LED array current regulation

[DJ Delorie]

I'm trying to decide how to drive ~100 UV leds. The Vf is 3.2-3.4 V
typ, 3.8 V max. I have a 36.5 VDC power supply (measured @ 0.1A). I
need If of 20 mA (no control needed, just on or off) through each LED.
Simple and cheap are preferred, provided the results are "good
enough". The application is PCB photolithography.

So far, I've looked into a few options:

1. Resistors. 10 LEDs in a string, 175 ohm resistor. Cheap and easy,
but slight variations on Vf would cause big variations in If.
Bigger resistors and shorter strings perhaps? 470 ohm w/ 8 LED
strings? Still only 10% regulation, not sure if that's OK for LEDs
(I don't have much info on the specific LEDs I got, other than what
I've listed above).

2. Infineon BCR402UE6182. This is a discrete single-channel current
regulator, but doesn't seem to guarantee a low enough If.

3. MAX6971 (or 2xMAX6970). Only needs a resistor and clock source to
drive up to 16 strings at 20 mA, but the lead time is 4-5 weeks.

4. MAX6957. In stock at Digikey, but requires an MCU to program it
and common-base transistors to isolate it from the high voltages.

5. Redoing the power supply for 18VDC to accomodate some other
solution.

Suggestions? Alternatives?

 

[nospam]

I'm trying to decide how to drive ~100 UV leds. The Vf is 3.2-3.4 V
typ, 3.8 V max. I have a 36.5 VDC power supply (measured @ 0.1A). I
need If of 20 mA (no control needed, just on or off) through each LED.

An LM317 and a 62 ohm resistor makes a 20mA constant current series
element. 9 LEDs in a chain leaves enough voltage for it.

Digikey have 100mA TO92 LM317s at $0.43.

--

 

[ehsjr]

I'm trying to decide how to drive ~100 UV leds. The Vf is 3.2-3.4 V
typ, 3.8 V max. I have a 36.5 VDC power supply (measured @ 0.1A). I
need If of 20 mA (no control needed, just on or off) through each LED.
Simple and cheap are preferred, provided the results are "good
enough". The application is PCB photolithography.

So far, I've looked into a few options:

1. Resistors. 10 LEDs in a string, 175 ohm resistor. Cheap and easy,
but slight variations on Vf would cause big variations in If.
Bigger resistors and shorter strings perhaps? 470 ohm w/ 8 LED
strings? Still only 10% regulation, not sure if that's OK for LEDs
(I don't have much info on the specific LEDs I got, other than what
I've listed above).

2. Infineon BCR402UE6182. This is a discrete single-channel current
regulator, but doesn't seem to guarantee a low enough If.

3. MAX6971 (or 2xMAX6970). Only needs a resistor and clock source to
drive up to 16 strings at 20 mA, but the lead time is 4-5 weeks.

4. MAX6957. In stock at Digikey, but requires an MCU to program it
and common-base transistors to isolate it from the high voltages.

5. Redoing the power supply for 18VDC to accomodate some other
solution.

Suggestions? Alternatives?

Here's one way:
4 strings of 9 LEDs & 8 strings of 8.
Each string gets an LM317 and a 62 ohm resistor
to give you very close to 20 mA.

+36.5 ---+--Vin[LM317]Vout---+
| Adj |
| | [62R]
[.1uF] | |
| +----------+---[LEDstring]--+
| |
Gnd -----+------------------------------------+

If all the LM317's are physically close, you can use
a single .1 uF cap near them.

Ed

 

[Fred Bartoli]

Paul Mathews a écrit :

How I'd do it if you really want identical current: all LEDs in series
and use boost regulator. Of course, the switcher will develop over 300
volts, so this approach has its disadvantages. You could also consider
using, say, 4 strings of 25 LEDs and 4 boost regulators. There are lots
of control chips designed to provide constant current boost output.
OTOH, if you reduce the number of emitters in a series string so that
there is more voltage across the resistor, you'll find that resistive
current limiting actually works pretty well. Emitter follower drive,
with a current-setting resistor in the emitter, also provides good
current regulation for low cost if you can drive with a constant
voltage.
Paul Mathews

Or use this to share the boost current between the leds strings:

.-----------.
| BOOST |
| |-----+--------+--------.
| | | | |
| | V -> V -> V ->
'-----------' - - -
^ | | |
| . . . Leds strings
| . . .
| | | |
| V -> V -> V ->
| - - - R1
| | | | ___
| | | +--|___|-.
| | | | |
| | | | |
| |/ |/ |/ |
| +--------+--------+----------+
| |> |> |> |
| | | | |
'-----------+ | | |
.-. .-. .-. .-.
| | | | | | | | R2
| | | | | | | |
'-' '-' '-' '-'
| | | |
=== === === ===
GND GND GND GND

 

[DJ Delorie]

An LM317 and a 62 ohm resistor makes a 20mA constant current series
element. 9 LEDs in a chain leaves enough voltage for it. Digikey
have 100mA TO92 LM317s at $0.43.

That's just right for my purposes. Thanks!

 

[ehsjr]

I'm trying to decide how to drive ~100 UV leds. The Vf is 3.2-3.4 V
typ, 3.8 V max. I have a 36.5 VDC power supply (measured @ 0.1A). I
need If of 20 mA (no control needed, just on or off) through each LED.
Simple and cheap are preferred, provided the results are "good
enough". The application is PCB photolithography.

So far, I've looked into a few options:

1. Resistors. 10 LEDs in a string, 175 ohm resistor. Cheap and easy,
but slight variations on Vf would cause big variations in If.
Bigger resistors and shorter strings perhaps? 470 ohm w/ 8 LED
strings? Still only 10% regulation, not sure if that's OK for LEDs
(I don't have much info on the specific LEDs I got, other than what
I've listed above).

2. Infineon BCR402UE6182. This is a discrete single-channel current
regulator, but doesn't seem to guarantee a low enough If.

3. MAX6971 (or 2xMAX6970). Only needs a resistor and clock source to
drive up to 16 strings at 20 mA, but the lead time is 4-5 weeks.

4. MAX6957. In stock at Digikey, but requires an MCU to program it
and common-base transistors to isolate it from the high voltages.

5. Redoing the power supply for 18VDC to accomodate some other
solution.

Suggestions? Alternatives?

Here's one way:
4 strings of 9 LEDs & 8 strings of 8.
Each string gets an LM317 and a 62 ohm resistor
to give you very close to 20 mA.

+36.5 ---+--Vin[LM317]Vout---+
| Adj |
| | [62R]
[.1uF] | |
| +----------+---[LEDstring]--+
| |
Gnd -----+------------------------------------+

If all the LM317's are physically close, you can use
a single .1 uF cap near them.

Ed

36.5V - 1.25 - 3.8V * 9 = 1.05V, which isn't a heck of a lot of overhead
for an LM317.

True. But 3.8 is max vf - typical is 3.2 to 3.4.
At 3.4*9 + 1.25 in the 317, you still have 4.65
overhead - plenty for the LM317. And, as you
suggest, it's easy enough to add another LM317
and drive 13 strings.

If that 36.5V turns out to be nominal instead of actual,
then you're really in the suds.

Right. If we can't accept the 36.5 figure, then all bets are
off. He appears to be trying to get by with the supply he
has on hand. He measured 36.5 at 100 mA. If it drops too low
when more current (240 mA, 12 strings) is drawn, he needs a
different supply or he needs to reduce the current to what the
supply can provide. Fewer strings means less total current -
and less available overhead for the current regulator. He said
he wanted results that are "good enough" - he'll need to decide
what that means.

Paranoia suggests making it strings of 8, and then only after double
checking the minimum _ever_ voltage from the supply.

Alternately you could use transistors at the base of the string, with
current regulation via op-amp. See below.

Regarding the information below:
Perhaps less costly, but you need to provide power for the
op-amp and match the transistors for the single op-amp design
and provide for the Vref so it's a bit more complex than
shown below. Still, it might be worth the complexity if the
extra overhead turns out to be necessary.

Ed

 

[nospam]

That's just right for my purposes. Thanks!

I mistakenly read the 3.2-3.4v as the diode spread. As someone else pointed
out 9 in a chain is marginal if you get unlucky with the LEDs. The LM317
will need about 3.5v across it to regulate.
--

 

[MooseFET]

I'm trying to decide how to drive ~100 UV leds. The Vf is 3.2-3.4 V
typ, 3.8 V max. I have a 36.5 VDC power supply (measured @ 0.1A). I
need If of 20 mA (no control needed, just on or off) through each LED.
Simple and cheap are preferred, provided the results are "good
enough". The application is PCB photolithography.

The LT3080 looks interesting for this sort of thing:

Last LED ------------
Many LEDs ---+--->!-----! IN OUT !---------
! ! SET !--- !
----------! Vcontrol ! ! !
------------ \ \
/ /
\ \
! !
GND GND

 

[DJ Delorie]

I mistakenly read the 3.2-3.4v as the diode spread.

That was listed as "nominal Vf" and I measured one at 3.3v @20mA.
Anyway, I can prototype it and see what happens, before committing to
anything.

 

[JosephKK]

Paul Mathews a écrit :


Or use this to share the boost current between the leds strings:

.-----------.
| BOOST |
| |-----+--------+--------.
| | | | |
| | V -> V -> V ->
'-----------' - - -
^ | | |
| . . . Leds strings
| . . .
| | | |
| V -> V -> V ->
| - - - R1
| | | | ___
| | | +--|___|-.
| | | | |
| | | | |
| |/ |/ |/ |
| +--------+--------+----------+
| |> |> |> |
| | | | |
'-----------+ | | |
.-. .-. .-. .-.
| | | | | | | | R2
| | | | | | | |
'-' '-' '-' '-'
| | | |
=== === === ===
GND GND GND GND

That is slick. I will remember this one.

 

[DJ Delorie]

To follow up on this - I got my order of LM317s and built a supply
using them - 12 independent 20mA supplies. The bulk voltage drops to
35.5 VDC with an 0.22 amp load (9x11 LED matrix), and a string of 9
LEDs drops 29.4 V, or 3.27 V per LED, leaving 1.25 V for the 62 ohm
resistor and 4.85 V across the LM317s. Even at 3.4 V per LED that
still leaves 3.6 V across the LM317s. I think this will do just fine
for my purposes. Thanks!

Photos: http://www.delorie.com/pcb/uvled/

 

[[email protected]]

I'm trying to decide how to drive ~100 UV leds. �The Vf is 3.2-3.4V
typ, 3.8 V max. �I have a 36.5 VDC power supply (measured @ 0.1A).�I
need If of 20 mA (no control needed, just on or off) through each LED.
Simple and cheap are preferred, provided the results are "good
enough". �The application is PCB photolithography.

So far, I've looked into a few options:

1. Resistors. �10 LEDs in a string, 175 ohm resistor. �Cheap and easy,
� �but slight variations on Vf would cause big variations in If.
� �Bigger resistors and shorter strings perhaps? �470 ohm w/ 8 LED
� �strings? �Still only 10% regulation, not sure if that's OK for LEDs
� �(I don't have much info on the specific LEDs I got, other than what
� �I've listed above).

2. Infineon BCR402UE6182. �This is a discrete single-channel current
� �regulator, but doesn't seem to guarantee a low enough If.

3. MAX6971 (or 2xMAX6970). �Only needs a resistor and clock sourceto
� �drive up to 16 strings at 20 mA, but the lead time is 4-5 weeks.

4. MAX6957. �In stock at Digikey, but requires an MCU to program it
� �and common-base transistors to isolate it from the highvoltages.

5. Redoing the power supply for 18VDC to accomodate some other
� �solution.

Suggestions? �Alternatives?

A simple BJT constant current source does it for about 10c for each
chain of leds.