Favourite circuits for CC generator ?

C

ChrisGibboGibson

Guest
I want a constant current generator that will produce 1 to 2 mA over an output
range of 2 to 6 volts running from a 10 to 15 volt supply.

It seems quite an easy task but here are the full requirements.

The exact tolerance of the current isn't that critical. 1mA would be fine,
1.2mA would be fine 1.6mA would be fine. Whatever is settled on, the tolerance
between different units could be as high as 10% without it causing any
problems.

Drift over time, in the long term, or with temperature isn't a problem either.
Even wild drift of up to 20% can be tolerated.

The difficult part is that over a 20mS period, and with the load varying such
that the CC output will swing from 2 to 6 volts, the current must not vary by
more than 0.05%

I'm up against a wall trying to come up with a circuit that meets this spec. No
doubt one of you lot knows one off the top of your head.

Gibbo
 
John Larkin wrote:

On 30 Sep 2004 20:07:32 GMT, chrisgibbogibson@aol.com
(ChrisGibboGibson) wrote:

I want a constant current generator that will produce 1 to 2 mA over an
output
range of 2 to 6 volts running from a 10 to 15 volt supply.

It seems quite an easy task but here are the full requirements.

The exact tolerance of the current isn't that critical. 1mA would be fine,
1.2mA would be fine 1.6mA would be fine. Whatever is settled on, the
tolerance
between different units could be as high as 10% without it causing any
problems.

Drift over time, in the long term, or with temperature isn't a problem
either.
Even wild drift of up to 20% can be tolerated.

The difficult part is that over a 20mS period, and with the load varying
such
that the CC output will swing from 2 to 6 volts, the current must not vary
by
more than 0.05%

I'm up against a wall trying to come up with a circuit that meets this spec.
No
doubt one of you lot knows one off the top of your head.

Gibbo

Small mosfet, source-lead current sampling resistor, bandgap voltage
ref, opamp closing the loop. I could post a schematic to abse if you
want.

Oh, source or sink?
Click to expand...
Source.

Will that meet the spec ?

Simulations say not though I haven't tried it in real life.

Gibbo
 
"ChrisGibboGibson" <chrisgibbogibson@aol.com> a écrit dans le message de
news:20040930185939.18603.00001764@mb-m07.aol.com...
"Fred Bartoli" wrote:

"ChrisGibboGibson" <chrisgibbogibson@aol.com> a écrit dans le message de
news:20040930160732.17863.00000014@mb-m01.aol.com...

Looks too easy to true to me though :)

Nah, the "trick" is to use a mosfet to avoid the output transistor base
Click to expand...
current variation due to base width modulation.


--
Thanks,
Fred.
 
ChrisGibboGibson wrote:

John Larkin wrote:


Small mosfet, source-lead current sampling resistor, bandgap voltage
ref, opamp closing the loop. I could post a schematic to abse if you
want.

Oh, source or sink?


Source.

Will that meet the spec ?

By a couple orders of magnitude!

Simulations say not though I haven't tried it in real life.

What do the sims show? Where is the error?


About 0.1% variation in current from 2 to 6 volts with a 10V supply. At 15V the
error is less.
Click to expand...
The DC error is limited by the open loop gain of the opamp.

Rene
--
Ing.Buero R.Tschaggelar - http://www.ibrtses.com
& commercial newsgroups - http://www.talkto.net
 
Jim Thompson wrote...
Replace "whatever" NPN with a TL431 or the low-voltage (1.25V)
equivalent.
Click to expand...
| +V |
| | |
| 100k ||-' 2N7000
| | ||<- 2N7002
| +---||-+
| \__|__ | 1.23
| / \_____| I = ---- = 1.0 mA
| LM385-adj /___\ | R R
| | 1.2k
| | |
| === ===
| GND GND


--
Thanks,
- Win

(email: use hill_at_rowland-dotties-org for now)
 
Winfield Hill wrote:

Jim Thompson wrote...

Replace "whatever" NPN with a TL431 or the low-voltage (1.25V)
equivalent.

| +V |
| | |
| 100k ||-' 2N7000
| | ||<- 2N7002
| +---||-+
| \__|__ | 1.23
| / \_____| I = ---- = 1.0 mA
| LM385-adj /___\ | R R
| | 1.2k
| | |
| === ===
| GND GND


--
Thanks,
- Win

(email: use hill_at_rowland-dotties-org for now)
Click to expand...
I'll go work out how to turn it into a source instead of a sink.

Thanks
Gibbo
 
"Fred Bartoli" wrote:

"Winfield Hill" <Winfield_member@newsguy.com> a écrit dans le message de
news:cjjrnb02aea@drn.newsguy.com...
Jim Thompson wrote...

Replace "whatever" NPN with a TL431 or the low-voltage (1.25V)
equivalent.

| +V |
| | |
| 100k ||-' 2N7000
| | ||<- 2N7002
| +---||-+
| \__|__ | 1.23
| / \_____| I = ---- = 1.0 mA
| LM385-adj /___\ | R R
| | 1.2k
| | |
| === ===
| GND GND



Hey, is it a componant count contest, or transistor count contest ?

We have the same count on the first one, I win on the second :)

Oh... What Chris need is a current *source*. I win the third :)
Well, hum, unless you can use the 385/431 in reverse or have a nice trick to
share.
Click to expand...
Yeah I'm still working on that and making no progress.

Fred wins so far.

Gibbo
 
On Fri, 01 Oct 2004 12:16:26 +0200, Rene Tschaggelar <none@none.net>
wrote:

ChrisGibboGibson wrote:

John Larkin wrote:


Small mosfet, source-lead current sampling resistor, bandgap voltage
ref, opamp closing the loop. I could post a schematic to abse if you
want.

Oh, source or sink?


Source.

Will that meet the spec ?

By a couple orders of magnitude!

Simulations say not though I haven't tried it in real life.

What do the sims show? Where is the error?


About 0.1% variation in current from 2 to 6 volts with a 10V supply. At 15V the
error is less.


The DC error is limited by the open loop gain of the opamp.
Click to expand...
To, maybe, a few PPM.

John
 
I read in sci.electronics.design that Winfield Hill
<Winfield_member@newsguy.com> wrote (in <cjkena0171i@drn.newsguy.com>)
about 'Favourite circuits for CC generator ?', on Fri, 1 Oct 2004:

Sorry, folks, I meant to say the TLV431, which controls its HI pin:
Click to expand...
Do these arrangements produce a predictable source resistance or is it
just 'very, very high'?
--
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
 
"John Woodgate" <jmw@jmwa.demon.contraspam.yuk> a écrit dans le message de
news:zeN5hsA9+cXBFwBv@jmwa.demon.co.uk...
I read in sci.electronics.design that Winfield Hill
Winfield_member@newsguy.com> wrote (in <cjkena0171i@drn.newsguy.com>)
about 'Favourite circuits for CC generator ?', on Fri, 1 Oct 2004:

Sorry, folks, I meant to say the TLV431, which controls its HI pin:

Do these arrangements produce a predictable source resistance or is it
just 'very, very high'?
Click to expand...
These source resistances are very, very, very, very very, very high.

Even the simple one with a single transistor is over 1G.

You still can render them predictable with and additional resistor :)


--
Thanks,
Fred.
 
On Friday 01 October 2004 02:05 pm, Fred Bartoli did deign to grace us with
the following:

Bummer! Guess what : I've just discovered I've always assumed the 385-adj
controled its top pin like the 431, and never bothered to check the DS,
just working from bad memory.

Still I win on transistor count and yours is vastly over engineered :)

But I thought it was a transient response contest!
Click to expand...

Cheers!
Rich
 
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