Hello Experts

I am trying to build a charging circuit for a specific task - both for learning and also for need. It is possible to control the output of op amp L272d by switching Vcc off via a relay?
If not possible, I have to add small signal relays and the relay count is allready quite high.

See attached scematic where the voltage switches is meant to switch Vcc off. LTspice does not include a no / nc relay so you have to imagine the voltage swithes, where the above is feeding the next op amp, until turned on (and cutting Vcc). The signal will still be on Vin+ approx 14.3 V but Vcc will go to 0 - and hopefully also the output of L272d. I have used op777 because could not find spice model.

I attach screenshot of data sheet too.



Any help or suggestion is most welcome.

 

In general, nothing about an opamp can be considered "controlled" when Vcc is in transition. Excuse me for living, but this appears to be a profoundly bad idea. There is doubtless a period of time where the device is not operating properly during those Vcc transitions. You should try for a solution where you are in control of all devices all the time.

LTspice has relay models but they might not be included in the standard download from Analog Devices. One of the best places to look is here:

http://bordodynov.ltwiki.org/

If you have trouble finding a particular opamp model you should use the Universal Opamp symbols along with their associated parameters that let you pick the important properties for your simulation. You can substitute actual parts at a later time.

 

In general, it is bad design practice to switch off a power pin of an IC when the rest of the circuit is still powered.

 

In general, nothing about an opamp can be considered "controlled" when Vcc is in transition. Excuse me for living, but this appears to be a profoundly bad idea. There is doubtless a period of time where the device is not operating properly during those Vcc transitions. You should try for a solution where you are in control of all devices all the time.

LTspice has relay models but they might not be included in the standard download from Analog Devices. One of the best places to look is here:

http://bordodynov.ltwiki.org/

Thanks for thoughts, I am convinced not to go that road. And thanks for the link. Best wishes.

 

One thing you can try for your application would be a 4:1 analog multiplexer. Here is a link to a datasheet for a basic part:

https://www.ti.com/lit/ds/symlink/s...ps%3A%2F%2Fwww.ti.com%2Fproduct%2FSN74LV4052A

 

In general, it is bad design practice to switch off a power pin of an IC when the rest of the circuit is still powered.

I will take that advice in general and not do that practice. Just seemed as a shortcut in component count. Thanks for reply. Best wishes.

 

One thing you can try for your application would be a 4:1 analog multiplexer. Here is a link to a datasheet for a basic part:

https://www.ti.com/lit/ds/symlink/sn74lv4052a.pdf?ts=1695483480537&ref_url=https%3A%2F%2Fwww.ti.com%2Fproduct%2FSN74LV4052A

It is genious. I did not think of that. Thanks for this suggestion. It can cut down parts I think. But have never worked with them, only know they exist. Only thougth is - this is going to be used in a car, high and low temp, vibration etc. And Vcc is between 12,5 to 14,3 when charging is on. A side note is, there will be 5 channels - just only created 3 for simplicity. Anyway - I will look into multiplexers - best wishes and thanks again.

 

If you would tell us what you are trying to do, we might be able to do better than shooting down your proposed solution for an unknown problem.

One generally controls the output of an opamp by controlling the inputs.

 

It is genious. I did not think of that. Thanks for this suggestion. It can cut down parts I think. But have never worked with them, only know they exist. Only thougth is - this is going to be used in a car, high and low temp, vibration etc. And Vcc is between 12,5 to 14,3 when charging is on. A side note is, there will be 5 channels - just only created 3 for simplicity. Anyway - I will look into multiplexers - best wishes and thanks again.

The 74LV4052 is a TTL like "clone" of the CD4052B, an original 4000 series CMOS part. Pull the datasheet for the CMOS part and note the VCC range should be something like 3-18 Volts. Should work fine in the automotive application.

 

If you would tell us what you are trying to do, we might be able to do better than shooting down your proposed solution for an unknown problem.

One generally controls the output of an opamp by controlling the inputs.

I am trying to build a charger that can charge an average lead acid car battery with maybe around 20 A/h. The must is:
It will only be charged from a cigaret lighter, no wirerun to engine compartment. The cigaret lighter is fused with 10 A. Charging time and so is minor priority. I have consideret a switching regulation and are 100 % aware of that it can be bought very cheaply from Aliexpress. But I want to both keep it simple and learn, though still get a good reliable product for use in my car. If I try to regulate the max A, which is chossen in the 5 to 6 amp range, with linear regulator I get a voltage drop. This is why relays are selected. Then simply calculated 5.6 V, 8.6 V, 10.5 V, 12 V, 12,85 V for the swich over relays to get a charging current between 4 - 6 A. There is an end resistor of 0.25 for safety matters and might be used for current shunt measurement.
It is pretty much the circuit. It is clumsy I know. But it is how far my skills reach at the moment. It is also awfull expensive in automotive reliable relays, power op amps, etc. And the powerloss on these resistors, especially in the start fase is best to forget about. But in normal operation it will only charge from between 10.5 V and up.

 

The 74LV4052 is a TTL like "clone" of the CD4052B, an original 4000 series CMOS part. Pull the datasheet for the CMOS part and note the VCC range should be something like 3-18 Volts. Should work fine in the automotive application.

Thanks for advice, I ordered the CD4052BE from Digikey - dip - easy soldering for beginner. Best wishes.