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### Topic: Help with schematic (changing GND to 6v) (Read 3997 times)previous topic - next topic

#### Pancra85

##### May 03, 2013, 12:32 am
Ok, I needed a driver for a 12v led and did this:
Vcc connected to 12v and of course GND to 0v.

It works fine, but now I need the same thing but instead of driving a 12v led I need it to be 6v, but the thing is that because of design Vcc still needs to be connected to 12v.
My question is: Can I connect GND to 6v so the voltage difference of the led is 6v??? (12v-6v=6v)
Also do I need to change anything on the circuit??

Thanks!

#### cjdelphi

#1
##### May 03, 2013, 01:36 am
question is...

how much current to power the led is required?

that circuit is overkill... and yes the same circuit would be fine, r2 is doing the work to protect the diode.

#### DVDdoug

#2
##### May 03, 2013, 01:49 am
Quote
My question is: Can I connect GND to 6v so the voltage difference of the led is 6v??? (12v-6v=6v)
What?   I'm pretty sure the answer is No.  But, I'm not sure I understand.  Where's the 6V coming from?  If you have a 6V supply, why don't you use it directly instead of trying to subtract?

If the 6V & 12V power supplies share a common ground and you ground the 6V supply, you are shorting it out.

You can measure the voltage difference between two power supplies (if they have a common ground), but if you try to pull power (current) from the difference you might have trouble (because you'd be trying to run current "backwards" into  the low-voltage supply.)

#### Pancra85

#3
##### May 03, 2013, 04:48 am
Quote
question is...

how much current to power the led is required?

that circuit is overkill... and yes the same circuit would be fine, r2 is doing the work to protect the diode.

Ok, the led is this one: http://www.alibaba.com/product-gs/712130845/10_Watt_RGB_LED_high_power.html
it is a high power RGB and as you can see on the table you need around 12v for BLUE and GREEN but you need around 6v for RED, and also the leds have a common positive pin.
As this is the case, I need to connect 12v on the positive side, and I was thinking that I could connect 6v on the negative side of RED instead of 0v so it has the required voltage.

Quote
Where's the 6V coming from?  If you have a 6V supply, why don't you use it directly instead of trying to subtract?

I only have a 12v power source and I was thinking getting 6v from it with a voltage divisor (resistors), the question is if it will work connecting the RED driver to Vcc->12v and GND->6v.

The schematic I referenced on the first post is a led driver and I need one for each color (RGB, so three of them)

#4
##### May 03, 2013, 04:59 am
Why so convoluted?
Just drive with 3 logic level, low Rds, low Gate capacitance, N-channel MOSFETs
http://www.digikey.com/product-detail/en/NTD5867NL-1G/NTD5867NL-1GOS-ND/2401422
10k pulldown resistor to hold gate off while arduino starts up.
Designing & building electrical circuits for over 25 years.  Screw Shield for Mega/Due/Uno,  Bobuino with ATMega1284P, & other '328P & '1284P creations & offerings at  my website.

#### Pancra85

#5
##### May 03, 2013, 09:07 pm
I think this is the way to go, as someone in another forum told me:
I was thinking in getting 6v from 12v with something like this: http://schematic-audio.blogspot.com.ar/2011/10/dc-converter-using-7808-regulator-ic.html

Quote
Why so convoluted?
Just drive with 3 logic level, low Rds, low Gate capacitance, N-channel MOSFETs
http://www.digikey.com/product-detail/en/NTD5867NL-1G/NTD5867NL-1GOS-ND/2401422
10k pulldown resistor to hold gate off while arduino starts up.

well... that will replace the whole circuit, right? I am certain that will not be able to find it here on my country (Argentina) :-(

#6
##### May 03, 2013, 11:18 pm
That will work better using a voltage regulator for the Red.
Can you snip off the top of the device so you have seperate anodes for the 3 LEDs?
Then you can have 12V for Green & Blue, and regulated 8V for the Red, and the cathodes can connect to your circuit above.
Designing & building electrical circuits for over 25 years.  Screw Shield for Mega/Due/Uno,  Bobuino with ATMega1284P, & other '328P & '1284P creations & offerings at  my website.

#### Grumpy_Mike

#7
##### May 03, 2013, 11:27 pm
Quote
I think this is the way to go, as someone in another forum told me:

I would switch to a forum where they didn't tell you such rubbish.

For a start that is not a proper circuit and second you always need one resistor per LED.

#8
##### May 04, 2013, 12:05 am
Those are 350mA parts tho, constant current source probably better way to go.
Quote

Ok, the led is this one: http://www.alibaba.com/product-gs/712130845/10_Watt_RGB_LED_high_power.html
Designing & building electrical circuits for over 25 years.  Screw Shield for Mega/Due/Uno,  Bobuino with ATMega1284P, & other '328P & '1284P creations & offerings at  my website.

#### Pancra85

#9
##### May 04, 2013, 02:54 am
Quote
That will work better using a voltage regulator for the Red.
Can you snip off the top of the device so you have seperate anodes for the 3 LEDs?
Then you can have 12V for Green & Blue, and regulated 8V for the Red, and the cathodes can connect to your circuit above.

You know what? I never tought trying to separate the anode of each led, but it can be done!!! That solves a problem. I tryed and now each color has it's own anode.
Sorry I forgot to add that it was a simplified circuit, instead of a resistor each color has it's own driver.

Quote
I would switch to a forum where they didn't tell you such rubbish.
For a start that is not a proper circuit and second you always need one resistor per LED.

hahaha, a genuine Grumpy Mike! I think that forum is this one! Great community!

Thanks, you all!! I think I have everything covered!

#### Riva

#10
##### May 04, 2013, 10:13 am
The circuit you described is a constant current supply, I used this same circuit and LED to create a colour cycle mood lamp
Don't PM me for help as I will ignore it.

#### sonnyyu

#11
##### May 04, 2013, 01:51 pmLast Edit: May 04, 2013, 01:56 pm by sonnyyu Reason: 1
The circuit is PWM controlled linear constant current sink supply, at 12V LED case it achieve >90% efficiency and without EMI. You might upgrade BUZ71 to logic type MOSFET to make it more Arduino friendly.

Now for 6V LED, all you need to do is upgrading heat sink of Q1, efficiency will drop to ~50%. That's all.

#### MarkT

#12
##### May 04, 2013, 03:30 pm

The circuit is PWM controlled linear constant current sink supply, at 12V LED case it achieve >90% efficiency and without EMI. You might upgrade BUZ71 to logic type MOSFET to make it more Arduino friendly.

Now for 6V LED, all you need to do is upgrading heat sink of Q1, efficiency will drop to ~50%. That's all.

Yes, pretty standard constant-current drive.  R2 and the forward Vbe of T1 set the current - there is a loss of 0.6V or so
due to Vbe,  You might need to be careful substituting other values and devices in case the configuration is unstable
and oscillates, I suspect the phase margin is a bit tight.

By "without EMI" you mean "low EMI", due to the limited slew-rate.
[ I will NOT respond to personal messages, I WILL delete them, use the forum please ]

#### sparkylabs

#13
##### May 04, 2013, 03:56 pm
I can't understand what all the fuss is about, run your LED with the correct resistor, switched by the mosfet (or BJT your choice) and change the PWM duty to alter the brightness - Done!

#### sonnyyu

#14
##### May 04, 2013, 04:11 pm
for my understanding most part of E and M from inductor which use at switch constant-current supply. now  we cut the inductor then EMI get cut as well.

"Ohm's Law" for an inductor
v=L*(di/dt)

from on to off state, since device MOSFET is able to provide huge di/dt, The v is huge.

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