Zeiss Microscope Nf stand - incandescent lamp to a 3 W power LED conversion 
By Dushan Grujich, on March 15th. 2013

The burned out lamp, Narva 67273 LWT-P5 
Table with incandescent-lamp dimensions critical for design of replacement
Drawing of the Narva 67273 LWT-P5 incandescent-lamp indicating critical dimensions  
Machining the body of the replacement
The new body with the burned out lamp 
Milling the recess for the V+ wire going through the body
The resulting groove 
The power LED mounted on a star heat sink with machined teflon insulators ready to mount on to the body
Completed LED replacement with burned out lamp shown for comparison
Power LED mounted on to the body 
The bottom part of the body threaded for screwing into the lamp socket, with a centre pin mounted into the Teflon insulator
The replacement LED lamp screwed into the socket of the lamp holder
To drive the power LED I have used a low cost 10 W DC-DC Step Down Converter, KIS-3R33S module, capable of delivering up to 3A continuously. This module comes with output voltage preset to 3.3V. However, it is easy to readjust and have output set at any voltage from 1.21 V - 10.43 V, if the circuit is modified then the output voltage can be varied up to 20V.
The final choice was 3W "Pure White" 235 lm LED with colour temperature of 6000 K, replacing 15 W incandescent lamp rated 220 lm. 

Different LEDs from different manufacturers operate at different maximum continuous voltage, which in turn determines the maximum current flowing through the LED. To protect LED and to provide stable light characteristics it is necessary to limit the voltage output from the KIS-3R33S module according to the specific requirements for each type of power LED, as specified by the manufacturer.

For power LED, which I have used, manufacturer has specified maximum continuous forward voltage at 3.7 V for the 235 lm three-watt LED HPB8b-49K3WHB. Following manufacturer's instructions, I have limited the maximum output voltage of the module so that it will suit the LED. I also wanted to have the means of continuous brightness adjustment so I used a potentiometer to vary the brightness.

Necessary modification of the module consisted in removal and replacement of three SMD resistors, done as follows.

The module is very small, measuring 21x22x10 mm total. To modify it one must remove the bottom cover and then remove module from its housing, not difficult, the cover is snap-on type.

Resistors R3, R4 (both marked 513 i.e. 51 k Ohm) and R6 (marked 332 i.e. 3.3 k Ohm) should be removed, and replaced. R3 and R4 with 39 k Ohm and 180 k Ohm (order of placement is unimportant as they are connected in parallel). Resistor R6 should be replaced with one of the 51 k Ohm resistors removed from R3 or R4 positions.

The additional components are the logarithmic tracking potentiometer VR1 250 k Ohm, connected externally, with integral switch, which when the switch is engaged turns the DC-DC converter ON. The two ceramic capacitors, 10uF /25V, one should be connected across VIN and GND and another across VOUT and GND.

The value of the resistor R6 (51 kOhm), chosen to prevent voltage output falling below 2.70 Volts, as below that value LED will cut off. Combination of resistors R3, R4, R5, R6 and VR1 determines the output voltage according to the following formula:

Vout = (((R3||R4||(R6+VR1))+R5)/R5)*0.925

Resistors R3, R4, R6+VR1 (R6 is connected in series with VR1) are connected in parallel and are actually acting as one resistor. To calculate value of two or more resistors in parallel:


in our case Rx=1/(1/R3+1/R4+1/(R6+VR1)) so Vout becomes:

Vout = ((Rx+R5)/R5)*0.925 (where R5 stays unchanged, as is, 10 k Ohm)

I have machined body out of Duraluminium from a stock bar OD 7/8 (22.23 mm) that I already had which was nicely fitting the need. The free end that originally was the glass envelope of the lamp I have machined a number of fins increasing the surface area to serve as a heat sink. I drilled the body through its length to accept a wire supplying V+ to the power LED. I let the body be at ground potential, thus eliminating need for a second wire.

The bottom part, I drilled larger hole and then pressed in a short length of Teflon rod which I drilled partly through and press fitted a short brass rod to which I soldered one end of a short length of Teflon insulated 22 SWG silver plated wire to it. The other end of the wire was soldered to a plus terminal of the LED.

The use of original power supply was retained with the addition of a full wave rectifier and a filter capacitor placed in the same box with the DC-DC converter, thus allowing for a quick return to use of incandescent lamp if the need ever arises.  

Note: If LED flicker is experienced, then the 'Soft Start' capacitor C3, part of the module, should be removed.

KIS-3R33S module underside view shoving bottom cover with terminal markings
The component layout
Module KIS-3R33S modified 
Control box showing the power control module, the intensity control potentiometer with integral  switch on the front panel and the AC power inlet and DC LED driving outlet on the back panel
The front panel intensity control potentiometer 
Back panel with AC Power input and DC LED driving outlet
This is alternative housing for the LED driver 

I found this box offered on eBay from a Chinese seller, at the cost of only US$5, free shipping. A power LED driver requiring DC input and giving DC output, a pulse width modulated unit based on NE555. Although not at all suitable for driving microscope LED illumination I bought it and used the housing only. It can be seen from the images below how the Chinese Dimmer looked like originally, inside. I have removed the original PC board that was in it, replaced potentiometer as well, as original was unsuitable with the resistance value of only 1 kOhm, thus I placed my protoboard with KIS module inside using the two existing mounting posts to fasten protoboard to the bottom half of the box.

The housing is rather smallish and perhaps more desirable that way, as the bench with the 'scope is almost always crowded with many things. Size of the housing is 60x65x35 mm (WxBxH), height with the pot knob is 57 mm, and the base is 60x90 mm with two 4 mm holes on each side should fixing it to the bench be desired.

Insides before
Insides after
Finished LED driver in a new housing
Parts required:
KIS-3R33S module 1 pc.
Power LED 3W 1 pc.
Resistor SMD 603 56k 1 pc.
Resistor SMD 603 68k 1 pc.
Potentiometer with switch 250k log 1 pc.
C* Capacitor X5R SMD 1206 10uF/25V 2 pcs.
Proto board
3. MP2307_r1.9 data sheet - Monolithic Power Systems, Inc.
Published in the May 2013 edition of Micscape Magazine.
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