Theory .. LED Current reg

[cjdelphi]

A few months ago i built a little (5 watt little?) Light which uses a LDR, it's worked flawlessly, but i don't have current limiters in place..

It dawned on me a few days ago, it does!... using a light diode resistor does do that

Come darkness the light comes on slowly 0.6v.. 1v... 3v to the gate, it starts off dim and the darker it gets the more saturated the mosfet is, but i just realized if the current increases due to heat more light output happens, more light starts to switch off the mosfet, and a balance happens

For a dollar or so that's 20 LDRs! All that's required then is the voltage divider/pot ..

Does this sound like a reasonable way to control current or am i over looking something?

 

[Harald Kapp]

light diode resistor

Do you mean a Light Dependent Resistor (LDR)? There no diode in it.

Does this sound like a reasonable way to control current or am i over looking something?

Hard to tell from the explanation only. Show us your circuit diagram, please.

the darker it gets the more saturated the mosfet is

Even not knowing your circuit details I doubt this is what's happening. A MOSFET works differently than a bipolar transistor. A MOSFET is in saturation when Vds >= (Vgs-Vt) whereas for a bipolar transistor in saturation Vce<Vbe. In your circuit Vgs very probably rises with sinking level of light which in turn will make the MOSFET more conductive (smaller Vds) so teh MOSFET is in the linear region of operation.

 

[cjdelphi]

Haha, i keep calling them diodes!

The circuit is in use right now, i'll get back to you on it..

 

[(*steve*)]

The main problem I could see is that someone placing their finger over the LDR will make the current go up. The other problem is that differences in the characteristics of the mosfet and LDR could mean you need to trim the circuit.

Other than that it might well be workable.

I wouldn't call it "constant current", but it has feedback which will serve to limit the current based on the combination of ambient and LED light seen by the LDR.

 

[cjdelphi]

Yup, i always cover it up to get more light, it has a 100w heatsink for a 5w, it's been running for 12 months works fantastic, but sure it's not steady, but close enough using the ldr as feedback to control the current


Yeah, not for use in the day (as sunlight switches it off) but for a night light with some kind of regulator, it does work