Maker Pro
Maker Pro

repairing a remote voltage monitor circuit.

A

Andy Baxter

Jan 1, 1970
0
I've been helping out a local environmental centre by trying to fix a
remote voltage monitor circuit they have with their small windmill /
battery bank. It runs off the 24v battery bank they have, and relays
readings of the current voltage and charge / discharge rate down to some
meters down in the valley where they live. (The windmill is on top of a
hill, with the battery and inverter in a shed next to it.)

The voltage is measured directly from the battery, and the current by
measuring the voltage drop across a short length of the cable that the
inverter and windmill charger connect to the batteries through.

The original circuit was designed and installed by someone else - he's
left an instruction manual with a complete wiring diagram, but no internal
circuit diagram for the electronics. I'm still trying to get in touch with
him to ask if he can help, but without luck so far. According to the
manual, the readings are relayed down to the meters in the valley using
standard current loop driver chips. The current loop runs down some eight
core signal cable that was installed for this purpose.

So far, what I've done is just rewire the whole thing following the
instructions in the manual, and replace all the fuses. (It had been partly
disconnected after someone else had a go at fixing it a few years ago.)
The story is that it originally stopped working after a lightning strike,
but there's no visible sign of this.

Now I've got to a point where everything obvious has been tested (except
the continuity of the signal cable down to the valley, which I'll do next
week) and it's still not working. The power light comes on, and the two
fault lights come on for 10 seconds (a self test mode?) then go off, but
the meters at the bottom of the hill are stuck against their stops (on the
'no current' side).

Does anyone have any general advice on how to approach something like
this? E.g. I'm not sure what's the best thing to do if the fault is in
the electronics - is it best just to replace all the chips and try to test
the other components in situ, or take the whole thing home with me and
build a test rig? Also, what kind of precautions should I be taking about
working with 24V DC? So far I've been wearing rubber gloves and making
sure everything is disconnected before I touch anything to do with the
battery bank. I have some photos of the setup if anyone wants me to post
them.

cheers, andy
 
J

Jasen Betts

Jan 1, 1970
0
The original circuit was designed and installed by someone else - he's
left an instruction manual with a complete wiring diagram, but no internal
circuit diagram for the electronics. I'm still trying to get in touch with
him to ask if he can help, but without luck so far. According to the
manual, the readings are relayed down to the meters in the valley using
standard current loop driver chips. The current loop runs down some eight
core signal cable that was installed for this purpose.

So far, what I've done is just rewire the whole thing following the
instructions in the manual, and replace all the fuses. (It had been partly
disconnected after someone else had a go at fixing it a few years ago.)
The story is that it originally stopped working after a lightning strike,
but there's no visible sign of this.
Does anyone have any general advice on how to approach something like
this? E.g. I'm not sure what's the best thing to do if the fault is in
the electronics - is it best just to replace all the chips and try to test
the other components in situ, or take the whole thing home with me and
build a test rig? Also, what kind of precautions should I be taking about
working with 24V DC? So far I've been wearing rubber gloves and making
sure everything is disconnected before I touch anything to do with the
battery bank. I have some photos of the setup if anyone wants me to post
them.

if there's only a few cheap chips in there replace them.


if it was a lightning strike the bits that would have got hit the hardest
are those nearest the cables, at both ends of the cable.
so check the circuitry behind the meters and that the
meters work...

damaged parts aren't always visibly damaged.

Bye.
Jasen
 
In sci.electronics.repair Andy Baxter said:
I've been helping out a local environmental centre by trying to fix a
remote voltage monitor circuit they have with their small windmill /
battery bank. [...] Does anyone have any general advice on how to
approach something like this?

Here's what I'd try. First, the meters can be loop powered (two wires)
or locally powered (four wires) - look at the meter connections. If
they're supposed to be locally powered, make sure that power supply is
working. It also wouldn't surprise me if this setup brings +24 V from
the top of the hill to power the meters separately from the signaling
loop; if the +24 V doesn't make it down there, the meters won't work.

If that checks out, then: Assuming it's 4 to 20 mA signaling, get a 24 V
power supply or 24 V worth of (small) batteries. Connect a 1 K resistor
and 5 K pot in series, and connect this across the 24 V supply. You can
now draw 4 to 24 mA (at 24.0 V nominal) from the supply. (If you don't
like going over 20 mA, use a 1.2 K resistor.) Go to where the meters are,
disconnect one of the meters from the signal wires that go up the hill,
and put the meter in series with the pot, resistor, and batteries. The
meter will have some internal resistance, so you won't be able to go to
the full 24 mA, but you should be able to vary the reading on the meter
by adjusting the pot. If you can't make the meter respond, then there
is a fault in the meter.

If you can make all of the meters move with your test rig connected
directly to the meters, try the same test, but from the top of the hill.
Disconnect the signal cable from the electronics up there and use your
battery-resistor-pot rig to send a varying current down the cable. If
the meters don't move, suspect the signal cable. If they do move, you
probably won't get exactly the same readings as you did before, due to
the added resistance of the signal cable, but you should get some kind of
varying reading.

If all of that checks out, you're left with the electronics at the top
of the hill. You might check the input wires from the battery bank and
the shunt to make sure the right signals are getting to the box. If
that's OK, then connect your 1 K resistor across the electronics box in
place of the wires to one of the meters, and measure the voltage across
it to see if the box is trying to drive the meter. If you can, put a
load on the batteries to see if the voltage across the 1 K resistor
changes. If you can't get a reasonable output from the electronics,
I would then vote for taking the electronics box home where you can test
it at your leisure.
Also, what kind of precautions should I be taking about working with
24V DC? So far I've been wearing rubber gloves and making sure
everything is disconnected before I touch anything to do with the
battery bank.

I wouldn't worry about getting shocked as much as I would about shorting
the output of the battery bank. It's possible to shock or burn yourself
on 24 V but you have to work at it a bit. It's pretty easy to melt a
wrench by dropping it across the terminals of a battery. If there are
no fuses in the system, now would be an excellent time to add some.

Matt Roberds
 
A

Andy Baxter

Jan 1, 1970
0
mroberds said:
In sci.electronics.repair Andy Baxter said:
I've been helping out a local environmental centre by trying to fix a
remote voltage monitor circuit they have with their small windmill /
battery bank. [...] Does anyone have any general advice on how to
approach something like this?

Here's what I'd try. First, the meters can be loop powered (two wires)
or locally powered (four wires) - look at the meter connections.

They are loop powered, according to the manual I have.
If that checks out, then: Assuming it's 4 to 20 mA signaling, get a 24 V
power supply or 24 V worth of (small) batteries. Connect a 1 K resistor
and 5 K pot in series, and connect this across the 24 V supply. You can
now draw 4 to 24 mA (at 24.0 V nominal) from the supply. (If you don't
like going over 20 mA, use a 1.2 K resistor.) Go to where the meters are,
disconnect one of the meters from the signal wires that go up the hill,
and put the meter in series with the pot, resistor, and batteries. The
meter will have some internal resistance, so you won't be able to go to
the full 24 mA, but you should be able to vary the reading on the meter
by adjusting the pot. If you can't make the meter respond, then there
is a fault in the meter.

Thanks, I'll try this.
If all of that checks out, you're left with the electronics at the top
of the hill. You might check the input wires from the battery bank and
the shunt to make sure the right signals are getting to the box.

I've tested all these cables for continuity, and they're OK.

Thanks for the detailed reply.

andy.
 
A

Andy Baxter

Jan 1, 1970
0
Jamie said:
put a wireless system in.

Why? It was working perfectly well before it broke, and the signal wire is
still in place. (If it's damaged we can use a different set of wires, or
install a new one which they have anyway). so as far as I can see there's
no need to rebuild the whole thing, which is adding extra complications
and costing them a lot in parts at the same time.
 
J

Jamie

Jan 1, 1970
0
Andy said:
I've been helping out a local environmental centre by trying to fix a
remote voltage monitor circuit they have with their small windmill /
battery bank. It runs off the 24v battery bank they have, and relays
readings of the current voltage and charge / discharge rate down to some
meters down in the valley where they live. (The windmill is on top of a
hill, with the battery and inverter in a shed next to it.)

The voltage is measured directly from the battery, and the current by
measuring the voltage drop across a short length of the cable that the
inverter and windmill charger connect to the batteries through.

The original circuit was designed and installed by someone else - he's
left an instruction manual with a complete wiring diagram, but no internal
circuit diagram for the electronics. I'm still trying to get in touch with
him to ask if he can help, but without luck so far. According to the
manual, the readings are relayed down to the meters in the valley using
standard current loop driver chips. The current loop runs down some eight
core signal cable that was installed for this purpose.

So far, what I've done is just rewire the whole thing following the
instructions in the manual, and replace all the fuses. (It had been partly
disconnected after someone else had a go at fixing it a few years ago.)
The story is that it originally stopped working after a lightning strike,
but there's no visible sign of this.

Now I've got to a point where everything obvious has been tested (except
the continuity of the signal cable down to the valley, which I'll do next
week) and it's still not working. The power light comes on, and the two
fault lights come on for 10 seconds (a self test mode?) then go off, but
the meters at the bottom of the hill are stuck against their stops (on the
'no current' side).

Does anyone have any general advice on how to approach something like
this? E.g. I'm not sure what's the best thing to do if the fault is in
the electronics - is it best just to replace all the chips and try to test
the other components in situ, or take the whole thing home with me and
build a test rig? Also, what kind of precautions should I be taking about
working with 24V DC? So far I've been wearing rubber gloves and making
sure everything is disconnected before I touch anything to do with the
battery bank. I have some photos of the setup if anyone wants me to post
them.

cheers, andy
put a wireless system in.
 
F

Franc Zabkar

Jan 1, 1970
0
In sci.electronics.repair Andy Baxter said:
I've been helping out a local environmental centre by trying to fix a
remote voltage monitor circuit they have with their small windmill /
battery bank. [...] Does anyone have any general advice on how to
approach something like this?

Here's what I'd try. First, the meters can be loop powered (two wires)
or locally powered (four wires) - look at the meter connections. If
they're supposed to be locally powered, make sure that power supply is
working. It also wouldn't surprise me if this setup brings +24 V from
the top of the hill to power the meters separately from the signaling
loop; if the +24 V doesn't make it down there, the meters won't work.

If that checks out, then: Assuming it's 4 to 20 mA signaling, get a 24 V
power supply or 24 V worth of (small) batteries. Connect a 1 K resistor
and 5 K pot in series, and connect this across the 24 V supply. You can
now draw 4 to 24 mA (at 24.0 V nominal) from the supply. (If you don't
like going over 20 mA, use a 1.2 K resistor.) Go to where the meters are,
disconnect one of the meters from the signal wires that go up the hill,
and put the meter in series with the pot, resistor, and batteries. The
meter will have some internal resistance, so you won't be able to go to
the full 24 mA, but you should be able to vary the reading on the meter
by adjusting the pot. If you can't make the meter respond, then there
is a fault in the meter.

If you can make all of the meters move with your test rig connected
directly to the meters, try the same test, but from the top of the hill.
Disconnect the signal cable from the electronics up there and use your
battery-resistor-pot rig to send a varying current down the cable.

I'd leave the rig at the bottom of the hill. At the top of the hill
I'd put a 100 ohm resistor between the two ends of the signal cable.
That would enable me to detect shorts in the cable, and to watch the
meter while I'm adjusting the loop current.

-- Franc Zabkar

Please remove one 'i' from my address when replying by email.
 
F

Franc Zabkar

Jan 1, 1970
0
I've been helping out a local environmental centre by trying to fix a
remote voltage monitor circuit they have with their small windmill /
battery bank. It runs off the 24v battery bank they have, and relays
readings of the current voltage and charge / discharge rate down to some
meters down in the valley where they live. (The windmill is on top of a
hill, with the battery and inverter in a shed next to it.)

It may help us visualise your setup if you described the various chips
at both ends, particularly the interface ICs. One current loop
implementation that gave problems for me involved a 7406 open
collector driver at one end and an optocoupler at the other.
The voltage is measured directly from the battery, and the current by
measuring the voltage drop across a short length of the cable that the
inverter and windmill charger connect to the batteries through.

The original circuit was designed and installed by someone else - he's
left an instruction manual with a complete wiring diagram, but no internal
circuit diagram for the electronics. I'm still trying to get in touch with
him to ask if he can help, but without luck so far. According to the
manual, the readings are relayed down to the meters in the valley using
standard current loop driver chips. The current loop runs down some eight
core signal cable that was installed for this purpose.

So far, what I've done is just rewire the whole thing following the
instructions in the manual, and replace all the fuses. (It had been partly
disconnected after someone else had a go at fixing it a few years ago.)
The story is that it originally stopped working after a lightning strike,
but there's no visible sign of this.

Now I've got to a point where everything obvious has been tested (except
the continuity of the signal cable down to the valley, which I'll do next
week) and it's still not working. The power light comes on, and the two
fault lights come on for 10 seconds (a self test mode?) then go off, but
the meters at the bottom of the hill are stuck against their stops (on the
'no current' side).

I'd be interested in knowing what circuitry is between the meter
movement and the interface. Does the current loop interface chip
merely translate 4mA and 20mA currents to 0 and 1 logic levels? Does
the meter operate in PWM mode, ie does it average the current pulses
to get an analogue reading? Does it possess any "smarts"?

What do you see on the meter if you drive it directly from the diode
range of a DMM? My DMMs produce 1mA on this range. You could use a
second series connected DMM to measure the current. Or you could
replace the meter with your DMM and measure the loop current directly.
Does anyone have any general advice on how to approach something like
this? E.g. I'm not sure what's the best thing to do if the fault is in
the electronics - is it best just to replace all the chips and try to test
the other components in situ, or take the whole thing home with me and
build a test rig? Also, what kind of precautions should I be taking about
working with 24V DC? So far I've been wearing rubber gloves and making
sure everything is disconnected before I touch anything to do with the
battery bank. I have some photos of the setup if anyone wants me to post
them.

cheers, andy

-- Franc Zabkar

Please remove one 'i' from my address when replying by email.
 
J

Jasen Betts

Jan 1, 1970
0
put a wireless system in.
Why? It was working perfectly well before it broke, and the signal wire is
still in place. (If it's damaged we can use a different set of wires, or
install a new one which they have anyway). so as far as I can see there's
no need to rebuild the whole thing, which is adding extra complications
and costing them a lot in parts at the same time.

Next time a lightning bolt strikes that hilltop,a wireless
system will survive a whole lot better than the wired system.
(especially if the aerial is mounted low) fibre-optic is another
option.

it's a hill top, they get struck quite regularly.

It all depends on how often you get electric storms in that area, and how
often you want to be repairing that device.

Bye.
Jasen
 
A

Andy Baxter

Jan 1, 1970
0
Franc Zabkar said:
It may help us visualise your setup if you described the various chips
at both ends, particularly the interface ICs. One current loop
implementation that gave problems for me involved a 7406 open
collector driver at one end and an optocoupler at the other.

There are some pictures at www.niftybits.ukfsn.org/electronics/

p1010087.jpg is a photo of the main circuit board from the top. The main
part of the circuit is two AD694 current loop drivers, each fed by a
IMA118 instrumentation amplifier - one for the voltage measurement and one
for the current shunt.

I think I can probably work it out - I just got spooked by the thought of
having to reconstruct the circuit diagram from the circuit itself, and
I've never done any diagnostic / repair work before, so I'm not sure
what's the best way to get started on something like this.

It looks like the bloke who built it should be sending me a circuit
diagram, so it shouldn't be as hard as I thought.
I'd be interested in knowing what circuitry is between the meter
movement and the interface. Does the current loop interface chip
merely translate 4mA and 20mA currents to 0 and 1 logic levels? Does
the meter operate in PWM mode, ie does it average the current pulses
to get an analogue reading? Does it possess any "smarts"?

It's just a standard 4-20 mA current loop. Not PWM, looking at the
datasheet for the driver. The meter circuitry just has a couple of
resistors and a diode on each meter. The meters are driven directly from
the current loop, not by any active circuitry.
What do you see on the meter if you drive it directly from the diode
range of a DMM? My DMMs produce 1mA on this range. You could use a
second series connected DMM to measure the current. Or you could
replace the meter with your DMM and measure the loop current directly.

Don't know, but I'll try some test like this next time I go up.

Thanks for your help - I'll post again to say how I get on with it.
 
J

Jamie

Jan 1, 1970
0
Andy said:
Franc Zabkar said:




There are some pictures at www.niftybits.ukfsn.org/electronics/
with the work they put into the manual, you would have thought they
could had made a real board or at least a hand sketched one.
oh well.
in any case, i would suggest looking at the op-amps and input devices.
and even the regulator.
 
W

w_tom

Jan 1, 1970
0
By looking at ICs only, I believe this is a voltage and a
current or a two channel voltage monitor. I suspect that two
4-20 cables carry analog data down the hill.

Two inputs would be the INA118 Op amps. Two 4-20 ma wires
would be driven by pin 11 of the AD694.

Step one: what does AD694 pin one connect to? If to pin 2
or 7, then 0 to 10 volts on pin 3 cause the 4-20 ma output.
IOW make changes to the circuit board voltage input (to inputs
of INA118) and observe corresponding voltage changes on pin 3
of AD694. Now you know the input is good (or bad).

Step two: see what comes out of AD694. Temporarily replace
the cable going down hill with a 100 ohm resistor in series
with a digital multimeter in 200 ma range. As those inputs to
INA118 change, then current measured by multimeter will change
from 4 to 20 ma.

Step three: Assuming the above tests have worked, then
perform the same step two test, this time by replacing the 100
ohm resistor (even sold in Radio Shack) with the cable going
downhill. IOW restore the connections except place the
multimeter in 200 ma mode in series with one wire of that
downhill cable. Again, changes to the input should measure
same 4-20 ma numbers on meter.

Step four: report back everything no matter whether problem
is solved or not. This is a two way street. When done, all
should have learned something.

Step five: warning - the most common way to cause meter
damage is to leave it in current mode when done. As a rule,
always switch meter back to voltage measurement after taking
current. Just a good habit to get into.
 
A

Andy Baxter

Jan 1, 1970
0
w_tom said:
By looking at ICs only, I believe this is a voltage and a
current or a two channel voltage monitor. I suspect that two
4-20 cables carry analog data down the hill.

Two inputs would be the INA118 Op amps. Two 4-20 ma wires
would be driven by pin 11 of the AD694.

Step one: what does AD694 pin one connect to? If to pin 2
or 7, then 0 to 10 volts on pin 3 cause the 4-20 ma output.
IOW make changes to the circuit board voltage input (to inputs
of INA118) and observe corresponding voltage changes on pin 3
of AD694. Now you know the input is good (or bad).

Step two: see what comes out of AD694. Temporarily replace
the cable going down hill with a 100 ohm resistor in series
with a digital multimeter in 200 ma range. As those inputs to
INA118 change, then current measured by multimeter will change
from 4 to 20 ma.

Step three: Assuming the above tests have worked, then
perform the same step two test, this time by replacing the 100
ohm resistor (even sold in Radio Shack) with the cable going
downhill. IOW restore the connections except place the
multimeter in 200 ma mode in series with one wire of that
downhill cable. Again, changes to the input should measure
same 4-20 ma numbers on meter.

Step four: report back everything no matter whether problem
is solved or not. This is a two way street. When done, all
should have learned something.

Step five: warning - the most common way to cause meter
damage is to leave it in current mode when done. As a rule,
always switch meter back to voltage measurement after taking
current. Just a good habit to get into.

Thanks - I'll let you know how I get on.
 
F

Franc Zabkar

Jan 1, 1970
0
Franc Zabkar said:


There are some pictures at www.niftybits.ukfsn.org/electronics/

p1010087.jpg is a photo of the main circuit board from the top. The main
part of the circuit is two AD694 current loop drivers, ...

I found this datasheet:
http://www.analog.com/UploadedFiles/Data_Sheets/390142776AD694_b.pdf
... each fed by a
IMA118 instrumentation amplifier - one for the voltage measurement and one
for the current shunt.

I think I can probably work it out - I just got spooked by the thought of
having to reconstruct the circuit diagram from the circuit itself, and
I've never done any diagnostic / repair work before, so I'm not sure
what's the best way to get started on something like this.

It looks like the bloke who built it should be sending me a circuit
diagram, so it shouldn't be as hard as I thought.

It looks simple enough to me now. Until I saw the photo I was thinking
that the flashing LEDs may have been driven by a microcontroller as
part of a POST. The 4mA-20mA current loop threw me as well, until I
saw that the 4mA offset could be switched off at pin 9 of the AD694. A
0-20mA current loop makes more sense than 4-20mA when driving an
analogue meter.

I suspect the LEDS may be connected to the ALARM pins (10) of the
AD694s. The alarm goes low when the output voltage (11) has run out of
headroom, ie when Vout = Vsupply - 2V. This could occur if the IC is
driving a loop with an excessively high resistance. I'm guessing that
the 555 timer may be wired as a one-shot, and that it applies a
momentary test voltage to each of the AD564 inputs via the relay. Or
maybe it just momentarily disconnects the current loops. This could
explain the flash of LEDs at power-on.

It seems to me that the easiest test would be to substitute your DMM
(in ammeter mode) for the load at various points in the circuit,
firstly at the output of the PCB, then at the end of the cable.

Just one other thought. Does the meter display 0-30V, say, or does it
use an expanded scale, say 20V-30V? It seems to me that anything below
about 20V (for a 24V battery) would be uninteresting. I reckon it
would be much better to see the detail at the top end of the voltage
range.

-- Franc Zabkar

Please remove one 'i' from my address when replying by email.
 
A

Andy Baxter

Jan 1, 1970
0
Franc Zabkar said:
It looks simple enough to me now. Until I saw the photo I was thinking
that the flashing LEDs may have been driven by a microcontroller as
part of a POST. The 4mA-20mA current loop threw me as well, until I
saw that the 4mA offset could be switched off at pin 9 of the AD694. A
0-20mA current loop makes more sense than 4-20mA when driving an
analogue meter.

From the manual, it's a 4-20 mA loop, but I don't know how this works at
the meter end.
I suspect the LEDS may be connected to the ALARM pins (10) of the
AD694s. The alarm goes low when the output voltage (11) has run out of
headroom, ie when Vout = Vsupply - 2V. This could occur if the IC is
driving a loop with an excessively high resistance. I'm guessing that
the 555 timer may be wired as a one-shot, and that it applies a
momentary test voltage to each of the AD564 inputs via the relay. Or
maybe it just momentarily disconnects the current loops. This could
explain the flash of LEDs at power-on.

That makes sense. There's also another timing element though - the system
was designed to operate a solenoid to turn the inverter on and off
remotely. This worked by breaking the circuit of the voltage measuring
current loop, and triggering a deliberate fault condition. I think he did
it that way to save on wires. That has some kind of timing associated
with it, according to the manual, so the 555 might be for that.
Just one other thought. Does the meter display 0-30V, say, or does it
use an expanded scale, say 20V-30V? It seems to me that anything below
about 20V (for a 24V battery) would be uninteresting. I reckon it
would be much better to see the detail at the top end of the voltage
range.

It's using an expanded scale - something like 20-30V but I'm not sure.
 
Top