2009
825
The output impedance is not a measure of the voltage, it is normally a measure of the accuracy of the constant current, ie compare to a constant current source formed by a voltage source and a resistor. That would have a high voltage.

#### Ron H

Joined Apr 14, 2005
7,014
The output impedance is not a measure of the voltage, it is normally a measure of the accuracy of the constant current, ie compare to a constant current source formed by a voltage source and a resistor. That would have a high voltage.
Here's a quote from the OP:
my project was to construct a insulator surface resistivity meter using 4 point probe methord,
the load ranges around 5mega ohms
And another:
the load would be in mega ohms range

#### russ_hensel

Joined Jan 11, 2009
825
"the load would be in mega ohms range"

Your are right (Ron H), I was not paying attention to that part.