Extech Ex505 (also Craftsman 82003) calibration/ mA ammeter issue - Page 1
Forum section: electronics / testgear

author: hunter
author_rank: Contributor
timestamp: March 30, 2013, 11:15:09 am
content: Hello, I have a trusty Craftsman 82003 DMM which happens to have the same board with Extech Ex505. Recently, I just found out that the mA current measurement mode is way inaccurate ( eg: reports 50mA while it should be 150mA as checked with other DMM). It seems to have oddly large internal resistance in mA mode (5 ohms when measured with another DMM) . This is an issue with mA mode, the 10A mode is fine and accurate. All other functions work ok. Fuse is ok.I wonder what sort of problems could lead to this inaccuracy, say I am trying to find out if this DMM is simply out of calibration or the mA mode is toasted. I wasn't sure if this was all along, but the only I could remember I once put a Li-ion cell across the mA mode, as soon as I saw the OL display I quickly disconnected, after checked, the fuse wasn't blow and the meter appeared to have no damage.I see there are about 7 micro pots on the board, how do you about figuring out which one is for the mA , if that is likely possible ?Thanks !

author: amspire
author_rank: Super Contributor
timestamp: March 30, 2013, 12:19:26 pm
content: It is extremely unlikely a pot would adjust from 50mA to 150mA - they probably only adjust +/- 10% or less.It is more likely something has happened such as the mA sense resistor got cooked and tripled in value. If this is the case, the resistor probably should be something like 1.5 ohms instead of 4.5 ohms (or whatever the resistor is now).Is the 40mA range also out or is it just the 400mA range?I would look at the fuses in the meter and see if someone has replaced the 0.5A ceramic fast blow fuse with a higher value such as a 2A fuse.This would allow the mA sense resistor to be overloaded. If it has been, it probably should look a little burnt.Richard.

author: ModemHead
author_rank: Frequent Contributor
timestamp: March 30, 2013, 12:36:09 pm
content: Did you compare meters simultaneously, with both meters in series with the current source? Otherwise differences in burden voltage may have altered the current you were trying to measure.4.99 ohms is a reasonable mA shunt value.

author: Wytnucls
author_rank: Super Contributor
timestamp: March 30, 2013, 12:58:36 pm
content: As mentioned earlier, there is probably something wrong with the shunt resistors or the fuse. Check both.

author: Lightages
author_rank: Unknown
timestamp: March 31, 2013, 12:06:07 am
content: Another failed Extech EX series? What a surprise! Open it up and see if it has cold solder joints, or is falling apart, or a bodged repair job like Dave had.

author: hunter
author_rank: Contributor
timestamp: March 31, 2013, 03:20:21 am
content: I did further testing and I think I know where the problem was , I had to do with its excessive internal resistance in mA mode. All I would like to know now is weather this is normal for this Extech model or my unit is defected..I measured current through a 270 ohm resistor, All results were correct up to 270mA until the resistor burned.I did the same thing with a 2 ohm resistor. All results were about half of the values they were supposed to be in all ranges. It seems reasonable but c'mon is it about the limit already ?Before I became aware of this, I was building a circuit that draws 250mA from 4V (values as displayed on Power supply). So impedance was about 16 ohms. After the DMM in mA mode was connected, the current drawn was 130mA which was way off from 250mA. Why does Extech meter fail in this case ? Is it because this is "Heavy duty industrial model" which is supposed to work on line voltage and things like that , not on low voltage measurements ?I have taken 2 pics of the board, can you tell which resistor is the shunt resistor ?In the 2nd pic, there are R36 (white-white-black-gold) and R37(black-black-brown). I noticed R36 solder joints were almost bridged together, leaving a tiny gap as you can see in the pic. The interesing thing is that where the mid point broke off , the part of the soldering is white ceramic instead of silver ! Should I do something about this ?Lastly,In the manual there is no specification on internal resistance, say I would buy a new meter, what sort of specs should I look for if I want to use it for low voltage, low impedance circuit ?

author: ivan747
author_rank: Super Contributor
timestamp: March 31, 2013, 03:51:02 am
content: Is R36 alright?

author: hunter
author_rank: Contributor
timestamp: March 31, 2013, 04:07:16 am
content: R36 is ok (99ohm) but R37 is a bit off (2 ohm vs brown-black-black) but R37 looks fine, It was squeezed in there so I might not get accurate reading for R37.

author: Rerouter
author_rank: Super Contributor
timestamp: March 31, 2013, 04:25:19 am
content: i think he is referring to how it looks to be shattered in half from the image... or at-least blown out on one side...

author: amspire
author_rank: Super Contributor
timestamp: March 31, 2013, 04:38:16 am
content: Definitely replace r36 if it is chipped.What if the rating of the fuse plugged into the 500mA socket? If it is not 500mA, it is significant. The 400mA shunt resistor is only protected as long as the correct 500mA is installed.Now to the values. I think you will find that R37 is not brown-black-black (10 ohms) - it is brown-black-black-silver (1 ohm) or it could possibly red-black-black-silver (2 ohms) which has been cooked enough to turn the red band to brown. 1 ohm makes a lot of sense for the correct value. It would mean 400mV full scale at 400mW. The 99 ohm in series with the 1 ohm makes 100 ohms so 100ohms is probably the resistance used for the microamp range.When you are checking the mA range wiring, put the range switch in mA mode. One end of R37 should connect either to common directly, or to the high end of the 10A range resistor. The other end should connect directly to one end of the 500mA fuse (taking the fuse out will make it easier to get the right end). The other end of the fuse should connect to the mA/uA current input.400mW is pushing it a bit for a small smd resistor, so if you replace it, you could get a bigger 1 ohm resistor and connect it via flying leads. 400mW means the resistor will heat significantly under full load, and it will drift. At a guess, the meter spec for current may be 1%, and they probably didn't care if it drifted as long as it stayed within specs. Find a safe place (if possible) to tack the resistor in place. Not pretty, but if you get a good quality 3W resistor, it may give a much more stable current measurement.Since you do not know the right pot to adjust current, I would probably try and find a 1 ohm resistor that gives a high reading and then calibrate with parallel resistors. If you try and find the right pot through trial and error, you will mess up the rest of the multimeter calibration. The calibration may only work if a corret procedure is followed, so if you change the pot settings, you may never get it right again.Check that fuse.Richard

author: Lightages
author_rank: Unknown
timestamp: March 31, 2013, 04:45:48 am
content: Is that a crack I see at the same place Dave had his with the horrible bodge job? The spot I am talking about is the thin bit of circuit board right beside the input jack at the far lower right.

author: hunter
author_rank: Contributor
timestamp: March 31, 2013, 05:41:20 am
content: Wow, yes it turns out the fuse is bad one. I bought 5 fast 500ma fuses from a place called Divine lightning and all of them have 4 ohms resistance ! I never imagined thisRegarding the R37, I don't know how it ended up like that, but like I said, the resistor is fine in perfect shape, BUT the 2 ends with the solder blob extends all the way to the middle of resistor leaving only a tiny gap. AND the those 2 blobs turn on a tiny ceramic plate (that's way it looked white in the photo). I just used a plier to break off the ceramic piece and solder on it. Now it's good.and yes, @amspire , R37 is brown-black-black-silver (2.7ohm).@Lightages, that is the thin pcb that goes from mA input to mA fuse. Mine has good trace unlike Dave's.

author: amspire
author_rank: Super Contributor
timestamp: March 31, 2013, 05:55:12 am
content: If R37 measures 2 ohms or 2.7 ohms, then it does need replacing with a 1 ohm resistor.Fuses need resistance otherwise they would never blow but 4 ohms is a little high. You should be able to get 500mA fuses with more like 0.5 ohms resistance.

author: hunter
author_rank: Contributor
timestamp: March 31, 2013, 06:01:25 am
content: Good point, I will check back with R37 after I find new fuses

author: amspire
author_rank: Super Contributor
timestamp: March 31, 2013, 06:07:32 am
content: The fuse resistance has no effect on accuracy - just the voltage drop across the meter.About 2V drop at 400mA is pretty ugly, and that 4 ohms will increase at 400mA as the fuse wire heats.

author: metalphreak
author_rank: Frequent Contributor
timestamp: March 31, 2013, 07:58:44 am
content: My multimeters (rebranded EX520s) have 2ohms of resistance across the inputs on the mA range. This is with the standard HRC fuses.