FLUKE 8050A searching adjusting tip to set DC at 0000

[Kiriakos GR]

Recently I did a repair on my FLUKE 8050A bench top DMM.
I have all the available documentation and I did an complete calibration & adjustments.

Funny enough there is no info about zeroing out the display at DCV.
I get 2-3 additional counts at the last digit depending the range.

I can use REL if I need the most of it accuracy, but I was hopping for a repair tip, as it looks that this adjustment is totally undocumented.

[modemhead]

Re: FLUKE 8050A searching adjusting tip to set DC at 0000

If you short the input and it reads zero within about 1 count, I wouldn't worry about it. I also would not use the Rel function to attempt to null a residual reading, because there is no guarantee that any small offset is constant once a low-impedance voltage source has been applied to the inputs.

There is no zeroing adjustment that I know of in an 8050A. The A/D converter includes an auto-zeroing phase that stores any non-zero error offsets in the system on tantalum capacitors C8 and C33, and subtracts them from the reading during the de-integration phase. You can check those caps to be thorough.

[Kiriakos GR]

Re: FLUKE 8050A searching adjusting tip to set DC at 0000

Thanks for the tip modemhead.
Crossing the leads it does has an effect and I recorded the details.

200mV = -00.01
2V = + .0000
20V = - 0.001
200V = + 00.00
1000V = + 000.0

Now it looks that the differences are not significant to be called as damage,
but the 20V range definably differentiates it behavior in contrast with the other ranges.

[modemhead]

Re: FLUKE 8050A searching adjusting tip to set DC at 0000

I have several 8050As, and that looks perfectly normal to me. The one I use on a regular basis stays powered up 24/7, usually on the 20V range, and it bobbles constantly between 0.000 and 0.001, even with the input shorted. I have found it to be very trustworthy in terms of accuracy.

[Kiriakos GR]

Re: FLUKE 8050A searching adjusting tip to set DC at 0000

Well I have just one 8050A but I have its one from every flavor .. LOL
I was had order the 8050A with the battery option, but this one came instead.
It turned as seller error, and I got back one small compensation, as it was expensive to ship it back.

On this design there is a 9 Ohm resistor which came burned out and I had to replace it with two in parallel due the fact that there is no such value available at the stores.

Since this first repair the meter was good for some time.
Well I would feel much better if I had a spare PCB for the LCD.
If you keep spares, drop me one PM regarding an cost estimation.

[modemhead]

Re: FLUKE 8050A searching adjusting tip to set DC at 0000

From your pictures, it appears that you do have the battery-powered version (option 01), except that the batteries have been removed. The small toroidal transformer, and also R26, do not exist in the AC-only model.

It is not good to run this meter without the batteries. The power supply consists of an LM317 configured as a constant-current source. The design depends on the batteries being there to regulate the voltage to near, or just above, the nominal 4.8V. Without the batteries, the voltage will rise much higher, and may cause problems.

[Kiriakos GR]

Re: FLUKE 8050A searching adjusting tip to set DC at 0000

You did send me a travel to the moon with out ticket.

The meter came with out batteries and clips, just the plastic covers of the batteries was attached.

The 8012A was not power up with low batteries, and assumed that the 8050A it can power up because is not a true (option 01).

Nor the Fluke schematic was helping, in my understanding the schematic was indicating that if there is an active mini inverter instead of the transformer, it would act to charge the batteries.

Any way, thanks for pushing my curiosity so to retest and find the truth,
regarding batteries I had to improvise with what ever I have available,
but it did worked.

Now as new owner of the (option 01) may I ask if the CHG LED turns off at a certain point ?

[modemhead]

Re: FLUKE 8050A searching adjusting tip to set DC at 0000

It is a very simple charging scheme. With the power switch off, the LM317 provides about 140mA of charging current via R26. This is a reasonable trickle-charge rate for basic 1300mAH NiCd cells, and should bring up a flat pack overnight.

With the power switch on, the current is increased by another 100mA via R25, to power the meter and continue charging the battery at a slightly reduced rate. If I recall correctly, the meter uses about 150mA.

The charging LED is always illuminated when the power switch is off. It's basically just an indicator of mains power, it is not related to the charge state of the battery pack at all.

If you plan to get some new cells to put in your meter, resist the urge to use NiMH. I'm not a battery expert, but I understand that they do not tolerate the constant trickle-charge as well as NiCd.

[Kiriakos GR]

Re: FLUKE 8050A searching adjusting tip to set DC at 0000

Thanks for the info!

In my first quick test, just to see if it powers up I did use NiMH in the one pair of the two.
I am aware that the charging voltage in NiCD are higher, and this would kill any NiMH which are unable to reach more than 1.44V as topped up.

The NiCD can handle 1.5 to 1.6V per cell with out a problem.

What looks interesting is the fact that even by using a lower rated 250 mA NiCD from a cordless phone, the overnight charging it killed it.

I will totally agree with you about the use of NiCD as must do .. and I will add that it must be also at the specified mA rating, or else the charging process will kill them.

Regarding the cost of those batteries, I have many friends (carpenters) and I will salvage some good cells from their dead power packs of their cordless screwdrivers. ;-)

[Kiriakos GR]

Re: FLUKE 8050A searching adjusting tip to set DC at 0000

I made a step to the future regarding batteries type.

While the meter uses 130 - 140mA at normal operation, at charging is capable for 200 - 240mA ( measured by Fluke 28II).
Which is very acceptable for those lithium-ion which have also internal over voltage protection.
I added also a 500mA mini fuse just in case! ( multiple insurance against bad luck). LOL

I did test this setup for two days, I can almost sign an warranty paper that this setup is truly capable even for 9 hours operation.

[modemhead]

Re: FLUKE 8050A searching adjusting tip to set DC at 0000

Just for reference, here is how the original pair of 2.4V packs (4.8V total) are connected.

[budm]

Re: FLUKE 8050A searching adjusting tip to set DC at 0000

So it is safe to use 7.2V (3.6 x 2) from Li-ion?

[tom66]

Re: FLUKE 8050A searching adjusting tip to set DC at 0000

Uhh... yeah. Don't charge lithium ion that way. The protection circuit is NOT to be relied upon. It is supposed to limit the chance of damage to the battery if the charger were to fail; it is supposed to be used once or twice maybe and not to be relied upon continuously. It will not necessary prevent thermal meltdown on every charge cycle.

You can buy li-ion charging kits based around MCP1630 or similar, which will happily charge single cells, for more cells, you can use advanced ICs or switch to NiMH which CAN be charged at a low current with only a resistor.

[Kiriakos GR]

Re: FLUKE 8050A searching adjusting tip to set DC at 0000

So it is safe to use 7.2V (3.6 x 2) from Li-ion?

Yes I believe that it is, the meter use to charge even NiCD up to 6.3V = Bat A+B
The Li-ion that I have are used ones, by pulling the AC cord, the battery voltage even as fully charged drops down to 6.66V with the load of 135mA.

The interesting part is the charging.
Those modern Li-ion are capable for slow and fast charge, and I am not worrying about the 240mA max as charging current.

Even so the 2.4V battery it can not get higher than 3.2V & the 3.6V battery is unable get higher than 4.5V.
When both batteries are close to be considered as fully charged the charging current is up to 200mA, suddenly the current drops down to zero, and the voltage of the A+B batteries reads as 15V.


The 8050A with out batteries connected at those pins there is a voltage reading as 30V.
The explanation is that the charging are made by pulses at 200Hz.

While this setup is safe but not easily replicable by others, at list I gained some time until to find a more suitable solution, that looks to be those 2400mA NiCD.

[tom66]

Re: FLUKE 8050A searching adjusting tip to set DC at 0000

A lithium ion battery (same for lithium polymer) is based on an extremely unstable chemistry. You must treat the battery -very- carefully. If you do not, the best case is you destroy it so it won't take a charge. Worst, sadly most common case is fire/smoke/explosion.

Essentially you must charge at a constant (or varying, but limited) current not exceeding 1C (for most cells) to about 4.2V. (If the battery starts below 2.7V, then charging at more than 0.05C is not recommended, until the 2.7V threshold is exceeded.) At the ~4.2V point (tolerance +/-0.025V), the charge circuit must switch to constant voltage with current limited to previous rate. When the current drops to around 1/5th to 1/20th the start current, the battery is fully charged. During discharge, the battery should not be continuously discharged below 2.7V for any extended period of time; doing so damages the battery.

The over-voltage circuit cuts off the charging typically above 4.4V for most batteries; however, this is merely the point of self-destruct of the battery. The under-voltage circuit will handle below about 2.2~2.5V. Going above 4.3V generally ruins a lithium ion battery, as does going below 2.7V for more than a few hours.

You can get ICs which do this charging all in one and require just 5V input and two ceramic capacitors for up to 500mA charge rate. If you want an easy to charge battery, you must use NiMH or NiCd. You cannot use lithium ion, or you will start a fire/explosion.

[Kiriakos GR]

Re: FLUKE 8050A searching adjusting tip to set DC at 0000

Even by considering the safeguard limits of 1C
the battery A=1200mA / 120mA & B=900mA / 90mA
A+B= 120 + 90 = 210mA the safe limits are reached and I will not see any explosion.

The lithium ion at fast charge they go up to 3C or 4C which this FLUKE charging circuitry is unable to deliver such current.

[tom66]

Re: FLUKE 8050A searching adjusting tip to set DC at 0000

Even 0.1C is too much to charge without a voltage limit!

Maybe it won't burn, maybe you'll be lucky (maybe not), but you REALLY need to be careful around them.

[Kiriakos GR]

Re: FLUKE 8050A searching adjusting tip to set DC at 0000

In the specific charging circuitry the inner resistance of the batteries works as balancing point, lithium polymer does not have the same resistance with NiCD and this difference triggers the overcharge safety.

Regarding luck, three days now, no Boom. LOL

[tom66]

Re: FLUKE 8050A searching adjusting tip to set DC at 0000

A lithium ion cell has VERY low resistance. Measured in single or double digit milliohms. From a 1000mAh 3S battery, you can easily jump-start a decently sized car; that's more than 150A for half a second. (My dad always carries one around with him now in the car; a great jump-starter when you're in a pinch.)

You sound like you do not understand lithium ion cells, so I invite you to research before you either a) destroy a nice meter due to a battery fire, or b) destroy everything you might own due to a battery fire, or c) kill yourself in a battery fire.

Some popular results for "lipo fire" (on rcgroups, a popular forum using a lot of lipos.)
https://www.google.co.uk/search?q=li...K4bR0QW_64DQCg

[modemhead]

Re: FLUKE 8050A searching adjusting tip to set DC at 0000

So it is safe to use 7.2V (3.6 x 2) from Li-ion?

The front end of the power supply in this meter is a constant-current source. Despite its simplicity, it is safe and effective for trickle charging NiCd cells in the 1200mAh to 1800mAh range. A healthy 4.8V pack will develop a voltage generally between 4.8 to 5.6V depending on its state of charge. This voltage is used as the sole means to regulate the difference between the meter's -5V DC rail and its -10V rail.

Operation outside these parameters is therefore, in a phrase, not recommended.