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Topic: The BEST way to control white stair lights? (Read 5 times) previous topic - next topic

Grumpy_Mike

That circuit is alright, the main problem with it is that all the current has to go through the resistor at the bottom, meaning it has to be high wattage. Depending on the type of FET you use you might not be able to run it at voltages lower than 10V. It is not very stable with temperature. More complex circuits can over come these limitations.

AndrewStone

@Grumpy_mike: Feel free to poke holes in this design as well!  Of course all designs are a trade off between price and performance.  If you have a link to a design that does not require a current sense resistor then I'd love to see it as all designs that I've seen use them.

Don't be scared of all the connectors on the top.  The real circuit is in the middle row, and consists of (left to right):
1 pot that lets you pick the current
1 opamp
1 resistor (R13) that just stops drawing too much from the opamp
1 connector (T1) Where you would pull down to turn off the circuit (i.e. PWM)
1 darlington to burn away the extra voltage
1 resistor (RSENSE1) that senses the current (you can use very low values, .1 to 1 ohm for example so heat dissipation is not such an issue with this resistor)

And on the far right SV1 is where you hook up the LED.

In short, the opamp uses feedback to equalize the voltage across RSENSE1 and the pot.

The PCB is essentially 4 of these, but certain parts (namely the opamp) are quad so its just one chip...

http://www.toastedcircuits.com Lightuino LED driver: 16 sources, 70 sinks, remote controlled.  Also high powered LED drivers.

Grumpy_Mike

Quote
If you have a link to a design that does not require a current sense resistor then I'd love to see it as all designs that I've seen use them.

No I didn't say that, what I said was that the current in that design was DETERMINED by the value of the current sensing resistor. In that design you posted it is not, and therefore better.

AndrewStone

@grumpy_mike: I understand what you are saying now.  R3 is forced to drop approximately the activation voltage of the BJT, or .7 volts.  This is pretty high compared to a < 1 ohm resistor, especially at low high brightness LED currents of about 100 to 200mA (i.e < 1 amp).  And if you want the circuit to be variable current, R3 it would have to be the pot.  So now you have to find a high wattage pot!  Not pretty in that it provides basically no physical design advantage over the LED + resistor combination (by "physical" I am not including the constant-current property which is sort of a design feature).   

@orangeLearner: Anyway, you can buy the opamp pretty cheaply at digikey http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=MCP6044-E/P-ND or I have a PCB that uses the SMT version that I can send you assembled or not.  PM me if interested...

http://www.toastedcircuits.com Lightuino LED driver: 16 sources, 70 sinks, remote controlled.  Also high powered LED drivers.

Boffin1

You don't need a variable high power pot, just put a 10k pot across the high power sense resistor..
With my mobile phone I can call people and talk to them -  how smart can you get ?

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