I, like many other proud owners of Omax LED microscopes, fried my LED PCB -and more by plugging in a wrong voltage but plug-compatible wall-wart.
Many OMAX scopes including my nice M837ZL series scope have a proprietary board DXL80D which is easily damaged by over-voltage. Like others owners I was unable to find a replacement part, and anyway it seemed poor engineering to be so fragile -not even a simple fuse to guard the light source.
The scope is repaired now and is fused, fully functional and tolerant of any correct polarity input voltage 5-35v, so if you pick up the wrong power adapter or can't find the original, all's well; just plug it in and use the scope. (Hard to know why it wasn't made like this in the first place?)
I posted the details of the repair at https://www.eevblog.com/forum/repair/he ... msg3846572 , as it is about a repair rather than using a microscope. But if the microscope is broken, you can't use it!
The parts are referenced on Amazon for convenience, but I have no recommendation on sources. The 'scope is now fully functional and repairable with "off the shelf" readily available inexpensive parts. Skills needed? Simple soldering and use of a digital multimeter. If the posted repair details aren't entirely clear, please feel free to contact me.
That's it! An "unreparable" nice Omax better than new for the price of a haircut!
Omax fried LED driver PCB -Solved!
Omax fried LED driver PCB -Solved!
- Attachments
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- Dimmer control with added calibration adjustment.
- omax-repair_bridged_dimmer_sm.jpg (73.15 KiB) Viewed 4344 times
Re: Omax fried LED driver PCB -Solved!
Well done … and thanks for sharing the solution.
Just one suggestion : if you want to make it even-nearer to ‘bulletproof’ … put a bridge rectifier on the input
Then, even a supply with the wrong polarity should be O.K.
MichaelG.
Just one suggestion : if you want to make it even-nearer to ‘bulletproof’ … put a bridge rectifier on the input
Then, even a supply with the wrong polarity should be O.K.
MichaelG.
Too many 'projects'
Re: Omax fried LED driver PCB -Solved!
Yes, a rectifier, even just a Si diode, Great idea!
A full bridge would drop the input voltage by 2 x 0.6v (forward Si voltage), making a commonly available 5v supply now appear as 3.8v at the LM317. This is just 0.8v above the target 3.0v LED voltage.
I think the LM317 needs at least 1.25v above its target to work, so not so good.
A single single diode would still leave 5 - 0.6 = 4.4v at the LM317, allowing a "hefty" 1.4v above the target 3.0v. I have fitted a single 10A10 diode in the +v supply and can confirm that it works as described. As MichaelG said, now it's really near "bulletproof".
(Maybe ceramic capacitors should be substituted for the electrolytics across LM317 In and Out... Worst enemy of Good is Better! )
So good to be enjoying my microscope again, and instead of a hugely expensive dealer repair, the parts for this substitute board came to under $5. -And it's better!
I 'tidied" up the fix by moving the calibration adjustment to the board. Neater and easier all round. Functionally exactly the same:
Keywords: Omax, LED, driver, pcb, power supply, repair.
A full bridge would drop the input voltage by 2 x 0.6v (forward Si voltage), making a commonly available 5v supply now appear as 3.8v at the LM317. This is just 0.8v above the target 3.0v LED voltage.
I think the LM317 needs at least 1.25v above its target to work, so not so good.
A single single diode would still leave 5 - 0.6 = 4.4v at the LM317, allowing a "hefty" 1.4v above the target 3.0v. I have fitted a single 10A10 diode in the +v supply and can confirm that it works as described. As MichaelG said, now it's really near "bulletproof".
(Maybe ceramic capacitors should be substituted for the electrolytics across LM317 In and Out... Worst enemy of Good is Better! )
So good to be enjoying my microscope again, and instead of a hugely expensive dealer repair, the parts for this substitute board came to under $5. -And it's better!
I 'tidied" up the fix by moving the calibration adjustment to the board. Neater and easier all round. Functionally exactly the same:
Keywords: Omax, LED, driver, pcb, power supply, repair.
Re: Omax fried LED driver PCB -Solved!
A 5V DC supply might have a ±5% output voltage tolerance and if, IIRC, an LM317 can have a fairly high dropout voltage, up to perhaps 2–3V under load. The LM317 just might, a little, with some not terrible unlikely, bad luck. If it should, I suppose you could substitute a 7.5V supply – although you then might have to safeguard your other gizmos against the newcomer.
-John
Re: Omax fried LED driver PCB -Solved!
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That ^^^ is succinctly put
I think you have found the best compromise.
MichaelG.
Too many 'projects'
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Re: Omax fried LED driver PCB -Solved!
Thanks for this great solution. Useful article and I appreciate it duck life