My 1st Fluke Mod - Simple DIY Fluke 175 backlight! - Page 1
Forum section: electronics / testgear

author: sambonator
author_rank: Regular Contributor
timestamp: February 27, 2019, 11:33:42 am
content: Inspired by this thread, I did my 1st Fluke Mod!https://www.eevblog.com/forum/testgear/fluke-175-and-179-side-by-side-backlight-hack-on-fluke-175/msg435984/#msg435984This is a super simple mod for the Fluke 175... just an ebay backlight cut to size, placed behind the LCD, and wires soldered to pads on the back of the board. Meter draws 22.59mA in voltage measurement mode, up to 23.45mA in diode and continuity modes, from the battery.Trying to still figure out the resistors/transistor parts needed to make it work like a 177/179. I guess I could put in a latching pushbutton switch where the backlight pushbutton exist on the 177/179.Right now the backlight is always on when the meter is on. Auto power off continues to work to shut the meter down (5 mins?).All the voltage, current, resistance, diode, capacitance tests after the mod continues to match my Fluke 87v.Any suggestions for improvement greatly appreciated!

author: graybeard
author_rank: Frequent Contributor
timestamp: February 27, 2019, 04:44:05 pm
content: That looks nice. What is the current draw of the backlight? Could you post a link to the one you bought?

author: sambonator
author_rank: Regular Contributor
timestamp: February 27, 2019, 05:45:49 pm
content: This is the backlight I used.https://www.ebay.com/itm/FuJing-model-LCD-Backlight-Kit-Glows-White-Turnigy-9x-FlySky-FS-TH9X-etc/183689720794They have various color modules if you want green, amber, red, blue, if you search for "Turnigy LCD Backlight."I removed the LED backlightmodule and leads from the harness it comes with, and soldered it to two of the four pads at the top middle of the board.Counting the pads from left to right, I used pad 2 (ground) and pad 4 (+3.2v when on). Honestly I have no idea what these pads are for but it was convenient

author: sambonator
author_rank: Regular Contributor
timestamp: February 27, 2019, 06:24:48 pm
content: P.S. Current draw of the backlight is 20mA. It seems to draw equivalent current to the Fluke 87v backlight on brightest mode...maybe slightly less, but looks brighter and more uniform (except the LED covered section). To shift it further right I'd have to cut into the display holder... might try that in the next iteration with an amber or green backlight, along with a switch.

author: ModemHead
author_rank: Frequent Contributor
timestamp: February 28, 2019, 10:27:47 pm
content: Quote from: sambonator on February 27, 2019, 05:45:49 pmThe normal operating current of this meter is just a couple of milliamps. Increasing the load by an order of magnitude on the 3.3V regulator may or may not be a great idea, can't say for sure. The original backlight LEDs run straight off the unregulated battery voltage, via the 475Ω current-limiting resistor.

author: sambonator
author_rank: Regular Contributor
timestamp: March 02, 2019, 12:36:01 am
content: Quote from: ModemHead on February 28, 2019, 10:27:47 pmThank you ModemHead for your input.I took a few more measurements and thought I'd post. The actual voltage on pad 4 is 3.273v, and measured 60mA available between pad 2 and 4. Even though the 20mA backlight uses just 1/3rd of available (is that the right term?), you're right who knows what stress that might be causing the circuit/regulator.I just received a used ebay Fluke 179 for my investigation. I'm surprised by the very dim backlight! The 179 backlight uses just 10mA vs. my 20mA hack. I think the 179 is much less than half as bright.The LED's on the board also seem fairly dim (see pics with them on and off). The plastic backlight with lightpipe also seem pretty inefficient (see pics). Maybe modding the backlight/lightpipe will improve things... thinking of giving the lightpipe section a mirror coating.Placing reflective sheet or tape behind this lens might also improve brightness, as all that's behind the backlight lens is white plastic.

author: sambonator
author_rank: Regular Contributor
timestamp: March 13, 2019, 06:11:41 am
content: Modded the 179 by putting chrome mirror tape around the light pipe and LED slots to capture as much light to the display as possible. I think light output improved just slightly.I also modded the 175 to use less current with a 47Ω resistor, and now it draws less than the 179 with backlight (10.02mA 175 vs 10.24mA 179 in DCV mode).The 175 is still much brighter.