Switch with large hysteresis - Page 1
Forum section: electronics / beginners

author: raff5184
author_rank: Regular Contributor
timestamp: February 27, 2020, 02:46:54 pm
content: 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

author: Tom45
author_rank: Frequent Contributor
timestamp: February 27, 2020, 04:59:33 pm
content: See design tool at:http://sim.okawa-denshi.jp/en/compkeisan.htmI 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.

author: raff5184
author_rank: Regular Contributor
timestamp: February 27, 2020, 06:26:44 pm
content: 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.

author: Tom45
author_rank: Frequent Contributor
timestamp: February 27, 2020, 07:05:23 pm
content: 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.

author: ender4171
author_rank: Contributor
timestamp: March 05, 2020, 07:41:24 pm
content: 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.

author: raff5184
author_rank: Regular Contributor
timestamp: March 05, 2020, 07:59:35 pm
content: it's basically what happens on page 6:https://www.analog.com/media/en/technical-documentation/data-sheets/2934f.pdfThe thing is this is a fixed small hysteresis and I need a larger one, of a couple of volts.Quote from: ender4171 on March 05, 2020, 07:41:24 pmHow? 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

author: Brumby
author_rank: Unknown
timestamp: March 05, 2020, 08:38:49 pm
content: 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.

author: Tom45
author_rank: Frequent Contributor
timestamp: March 05, 2020, 10:12:19 pm
content: Did you look at the tool at the link in my first post?http://sim.okawa-denshi.jp/en/compkeisan.htmUsing 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.

author: wizard69
author_rank: Super Contributor
timestamp: March 06, 2020, 05:07:06 am
content: 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.

author: raff5184
author_rank: Regular Contributor
timestamp: March 06, 2020, 09:23:57 pm
content: Quote from: Tom45 on March 05, 2020, 10:12:19 pmyes 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.pdfor a 74HC14?

author: raff5184
author_rank: Regular Contributor
timestamp: March 06, 2020, 09:26:56 pm
content: Quote from: Brumby on March 05, 2020, 08:38:49 pmThanks. 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

author: Zero999
author_rank: Super Contributor
timestamp: March 06, 2020, 11:10:18 pm
content: Quote from: raff5184 on March 06, 2020, 09:23:57 pmThe 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.pdfhttp://www.ti.com/lit/ds/symlink/tlc3702.pdf

author: Tom45
author_rank: Frequent Contributor
timestamp: March 06, 2020, 11:28:33 pm
content: Quote from: raff5184 on March 06, 2020, 09:23:57 pmThose 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.