Fluke 731B battery & charge circuit
So I came by a 731B for a low price and no surprise, the battery is
missing. No big deal, the archives here tell that it's 12 2/3A cells. The
local Batteries Plus can do that for less than $30... and they did (less
than the cost of 12 individual cells and they had them in stock).
FWIW, my in-cal 3456A read 9.99999 with the 731B on AC power and no battery
installed.
It was a little work to install the battery as one of the pins in the 731B
had broken off and I had no connectors to fit the pins anyway. I soldered
wires to the remaining pin and trace next to the broken pin. I found a 2
pin inline connector in my parts stash and put used it to make the new
battery disconnectable.
Now, being paranoid, I wanted to make sure the battery was charging
properly and powering the reference properly. So I watched the battery
voltage rise and also checked the 731B's power supply output.
Now I see problems... the switch between the battery and the internal
regulator is simply a couple of diodes. In theory, I suppose, the
regulator always supplies a greater voltage than the battery can charge
to. Wrong. The regulator is an 18V zener (1N967B as discussed here
before) supplied by a 1mA current source and driving an emitter follower.
My zener is showing 16.8V and the battery has charged to over 16.2V. The
charging circuit/battery is now powering the instrument! In fact, with no
battery in place, the charging circuit completely overpowers the regulator
and powers the instrument! (I had noted over 20V at the battery terminals
before installing the battery).
The manual says the voltage should be about 17V at the collector of Q1 when
on AC power. That is after the switch diodes. I don't consider 15.6V
(measured) about 17V and looking at the circuit, 17V isn't particularly
likely since the 1N967B is spec'd at 7mA, it's driven by 1mA and then there
are two diode drops.
I put my 'scope after the switch diodes and there is at least 20mV pk-pk
ripple. At the emitter of the emitter follower, it's clean.
So, I added a couple of 1N4148 diodes in series with the 1N967B. That
raised the effective zener voltage to 18V. The regulated supply now
supplies the instrument and it's clean on the 'scope. But, the 3456A is
reading 10.00002... There is a handwritten comment in the manual (found
online at one of the usual places) that a 1.3V difference between battery
and AC operation can result in a 10uV difference at the output. That could
explain the difference since the instrument is supplied with 16.8V now and
it was > 20V before.
After the base-emitter drop, the battery can now charge to 17.3V before
I'll see a problem again. It's still charging so I don't know if it will
get that high.
Conclusions:
If there is no battery installed, the 731B runs off the battery charge
circuit. It's noisy and > 20V when it should be "around 17V". So if you
have a 731B with no battery, I'd suggest clipping CR8 - unless you
absolutely know it was calibrated in this state in which case, I'd be
tempted to leave it alone (see above comment about difference between AC
and battery operation).
If a battery is installed, it's still possible that the 731B is running off
the battery charge circuit. I'm thinking about how to fix this... (a
normally closed relay driven by the AC supply in line with CR8 would do the
trick, but might take too much current from the supply. I could achieve
the same with a couple of transistors, but it would cost a little current
when running on battery... perhaps a MOSFET.).
Orin.
You might want to check/replace the current limiting resistor in the
batt charge circuit, and the zener.
Both of my 731Bs had low zener output.
Had to replace the zeners in both and that 5ma jfet regulator in one.
Both do vary well, and do not show any change switching from AC (with
battery floating at full charge) to DC according to the 3456es, at least
until the batter starts dropping.
On 6/16/13 8:15 PM, Orin Eman wrote:
So I came by a 731B for a low price and no surprise, the battery is
missing. No big deal, the archives here tell that it's 12 2/3A cells. The
local Batteries Plus can do that for less than $30... and they did (less
than the cost of 12 individual cells and they had them in stock).
FWIW, my in-cal 3456A read 9.99999 with the 731B on AC power and no battery
installed.
It was a little work to install the battery as one of the pins in the 731B
had broken off and I had no connectors to fit the pins anyway. I soldered
wires to the remaining pin and trace next to the broken pin. I found a 2
pin inline connector in my parts stash and put used it to make the new
battery disconnectable.
Now, being paranoid, I wanted to make sure the battery was charging
properly and powering the reference properly. So I watched the battery
voltage rise and also checked the 731B's power supply output.
Now I see problems... the switch between the battery and the internal
regulator is simply a couple of diodes. In theory, I suppose, the
regulator always supplies a greater voltage than the battery can charge
to. Wrong. The regulator is an 18V zener (1N967B as discussed here
before) supplied by a 1mA current source and driving an emitter follower.
My zener is showing 16.8V and the battery has charged to over 16.2V. The
charging circuit/battery is now powering the instrument! In fact, with no
battery in place, the charging circuit completely overpowers the regulator
and powers the instrument! (I had noted over 20V at the battery terminals
before installing the battery).
The manual says the voltage should be about 17V at the collector of Q1 when
on AC power. That is after the switch diodes. I don't consider 15.6V
(measured) about 17V and looking at the circuit, 17V isn't particularly
likely since the 1N967B is spec'd at 7mA, it's driven by 1mA and then there
are two diode drops.
I put my 'scope after the switch diodes and there is at least 20mV pk-pk
ripple. At the emitter of the emitter follower, it's clean.
So, I added a couple of 1N4148 diodes in series with the 1N967B. That
raised the effective zener voltage to 18V. The regulated supply now
supplies the instrument and it's clean on the 'scope. But, the 3456A is
reading 10.00002... There is a handwritten comment in the manual (found
online at one of the usual places) that a 1.3V difference between battery
and AC operation can result in a 10uV difference at the output. That could
explain the difference since the instrument is supplied with 16.8V now and
it was > 20V before.
After the base-emitter drop, the battery can now charge to 17.3V before
I'll see a problem again. It's still charging so I don't know if it will
get that high.
Conclusions:
If there is no battery installed, the 731B runs off the battery charge
circuit. It's noisy and > 20V when it should be "around 17V". So if you
have a 731B with no battery, I'd suggest clipping CR8 - unless you
absolutely know it was calibrated in this state in which case, I'd be
tempted to leave it alone (see above comment about difference between AC
and battery operation).
If a battery is installed, it's still possible that the 731B is running off
the battery charge circuit. I'm thinking about how to fix this... (a
normally closed relay driven by the AC supply in line with CR8 would do the
trick, but might take too much current from the supply. I could achieve
the same with a couple of transistors, but it would cost a little current
when running on battery... perhaps a MOSFET.).
Orin.
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On Sun, Jun 16, 2013 at 11:20 PM, Christopher Brown cbrown@woods.netwrote:
You might want to check/replace the current limiting resistor in the
batt charge circuit, and the zener.
I'll check the current limiting resistor for sure. It's a carbon comp, so
it could well have drifted.
Both of my 731Bs had low zener output.
Had to replace the zeners in both and that 5ma jfet regulator in one.
The E505 has already been replaced with I presume a J505 as it's in a TO92
case. Do you remember what your good zener voltage was? I know they are
spec'd at +/- 5% and the J505 is +/- 20% (don't know about the E505, I
can't find a datasheet) so there could be quite a range. There is no
indication on the part list that they were selected. I have some zeners on
order and I'm going to see what I get with 1mA through them.
Thanks for the info.
Orin.
They are not selected, think of that whole assembly as the "pre supply",
the close regulation and the standard are later.
I suspect noise would be had, but exact voltages come later.
I ordered 100 Fairchild 1N5248B (Mouser 512-1N5248B) for $3.70
Not exactly critical, but this is a fun hobby, so I setup a quicky test
setup, ran em all, picked the five closest to 18V, then breadboarded em
with 24v through dropping resistor and let em run for a week (while I
was doing other work on the 731s).
Didn't see any change, picked 2 and used em. Rest are in the parts bin.
That jfet and the shielded transformer are hard to get. Everything else
on the power supply board is nothing special. All of the specially
selected stuff is on the daughter board.
My take on things...
The highest float charging battery voltage should be ~ .8v lower than
the zener.
The "settled" voltage of the battery (where the pack will sit after
12hrs of no charge and where it will sit until the final discharge
slope) should be high enough that there is no change in the output as
read by a 3456A in 6 digit.
The point here is to have a short term stable noise free power source
going to the actual ref circuit. Longer term, you just need to keep the
changes small enough to minimize output changes.
Based on what I saw with both of mine (came without batteries), that ~
20v actually included about 3V AC, and the meter was calibrated without
the pack. (Take a reading of pre-supply output with the battery
disconnected with and without filter DC and then ACV on the 3456A)
Both of mine were last calibrated before their batteries were pulled
and were low when running off of the battery charger. I did not find
the problems with the zener, jfet and current limit resisters until I
built replacement packs and was trying to verify circuit functions.
On 6/16/13 10:48 PM, Orin Eman wrote:
On Sun, Jun 16, 2013 at 11:20 PM, Christopher Brown cbrown@woods.netwrote:
You might want to check/replace the current limiting resistor in the
batt charge circuit, and the zener.
I'll check the current limiting resistor for sure. It's a carbon comp, so
it could well have drifted.
Both of my 731Bs had low zener output.
Had to replace the zeners in both and that 5ma jfet regulator in one.
The E505 has already been replaced with I presume a J505 as it's in a TO92
case. Do you remember what your good zener voltage was? I know they are
spec'd at +/- 5% and the J505 is +/- 20% (don't know about the E505, I
can't find a datasheet) so there could be quite a range. There is no
indication on the part list that they were selected. I have some zeners on
order and I'm going to see what I get with 1mA through them.
Thanks for the info.
Orin.
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On Mon, Jun 17, 2013 at 12:29 AM, Christopher Brown cbrown@woods.netwrote:
My take on things...
The highest float charging battery voltage should be ~ .8v lower than
the zener.
I agree with that. My battery seemed to float at 17.0V last night, but I
turned it all off before going to bed - I want to be sure it doesn't
overcharge before leaving it on all the time. After 19 hours off, the
battery has self-discharged down to 16.5V.
The "settled" voltage of the battery (where the pack will sit after
12hrs of no charge and where it will sit until the final discharge
slope) should be high enough that there is no change in the output as
read by a 3456A in 6 digit.
This is yet to be determined with my battery pack. I'm hopeful.
The point here is to have a short term stable noise free power source
going to the actual ref circuit. Longer term, you just need to keep the
changes small enough to minimize output changes.
Based on what I saw with both of mine (came without batteries), that ~
20v actually included about 3V AC, and the meter was calibrated without
the pack. (Take a reading of pre-supply output with the battery
disconnected with and without filter DC and then ACV on the 3456A)
I was curious about the waveform as well as the AC on the pre-supply
output, so I put the 'scope on the pre-supply output with battery
disconnected. A base of ~17V as expected with bumps to ~28V at 120 Hz.
The 'scope reported 22V RMS which is in line with the 3V AC you reported.
(The 'scope is a Tek TDS210 that I got new about 15 years ago. It just got
its first cal... in cal and returned as received.)
I suppose it's to the credit of the actual reference circuit that it does
so well with such a 'noisy' supply. If I were to want to run AC only with
no battery installed, I'd clip CR8 for sure to isolate the charging circuit.
Both of mine were last calibrated before their batteries were pulled
and were low when running off of the battery charger. I did not find
the problems with the zener, jfet and current limit resisters until I
built replacement packs and was trying to verify circuit functions.
The history of mine is uncertain. Certainly there is a 3 count difference
on the 3456A with 6 digit resolution between battery connected/disconnected
with it reading lower with the battery disconnected.
Since it doesn't seem to much mind 11V of 'ripple', I'm tempted to stick an
LM317L in there instead of the fussy zener/jfet/2n3904 circuit. I just
have to go work out how much current it's drawing so I don't overheat the
LM317L. I figure any slight drift from the LM317's voltage setting
resistors isn't going to matter at all. I might just do this to add to the
knowledge base here.
Orin.
On 6/17/13 6:50 PM, Orin Eman wrote:
Since it doesn't seem to much mind 11V of 'ripple', I'm tempted to stick an
LM317L in there instead of the fussy zener/jfet/2n3904 circuit. I just
have to go work out how much current it's drawing so I don't overheat the
LM317L. I figure any slight drift from the LM317's voltage setting
resistors isn't going to matter at all. I might just do this to add to the
knowledge base here.
Orin.
If the jfet is good (sounds like it is).
Check the resisters on the PS board, IIRC it is the 510ohm one that is
the current limiter for the charging circuit.
Replace the zener...pennies
Make sure the batteries are NiCd of about the right capacity (400 -
1000mah).
The charge circuit is dead simple and should float the pack at 40 - 60ma.
From you description it just sounds like the zener voltage is low, just
like both of mine.
On Tue, Jun 18, 2013 at 12:30 AM, Christopher Brown cbrown@woods.netwrote:
On 6/17/13 6:50 PM, Orin Eman wrote:
Since it doesn't seem to much mind 11V of 'ripple', I'm tempted to stick
an
LM317L in there instead of the fussy zener/jfet/2n3904 circuit. I just
have to go work out how much current it's drawing so I don't overheat the
LM317L. I figure any slight drift from the LM317's voltage setting
resistors isn't going to matter at all. I might just do this to add to
the
knowledge base here.
Orin.
If the jfet is good (sounds like it is).
Check the resisters on the PS board, IIRC it is the 510ohm one that is
the current limiter for the charging circuit.
Just did that - 530 ohms and the other 180 ohm series resistor was 183 ohms
so I think I'm OK there.
Replace the zener...pennies
Zeners are on order...
Make sure the batteries are NiCd of about the right capacity (400 -
1000mah).
The Batteries Plus NiCd cells are 750 mAH so they are right in there.
The charge circuit is dead simple and should float the pack at 40 - 60ma.
From you description it just sounds like the zener voltage is low, just
like both of mine.
Sure does. Seems OK at the moment with the two 1N4148s in series with the
'original' zener.
Orin.
The LM317L is self protected for dissipation. As both it and the 2N3904 are TO92 the temperature should be similar. The 317L has slightly lower thermial resistance so the chip should run cooler.
The unfiltered Nicad charging is deliberate, there is a school of thought that pulse charging improves life.
Robert G8RPI.
From: Orin Eman orin.eman@gmail.com
To: Discussion of precise voltage measurement volt-nuts@febo.com
Sent: Tuesday, 18 June 2013, 3:50
Subject: Re: [volt-nuts] Fluke 731B battery & charge circuit
On Mon, Jun 17, 2013 at 12:29 AM, Christopher Brown cbrown@woods.netwrote:
My take on things...
The highest float charging battery voltage should be ~ .8v lower than
the zener.
I agree with that. My battery seemed to float at 17.0V last night, but I
turned it all off before going to bed - I want to be sure it doesn't
overcharge before leaving it on all the time. After 19 hours off, the
battery has self-discharged down to 16.5V.
The "settled" voltage of the battery (where the pack will sit after
12hrs of no charge and where it will sit until the final discharge
slope) should be high enough that there is no change in the output as
read by a 3456A in 6 digit.
This is yet to be determined with my battery pack. I'm hopeful.
The point here is to have a short term stable noise free power source
going to the actual ref circuit. Longer term, you just need to keep the
changes small enough to minimize output changes.
Based on what I saw with both of mine (came without batteries), that ~
20v actually included about 3V AC, and the meter was calibrated without
the pack. (Take a reading of pre-supply output with the battery
disconnected with and without filter DC and then ACV on the 3456A)
I was curious about the waveform as well as the AC on the pre-supply
output, so I put the 'scope on the pre-supply output with battery
disconnected. A base of ~17V as expected with bumps to ~28V at 120 Hz.
The 'scope reported 22V RMS which is in line with the 3V AC you reported.
(The 'scope is a Tek TDS210 that I got new about 15 years ago. It just got
its first cal... in cal and returned as received.)
I suppose it's to the credit of the actual reference circuit that it does
so well with such a 'noisy' supply. If I were to want to run AC only with
no battery installed, I'd clip CR8 for sure to isolate the charging circuit.
Both of mine were last calibrated before their batteries were pulled
and were low when running off of the battery charger. I did not find
the problems with the zener, jfet and current limit resisters until I
built replacement packs and was trying to verify circuit functions.
The history of mine is uncertain. Certainly there is a 3 count difference
on the 3456A with 6 digit resolution between battery connected/disconnected
with it reading lower with the battery disconnected.
Since it doesn't seem to much mind 11V of 'ripple', I'm tempted to stick an
LM317L in there instead of the fussy zener/jfet/2n3904 circuit. I just
have to go work out how much current it's drawing so I don't overheat the
LM317L. I figure any slight drift from the LM317's voltage setting
resistors isn't going to matter at all. I might just do this to add to the
knowledge base here.
Orin.
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On Wed, Jun 19, 2013 at 6:26 AM, Robert Atkinson robert8rpi@yahoo.co.ukwrote:
The LM317L is self protected for dissipation. As both it and the 2N3904
are TO92 the temperature should be similar. The 317L has slightly lower
thermial resistance so the chip should run cooler.
Thanks for the note.
I'd considered that the LM317L and 2N3904 were in the same case, but
guessed that the 317L is going to have some overhead that would increase
it's dissipation. However, as you note, the 317L has self-protection and
that should make any extra dissipation harmless. After all, if the 317
shuts down, we are no worse off than before. I hadn't compared the thermal
resistance of the 2N3904 vs. the LM317L. I thought about it some more and
decided I'd just have to believe the datasheets!
The unfiltered Nicad charging is deliberate, there is a school of thought
that pulse charging improves life.
I'd noticed that when looking for battery specs so I wasn't particularly
surprised that the battery was charged with from an unfiltered source.
My concern was that unfiltered source powering the actual voltage reference
when the pre-regulator zener voltage is low... then I looked up the 731A.
The only filtering in the 731A is the battery and I had noted that at least
in the 731B, it resulted in 20mV of noise when the battery voltage was
greater than that of the pre-regulator.
I'm still temped to try a 317L, though I have replacement zeners and jfet
current regulator on order. As I said before, just to add to the knowledge
base. I'm sure it's going to be easier (and cheaper) to get an LM317 of
some type or another than the jfet regulator - certainly, I can go down to
the local electronics 'surplus' store (vetco.net) and get an NTE LM317L
equivalent any day of the week for less than $2*. NTE do not have an
equivalent for either E505 or J505.
Orin.
*as long as nothing else 'follows me home'.
On 06/17/2013 02:29 AM, Christopher Brown wrote:
<snip>That jfet and the shielded transformer are hard to get. Everything else
on the power supply board is nothing special. All of the specially
selected stuff is on the daughter board.
I needed the current diode to repair a 731B as well. Try EBay.com. Search for
"j505 current" and you will find them.
--
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