Need Op Amp for design

I have a little project am trying to design, and I am running into a little difficulty in the concept stage.

I have a device that needs to consume essentially zero power when shut off.  A few nanoamps is fine.  The device uses a 6S LiPo battery that must be monitored during operation to make sure the battery is not discharged beyond critical limits.  To this end, the battery is connected to a 6 input digital battery monitor which constantly monitors the health of each cell.  During operation, the battery monitor consumes an insignificant amount of power, but it is in the milliamp range, making it too large to be able to remain connected when the device is stored for moderately extended period of time.

I solved the problem by using 7 reed relays between the battery and the monitor, energizing the relays when the main power is switched on.  This works perfectly, but there is an issue.  Reed relays are comparatively large, and the resulting design cannot be shrunk down to much less than 3" x 3" after adding the additional components.

What I am considering is replacing the reed relays with unity gain operational amplifiers.  A pair of TL074CDR op amps is much smaller than the red relays, and mechanically I should be able to reduce the board to a much smaller footprint.  The problem is when the mains power is shut off, the op amps will still be attached to ground, while their inputs will still be attached to various voltages up to 25V with no Vcc.  I am thinking this may destroy the JFET inputs of the devices.  Looking at the logical design, I'm not sure, however.  Both input sources are attached to the collector of a PNP transistor, which most certainly could carry current to its base, but the base is attached to the collector of an NPN transistor which should exhibit a very high impedance when the power is off.  The base is also attached to the bases of a pair of PNP transistors, but those junctions should also offer a very high impedance to ground.  The drains of both JFETs are attached to the bases of NPN transistors which offer a fairly low impedance to ground.  Given that, I am thinking the JFETs would be destroyed by an input voltage greater than 15V when the power is shut off.

Am I wrong?  If not, is there a different choice of high input impedance op amp that can withstand up to 25V when power is shut off?

"Leslie Rhorer"  wrote in message 
news:436c895c-a0bc-4cac-aaca-44acc4662e36@googlegroups.com...

> I have a little project am trying to design, and I am running into a 
> little difficulty in the concept stage.

> I have a device that needs to consume essentially zero power when shut 
> off.  A few nanoamps is fine. The device uses a 6S LiPo battery that must 
> be monitored during operation to make sure the battery is not discharged 
> beyond critical limits.  To this end, the battery is connected to a 6 
> input digital battery monitor which constantly monitors the health of each 
> cell.  During operation, the battery monitor consumes an insignificant 
> amount of power, but it is in the milliamp range, making it too large to 
> be able to remain connected when the device is stored for moderately 
> extended period of time.

> I solved the problem by using 7 reed relays between the battery and the 
> monitor, energizing the relays when the main power is switched on.  This 
> works perfectly, but there is an issue.  Reed relays are comparatively 
> large, and the resulting design cannot be shrunk down to much less than 3" 
> x 3" after adding the additional components.

> What I am considering is replacing the reed relays with unity gain 
> operational amplifiers.  A pair of TL074CDR op amps is much smaller than 
> the red relays, and mechanically I should be able to reduce the board to a 
> much smaller footprint.  The problem is when the mains power is shut off, 
> the op amps will still be attached to ground, while their inputs will 
> still be attached to various voltages up to 25V with no Vcc.  I am 
> thinking this may destroy the JFET inputs of the devices.  Looking at the 
> logical design, I'm not sure, however.  Both input sources are attached to 
> the collector of a PNP transistor, which most certainly could carry 
> current to its base, but the base is attached to the collector of an NPN 
> transistor which should exhibit a very high impedance when the power is 
> off.  The base is also attached to the bases of a pair of PNP transistors, 
> but those junctions should also offer a very high impedance to ground. 
> The drains of both JFETs are attached to the bases of NPN transistors 
> which offer a fairly low impedance to ground.  Given that, I am thinking 
> the JFETs would be destroyed by an input voltage greater than 15V when the 
> power is shut off.

> Am I wrong?  If not, is there a different choice of high input impedance 
> op amp that can withstand up to 25V when power is shut off?

You might want to look at my recent posts on various BMS designs. The lowest 
power version would use a DG408 analog MUX that is powered from the full 
pack voltage (up to 40V) and draws only about 10 uA standby current. it is 
best to measure each cell using a difference amplifier, but you might be 
able to use a voltage divider to read the taps on the pack with some loss of 
precision. I will use a PIC to select and read the cell voltages and perform 
other functions. I plan to power it from the bottom cell. A PIC16LF1825 
draws only 30 nA to 2 uA in sleep. It can wake itself up for a few mSec to 
take readings and then sleep for a few minutes.

Another design I may pursue uses TLP175 opto-MOS devices, which could 
replace your reed relays, and only need about 1 mA to turn on. My 
preliminary design:
http://enginuitysystems.com/pix/electronics/BMS_TLP222.png

You might also look through an active thread on BMS design:
http://www.diyelectriccar.com/forums/showthread.php?p=926570&posted=1#post926570

Good luck!

Paul 

Thanks.  Your designs really don't match my design needs.  First of all, this needs to be a very inexpensive and simple device.  It is a flashlight.  Yours seem much more suited to monitoring large scale devices like motor vehicles.  Furthermore, I am really not concerned about the operational current, and I am concerned about the measurement period.  Finally, the measurement device itself is already in place.  I just need to shut down completely when the flashlight is sitting on the shelf.

The question here is, "What device can I use to connect the battery to the meter that won't be destroyed by shutting off the power?

Worst case, I could go ahead and use a single reed relay attached to the ground of all the op-amps, which would cause them to float and prevent destruction by the input voltages, but once again that will take up some space.

On Sunday, 19 November 2017 10:43:01 UTC, Leslie Rhorer  wrote:

> Thanks.  Your designs really don't match my design needs.  First of all, this needs to be a very inexpensive and simple device.  It is a flashlight.  Yours seem much more suited to monitoring large scale devices like motor vehicles.  Furthermore, I am really not concerned about the operational current, and I am concerned about the measurement period.  Finally, the measurement device itself is already in place.  I just need to shut down completely when the flashlight is sitting on the shelf.
> 
> The question here is, "What device can I use to connect the battery to the meter that won't be destroyed by shutting off the power?
> 
> Worst case, I could go ahead and use a single reed relay attached to the ground of all the op-amps, which would cause them to float and prevent destruction by the input voltages, but once again that will take up some space.

The usual solution is a diode or 2 from inputs to power rail. With a suitable opamp/comparator that hopefully can eat...
LMC6061 20uA
LMC6442 2uA


NT

On Sun, 19 Nov 2017 01:29:14 -0800 (PST), Leslie Rhorer
<rhorerles@gmail.com> wrote:

>I have a little project am trying to design, and I am running into a little difficulty in the concept stage.
>
>I have a device that needs to consume essentially zero power when shut off.  A few nanoamps is fine.  The device uses a 6S LiPo battery that must be monitored during operation to make sure the battery is not discharged beyond critical limits.  To this end, the battery is connected to a 6 input digital battery monitor which constantly monitors the health of each cell.  During operation, the battery monitor consumes an insignificant amount of power, but it is in the milliamp range, making it too large to be able to remain connected when the device is stored for moderately extended period of time.
>
>I solved the problem by using 7 reed relays between the battery and the monitor, energizing the relays when the main power is switched on.  This works perfectly, but there is an issue.  Reed relays are comparatively large, and the resulting design cannot be shrunk down to much less than 3" x 3" after adding the additional components.
>
>What I am considering is replacing the reed relays with unity gain operational amplifiers.  A pair of TL074CDR op amps is much smaller than the red relays, and mechanically I should be able to reduce the board to a much smaller footprint.  The problem is when the mains power is shut off, the op amps will still be attached to ground, while their inputs will still be attached to various voltages up to 25V with no Vcc.  I am thinking this may destroy the JFET inputs of the devices.  Looking at the logical design, I'm not sure, however.  Both input sources are attached to the collector of a PNP transistor, which most certainly could carry current to its base, but the base is attached to the collector of an NPN transistor which should exhibit a very high impedance when the power is off.  The base is also attached to the bases of a pair of PNP transistors, but those junctions should also offer a very high impedance to ground.  The drains of both JFETs are attached to the bases of NPN
>transistors which offer a fairly low impedance to ground.  Given that, I am thinking the JFETs would be destroyed by an input voltage greater than 15V when the power is shut off.
>
>Am I wrong?  If not, is there a different choice of high input impedance op amp that can withstand up to 25V when power is shut off?

The ancient LM324 has PNP input transistors, and the safe input
voltages can go to +32 irrespective of V+.

The working input common-mode voltage does not go to the V+ rail.

We like the Fujitsu FTR-B3GA relays, tiny DPDT things, cheap in
quantity.

Do you really need nA standby currents? Self-discharge of the
batteries is probably lots of microamps.

Reed relays are big, expensive, and unreliable.




-- 

John Larkin         Highland Technology, Inc

lunatic fringe electronics 

On Sun, 19 Nov 2017 10:00:13 -0800, John Larkin
<jjlarkin@highlandtechnology.com> wrote:

[snip]
>
>The ancient LM324 has PNP input transistors, and the safe input
>voltages can go to +32 irrespective of V+.

+32V Relative to V-

>
>The working input common-mode voltage does not go to the V+ rail.
>
>We like the Fujitsu FTR-B3GA relays, tiny DPDT things, cheap in
>quantity.
>
>Do you really need nA standby currents? Self-discharge of the
>batteries is probably lots of microamps.
>
>Reed relays are big, expensive, and unreliable.
		
                                        ...Jim Thompson
-- 
| James E.Thompson                                 |    mens     |
| Analog Innovations                               |     et      |
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    |
| STV, Queen Creek, AZ 85142    Skype: skypeanalog |             |
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  |
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     It's what you learn, after you know it all, that counts.

"Leslie Rhorer"  wrote in message 
news:ec862905-d0a9-43c5-abe3-4eed9574f841@googlegroups.com...

> Thanks.  Your designs really don't match my design needs.  First of all, 
> this needs to be a very inexpensive and simple device.  It is a 
> flashlight.  Yours seem much more suited to monitoring large scale devices 
> like motor vehicles.  Furthermore, I am really not concerned about the 
> operational current, and I am concerned about the measurement period. 
> Finally, the measurement device itself is already in place.  I just need 
> to shut down completely when the flashlight is sitting on the shelf.

. The question here is, "What device can I use to connect the battery to the 
meter that won't be destroyed by shutting off the power?

> Worst case, I could go ahead and use a single reed relay attached to the 
> ground of all the op-amps, which would cause them to float and prevent 
> destruction by the input voltages, but once again that will take up some 
> space.

There are analog multiplexers that shut down the inputs under power-off 
conditions, but they are expensive;
http://www.analog.com/en/products/switches-multiplexers/fault-protected-switches-multiplexers.html
http://www.analog.com/media/en/technical-documentation/data-sheets/ADG5208F_5209F.pdf

There may be others, and the DG408 may also act the same way.

The opto-MOS SSRs (TLP175) are only about 60 cents in 100 piece quantity and 
are in a small 4x5mm SMT package. You could also use discrete MOSFETs 
instead of relays (SSR, reed, or other).

It would help if you shared a schematic and other details of your present 
design.

Paul 

This will not be in any significant quantity.  Probably 20 at most.

A diode to the power rail won't work.  That will cause the Op Amp to power up and send signal to the battery monitor, powering it up.  The battery monitor is powered from its input leads.

The 324 can survive an input 32V (actually, around 25V in this case) higher than its V+?  I thought not.  Looking at the design, and the specs, it sure enough looks like it can.  That may be my solution.

A schematic?  Really?  OK.  I don't see how to attach a file to the thread, but I will put one up on my web site.

http://fletchergeek.com/images/Battery%20Monitor.png

Yes, I think the battery leakage is on the order of a microamp or so.  That's why I specified a few nanoamps as the off current.  Let's see, a typical worst case shelf discharge for a LiPo battery is about 5% per month.  Given this is a 20AH battery, that amounts to over a milliamp.  Good heavens!  So you are correct, I don't really need much better than a few microamps.  The battery monitor pulls several milliamps, though, so a switch is definitely needed.

The FTR-B3GA is not all that cheap, nor all that small.  They run about $3.75 each, and I would need 4 per light.  Compare that to a quad LM324 for under $.10, and I only need 2 per light.  The FTR-B3GA is 10.6 mm x 7.2 mm, for a total of .305 square cm.  That's not too bad, but the LM324 is only 8.65 mm x 6.5 mm for a total of .112 square cm.  It's true the relay solution offers virtually zero off-current, but the tiny leakage into the op amp inputs is not significant, as one can see above.  I think the op-amps win hands down in this case.

I looked at the TLP175, and electrically it is a great solution.  Size and cost put it behind the LM324, however.  I would need at least 6 (preferably 7) per unit, which brings the cost and space way above the LM324.  If they make a quad package of the device, even at $.78 per chip at the 10 quantity volume, it would be attractive.  I am not contemplating buying hundreds of these at this time.

On Monday, 20 November 2017 07:50:48 UTC, Leslie Rhorer  wrote:
> This will not be in any significant quantity.  Probably 20 at most.
> 
> A diode to the power rail won't work.  That will cause the Op Amp to power up and send signal to the battery monitor, powering it up.  The battery monitor is powered from its input leads.
> 
> The 324 can survive an input 32V (actually, around 25V in this case) higher than its V+?  I thought not.  Looking at the design, and the specs, it sure enough looks like it can.  That may be my solution.
> 
> A schematic?  Really?  OK.  I don't see how to attach a file to the thread, but I will put one up on my web site.
> 
> http://fletchergeek.com/images/Battery%20Monitor.png
> 
> Yes, I think the battery leakage is on the order of a microamp or so.  That's why I specified a few nanoamps as the off current.  Let's see, a typical worst case shelf discharge for a LiPo battery is about 5% per month.  Given this is a 20AH battery, that amounts to over a milliamp.  Good heavens!  So you are correct, I don't really need much better than a few microamps.  The battery monitor pulls several milliamps, though, so a switch is definitely needed.
> 
> The FTR-B3GA is not all that cheap, nor all that small.  They run about $3.75 each, and I would need 4 per light.  Compare that to a quad LM324 for under $.10, and I only need 2 per light.  The FTR-B3GA is 10.6 mm x 7.2 mm, for a total of .305 square cm.  That's not too bad, but the LM324 is only 8.65 mm x 6.5 mm for a total of .112 square cm.  It's true the relay solution offers virtually zero off-current, but the tiny leakage into the op amp inputs is not significant, as one can see above.  I think the op-amps win hands down in this case.

You're not giving us the relevant info here. What signal a powered opamp sends depends entirely on your circuit. Why your schematic shows power switched to the opamps is another mystery. I don't see how can we help if you don't explain why you've made the slightly puzzling choices you have.


NT