Omax fried LED driver PCB -Solved!

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kx4qc
Posts: 2
Joined: Wed Dec 01, 2021 2:54 pm

Omax fried LED driver PCB -Solved!

#1 Post by kx4qc » Wed Dec 01, 2021 3:46 pm

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.
The fried Omax PCB. Incinerated chip at center.
The fried Omax PCB. Incinerated chip at center.
omax_repair_fried_pcb_q1_sm.jpg (48.73 KiB) Viewed 4047 times
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!

Overview of simple repair.
Overview of simple repair.
omax_repair_sm.jpg (75.99 KiB) Viewed 4047 times
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.

Just 4 parts on a board.
Just 4 parts on a board.
omax_repair_LM317_sm.jpg (87.59 KiB) Viewed 4047 times
That's it! An "unreparable" nice Omax better than new for the price of a haircut!
Attachments
Dimmer control with added calibration adjustment.
Dimmer control with added calibration adjustment.
omax-repair_bridged_dimmer_sm.jpg (73.15 KiB) Viewed 4047 times

MichaelG.
Posts: 3976
Joined: Mon Apr 10, 2017 8:24 am
Location: North Wales

Re: Omax fried LED driver PCB -Solved!

#2 Post by MichaelG. » Wed Dec 01, 2021 4:44 pm

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.
Too many 'projects'

kx4qc
Posts: 2
Joined: Wed Dec 01, 2021 2:54 pm

Re: Omax fried LED driver PCB -Solved!

#3 Post by kx4qc » Fri Dec 03, 2021 2:29 am

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:

The replacement board, new ver with adjustment
The replacement board, new ver with adjustment
omax_newboard_sm.jpg (76.88 KiB) Viewed 3980 times
Keywords: Omax, LED, driver, pcb, power supply, repair.

jfiresto
Posts: 342
Joined: Fri Feb 15, 2019 8:19 am
Location: Northern Germany

Re: Omax fried LED driver PCB -Solved!

#4 Post by jfiresto » Fri Dec 03, 2021 9:10 am

kx4qc wrote:
Fri Dec 03, 2021 2:29 am
... 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....
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

MichaelG.
Posts: 3976
Joined: Mon Apr 10, 2017 8:24 am
Location: North Wales

Re: Omax fried LED driver PCB -Solved!

#5 Post by MichaelG. » Fri Dec 03, 2021 12:07 pm

kx4qc wrote:
Fri Dec 03, 2021 2:29 am
Yes, a rectifier, even just a Si diode, Great idea!
[…]

... Worst enemy of Good is Better!
.

That ^^^ is succinctly put :idea:

I think you have found the best compromise.

MichaelG.
Too many 'projects'

prasedenica
Posts: 1
Joined: Fri Jul 29, 2022 7:09 am

Re: Omax fried LED driver PCB -Solved!

#6 Post by prasedenica » Fri Jul 29, 2022 7:13 am

Thanks for this great solution. Useful article and I appreciate it duck life

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