LED bulb for Wild M20

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cmug
Posts: 11
Joined: Mon Oct 29, 2018 8:36 pm

LED bulb for Wild M20

#1 Post by cmug » Thu Mar 26, 2020 10:27 am

Original 6V 15 W bulb for this Microsscope is a bit rare. So make an LED light as replacement seems better.
I made the holder on my lathe. Thought the plastic was Polypropylene. In front a disc aluminium as cooling for the LED. As an LED I used the much too expensive Philips car LED lamp 6000K (nr 129416000KX1) . Inside there was a diode bridge plus an 47 Ohm resistor in series with the LED. I measured 28 mA at 12 Volt . So approximately 0,35 Watt
I made an current source from 28 mA in the supply wire, so less change to damage the LED.
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daruosha
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Location: Tehran, Iran

Re: LED bulb for Wild M20

#2 Post by daruosha » Thu Mar 26, 2020 9:40 pm

cmug,

Nice job.

47Ω resistor on a 12V supply. limits the current somewhere around 250 mA, which is much higher than your LED power calculation. If you have a constant current source supplying at 28 mA, you don't need the current limiting resistor at all.

I assume 470Ω is the proper value on a 12V supply rail. I'm sure you know your stuff, however I encourage enthusiastic retrofitters to take a look at this article for current limiting calculation and the theory behind it: https://www.circuitspecialists.com/blog ... -lighting/
Daruosh.

cmug
Posts: 11
Joined: Mon Oct 29, 2018 8:36 pm

Re: LED bulb for Wild M20

#3 Post by cmug » Fri Mar 27, 2020 9:35 am

The 28 mA is the measured value (not calculated). There was a diode bridge between the 12V DC and the LED, which also has voltage drop. That is the reason I think that the calculation based on DC is different then what I did.
The advantage of the diode bridge is that you can use both AC or DC and the polarity (DC) does not matter.
The advantage of the current-source is that every voltage 12 to 35 volts the circuit works well and the LED is never overloaded.
Any forum member who can read electronic schematics. Here my note book:
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jfiresto
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Location: Northern Germany

Re: LED bulb for Wild M20

#4 Post by jfiresto » Fri Mar 27, 2020 11:39 am

They were festoon style, but I also tried those Philips LED automotive replacement bulbs, and then found something else that was brighter and better regulated.

ON Semiconductor makes some handy, two-terminal current regulators that you can simply drop between the power supply and the one or more series-connected LEDs. They are in fairly big, easy to solder SMD packages, take up to 45 or 120V in and output 15, 20, 30 or 50 +/-15% mA. I can find current some part numbers if they would be interesting.
-John

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daruosha
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Location: Tehran, Iran

Re: LED bulb for Wild M20

#5 Post by daruosha » Fri Mar 27, 2020 12:45 pm

cmug,

LM317 is a classic regulator can work both in constant current mode and constant voltage mode, it's not the most efficient nowadays, but it's OK as long as you don't burn too much power in the regulator itself.

I looked at your schematics, your original circuit doesn't need current limiting resistor (if you already supplying it with 28mA constant current) and you didn't account for the rectifier diodes voltage drop in your calculations as well. It works, but outside the optimum spec.

Your second circuit with LM317 in CC configuration mode is fine, but be careful about your heat dissipation and how much power is actually wasting. I'm not saying you're doing anything wrong, but keep the input voltage and LED forward voltage as close as possible (plus 1.2V for internal voltage reference of LM317) to minimize the power waste and heat generation(e.g. use 6V instead of 12V).

You can operate a LED in many ways, but if you want your circuit efficient amd maximum brightness without shortening the life of the LED and other components in your circuit, you have to consider lots of other parameters like what I have mentioned above.
Daruosh.

cmug
Posts: 11
Joined: Mon Oct 29, 2018 8:36 pm

Re: LED bulb for Wild M20

#6 Post by cmug » Mon Mar 30, 2020 2:32 pm

I now realise that I am also vintage. An LM317 is a longstanding part in electronics. Well, I also likes to work with electron tubes, then every semiconductor is soon 'modern'.
Your tip on the ON Semiconductor website is very valuable. There are indeed nice chips for LED applications, better then LM317

jfiresto wrote : "then found something else that was brighter and better regulated."
May I ask which LED (type Nr) you like to use nowadays ?

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