Calling any Fluke 8808A owners - Page 1
Forum section: electronics / testgear

author: diddly
author_rank: Newbie
timestamp: June 02, 2022, 10:23:17 pm
content: Hey all, long time reader, first time poster.I just picked up an 8808A used and it all seems to function as expected. I did notice though, after leaving it "off" overnight (that is, soft-off but still plugged in with the mains switch on), in the morning I could feel it was warm on the front panel in the general mid area of the screen.Is this normal? You could maybe convince me that it self-heats so that it is ready to use next time you power it up, but you could also convince me I need to repair it. Does anyone else with one of these meters observe this self-heating?Thanks,Dave

author: bdunham7
author_rank: Super Contributor
timestamp: June 02, 2022, 11:46:31 pm
content: Buy something broken to repair, leave your 8808A alone! That line of meters does exactly what you surmise--they keep the reference and inguard circuits powered up when you use the front panel button instead of the switch on the back. That way they are ready and accurate in 2 seconds instead of 30.

author: diddly
author_rank: Newbie
timestamp: June 03, 2022, 09:05:28 am
content: Excellent, that was what I was hoping to hear.Thanks

author: gamalot
author_rank: Super Contributor
timestamp: June 03, 2022, 09:12:13 am
content: I put my Fluke8088A8808A and Tek DMM4040 in place where I can reach their power switch on the rear panel.

author: diddly
author_rank: Newbie
timestamp: June 03, 2022, 09:34:41 am
content: I'll likely follow suit as it will not be used like on the production floor, and be off a lot.Any advice on using the really low current ranges on the ammeter? I mainly wanted this meter for its decent low-burden ammeter when measuring sleep currents on devices. However, sometimes it is nice to let a unit cycle where it runs in the low mA range, then sleeps in the (hopefully) sub-uA range.Ideally I could set the 8808A up to autorange, but I feel it might sometimes hunt around and choose the 200mA range with a higher burden, resetting the device. Probably not a good idea to set it to 200uA range and then walk away and overload the meter, but what does happen in the case? Will it overload and damage the circuit, or simply shunt the current elsewhere safely?

author: AVGresponding
author_rank: Super Contributor
timestamp: June 03, 2022, 12:04:17 pm
content: Quote from: diddly on June 03, 2022, 09:34:41 amThe low current ranges on meters like these are protected by back to back diodes after the fuse. The diodes prevent the voltage across the shunt from becoming too high for the downstream circuitry; if the current gets high enough the diodes can't clamp it, the input fuse will blow.All that will happen in your use case is you'll get an overrange reading that will spoil your graph/statistics.

author: gamalot
author_rank: Super Contributor
timestamp: June 03, 2022, 01:26:04 pm
content: Quote from: diddly on June 03, 2022, 09:34:41 amI've never paid attention to the low current range of the Fluke 8808A, just did a quick test and it seems to have good enough accuracy for your needs.butIf you manually choose the 200uA range, the value of shunt resistance is 900 ohms, and if your device operates at 10mA, the burden voltage will be 9V, which means it may not work properly.<---WRONG!!!

author: AVGresponding
author_rank: Super Contributor
timestamp: June 03, 2022, 01:48:50 pm
content: If the input protection is at all similar to the 8842A, and it probably is, it should clamp at around 4.5V. Still, that would be the same result as far as the DUT is concerned...

author: bdunham7
author_rank: Super Contributor
timestamp: June 03, 2022, 02:49:03 pm
content: Quote from: diddly on June 03, 2022, 09:34:41 amIt has a transimpedance-like circuit for the lowest two current ranges, thus the very low burden voltage. I don't know the details of it, but I would presume it is adequately protected against overloads. I don't know how high the voltage across it might get. As for autoranging, I don't see why that would create any significant burden--the shunt resistance only goes up to 1 ohm + the fuse, so a few mA would mean a few more mV drop. It specifies less than 50mV @ 20mA.

author: gamalot
author_rank: Super Contributor
timestamp: June 03, 2022, 03:00:23 pm
content: It is my BAD! I have a Fluke 8808A but I don't really know it. I didn't even notice when OP mentioned its low burden feature.I think it's about time I took a moment to really figure it out, the only thing I know at the moment is that manually choose the 200uA range and then running 200mA current through it won't cause damage.

author: AVGresponding
author_rank: Super Contributor
timestamp: June 03, 2022, 03:14:17 pm
content: Quote from: bdunham7 on June 03, 2022, 02:49:03 pmThe problem you might get when it shifts range is the DUT might reset due to the circuit being open while the relay switches over.

author: gamalot
author_rank: Super Contributor
timestamp: June 03, 2022, 03:42:46 pm
content: Because I don't know what kind of device the OP's DUT is, I simply used a 50 ohm resistor as the load, the power supply voltage is 5V, and tested the current and burden voltage in 4 ranges from 200mA to 200uA. It can be seen from the test results that if the setting is at 200uA or 2mA, when the actual current exceeds the range, the burden voltage may be much larger than the 5mV given in the manual.

author: bdunham7
author_rank: Super Contributor
timestamp: June 03, 2022, 03:46:41 pm
content: Quote from: AVGresponding on June 03, 2022, 03:14:17 pmI don't know for sure on this unit, but a fairly typical setup has the shunts arranged in series and the relay(s) bypass them as needed so there's never an open.

author: AVGresponding
author_rank: Super Contributor
timestamp: June 03, 2022, 03:47:01 pm
content: Yes, not to mention that any overrange "reading" is useless data in any case.

author: AVGresponding
author_rank: Super Contributor
timestamp: June 03, 2022, 03:54:35 pm
content: Quote from: bdunham7 on June 03, 2022, 03:46:41 pmThat makes sense. Should be fairly easy to test with a DSO.

author: AVGresponding
author_rank: Super Contributor
timestamp: June 03, 2022, 05:03:19 pm
content: Quote from: bdunham7 on June 03, 2022, 03:46:41 pmJust checked the schematic for the 34401A, and it doesn't work this way, it uses changeover contacts, so if you can't find a service manual for the 8808A (I can't), it would be best to test whether it does or not first.

author: bdunham7
author_rank: Super Contributor
timestamp: June 03, 2022, 06:53:27 pm
content: Quote from: AVGresponding on June 03, 2022, 05:03:19 pmSo of course I had to try it. Yeah, HP gets a D- for that implementation--the shunts are still in series but they apparently skimped on both shunts and relays there. Still, it only happens shifting to the highest two current ranges which means over 100mA. Pretty common issue in compact bench meters of the day and the 8840A/8842A only avoid it by only having a fixed 0R1 shunt. 8846A also doesn't impress me going from mA to uA ranges--there's some switching noise there. Old boat-anchor 8500-series seem to avoid this, but a capacitor across the inputs is a good idea in any case. I don't have an 8808A, it would be interesting to see what happens when it shifts up from the 2mA to the 20mA range.

author: AVGresponding
author_rank: Super Contributor
timestamp: June 04, 2022, 02:37:18 pm
content: I wonder if it's something as simple as board space? I see no reason why you'd need to synchronise the shunt changeover with what seems to be the ohms 2-W/4-W changeover.

author: MrYakimovYA
author_rank: Regular Contributor
timestamp: June 05, 2022, 02:11:06 am
content: Quote from: diddly on June 03, 2022, 09:34:41 amBy the way, there is almost the same topic since the linked message:https://www.eevblog.com/forum/testgear/any-review-about-tektronix-dmm4040/msg4198747/#msg4198747