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Modifying a Labophot for use with Hi CRI LED
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mawyatt



Joined: 22 Aug 2013
Posts: 1118
Location: Clearwater

PostPosted: Thu Dec 28, 2017 12:48 pm    Post subject: Reply with quote

aidanmoore wrote:
Quote:
Chris mentioned putting the electrolytics on the 60 Hz input side (frequency doubled to 120Hz because of full wave rectification), this is a good idea. The circuit will attempt to keep the LED current constant, but the input voltage will be following a 120Hz ripple with highly varying amplitude, this may actually fall out of regulation during the ripple minimums. When this happens the LED current will be "modulated" at 120Hz and the light output will also be modulated.The input filter caps will help create a somewhat average input voltage and reduce the ripple.


The input cap for ripple attenuation seemed a necessity as at the low brightness end of the pot setting I could clearly see flicker, and the cap fixed this. I agree that an electrolytic at the output probably isn't doing too much good.



Also, the MT3608 board that I used from AliExpress for $0.54 per board Smile matches the picture Rik references in his post, so they are all most likely built from the same schematic.


So your MT3608 schematic matches the top portion (printed) of the above schematic with the pot used as a proper potentiometer (3 terminal) rather than having the wiper shorted to a pin and used as a variable resistor (2 terminal)?
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glmory



Joined: 30 Dec 2016
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PostPosted: Thu Dec 28, 2017 12:57 pm    Post subject: Reply with quote

The boards I bought were these. They definitely don’t match the schematic in that two pins are shorted together. That seems to have been the primary reason I failed boards although bad wiring may have also failed some.

WINGONEER 5PCS MT3608 DC-DC Step Up Power Apply Module Booster 2A Power Module for Arduino https://www.amazon.com/dp/B06XHJCHX6/ref=cm_sw_r_cp_api_QUurAbT02NVGA
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rjlittlefield
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Joined: 01 Aug 2006
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Location: Richland, Washington State, USA

PostPosted: Thu Dec 28, 2017 1:05 pm    Post subject: Reply with quote

glmory wrote:
WINGONEER 5PCS MT3608 DC-DC Step Up Power Apply Module Booster 2A Power Module for Arduino https://www.amazon.com/dp/B06XHJCHX6/ref=cm_sw_r_cp_api_QUurAbT02NVGA

Interesting -- and perhaps the most bemusing product description I have ever read:
Quote:
Product Description
Matters needing attention:
...
2. The peak current output current no more than TV university.



--Rik
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mawyatt



Joined: 22 Aug 2013
Posts: 1118
Location: Clearwater

PostPosted: Thu Dec 28, 2017 1:23 pm    Post subject: Reply with quote

Seems these boards based upon the MT3608 are everywhere on eBay, different versions, and the sellers have no idea what they are or how they work!

Since these boards are all (one's I've found) configured as boost switch mode regulators the output voltage must always be greater than the input, but I didn't find that spec anywhere either.

Another interesting thing is the schematic shown allow the output voltage to rise to ~27.9 volts, which means the chip "sees" a voltage at pin 1 of Voutput + a forward diode drop or about ~28.7 volts. This exceeds the maximum voltage specification for the chip of 28 volts! If the 100K pot has a 10% tolerance the voltage could rise to ~ 31.4 volts, a simple worst case analysis with 5% resistor tolerance and 10% pot tolerance yields ~32.8 volts Shocked

https://www.olimex.com/Products/Breadboarding/BB-PWR-3608/resources/MT3608.pdf

Good news is they are dirt cheap, so smoking a couple doesn't cost much...other than hurt ego, no harm Rolling Eyes

Best,

Mike
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ChrisR
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PostPosted: Thu Dec 28, 2017 2:35 pm    Post subject: Reply with quote

Quote:
Also note the boards evidently do not have the 22uF capacitors
That's what I was wondering, though having looked things up capacitors these days are a lot smaller than I thought. 1206 package is 3.2mm x 1.6mm x 0.55mm. They could be?
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mawyatt



Joined: 22 Aug 2013
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PostPosted: Thu Dec 28, 2017 2:47 pm    Post subject: Reply with quote

ChrisR wrote:
Quote:
Also note the boards evidently do not have the 22uF capacitors
That's what I was wondering, though having looked things up capacitors these days are a lot smaller than I thought. 1206 package is 3.2mm x 1.6mm x 0.55mm. They could be?


Chris,

Don't think they can squeeze a 22uF cap with any reasonable breakdown voltage (would need to be >28 volts) into a 1206 package. My guess would be these are just decoupling for the ~1MHz switching frequency, so 0.1 or 0.22uF.

Best,

Mike
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aidanmoore



Joined: 24 Sep 2015
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Location: Toronto

PostPosted: Fri Dec 29, 2017 6:08 pm    Post subject: Reply with quote

Quote:
So your MT3608 schematic matches the top portion (printed) of the above schematic with the pot used as a proper potentiometer (3 terminal) rather than having the wiper shorted to a pin and used as a variable resistor (2 terminal)?


I photographed a new board from AliExpress below, and it appears to match the reference schematic, with a direct connection of the pot wiper to pin 3


Here is the schematic traced from the board and resistor values referencing the schematic



This is the connection to external R1 and R2 resistors:



To get the vin/vout ratio below I measured R1 = 98K and R2 =2.4K + 2.2K = 4.6K




I still have not tried the series 5 ohm resistor as I am looking for a suitable power rated resistor.
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glmory



Joined: 30 Dec 2016
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PostPosted: Sat Dec 30, 2017 2:18 pm    Post subject: Reply with quote

aidanmoore wrote:

I still have not tried the series 5 ohm resistor as I am looking for a suitable power rated resistor.


The resistors I am using are:
https://www.amazon.com/gp/product/B0087ZCSQ4/ref=oh_aui_detailpage_o06_s01?ie=UTF8&psc=1

They are 50W resistors which are obviously over-kill. There is a lot of room in the Labophot though so it doesn't really matter that they are too large.

Interesting that the board I bought on Amazon is somewhat different. If I remember right Amazon delivered much quicker so I bought that one. Eventually it worked out, but was more trouble than I expected.

I increased the 5 ohm resistor to 10 ohms, reduced the resistance to ground and added some additional capacitance. Here is what I have now:



Overall this runs quite well. It now runs from about 0.06-0.5 amperes and is cooler than before the LED upgrade while being a lot brighter. There are three issues, but they are relatively minor and probably can be fixed if I decide to put in the effort:

1. It is still a bit too bright at the lowest setting and its color is a bit off from what I would prefer, a blue filter should be able to fix both problems.

2. I still get ripple when using an electronic shutter at very high shutter speeds and high LED power. I can get about 1/1600 second with bright field and about 1/60 seconds with phase contrast on a 10X 0.25 NA objective though. This is really good enough I might just leave it as is. Adding some more capacitance may be able to fix the problem though so I will likely try.

3. It runs backwards. 1 is high and 6 is low. This isn't as annoying as I anticipated so I may not bother fixing it. Rewiring the pot should be able to fix it, but the one time I tried it seemed like the rewiring burned up my board. The pot is connected to some other circuitry which I haven't investigated, some adjustments may need to be made to that circuitry to make it run normally.
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