EEVblog Electronics Community Forum
Electronics => Beginners => Topic started by: Barry_2022 on May 23, 2024, 11:26:51 am
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Hi, I have a Keithley 2110 with an LCD display and the backlight LEDs have blown. Not for the first time by the look of it and the small backlight board on one of the backlight panels looks looks unrepairable. The LEDs are 603 size and are wired 2 sets of 3 in parallel. These are connected in series to a 5V supply to the main human interface board (LCD and keyboard switches). As far as I can see there are no ballast resistors and the backlights modules are near zero ohms to the 5v supply.
At first I was concerned that the resistor R303 appears to be absent (may have been removed by a previous owner to increase the brightness?) , but as its wired in parallel then its not going to control the current to the back panel anyway.
So my question is what voltage colour LEDs should I use to replace the blown ones with. If I use white the typical Vf is 3.4V and the total V for the backlight is 6.8V not the 5V supplied. Is this an issue?
Thanks Barry
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White LEDs typically have a forward voltage (Vf) of around 3.0V to 3.4V. If you use white LEDs with a Vf of 3.4V, the series configuration (2 LEDs in series) will require 6.8V, which is higher than the 5V supply.
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Hi thanks. The working LED bank (3 x 603 in Parallel) appears to be white and measure 2.5v DC on the diode tester on a Fluke 289 DMM. The LEDs fitted are TY-L0603PW1 with a data sheet Fv of between 2.8 and 3.6V and Ifmax of 30 mA.
So I guess the ones I have are on the lower end of that
Cheers
Barry
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I just fitted some random white 0603 LEDs (https://www.eevblog.com/forum/repair/keithley-2110-back-lit/msg4492477/#msg4492477) I had lying around and they worked fine.
Perhaps the the 5V supply is actually a current source, dunno?
Anyway, doesn't seem too critical.
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Hi thanks every one I ended up using six 603 white LEDs 2 x 3 in parallel and working ok.
Cheers
Barry