Baffling simple circuit

B

Bob Engelhardt

Guest
https://i.imgur.com/OWbZIrH.jpg

The circuit monitors a battery & when its voltage is above an adjustable
threshold, it closes a relay. When the voltage drops below the
threshold, it turns off and latches off (the feedback diode).

What's baffling is that it works for a second or 2 and turns off.
Particularly baffling is when I measure the voltage at the arrow, it
stays on. The voltmeter is a Fluke 8000A with 10M input impedance.
When I turn the Fluke _power_ off, the circuit turns off!?

The supply voltages are +- 12v, the threshold is 1.5v, and the voltage
at the arrow is 1.8v. The op amp "on" voltage is 10.8. All resistors
are 10k order of magnitude.

All help will be keenly appreciated,
Bob
 
On 11/20/2017 9:47 PM, Bob Engelhardt wrote:
https://i.imgur.com/OWbZIrH.jpg

The circuit monitors a battery & when its voltage is above an adjustable
threshold, it closes a relay.  When the voltage drops below the
threshold, it turns off and latches off (the feedback diode).

What's baffling is that it works for a second or 2 and turns off.
Particularly baffling is when I measure the voltage at the arrow, it
stays on.  The voltmeter is a Fluke 8000A with 10M input impedance. When
I turn the Fluke _power_ off, the circuit turns off!?

The supply voltages are +- 12v, the threshold is 1.5v, and the voltage
at the arrow is 1.8v.  The op amp "on" voltage is 10.8.  All resistors
are 10k order of magnitude.

All help will be keenly appreciated,
Bob
Click to expand...

Is the ground, measured at each ground symbol, really 0V?
It "smells" like something is floating.

Ed
 
On 2017-11-21, Bob Engelhardt <BobEngelhardt@comcast.net> wrote:
https://i.imgur.com/OWbZIrH.jpg

The circuit monitors a battery & when its voltage is above an adjustable
threshold, it closes a relay.
Click to expand...

It looks to me look like that diode in the positive feedback path stops
it from monitoring the battery until the relay turns on.

--
This email has not been checked by half-arsed antivirus software
 
On Monday, November 20, 2017 at 9:48:12 PM UTC-5, Bob Engelhardt wrote:
https://i.imgur.com/OWbZIrH.jpg

The circuit monitors a battery & when its voltage is above an adjustable
threshold, it closes a relay. When the voltage drops below the
threshold, it turns off and latches off (the feedback diode).

What's baffling is that it works for a second or 2 and turns off.
Particularly baffling is when I measure the voltage at the arrow, it
stays on. The voltmeter is a Fluke 8000A with 10M input impedance.
When I turn the Fluke _power_ off, the circuit turns off!?

The supply voltages are +- 12v, the threshold is 1.5v, and the voltage
at the arrow is 1.8v. The op amp "on" voltage is 10.8. All resistors
are 10k order of magnitude.

All help will be keenly appreciated,
Bob
Click to expand...

Sorry, (I'm confused) the circuit works with the DMM monitor in place?

If not a ground issue (Re Ed.) then maybe it's oscillating?? Try some
capacitance??

George H.
 
On 11/21/2017 1:37 AM, ehsjr wrote:

Is the ground, measured at each ground symbol, really 0V?
It "smells" like something is floating.

Ed
Click to expand...

Thanks - that's just the kind of tip that I was looking for.
 
On 11/21/2017 3:32 AM, Jasen Betts wrote:
It looks to me look like that diode in the positive feedback path stops
it from monitoring the battery until the relay turns on.
Click to expand...

I should have said: the relay powers a timer and does not affect the
monitor circuit.

Also, the diode is there to pull down the input when the output goes
negative due to falling battery voltage, and keep the op amp from going
high when the battery recovers. That was my intent, anyhow.

Bob
 
On 11/21/2017 9:37 AM, George Herold wrote:

Sorry, (I'm confused) the circuit works with the DMM monitor in place?

If not a ground issue (Re Ed.) then maybe it's oscillating?? Try some
capacitance??
Click to expand...

Yes, you should be confused: connecting the DMM "causes" the circuit to
work.

Right after I check the grounds I'm going to get out the scope and look
at things.
 
In article <ov286q026fn@news6.newsguy.com>, BobEngelhardt@comcast.net
says...
On 11/21/2017 1:37 AM, ehsjr wrote:

Is the ground, measured at each ground symbol, really 0V?
It "smells" like something is floating.

Ed

Thanks - that's just the kind of tip that I was looking for.
Click to expand...

I am guesing you may have created a RF detector at the input using that
diode..

also take note that you did say before you are using a +/- 12 supply
or close to that? That would mean the output is going to be -10/+10 or
there abouts. Looking at that diode in the loop back makes me think the
output when -10 is going to bleed back to the input divider and thus
current will be present and since small diodes like the generic 914 etc
love to act as a detector, you could have noise etc..

Try placing a cap around the diode.
Jamie
 
On 11/21/2017 5:10 PM, Bob Engelhardt wrote:
On 11/21/2017 9:37 AM, George Herold wrote:

Sorry, (I'm confused) the circuit works with the DMM monitor in place?

If not a ground issue (Re Ed.) then maybe it's oscillating?? Try some
capacitance??


Yes, you should be confused: connecting the DMM "causes" the circuit to
work.

Right after I check the grounds I'm going to get out the scope and look
at things.
Click to expand...

OK ... I didn't do full disclosure - there was circuitry that I didn't
show 'cause I thought that it was irrelevant but wasn't. Not the 1st
time - I should know by now. The whole circuit:
https://i.imgur.com/9bshfZN.jpg
The part on the left is a constant current load.

There was oscillation & ringing on the MOSFET gate. 130kHz base + 10x
that ringing. -2v to +5v.

I grabbed a TLAR* capacitor and put it on the gate, to ground.
Stabilized it perfectly & the whole thing works. A 10v 6.8uF tantalum
electro.

I don't know anything about capacitor choice (this is S.E.Basics) and
I've used electros in power supplies to smooth full wave rectified. But
I think that I've read that they shouldn't be used on AC (+&-
excursions) - they are polarized.

What would be a proper cap to use here?

Thanks,
Bob



* - TLAR - That Looks About Right
 
In article <ov54e808gm@news6.newsguy.com>, BobEngelhardt@comcast.net
says...
On 11/21/2017 5:10 PM, Bob Engelhardt wrote:
On 11/21/2017 9:37 AM, George Herold wrote:

Sorry, (I'm confused) the circuit works with the DMM monitor in place?

If not a ground issue (Re Ed.) then maybe it's oscillating?? Try some
capacitance??


Yes, you should be confused: connecting the DMM "causes" the circuit to
work.

Right after I check the grounds I'm going to get out the scope and look
at things.



OK ... I didn't do full disclosure - there was circuitry that I didn't
show 'cause I thought that it was irrelevant but wasn't. Not the 1st
time - I should know by now. The whole circuit:
https://i.imgur.com/9bshfZN.jpg
The part on the left is a constant current load.

There was oscillation & ringing on the MOSFET gate. 130kHz base + 10x
that ringing. -2v to +5v.

I grabbed a TLAR* capacitor and put it on the gate, to ground.
Stabilized it perfectly & the whole thing works. A 10v 6.8uF tantalum
electro.

I don't know anything about capacitor choice (this is S.E.Basics) and
I've used electros in power supplies to smooth full wave rectified. But
I think that I've read that they shouldn't be used on AC (+&-
excursions) - they are polarized.

What would be a proper cap to use here?

Thanks,
Bob



* - TLAR - That Looks About Right
Click to expand...

i woupd put a .1uf at least around the diode.
 
On Wednesday, November 22, 2017 at 7:18:06 PM UTC-5, Bob Engelhardt wrote:
On 11/21/2017 5:10 PM, Bob Engelhardt wrote:
On 11/21/2017 9:37 AM, George Herold wrote:

Sorry, (I'm confused) the circuit works with the DMM monitor in place?

If not a ground issue (Re Ed.) then maybe it's oscillating?? Try some
capacitance??


Yes, you should be confused: connecting the DMM "causes" the circuit to
work.

Right after I check the grounds I'm going to get out the scope and look
at things.



OK ... I didn't do full disclosure - there was circuitry that I didn't
show 'cause I thought that it was irrelevant but wasn't. Not the 1st
time - I should know by now. The whole circuit:
https://i.imgur.com/9bshfZN.jpg
The part on the left is a constant current load.
Click to expand...

Right (That circuit's an old friend of mine),
put ~10 k ohm in the feedback path from the top of
the sense resistor to the inverting input. Then roll
off the opamp gain with an integrating cap ~100pF is a
good start/gyess. Cap from output to inverting input.

George H.
There was oscillation & ringing on the MOSFET gate. 130kHz base + 10x
that ringing. -2v to +5v.

I grabbed a TLAR* capacitor and put it on the gate, to ground.
Stabilized it perfectly & the whole thing works. A 10v 6.8uF tantalum
electro.

I don't know anything about capacitor choice (this is S.E.Basics) and
I've used electros in power supplies to smooth full wave rectified. But
I think that I've read that they shouldn't be used on AC (+&-
excursions) - they are polarized.

What would be a proper cap to use here?

Thanks,
Bob



* - TLAR - That Looks About Right
Click to expand...
 
On 11/22/2017 7:58 PM, George Herold wrote:
Right (That circuit's an old friend of mine),
put ~10 k ohm in the feedback path from the top of
the sense resistor to the inverting input. Then roll
off the opamp gain with an integrating cap ~100pF is a
good start/gyess. Cap from output to inverting input.

George H.
Click to expand...

Thank you! It's the perfect reply: it's the answer to the question that
I should have asked.
 
On 11/22/2017 11:03 PM, Bob Engelhardt wrote:
On 11/22/2017 7:58 PM, George Herold wrote:

Right (That circuit's an old friend of mine),
put ~10 k ohm in the feedback path from the top of
the sense resistor to the inverting input. Then roll
off the opamp gain with an integrating cap ~100pF is a
good start/gyess. Cap from output to inverting input.

George H.

Thank you! It's the perfect reply: it's the answer to the question that
I should have asked.
Click to expand...

Thanks again - it's done and working perfectly.

For my edification, if you don't mind: what do the resistor and cap do?

Bob
 
Bob Engelhardt wrote on 11/23/2017 10:42 AM:
On 11/22/2017 11:03 PM, Bob Engelhardt wrote:
On 11/22/2017 7:58 PM, George Herold wrote:

Right (That circuit's an old friend of mine),
put ~10 k ohm in the feedback path from the top of
the sense resistor to the inverting input. Then roll
off the opamp gain with an integrating cap ~100pF is a
good start/gyess. Cap from output to inverting input.

George H.

Thank you! It's the perfect reply: it's the answer to the question that
I should have asked.

Thanks again - it's done and working perfectly.

For my edification, if you don't mind: what do the resistor and cap do?
Click to expand...

The gain of the circuit is set by the ratio of the feedback impedance to the
impedance to ground. The resistor is added to reduce the gain across the
board to something reasonable rather than the huge full gain of the opamp
and the capacitor reduces it further for higher frequencies since it only
needs to respond to lower frequencies.

--

Rick C

Viewed the eclipse at Wintercrest Farms,
on the centerline of totality since 1998
 
On 11/25/2017 9:13 AM, rickman wrote:

The gain of the circuit is set by the ratio of the feedback impedance to
the impedance to ground. The resistor is added to reduce the gain
across the board to something reasonable rather than the huge full gain
of the opamp and the capacitor reduces it further for higher frequencies
since it only needs to respond to lower frequencies.
Click to expand...

Thanks
 
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