Electronics > Beginners
6V LED driver - old microscope upgrade
(1/5) > >>
Tsou:
Hello!

I've an old microscope with an halogen bulb but I'd like to replace it with something more recent, a LED light.
The halogen bulb is taking 6V and a potentiometer makes this voltage change between 0V and 6V in order to make the halogen bulb more or less bright. (Image of the microscope below)

So, I read some schematics (my main field is Biomedical Sciences, not Electronics) and did my own (see LED.png). I also did the layout of the board (see LED-layout.png).
I've used the parts I've laying around, they may not be the best components to use...
The zener diode is a 3.3V, with the 1 ohm resistor it makes the current go up to 3.3A max. The LED takes 6V and max 20W, so I should stay within the limits (3.3A x 6V = 19.8W).
My plan is to use the 6V going in the potentiometer of the microscope to drive my circuit, and use the in-frame potentiometer to vary the 6V going in the op-amp.


I've added some component, in regards to the advices given in this post : https://www.eevblog.com/forum/projects/linear-led-driver-with-mosfet/msg1796558/#msg1796558

Do you think my design is alright? Does it need something else?

Thank you!
spec:
+  Tsou

Hell, this has been a long journey for you: I have just read through your other thread on this subject too. :)

About your latest circuit, attached above, it is a classic constant current generator and as such is a good architecture which has been used countless times before.

There are a few detailed issues with your implementation though. I will do an analysis and report back.

But just for starters, the TIL072 is not suitable for negative rail input signals and the frequency stabilizing capacitor value is way to low for safety.
flynwill:
Well it's not going to work off 6 volts.

According to the CREE datasheet the LED typically drops ~6.1V at 3.3 A.  Assuming your FET is fully saturated you still the 3.3V being dropped by your sense resistor you will need at least a 10V supply.

Second problem:  At 3.3A & 3.3V your sense resister is going to be dissipating ~10W.  From your layout I would guess that you've not left space for more than a 1/4W resistor. 

With a 6V available supply you probably need to be looking at 3V LED modules, and a much much smaller value for your sense resistor (I'd suggest 0.051 ohm 1/2 Watt resistor).  Also if you stick with the linear design your FET is going to need a heat sink.  A switching driver will be a lot more efficient but also more design work.

Also you probably don't need a 20W LED.  How may Watts is the halogen bulb you are replacing?  I would wager a 1 or 2 Watt LED will be plenty of light.
spec:
+ Tsou

Could you build the attached circuit for feasibility proving.

In your schematic you do not show the value of the potentiometer, so I have assumed 10K. The value is not that critical, but could you post the value you are using.

If you build the feasibility circuit use low ESR capacitors. The values of the capacitors are not critical, but the leads should be as short as possible and connected as close as possible to the MOSFET. Likewise with the gate resistor, R1.

Before you turn the feasibility circuit on,  make sure that the wiper of the potentiometer is fully at the 0V end of its track.

When you turn the circuit on, the LED should be off, but at about halfway travel of the wiper, with any luck, the LED should start illuminating.

Carry on turning the potentiometer until the wiper is fully at the 6V end. The LED brightness should increase. With the wiper fully at the 6V end, note if the LED is bright enough for your purposes.

Be interested to hear the results :)
spec:

--- Quote from: flynwill on November 04, 2018, 04:00:32 pm ---Well it's not going to work off 6 volts.

According to the CREE datasheet the LED typically drops ~6.1V at 3.3 A.  Assuming your FET is fully saturated you still the 3.3V being dropped by your sense resistor you will need at least a 10V supply.

Second problem:  At 3.3A & 3.3V your sense resister is going to be dissipating ~10W.  From your layout I would guess that you've not left space for more than a 1/4W resistor. 

With a 6V available supply you probably need to be looking at 3V LED modules, and a much much smaller value for your sense resistor (I'd suggest 0.051 ohm 1/2 Watt resistor).  Also if you stick with the linear design your FET is going to need a heat sink.  A switching driver will be a lot more efficient but also more design work.

Also you probably don't need a 20W LED.  How may Watts is the halogen bulb you are replacing?  I would wager a 1 or 2 Watt LED will be plenty of light.

--- End quote ---
I had similar thoughts. :)
Navigation
Message Index
Next page
There was an error while thanking
Thanking...

Go to full version
Powered by SMFPacks Advanced Attachments Uploader Mod