Reversed battery on Fluke 85 III and now dead - Page 1
Forum section: electronics / repair

author: tungsten2k
author_rank: Newbie
timestamp: September 13, 2014, 05:45:33 pm
content: So I had four 30C 250mAh LiPo batteries laying around that I bought for my Blade mCP-X helicopter that did not fit well.It seems like every 6 months, I get the low-Batt indicator on my Fluke 85-iii because is one of the earlier models that drains the battery quickly when in standby (and I tend to forget my meter on all the time).So when the low-Batt indicator lit up yesterday, I decided to replace the 9v with two of these LiPo batteries in series. I'm an active RC hobbiest so I always have a charger available, but never have 9v batteries. This LiPo arrangement would allow me to quick-charge the battery in 30min every 6 mos for my Fluke, and I'd stop having to buy these $4 9V alkalines (and have to stop my project to go to the store to get the battery).UNFORTUNATELY: When I soldered the 9v battery connector lead (Radio Shack #270-0324) onto the 2S1P 250mAh pack, I made the mistake of connecting the red lead to the postive and black to negative. AND in my infinite wisdom, failed to check that the polarity on the connector metered out correctly (my meter had a dead battery and I was too lazy to go find my cheap meter).Well, when you use the 9v battery connector lead as a source for a 9v battery eliminator, the polarity is reversed so the leads are the reversed colors. Connecting my LiPo pack in reverse polarity apparently took out my Fluke because it's dead. I opened it up and checked the fuses and they were fine. I can't imagine the Fluke doesn't have reverse battery protection given that it's very easy to connect a 9v battery reversed while trying to snap it on, so I'm not sure why it died. I can only speculate that the reverse protection circutry in the Fluke is designed for a large internal resistance battery like a 9v alkaline and when I connected the hi-C-rated LiPo battery reversed, it took the protection circutry out ?I opened it up and didn't smell any magic smoke. I downloaded the service manual but it didn't really have anything for testing the 9v source power circuitry and didn't contain a schematic on that area, so I'm not sure where to start. I did the standard things... checking voltage was making to the board from the leads, testing the fuses, etc.I'd appreciate any ideas some of you might have.Thanks for reading,-=dave

author: saturation
author_rank: Super Contributor
timestamp: September 13, 2014, 07:21:15 pm
content: Sorry to read.Just first thoughts before getting complicated: do you have a working 9V alkaline battery to reconnect to the 85-III and insure its really dead? This removes any issues with the DIY battery pack cells or connectors.If dead still, its possible the power protection circuitry for DMMs in the 80s series will be minimally changed from series 1. I'd download Series 1 manual and there is a schematic to get a general idea of its location on the mobo; it also has a detailed hardware troubleshooting guide particularly for the power supply.Keep us posted, good luck!

author: ModemHead
author_rank: Frequent Contributor
timestamp: September 13, 2014, 10:51:13 pm
content: +1 on the connecting the original 9V supply for testing.Not 100% sure, but I think the series III power supply is the same as the series I. There is indeed a reverse-polarity protection diode. If your modified battery popped it, the next check is the CMOS hex-inverter package (CD4069), which is connected across the battery all the time, regardless of switch position. Five of the gates are used for the beeper, but one is used for power logic.Be sure to check Vdd and Vss with respect to the COM jack, as I recall it's +3.0V and -3.2V, respectively.

author: tungsten2k
author_rank: Newbie
timestamp: September 14, 2014, 03:54:32 am
content: Yes, I have tried a good working 9v. I guess a lot of inexperienced people come through these forums, so I can understand the fundemental suggestions on the obvious.Thanks a bunch for the schematic on the series I - I hadn't thought of checking on the earlier series for it. ...as well as the further troubleshooting suggestions. I'll update here if I find out what blew.-=dave

author: macboy
author_rank: Super Contributor
timestamp: September 15, 2014, 03:58:05 pm
content: I have no experience with the 85 specifically, but I can offer a tip. A simple reverse-polarity protection that is commonly used is a reverse-biased diode across the supply leads. The intent is the short out the supply (battery) if connected backwards, to prevent the reverse voltage from reaching the device circuitry. Often enough, this causes that diode to fail shorted, which will prevent the device from working even with normal polarity. Check for a reversed diode across the battery inputs, and check if it is shorted.

author: lowimpedance
author_rank: Super Contributor
timestamp: September 16, 2014, 12:29:21 am
content: Quote from: macboy on September 15, 2014, 03:58:05 pmLooking at the cct posted for the 83 there is a diode used (assuming the same cct used in the 85). The LiPo pack would have enough grunt to burn the diode open, not just short it ? . Im afraid there will likely be more than just that.

author: ModemHead
author_rank: Frequent Contributor
timestamp: September 16, 2014, 01:45:36 am
content: Quote from: lowimpedance on September 16, 2014, 12:29:21 amHence my use of the technical term "popped it" above. Hat tip, PhotonicInduction.

author: lowimpedance
author_rank: Super Contributor
timestamp: September 16, 2014, 02:32:51 am
content: Quote from: ModemHead on September 16, 2014, 01:45:36 amAnd he would have finished it off with his "little hammer"

author: saturation
author_rank: Super Contributor
timestamp: September 17, 2014, 02:17:00 pm
content: There are many concerns in this mod that experience easily cures. It would be helpful to detail this issue in the forum so when archived, it'll be available for anyone else trying it. This expands on previous comments by others.Reverse Input Protection, See posted schematic.The diode CR4 is a cheap simple solution, and is the principle protection against sustained ADC in many designs powered by alkaline batteries. However CR5 is paralleled with CR4 is vulnerable to current surges as C35 will appear as a short for microsecond durations, as it charges. BAV99 tolerate <~ 4A in ms periods, IN4007 about 30A.Chemistry SubstitutionDrop in 9v NiMH and Primary Lithium are designed to mimic the internal resistance of 9v alkaline or less [ ZnCl]. Either they naturally have high internal resistance or a PTC is inserted in series to act as current limiters. If a LiPo or similar low internal resistance battery is used and connected in reverse polarity, very high currents will run in the protect diodes. As the protect diodes appear as shorts, its unhealthy for the diodes and LiPo leading to potential failures in both. LiPo surge currents can be very high even if it had built in PCM, as current limits are designed around the intended battery application.Comments on the OP:The 85-III has a bug so it never goes into sleep mode, but the draw is <= 1mA quiescent. LiPo 2S1P 250mAh pack will give the same lifespan as a 9V alkaline, ~ 200-250mA, ~ 10 days. The only difference is the battery is a secondary versus primary type.TwoLiPo packs? Each pack is ~ 7.4Vdc, if the designed had been assembled correctly it would provide the DMM with 14.8Vdc, 250mAh. This is far over expected input voltage and could damage the internal regulator at some point, which is harder to repair.Hopefully the protection diodes failed quickly, as the reverse input can already damage the regulator section of the DMM if the not main Fluke IC.Recall 9V alkaline cells are made of 6 AAA/A cells rated at 1.5Vdc each. Fresh open circuit about 1.7Vdc, for a total ~ 10V, new. NiMH typically 6-7 cells at 1.25Vdc each.Recall LiPo packs made for RC use can have much higher momentary surge over the sustained maximum output current. A typical small pack LiPo 2S1P 250mAh can provide 20-30C for some minutes but may burst up to 60C. That translates to sustained 1-3 min current of 5-7.5A and a burst as high as 15A ~< 1 sec.A simpler solution to the dilemma:Turn off the 85-III auto-off function with each power up. Thus the meter will not appear to be 'off' when its still 'on.' It serves as a reminder to turn it off.Replace the 9V with a NiMH LSD 9V, see the eevblog archives on a discussion on the Tenergy Centura 9V LSD. In the USA, there is a kit for $20 for 4 batteries plus charger, that's ~ 4 batteries paid for and the charger is 'free'. This kit is ~ the cost of a 1-2 S21P LiPo pack.http://www.amazon.com/Tenergy-T-228-Compact-Rechargeable-Batteries/dp/B007VIHEM6/ref=sr_1_2?ie=UTF8&qid=1410961608&sr=8-2&keywords=Tenergy+Centura+9V+200mAh+with+chargerBest wishes to the repairs.Quote from: tungsten2k on September 13, 2014, 05:45:33 pm