# Baffling simple circuit

Started by November 20, 2017
```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.&Acirc;&nbsp; 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.&Acirc;&nbsp; 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.&Acirc;&nbsp; The op amp "on" voltage is 10.8.&Acirc;&nbsp; All resistors
> are 10k order of magnitude.
>
> All help will be keenly appreciated,
> Bob
>

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.

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

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

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.
>

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??
>

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.

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.
>
>

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

i woupd put a .1uf at least around the diode.

```