# Need a regulator to power 700ma LED from 900ma constant current driver

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#### jpoopdog

##### Member
I know it seems silly but this is about being economical. Im comming up with a reliable solution that can be implemented by someone who is not particularly skilled. Although i myself have the same issue, i intend to build a driver which makes use of the monitors brightness PWM signal.

Is problem is this, wacom cintiq monitors, namely the 24HD, have faulty inverters or CCFLs, either way, the backlights die pretty young in some of them.
An LED replacement kit exists, however it turns out that the kits driver, is not a dual output driver, it has two output plugs, but, its a shared 500ma across two LEDs in parallel, and these LEDs can easily take 500ma each without needing a heatsink, im testing up to 700ma though if its not bright enough. I havent yet determined what the maximum current for the strips is, the manufacturers are no help.
I suspect the drivers output 500ma so that the kit may be used as is in monitors that only take 1 strip instead of 2, without blowing the 1 strip, since the total current is still less than its maximum, which is over 500ma i know that much at least.

Mike

#### jpoopdog

##### Member
i cant find any SMPS that run off 24v.
But it would be the same for me to just use a stepdown converter.
The monitor runs off 24v btw

#### jpoopdog

##### Member
I already have some in mind.
But im more interested in the main question here about a linear regulator to use with a constant current driver

#### Pommie

##### Well-Known Member
If your strips are 9V you can wire them in series and the constant current source should put out 18V at 500mA.

Mike.

#### alec_t

##### Well-Known Member
Is the current of your constant current source adjustable? If not, it will try to put 900mA through the LEDs and may not play nice with any downstream regulator.

#### Nigel Goodwin

##### Super Moderator
Is the current of your constant current source adjustable? If not, it will try to put 900mA through the LEDs and may not play nice with any downstream regulator.
Not really - although it would obviously be best to adjust the CC source correctly.

You simply need to add a shunt regulator, as you're regulating current not voltage - so the shunt regulator needs to bypass 200mA round the load LED's, leaving the other 700mA to go through them.

Good point.

#### schmitt trigger

##### Well-Known Member
As a matter of fact, a quick and dirty way would be a paralleled resistor which, at the loaded output voltage, will bypass 200mA.

Being quick and dirty means that these 200mA will be wasted as heat. It also means that the current regulation is a function of the voltage regulation.

By far, the best would be to modify the 900mA driver to output only 700mA. Very likely there is a current setting pin.
Do you have the spec sheet for the driver?

#### jpoopdog

##### Member
unfortunately no, and whats worse is the manufacturer deliberately scratched the part number off the IC
It has two SMD resistors and a capacitor, the resistors are 200 and 300 ohm, i think, they say r200 and r300 on them.
the capacitor however im not sure about, but one terminal is connected to the positive output so i think its just the resistors that determine current if its possible to change it at all.
Also i found these things while claiming to do 900, actually only reach about 800-850, so using a resistor could work i think.

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#### Nigel Goodwin

##### Super Moderator
Also i found these things while claiming to do 900, actually only reach about 800-850, so using a resistor could work i think.
A resistor is OK as long as the voltage is fairly constant, otherwise a constant current shunt (as I suggested) would be better.

However, in either case you will have to dissipate the unwanted energy as heat - so a BIG resistor, probably on a heatsink?, or a constant current shunt on a heatsink.

What voltage do you have across the LED's when they are running?.

#### tomizett

##### Active Member
the resistors are 200 and 300 ohm, i think, they say r200 and r300 on them
That could be 0.2R and 0.3R, ie 200mOhm and 300mOhm. In which case, these are probably current sense resistors and increasing them will decrease the output current. A photograph, or better still a traced-out schematic, of the existing driver would probably help.

#### schmitt trigger

##### Well-Known Member
That could be 0.2R and 0.3R, ie 200mOhm and 300mOhm. I.
Completely agree with you, these are the current sensing resistors.

A simple experiment would consist of replacing the r300 with a r390 resistor and then re-measure the output current again.

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