Searching \ for '[EE:] Constant current diodes or FETs?' in subject line. ()
Make payments with PayPal - it's fast, free and secure! Help us get a faster server
FAQ page: www.piclist.com/techref/power.htm?key=current
Search entire site for: 'Constant current diodes or FETs?'.

Exact match. Not showing close matches.
PICList Thread
'[EE:] Constant current diodes or FETs?'
2004\06\24@163707 by Harold Hallikainen

face picon face
I'm loooking for a two terminal constant current device that could be put
between an LED and a variable DC supply to hold the LED current at 5mA as
the supply varies over a 0-10V range (recognizing we're not going to get
it at 0V, but I'd like the widest range possible). So far i've found the
Central Semiconductor CCLM5750 which is a nominal 5.750mA, but it takes
4.5V to get 80% of the rated current. Anyone have other ideas?

THANKS!

Harold


--
FCC Rules Online at http://www.hallikainen.com

--
http://www.piclist.com hint: To leave the PICList
spam_OUTpiclist-unsubscribe-requestTakeThisOuTspammitvma.mit.edu

2004\06\24@164439 by Robert B.

flavicon
face
Have you considered biasing a BJT with a zener diode to create a crude
current source? It should work from about 1v on up, so long as the Zener can
handle the current at max voltage.  It would consume power at a fairly large
rate, so probably not great for battery powered projects.

{Original Message removed}

2004\06\24@170833 by Bob Blick

face picon face
> I'm loooking for a two terminal constant current device that could be put
> between an LED and a variable DC supply to hold the LED current at 5mA as
> the supply varies over a 0-10V range (recognizing we're not going to get
> it at 0V, but I'd like the widest range possible). So far i've found the
> Central Semiconductor CCLM5750 which is a nominal 5.750mA, but it takes
> 4.5V to get 80% of the rated current. Anyone have other ideas?

Hi Harold,

Would you settle for 8 mA? The 2N5951 (NS, Fairchild) I just tried works
great. My other choice at hand, the MPF102, runs at 15 mA.

Tie gate to either drain or source, use that as the more negative side.

2N5951 available everywhere, in small quantities at Jameco.

Cheers,

Bob

--
http://www.piclist.com hint: To leave the PICList
.....piclist-unsubscribe-requestKILLspamspam@spam@mitvma.mit.edu

2004\06\24@171129 by llile

flavicon
face
If you put a 5v 78L05 regulator across a resistor, it becomes a constant
current device.  A pretty cheap way of doing it, maybe easier to get than
a constant current diode.  You get the constant current, IIRC, out of the
ground pin with the resistor connected between the ground pin and 5V pin.


-- Lawrence Lile
Lile Engineering
Embedded solutions and industrial controls





Harold Hallikainen <haroldspamKILLspamHALLIKAINEN.COM>
Sent by: pic microcontroller discussion list <.....PICLISTKILLspamspam.....MITVMA.MIT.EDU>
06/24/2004 03:37 PM
Please respond to pic microcontroller discussion list


       To:     EraseMEPICLISTspam_OUTspamTakeThisOuTMITVMA.MIT.EDU
       cc:
       Subject:        [EE:] Constant current diodes or FETs?


I'm loooking for a two terminal constant current device that could be put
between an LED and a variable DC supply to hold the LED current at 5mA as
the supply varies over a 0-10V range (recognizing we're not going to get
it at 0V, but I'd like the widest range possible). So far i've found the
Central Semiconductor CCLM5750 which is a nominal 5.750mA, but it takes
4.5V to get 80% of the rated current. Anyone have other ideas?

THANKS!

Harold


--
FCC Rules Online at http://www.hallikainen.com

--
http://www.piclist.com hint: To leave the PICList
piclist-unsubscribe-requestspamspam_OUTmitvma.mit.edu



--
http://www.piclist.com hint: To leave the PICList
@spam@piclist-unsubscribe-requestKILLspamspammitvma.mit.edu

2004\06\24@172050 by Jose Da Silva

flavicon
face
On June 24, 2004 01:37 pm, Harold Hallikainen wrote:
>I'm loooking for a two terminal constant current device that could be put
>between an LED and a variable DC supply to hold the LED current at 5mA as
>the supply varies over a 0-10V range (recognizing we're not going to get
>it at 0V, but I'd like the widest range possible). So far i've found the
>Central Semiconductor CCLM5750 which is a nominal 5.750mA, but it takes
>4.5V to get 80% of the rated current. Anyone have other ideas?

Haven't tried this, but the theory seems okay.
Put the LED in parallel with a 1N4001 diode.
then put the both of them in series with your circuit.

The LED is expecting more than .7v to run full brilliance, so it will
probably run a bit on the dim side.
The 1N4001 should be able to handle 1amp and acts like a 0.7v clamp on the
LED.

Select something bigger than a 1N4005 if you need to deal with more
current.

--
http://www.piclist.com hint: To leave the PICList
KILLspampiclist-unsubscribe-requestKILLspamspammitvma.mit.edu

2004\06\24@172713 by Dave Dilatush

picon face
Harold Hallikainen wrote...

>I'm loooking for a two terminal constant current device that could be put
>between an LED and a variable DC supply to hold the LED current at 5mA as
>the supply varies over a 0-10V range (recognizing we're not going to get
>it at 0V, but I'd like the widest range possible). So far i've found the
>Central Semiconductor CCLM5750 which is a nominal 5.750mA, but it takes
>4.5V to get 80% of the rated current. Anyone have other ideas?

How about NSC's LM134?  It's actually billed as a temperature
sensor because its current output is proportional to absolute
temperature; but it seems to me that would be an advantage in
this application as it would tend to offset the LED's somewhat
negative temperature dependence.

AFAIK the LM134 requires about 1 volt of headroom to operate.  A
current programming resistor is also required; for 5 mA the value
would be about 12 ohms.

HTH...

Dave D.

--
http://www.piclist.com hint: To leave the PICList
RemoveMEpiclist-unsubscribe-requestTakeThisOuTspammitvma.mit.edu

2004\06\24@174202 by res0qrqr

picon face
Just curious, why the two-terminal restriction?
To me, it sounds like a dandy application for a TL431
(but then you've got a third terminal and two
resistors to deal with...)

{Quote hidden}

--
http://www.piclist.com hint: To leave the PICList
spamBeGonepiclist-unsubscribe-requestspamBeGonespammitvma.mit.edu

2004\06\24@180445 by Dave VanHorn

flavicon
face
>
> > I'm loooking for a two terminal constant current device that could be put
> > between an LED and a variable DC supply to hold the LED current at 5mA as
> > the supply varies over a 0-10V range (recognizing we're not going to get
> > it at 0V, but I'd like the widest range possible).

As you approach zero, the problems begin. In fact, with a red LED you'll
need at least 2V.
An LM317 regulator can be set as a constant current source, at a  penalty
of another couple volts.

--
http://www.piclist.com hint: To leave the PICList
TakeThisOuTpiclist-unsubscribe-requestEraseMEspamspam_OUTmitvma.mit.edu

2004\06\24@181104 by Harold Hallikainen

face picon face
The two terminal restriction is because I have no power supply available,
just the 0-10VDC signal that needs to power the LED. I'd like to get
constant current through the LED over as much of the voltage range as
possible. I'll accept 3 terminals as long as I don't need another power
supply...

THANKS!

Harold


{Quote hidden}

--
FCC Rules Online at http://www.hallikainen.com

--
http://www.piclist.com hint: To leave the PICList
piclist-unsubscribe-requestEraseMEspam.....mitvma.mit.edu

2004\06\24@181107 by Spehro Pefhany

picon face
At 05:04 PM 6/24/2004 -0500, you wrote:

>> > I'm loooking for a two terminal constant current device that could be put
>> > between an LED and a variable DC supply to hold the LED current at 5mA as
>> > the supply varies over a 0-10V range (recognizing we're not going to get
>> > it at 0V, but I'd like the widest range possible).
>
>As you approach zero, the problems begin. In fact, with a red LED you'll
>need at least 2V.
>An LM317 regulator can be set as a constant current source, at a  penalty
>of another couple volts.

If you go three terminal, and multiple components, it's not that hard to get
within some mV of the actual forward voltage, but kind of a pointless
design exercise..

Best regards,

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

--
http://www.piclist.com hint: To leave the PICList
RemoveMEpiclist-unsubscribe-requestEraseMEspamEraseMEmitvma.mit.edu

2004\06\24@181727 by Harold Hallikainen

face picon face
THANS! The LM134 DOES look interesting.

Harold


{Quote hidden}

--
FCC Rules Online at http://www.hallikainen.com

--
http://www.piclist.com hint: To leave the PICList
RemoveMEpiclist-unsubscribe-requestTakeThisOuTspamspammitvma.mit.edu

2004\06\24@181728 by Harold Hallikainen

face picon face
Thanks for the idea! However, here I need to also keep the supply current
to a minimum.

Harold


{Quote hidden}

--
FCC Rules Online at http://www.hallikainen.com

--
http://www.piclist.com hint: To leave the PICList
RemoveMEpiclist-unsubscribe-requestKILLspamspammitvma.mit.edu

2004\06\24@181937 by Harold Hallikainen

face picon face
Thanks! Forward voltage there is too high (5V + about 2V dropout of
regulator). I'm looking for 0V (ideally).

Thanks!

Harold

{Quote hidden}

--
FCC Rules Online at http://www.hallikainen.com

--
http://www.piclist.com hint: To leave the PICList
spamBeGonepiclist-unsubscribe-requestspamKILLspammitvma.mit.edu

2004\06\24@181938 by Dave VanHorn

flavicon
face
At 03:10 PM 6/24/2004 -0700, Harold Hallikainen wrote:
>The two terminal restriction is because I have no power supply available,
>just the 0-10VDC signal that needs to power the LED.

Ok, you were putting unneeded restrictions in place.
One option is to power your constant current circuit above the LED, so that
it's Vf isn't subtracted from the operating range.

--
http://www.piclist.com hint: To leave the PICList
.....piclist-unsubscribe-requestspam_OUTspammitvma.mit.edu

2004\06\24@183636 by Harold Hallikainen

face picon face
Sounds interesting. Neither Fairchild or National have data on the 2N5951.
Sounds like your setting Vgs to zero, right? That then results in 8mA?
Over what voltage range?

THANKS!

Harold


{Quote hidden}

--
FCC Rules Online at http://www.hallikainen.com

--
http://www.piclist.com hint: To leave the PICList
TakeThisOuTpiclist-unsubscribe-requestKILLspamspamspammitvma.mit.edu

2004\06\24@183845 by Harold Hallikainen

face picon face
Thanks for the idea! We'd have the voltage Vce drop plus the emitter
resistor drop, then have to get current to power the zener bias circuit. I
don't think I can get that whole thing to work on 1V or less. Thanks,
though!

Harold


> Have you considered biasing a BJT with a zener diode to create a crude
> current source? It should work from about 1v on up, so long as the Zener
> can
> handle the current at max voltage.  It would consume power at a fairly
> large
> rate, so probably not great for battery powered projects.
>
> {Original Message removed}

2004\06\24@185129 by Harold Hallikainen

face picon face
Thanks! I'm not seeing how I can get around the LED Vf requirement. What I
need is a dynamic resistor that sits between the LED and the variable
supply. The supply varies between 0 and 10V. No matter what the voltage, I
want the LED current to be 5mA. (recognizing, of course, that I won't get
that when the power supply voltage is below the LED Vf). This "dynamic
resistor" should adjust between 0 ohms and some higher resistance to limit
the LED current to 5mA. Ideally the circuit draws only 5mA (and all of
that goes through the LED).

What I'm trying to do is build a solid state relay that has a very low
turn on voltage, very low operating current, and the control current does
not increase with control voltage. Ideally, the SSR would turn on with a
control voltage of 500mV or so, but I don't see that happening. We have no
power supply available other than the control voltage. An MOC3023 fires
the output triac at about 1.1V across the LED with an LED current of 3mA.
The LED right across our control voltage is pretty good until the voltage
goes above 1.1V when, of course, the current increases. So, how can I keep
that current from increasing without losing my low turn-on voltage?

THANKS to all for the great ideas!

Harold


{Quote hidden}

--
FCC Rules Online at http://www.hallikainen.com

--
http://www.piclist.com hint: To leave the PICList
RemoveMEpiclist-unsubscribe-requestspamspamBeGonemitvma.mit.edu

2004\06\24@185337 by Bob Blick

face picon face
> Sounds interesting. Neither Fairchild or National have data on the 2N5951.
> Sounds like your setting Vgs to zero, right? That then results in 8mA?
> Over what voltage range?

Hi Harold,

Here's a link to some minimal 2N5951 data:
http://www.jameco.com/Jameco/Products/ProdDS/38527.pdf

Tie gate to either source or drain, JFETs don't really care, then just
treat it like a two-terminal device and put it in series with your LED. 8
mA is reached pretty quickly - the jfet has its on-resistance of about 100
ohms.

So your LED acts like it has a small resistor in series, until the point
the jfet starts to current limit(it's up to about 200 ohms with a volt
across it).

Current after that is solid at 8mA until I ran out of power supply at 15
volts.

I'm sure there are lots of jfets that would work, I only had two types on
hand to test.

If I had to guess, I'd bet the 2SK30A would work really well, but almost
all my analog stuff like that is in storage.

Cheers,

Bob

--
http://www.piclist.com hint: To leave the PICList
spamBeGonepiclist-unsubscribe-request@spam@spamspam_OUTmitvma.mit.edu

2004\06\24@191514 by Dwayne Reid

flavicon
face
At 04:52 PM 6/24/2004, Harold Hallikainen wrote:
>Thanks! I'm not seeing how I can get around the LED Vf requirement. What I
>need is a dynamic resistor that sits between the LED and the variable
>supply. The supply varies between 0 and 10V. No matter what the voltage, I
>want the LED current to be 5mA. (recognizing, of course, that I won't get
>that when the power supply voltage is below the LED Vf). This "dynamic
>resistor" should adjust between 0 ohms and some higher resistance to limit
>the LED current to 5mA. Ideally the circuit draws only 5mA (and all of
>that goes through the LED).

Classic 2 transistor current source is probably as close as you are going
to get: uses 2 transistors of same polarity plus 2 resistors.  I like to
use 2n4401.  Not your ideal zero parts solution but will work down to about
0.1V above the LEDs Vf.

Led anode to V+.  Led cathode to Q1c.  Q1e to V- via Iset resistor
(120R).  Q2e to V-.  Q2c to Q1b and also to 1 side of bias resistor
(10k).  Other side of bias resistor to V+.  Q2b to Q1e.  Bias resistor
tries to turn Q1 on as hard as it can.  Q2 monitors the voltage drop across
Iset resistor and throttles Q1 back.

I'm guessing that it should work down to about 2V with a red LED.

dwayne

--
Dwayne Reid   <TakeThisOuTdwaynerspamspamplanet.eon.net>
Trinity Electronics Systems Ltd    Edmonton, AB, CANADA
(780) 489-3199 voice          (780) 487-6397 fax

Celebrating 20 years of Engineering Innovation (1984 - 2004)
 .-.   .-.   .-.   .-.   .-.   .-.   .-.   .-.   .-.   .-
    `-'   `-'   `-'   `-'   `-'   `-'   `-'   `-'   `-'
Do NOT send unsolicited commercial email to this email address.
This message neither grants consent to receive unsolicited
commercial email nor is intended to solicit commercial email.

--
http://www.piclist.com hint: To leave the PICList
piclist-unsubscribe-requestEraseMEspammitvma.mit.edu

2004\06\24@192136 by Harold Hallikainen

face picon face
Sounds good! Samples on order!

Harold


{Quote hidden}

--
FCC Rules Online at http://www.hallikainen.com

--
http://www.piclist.com hint: To leave the PICList
@spam@piclist-unsubscribe-requestRemoveMEspamEraseMEmitvma.mit.edu

2004\06\24@193833 by Harold Hallikainen

face picon face
Thanks for all your effort! It looks like we'd need about 700mv across
Iset, then another 50mv across Q1 CE (assuming saturation), so that'd be a
minimum of 750mv. Pretty good! I'll try it out.

Thanks!

Harold



{Quote hidden}

--
FCC Rules Online at http://www.hallikainen.com

--
http://www.piclist.com hint: To leave the PICList
spamBeGonepiclist-unsubscribe-requestEraseMEspammitvma.mit.edu

2004\06\24@205411 by Spehro Pefhany

picon face
At 03:52 PM 6/24/2004 -0700, you wrote:
>Thanks! I'm not seeing how I can get around the LED Vf requirement. What I
>need is a dynamic resistor that sits between the LED and the variable
>supply. The supply varies between 0 and 10V. No matter what the voltage, I
>want the LED current to be 5mA. (recognizing, of course, that I won't get
>that when the power supply voltage is below the LED Vf). This "dynamic
>resistor" should adjust between 0 ohms and some higher resistance to limit
>the LED current to 5mA. Ideally the circuit draws only 5mA (and all of
>that goes through the LED).

You can do this with two active parts (an LM10C and a BJT) plus three
resistor in such a way as to get down to within 60mV or so of the Vf for
minimum supply voltage. Divide the 800mV reference down to 50mV with two
resistors, connect it to the non-inverting input of the op-amp. Connect
the op-amp output to the base of the transistor and feed the voltage
back from an emitter sense resistor to ground to the op-amp inverting input.
The LM10C is powered from the supply voltage, the LED goes from the
collector to the +ve supply voltage. Wasted current is a few hundred uA
typically, and it will work down to Vf + 60mV or so with any color of
LED. You use an emitter resistor to drop 50mV at 5mA, or 10 ohms.

The LM10C is a bit expensive though (1.60 in 100's), the other parts
cost just about nothing.

Best regards,

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

--
http://www.piclist.com hint: To leave the PICList
RemoveMEpiclist-unsubscribe-request@spam@spamspamBeGonemitvma.mit.edu

2004\06\25@101924 by David VanHorn

flavicon
face
At 04:11 PM 6/24/2004 -0500, .....llile@spam@spamEraseMESALTONUSA.COM wrote:

>If you put a 5v 78L05 regulator across a resistor, it becomes a constant
>current device.  A pretty cheap way of doing it, maybe easier to get than a constant current diode.  You get the constant current, IIRC, out of the ground pin with the resistor connected between the ground pin and 5V pin.

The LM317 and LM338 do it with lower voltage drop, since you can set it to 1.2V output. I think that's also a band-gap voltage. They are VERY flat with temperature, I use these for my NIMH battery station, on the dischargers.
They get pretty hot, dumping 7-14W each, but they remain flat.

IIRC, they make similar devices up to about 10A.

--
http://www.piclist.com hint: PICList Posts must start with ONE topic:
[PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads

2004\06\25@103915 by Harold Hallikainen

face picon face
Interesting idea! Seems like I could just use a rail to rail op amp with
the LED between output and inverting input. Current sense resistor between
inverting input and ground. Reference voltage on non-inverting input.

Thanks!

Harold


{Quote hidden}

--
FCC Rules Online at http://www.hallikainen.com

--
http://www.piclist.com hint: PICList Posts must start with ONE topic:
[PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads

2004\06\25@110330 by Spehro Pefhany

picon face
At 07:36 AM 6/25/2004 -0700, you wrote:
>Interesting idea! Seems like I could just use a rail to rail op amp with
>the LED between output and inverting input. Current sense resistor between
>inverting input and ground. Reference voltage on non-inverting input.
>
>Thanks!

Yes, most likely with poorer to much poorer dropout performance. Watch the
supply voltage of the op-amp too, many R-R types these days can't handle
your 10V. The rather mature (ca. 1976) LM10 will operate from as low as 1.2V
(which is lower than the Vf of any visible LED) up to 40V-45V. Really one of
Robert Widlar's finest pieces of work.

Here's a 22M TIF of him looking at the LM10 layout (DON'T download unless you
have a good connection or lots of time-- 22M):

http://appsrv.national.com/gallery1/231/231_rgb.tif

Best regards,

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

--
http://www.piclist.com hint: PICList Posts must start with ONE topic:
[PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads

More... (looser matching)
- Last day of these posts
- In 2004 , 2005 only
- Today
- New search...