Electronics > Beginners
6V LED driver - old microscope upgrade
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ratboy:
I retro fitted my old Reichert MicroStar IV with Led lighting using something like
Led: https://www.ebay.com/sch/sis.html?_nkw=6V%2F12V+LG+3535++4Chip+LED+Emitter+instead+of+CREE+XPG++LED+with+20MM+Copper+Star&_id=262224123950&&_trksid=p2057872.m2749.l2658

Driver: https://www.ebay.com/itm/5A-Constant-Current-Constant-LED-Driver-Lithium-ion-Batteries-Charging-Module-/321905738478?hash=item4af313c2ee

If i remember correctly, i mounted the Led on a heat sink and hacked away the existing halogen base and squeezed it it.
The led driver board, one of the fine tuning pot was removed and replaced using the wiring/pot of the existing controls.
You can play with it while outside and see which pot is the one.
The range of the existing pot + fix resistor, is sufficient to go from very dim to very bright.

can't remember how i powered the board though, but the scope still uses the existing 240v in to power the whole thing, so i guess i used the existing transformer and switch.

The above links are for similar items as the original ebay links are obsolete.


 
Zero999:

--- Quote from: spec on November 04, 2018, 11:07:25 pm ---
--- 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.

--- End quote ---
Another issue is the forward voltage of the LED has a negative temperature coefficient which will cause the current to increase, as it warms up. The LED current should be measured using a meter and the potentiometer adjusted downwards if it starts to get close to the LED's maximum rating.
Tsou:

--- Quote from: Hero999 on November 04, 2018, 09:55:35 pm ---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.

--- End quote ---

I did some improvements with your advices, I put an unity gain buffer, I also changed the zener diode for a voltage regulator and a resistor divider. I also add some capacitors to filter the supply voltage, because there's clearly no capacitors in the microscope, the supply voltage is coming in the potentiometer right from the rectifier.

I'm also thinking, wouldn't it be more correct to just put away all the electrical parts and take a new 6V power supply, so I have something more recent and so I'm sure what's inside the microscope?
I plan to make an epi-fluorescence module, so it could be interesting to get 6V from the power supply to power the fluorescent diodes, without the need to have multiple power supplies. A good 5A 6V power supply could be just enough I guess.
Or I should get a more powerful one, like a 9V? But this mean I've to dissipate the extra 3V in the resistor, so this seems not to be a better solution



--- Quote from: ratboy on November 04, 2018, 11:58:22 pm ---I retro fitted my old Reichert MicroStar IV with Led lighting using something like
Led: https://www.ebay.com/sch/sis.html?_nkw=6V%2F12V+LG+3535++4Chip+LED+Emitter+instead+of+CREE+XPG++LED+with+20MM+Copper+Star&_id=262224123950&&_trksid=p2057872.m2749.l2658

Driver: https://www.ebay.com/itm/5A-Constant-Current-Constant-LED-Driver-Lithium-ion-Batteries-Charging-Module-/321905738478?hash=item4af313c2ee

If i remember correctly, i mounted the Led on a heat sink and hacked away the existing halogen base and squeezed it it.
The led driver board, one of the fine tuning pot was removed and replaced using the wiring/pot of the existing controls.
You can play with it while outside and see which pot is the one.
The range of the existing pot + fix resistor, is sufficient to go from very dim to very bright.

can't remember how i powered the board though, but the scope still uses the existing 240v in to power the whole thing, so i guess i used the existing transformer and switch.

The above links are for similar items as the original ebay links are obsolete.

--- End quote ---

Thank you for your advice, I think I'll do my own circuit and also buy the circuit you showed, in case it doesn't work as good...
Zero999:
I think a new 6V power supply is a sensible option.

If there are no smoothing capacitors, i.e. the 6V PSU is just a rectifier on a transformer, then a 220µF capacitor will be far too small to provide adequate smoothing. You probably need 10mF, i.e. 10000µF or even more.
Tsou:

--- Quote from: Hero999 on November 07, 2018, 10:19:40 am ---I think a new 6V power supply is a sensible option.

If there are no smoothing capacitors, i.e. the 6V PSU is just a rectifier on a transformer, then a 220µF capacitor will be far too small to provide adequate smoothing. You probably need 10mF, i.e. 10000µF or even more.

--- End quote ---
Ouf, 10mF, that's a lot... I guess a new PSU would be better...

I think I've found a good solution, I can put a new 12V PSU, get a 12V LED and use my board with 12V. So, R1 = 22k, I get 0.65V between R1 and R2, this means 1.3A across R5. We have 0.05 * 1.3 = 0.65V across R5 and 0.845W to dissipate, with a 1W resistor it should be ok. If it's too high I can lower the max current, these LEDs seem to be powerful enough.  Someone on an microscopy forum told me that 10W LED are powerful enough for microscopy, this mean less than one amp, so I can lower the current if needed.
LM358 and P16NF06L can handle 12V, but the AMS1117 is rated to 15V max, and 12V in usual conditions, so it's at its limits, but it should be ok if I'm right?
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