Electronics > Beginners
6V LED driver - old microscope upgrade
spec:
--- Quote from: Tsou on November 04, 2018, 09:27:34 pm ---@spec I'll try your circuit when I can, but I don't have the LED for now, neither my microscope near me
--- End quote ---
OK :)
Zero999:
--- Quote from: spec on November 04, 2018, 06:55:38 pm ---+ 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 :)
--- End quote ---
Be careful with that circuit. It might blow up the LED with the power cranked up to the maximum, as there's nothing to limit the LED current, other than the on resistance of the MOSFET, especially if the 6V supply is higher than that.
--- Quote from: Tsou on November 04, 2018, 09:27:34 pm ---@spec I'll try your circuit when I can, but I don't have the LED for now, neither my microscope near me
@Hero999 I don't know the voltage with no load, I didn't try that and I don't have the microscope to try it, but I read somewhere that "The 0 - 6V output of the Nikon's internal PSU is DC so could be the basis of an LED supply, but a suitable dropping resistor is needed as it is 10V open circuit and would immediately blow an LED typically rated for 4.5V max" (http://www.microscopy-uk.org.uk/mag/indexmag.html?http://www.microscopy-uk.org.uk/mag/artaug07/dw-labophot.html).
So I guess we can say that the voltage under light/no loads is 10V, I don't know if this could have any effect on the circuit...
I changed the reference of the opamp, the value of R3 and C1 and I add a resistor divider to have ~0.125V on the positive input of the opamp (see LED2.png)
I'll also need to add some capacitors to smooth the 6V supply
--- End quote ---
Don't worry about the open circuit voltage, the LED needs the forward current to be regulated. not the voltage, which is what this circuit does.
When you calculated the values of R5 and R6, you missed that RV1 is in parallel with R6, so the output voltage will be lower than that. If you don't know the value of RV1, then you could use the spare op-amp on the LM358 IC as a unity gain buffer between R6 and RV1.
flynwill:
Indeed this is much closer. I would strongly recommend characterizing that "6V power supply" in the existing scope if you can. I may be just a small transformer and rectifier with no filter capacitor. In that case you may want either add a filter cap to the design, or a diode in series with VCC to the opamp which will let you get away with a smaller capacitor as it now only has to filter the op amp power supply.
While you are characterizing the PS, do check out the resistance of the existing pot. It might be quite low, and if you know it accurately you can eliminate R6.
spec:
--- Quote from: Hero999 on November 04, 2018, 09:55:35 pm ---
--- Quote from: spec on November 04, 2018, 06:55:38 pm ---+ 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 :)
--- End quote ---
Be careful with that circuit. It might blow up the LED with the power cranked up to the maximum, as there's nothing to limit the LED current, other than the on resistance of the MOSFET, especially if the 6V supply is higher than that.
--- End quote ---
Yes, I did say to ensure that the pot is at the 0V end before switching the supply on and to gradually move the wiper towards the 6V supply line end. The current will be limited by five factors: the slope resistance of the LED which is significant as shown on the LED data sheet, the Rdss of the MOSFET which I think has a positive temperature coefficient, temperature effect on VGDth, which has a positive temperature coefficient, the impedance of the 6V supply line, and the current limitations of the 6V power supply.
By the way, the purpose of the feasibility circuit is to establish, one way or another, if the approach of using the 6V Cree LED on the microscope 6V power supply is feasible. That is the fundamental enabling factor that needs to be established before building more complex circuits which will just cloud the issue at this stage.
On a similar theme, it seems, from the above posts that the microscope 6V supply itself is suspect and at the very least will need extra smoothing/reservoir capacitors.
spec:
--- Quote from: Tsou on November 04, 2018, 09:27:34 pm ---I changed ... the value of C1
--- End quote ---
100nF rather than the previous 100p for C1 looks more like it, especially with the relatively slow LM358. :)
But I suspect that a bit more work will be required to compensate the circuit.
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