On Tuesday, November 21, 2017 at 2:35:27 PM UTC-5, Winfield Hill wrote:> John Larkin wrote... > > > > The ancient LM324 has PNP input transistors, and the > > safe input voltages can go to +32 irrespective of V+. > > I also thought of PNP-input single-supply op-amps as > a good solution, but the O.P. seems not to notice. > > > -- > Thanks, > - WinWeren't those the original phase inversion cheapos? Meaning you could end up with anything powering them up with a live input.
Need Op Amp for design
Started by ●November 19, 2017
Reply by ●November 24, 20172017-11-24
Reply by ●November 24, 20172017-11-24
On Sunday, November 19, 2017 at 4:29:18 AM UTC-5, Leslie Rhorer 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?You should buy a battery management IC, some of them handle lots of cells, and this is especially true of Li batteries. There are physics based considerations that dwarf the triviality of the electronics interconnection topology.
Reply by ●November 24, 20172017-11-24
On Tuesday, November 21, 2017 at 1:35:27 PM UTC-6, Winfield Hill wrote:> John Larkin wrote... > > > > The ancient LM324 has PNP input transistors, and the > > safe input voltages can go to +32 irrespective of V+. > > I also thought of PNP-input single-supply op-amps as > a good solution, but the O.P. seems not to notice.
Reply by ●November 24, 20172017-11-24
On Fri, 24 Nov 2017 12:38:20 -0800 (PST), bloggs.fredbloggs.fred@gmail.com wrote:>On Tuesday, November 21, 2017 at 2:35:27 PM UTC-5, Winfield Hill wrote: >> John Larkin wrote... >> > >> > The ancient LM324 has PNP input transistors, and the >> > safe input voltages can go to +32 irrespective of V+. >> >> I also thought of PNP-input single-supply op-amps as >> a good solution, but the O.P. seems not to notice. >> >> >> -- >> Thanks, >> - Win > >Weren't those the original phase inversion cheapos? Meaning you could end up with anything powering them up with a live input.I don't know of any problems with the original LM324. Except the ghastly crossover distortion. And the pitiful slew rate. And the wimpy drive. And the horrible stuff that happens if any of the inputs go below ground. The LM709 had front-end zener quirks. -- John Larkin Highland Technology, Inc trk jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
Reply by ●November 24, 20172017-11-24
On Tuesday, November 21, 2017 at 1:35:27 PM UTC-6, Winfield Hill wrote:> John Larkin wrote... > > > > The ancient LM324 has PNP input transistors, and the > > safe input voltages can go to +32 irrespective of V+. > > I also thought of PNP-input single-supply op-amps as > a good solution, but the O.P. seems not to notice.I don't know why you say that. I agreed it was the best solution, given the space and cost parameters in my reply to John Larkin. Ever since then it has come up the winner in every comparison. The only real competitor is the TLP175, and it loses on both cost and space.
Reply by ●November 24, 20172017-11-24
On 11/24/2017 04:47 PM, John Larkin wrote:> On Fri, 24 Nov 2017 12:38:20 -0800 (PST), > bloggs.fredbloggs.fred@gmail.com wrote: > >> On Tuesday, November 21, 2017 at 2:35:27 PM UTC-5, Winfield Hill wrote: >>> John Larkin wrote... >>>> >>>> The ancient LM324 has PNP input transistors, and the >>>> safe input voltages can go to +32 irrespective of V+. >>> >>> I also thought of PNP-input single-supply op-amps as >>> a good solution, but the O.P. seems not to notice. >>> >>> >>> -- >>> Thanks, >>> - Win >> >> Weren't those the original phase inversion cheapos? Meaning you could end up with anything powering them up with a live input. > > I don't know of any problems with the original LM324. > > Except the ghastly crossover distortion. > > And the pitiful slew rate. > > And the wimpy drive. > > And the horrible stuff that happens if any of the inputs go below > ground. > > The LM709 had front-end zener quirks. > >I use a fair number of LM358s these days, but not for things requiring speed or great fidelity. The original FET-input amps (e.g. TL081, LF356, MC34081 and their relatives) had the so-called "phase inversion" problem. It wasn't really phase inversion, because what happens is that when the negative CM range is exceeded the output goes high irrespective of which input is higher or lower. It's most commonly a problem with noninverting stages, of course, which is why it looks like phase inversion. The old Motorola bipolar amps such as the MC33078 (which I also still use) were also bad for this because their output swing was wider than their input CM range, so the second stage of a noninverting cascade was liable to misbehave. It's one of those minor warts that you get used to. As Jim Williams used to say, "Always invert, except when you can't." Cheers Phil Hobbs -- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC / Hobbs ElectroOptics Optics, Electro-optics, Photonics, Analog Electronics Briarcliff Manor NY 10510 http://electrooptical.net https://hobbs-eo.com
Reply by ●November 24, 20172017-11-24
On Tuesday, November 21, 2017 at 2:18:46 PM UTC-6, Steve Wilson wrote:> That's ok, Win. These people are probably too young to know who you are, or > to have read AOE.By "these people", are you referring to me? I'm 58.
Reply by ●November 24, 20172017-11-24
On Friday, November 24, 2017 at 3:48:01 PM UTC-6, John Larkin wrote:> I don't know of any problems with the original LM324. > > Except the ghastly crossover distortion. > > And the pitiful slew rate. > > And the wimpy drive. > > And the horrible stuff that happens if any of the inputs go below > ground.None of which matter in this situation, which is one reason why your original suggestion is definitely the front runner. I've ordered a couple of Op Amps, including a 324, from Mouser for testing. They should arrive Monday.
Reply by ●November 24, 20172017-11-24
> > > > 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 should buy a battery management IC, some of them handle lots of cells, and this is especially true of Li batteries. There are physics based considerations that dwarf the triviality of the electronics interconnection topology.I am using an off-the shelf LiPo battery monitor specifically designed to monitor LiPo batteries during use. These are extremely common in the Radio Control hobby sector, and are readily available at very low cost. Below is an example. The only problem is these are designed to be manually disconnected when not in use. The reed relays work perfectly, but are too large to be able to shrink the design down to a desirable size for a lower power version of the light. The 100W version uses a battery that is nearly 4" x 4" x 12", so a smaller board is not of any value, but I would like to create a few 20W lights with a smaller footprint and less weight. https://www.ebay.com/itm/5pcs-Lipo-Alarm-Low-Voltage-1-8S-Battery-Buzzer-RC-Checker-Tester-LED-Indicator/263108054278?ssPageName=STRK%3AMEBIDX%3AIT&_trksid=p2060353.m2749.l2649
Reply by ●November 24, 20172017-11-24
On Fri, 24 Nov 2017 14:06:48 -0800 (PST), Leslie Rhorer <rhorerles@gmail.com> wrote:>On Friday, November 24, 2017 at 3:48:01 PM UTC-6, John Larkin wrote: >> I don't know of any problems with the original LM324. >> >> Except the ghastly crossover distortion. >> >> And the pitiful slew rate. >> >> And the wimpy drive. >> >> And the horrible stuff that happens if any of the inputs go below >> ground. > >None of which matter in this situation, which is one reason why your original suggestion is definitely the front runner. I've ordered a couple of Op Amps, including a 324, from Mouser for testing. They should arrive Monday.One other quirk: if one of the four opamps rails, it can mess up the other three. Shared current sources. I don't remember just how bad that is. -- John Larkin Highland Technology, Inc trk jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
