Switch with large hysteresis

[raff5184]

Hi all,
I need a switch or a switching circuitry with hysteresis.
Basically I have a supercapacitor (4F, 5.5V) connected to a load through this switch that needs to close and open in different conditions.
I want the switch to close when the voltage across the supercap is raising and becomes larger than 4.5V but it has to open when it goes down to 2V.
When I close the switch there will be a small voltage drop (not as low as 2V) so the voltage goes below 4.5V but I want the switch still to be closed, until the 2V.

Are there chips that already do this? What types? Or do I have to design my switching?
Thanks

[Tom45]

See design tool at: http://sim.okawa-denshi.jp/en/compkeisan.htm

I think your open vs. closed is backwards from normal practice. An open switch doesn't conduct current, ie, it is an open circuit. A closed switch does conduct current.

[raff5184]

Hey,

no, I meant that I want to connect the load (close the switch) only after the supercap has reached at least 4.5V. If I connect the load before I might not have enough power to charge the supercapacitor AND power the load at the same time. So, only after the supercap is charged enough I can discharge the energy on the load. Similarly, while discharging, if I go down to 2V I want to open the switch (disconnect the load) because there is not enough voltage to supply.

[Tom45]

Oh I see. I thought you were talking about a switch to control the charge. Instead you want a switch between the capacitor and the load.

[ender4171]

I've never seen a switch that smart (though that's not to say they don't exist somewhere), but what you are describing would be pretty trivial to accomplish with a relay and micro.

[raff5184]

it's basically what happens on page 6: https://www.analog.com/media/en/technical-documentation/data-sheets/2934f.pdf
The thing is this is a fixed small hysteresis and I need a larger one, of a couple of volts.

what you are describing would be pretty trivial to accomplish with a relay and micro.

How? Would I need an ADC or a micro with an integrated ADC to measure the voltage level?

I can't use active components unless they absorb current in the order of micro-amps

[Brumby]

It's not hard to find a micro with an ADC built in.  The Atmega328 series has them and those can be found in older Arduino boards.

[Tom45]

Did you look at the tool at the link in my first post?

   http://sim.okawa-denshi.jp/en/compkeisan.htm

Using a comparator and 3 resistors you can design what you are looking for. You would probably need a transistor or relay on the output of the comparator for switching your load.

[wizard69]

You might want to look at industrial controls vendors like Omron, A&B, SMC or others that sell various interface modules.   Many of the optical switch or pressure switch modules from these vendors have hysteresis settings.   SMC for example sells pressure switch modules that take a voltage reading from the transducer in a voltage range of 1-5 (if memory holds).  Going this route is only viable if you don't have a volume requirement and are in a hurry.

[raff5184]

Did you look at the tool at the link in my first post?

   http://sim.okawa-denshi.jp/en/compkeisan.htm

yes I did. But I am not sure what opamp to use for this. Any suggestions?

Also, what about these: http://www.ti.com/lit/ds/symlink/lm2903.pdf or a 74HC14?

[raff5184]

It's not hard to find a micro with an ADC built in.  The Atmega328 series has them and those can be found in older Arduino boards.

Thanks. Like I said, I can't use devices that absorbs milliampers to work, because the purpose of my voltage monitor is to connect a supercapacitor on the actual load after it has stored enough energy

[Zero999]

Did you look at the tool at the link in my first post?

   http://sim.okawa-denshi.jp/en/compkeisan.htm

yes I did. But I am not sure what opamp to use for this. Any suggestions?

Also, what about these: http://www.ti.com/lit/ds/symlink/lm2903.pdf or a 74HC14?

The 74HC14 is a Schmitt trigger IC, with fixed hysteresis, which isn't tightly controlled. The LM393 is a decent choice but is probably a little too power hungry for your application. Use a lower power comparator IC such as the  MCP6561 or TLC3702.
http://ww1.microchip.com/downloads/en/devicedoc/22139c.pdf
http://www.ti.com/lit/ds/symlink/tlc3702.pdf

[Tom45]

yes I did. But I am not sure what opamp to use for this. Any suggestions?

Those are comparators at that site, not opamps.

I don't use comparators enough to make a recommendation. Try starting with a search for low power comparators.