Constant Current Source Design

[confused soul]

I need to design a constant current source of 0.1 mA. The load varies
between 1Mohm and 1 Kohm. Using a constant voltage source, is it
possible to design a constant current source?
Any help in this is greatly appreciated.

--Confused Soul

 

[Jim Thompson]

On 24 Jan 2005 08:47:31 -0800, "confused soul"
<confused.mind@gmail.com> wrote:

I need to design a constant current source of 0.1 mA. The load varies
between 1Mohm and 1 Kohm. Using a constant voltage source, is it
possible to design a constant current source?
Any help in this is greatly appreciated.

--Confused Soul

0.1mA thru 1Meg is 100V. What supplies do you have available?

...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona Voice480)460-2350 | |
| E-mail Address at Website Fax480)460-2142 | Brass Rat |
| http://www.analog-innovations.com | 1962 |

I love to cook with wine. Sometimes I even put it in the food.

 

[John Popelish]

confused soul wrote:

I need to design a constant current source of 0.1 mA. The load varies
between 1Mohm and 1 Kohm. Using a constant voltage source, is it
possible to design a constant current source?
Any help in this is greatly appreciated.

--Confused Soul

Good enough approximations may be possible.
Is the load resistance connected to a positive supply, a negative
supply, ground, or none of the above?

What supply voltages are available?

What kind of frequency response must this current source have?

0.1 mA through 1 Mohm implies 100 volts drop across that resistance,
so a voltage supply greater than 100 volts will be necessary to feed
the current regulator.

--
John Popelish

 

[Winfield Hill]

John Woodgate wrote...

confused soul asked...

I need to design a constant current source of 0.1 mA. The load
varies between 1Mohm and 1 Kohm. Using a constant voltage source,
is it possible to design a constant current source?

Yes. It sounds like homework to me. We don't do homework but we
may give hints to polite enquirers. ...
0.1 mA through 1 Mohm means that there is 100 V across the 1 Mohm.

Hints: check out common bipolar-transistor current-source circuits,
with opamp feedback. Think about using two MPSA42 (npn) or MPSA92
(pnp) transistors, wired in the Darlington configuration.


--
Thanks,
- Win

 

[confused soul]

Looking at the previous replies, I kinda feel dumb that I didnt even
think about the possible voltage required. 100 V seems impractical in
my case. It is possible for me to change the constant current value to
0.01 mA or even 1 uA instead of 0.1 mA. In that case, what kind of
circuits can be used for building a constant current source.

 

[Nicolai]

Have a look at the Fixed Current Source schematic on page 10 of
http://cache.national.com/ds/LM/LM124.pdf
Exchange the PNP transistor with a P-channel MOSFET with low Vgs threshold
and recalculate the resistor values for your own needs and it should do the
trick. If you need better accuracy just use a better OP-AMP and resistors.

The High Compliance Current Sink on page 12 will do the same trick if you
need to sink instead of source the current. Again just substitute the NPN
transistor with an N-channel MOSFET and recalculate for your needs. The
current sink circuit has the advantage that it can be made to work with any
voltage as long as you use the right MOSFET.

You can keep the transistors if you don't like MOSFETs but then you will
have a small error contribution from the base current. Old datasheets are
abundant with similar circuits.

HTH

"John Woodgate" <jmw@jmwa.demon.contraspam.yuk> wrote in message
news:SVfBDwGd4W9BFw1n@jmwa.demon.co.uk...

I read in sci.electronics.design that confused soul
confused.mind@gmail.com> wrote (in <1106601781.005044.163500@z14g2000cw
z.googlegroups.com&gt about 'Constant Current Source Design', on Mon, 24
Jan 2005:

Looking at the previous replies, I kinda feel dumb that I didnt even
think about the possible voltage required. 100 V seems impractical in
my case. It is possible for me to change the constant current value to
0.01 mA or even 1 uA instead of 0.1 mA. In that case, what kind of
circuits can be used for building a constant current source.

A few weeks ago we had a thread here in which a FET-based current source
could give output source impedances of gigohms. That should be OK for
you! If you can't search the group archive, maybe someone will give you
a message ID to look up or a copy of one of the relevant posts.
--
Regards, John Woodgate, OOO - Own Opinions Only.
The good news is that nothing is compulsory.
The bad news is that everything is prohibited.
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk

 

[Paul Burridge]

On 24 Jan 2005 13:23:01 -0800, "confused soul"
<confused.mind@gmail.com> wrote:

Looking at the previous replies, I kinda feel dumb that I didnt even
think about the possible voltage required. 100 V seems impractical in
my case. It is possible for me to change the constant current value to
0.01 mA or even 1 uA instead of 0.1 mA. In that case, what kind of
circuits can be used for building a constant current source.

A FET-based solution is well worth considering!
--

"What is now proved was once only imagin'd." - William Blake, 1793.

 

[Spehro Pefhany]

On Tue, 25 Jan 2005 18:23:16 +0100, the renowned "Nicolai"
<nmahncke@tiscali.dk> wrote:

Have a look at the Fixed Current Source schematic on page 10 of
http://cache.national.com/ds/LM/LM124.pdf
Exchange the PNP transistor with a P-channel MOSFET with low Vgs threshold
and recalculate the resistor values for your own needs and it should do the
trick. If you need better accuracy just use a better OP-AMP and resistors.

The High Compliance Current Sink on page 12 will do the same trick if you
need to sink instead of source the current. Again just substitute the NPN
transistor with an N-channel MOSFET and recalculate for your needs. The
current sink circuit has the advantage that it can be made to work with any
voltage as long as you use the right MOSFET.

You can keep the transistors if you don't like MOSFETs but then you will
have a small error contribution from the base current. Old datasheets are
abundant with similar circuits.

HTH

With a bipolar op-amp such as the LM324, there's an error component
from the input bias current. At 1uA, it could be as much as 50% error
(typically 4%). It also varies with supply voltage and somewhat with
temperature. There are better op-amps.


Best regards,
Spehro Pefhany
--
"it's the network..." "The Journey is the reward"
speff@interlog.com Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog Info for designers: http://www.speff.com