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Help with designing a relay with adjustable hysteresis

Michael Berg

May 21, 2017
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Hello,

I am looking for a design or a component which will perform the following functions:
  • It will connect a power source like a battery or a super capacitor to a load, acting as a switch in between.
  • It will turn off the load when the input voltage drops below X
  • It will turn on the load when the input voltage reaches Y - X and Y serve as hysteresis lower and upper bounds
  • Very energy efficient, able to run without any external power source
For example:
  1. If the input voltage starts at 0, the load will be off (disconnected).
  2. When the voltage rises (for example if a solar panel is charging the capacitor or battery), the unit will wait until the voltage reaches Y.
  3. When the voltage reaches Y, it will enable the load.
  4. The load will now cause the voltage to drop below Y again - which should be allowed and not cause the load to be disconnected. Only as the load drains the battery or cap below X is the load disconnected.
  5. Go back to step 2.
Ideally both X and Y should be configurable in some way, for example using pots or something else. Doesn't have to be fancy, if I can calculate a resistor value and solder that into a board that will be fine too.

The thing about this component is that it must be "powered" by the input itself and must be able to start operating as the input voltage rises. It must use very little power itself since the application is for small scale solar/super capacitor operations with very little power to spare.

Anyone got some good ideas for this type of project?

-Michael
 

Harald Kapp

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Use a solid state relay for low control power (an electromechanical relay will draw comparatively high power).
Use a comparator circuit with adjustable hysteresis to turn the solid state relay on or off. Comparator circuits can be found galore on the internet using e.g. the venerable 555 chip, comparator chips like the LM239 etc.
 

Michael Berg

May 21, 2017
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Right... I see.

I found this: https://groups.google.com/forum/#!topic/diyrovers/XyMnvZqH3A8

The 555 used there switches on when the rising edge input reaches 1/3 of the VCC and off when the falling edge reaches 2/3 VCC:

Since i need kind of the opposite (rising edge input at 2/3 VCC = On, falling edge inpuit at 1/3 VCC = off) I suppose I could just use two of these circuits and switch my load depending on some sort of AND logic between the outputs from the two 555's. And I could use resistors ahead of VCC to make 2/3 VCC = a specific voltage.. I think.

Any more help you have to offer is appreciated :)

-Michael
 

BobK

Jan 5, 2010
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The 555 has a push pull output. It goes high and low, not on and off. The on and off depend on how you wire the relay. A relay wired between the output and ground will go on when the output is high. A relay wired between the output and Vcc will go on when the output is low.

That said, I would not use a 555 because of the set trigger points. You want a simple comparator which will allow you to set the thresholds to any voltage you want, and to make them adjustable as you said you wanted.

Bob
 

Michael Berg

May 21, 2017
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Are you aware of any pre-built or existing comparators with hysteresis that might be used in an Arduino project?

Also these comparators compare a signal to a reference voltage. In my case the VCC *is* the votage to measure. Imagine a super capacitor at 0V, connected to a solar panel. The panel will slowly charge the capacitor, and when the cap voltage reaches Y volts, I want to enable the load. Doesn't seem like a comparator is useful for that.

-Michael
 

BobK

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All comparators have some hysteresis. You can add more hytetesis by using positive feedback.

You would indeed need a votage reference to compare to. This can be as simple as s diode and resistor.

A 555 uses a reference of Vcc / 3, which is of no use at all for comparing Vcc.

Bob
 

Michael Berg

May 21, 2017
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"You would indeed need a votage reference to compare to. This can be as simple as s diode and resistor."

Yes that's the problem really. I don't have a VCC reference, only the voltage coming out of the super capacitor. This is a voltage that starts at 0 and rises until some point. Somehow the comparator would need to compare this voltage to an absolute number rather than a VCC.

It's not an easy problem after all .. :)

-Michael
 

BobK

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There is no problem with measuring the power supply voltage as long as it is high enough to power the comparator and the voltage reference. The difficulty is that you want it to work starting at 0V. I think a circuit could be designed to do this. A MOSFET with a gate threshold above the min voltage required for the compator circuit, but well below the min voltage you need for turn on, plus a pulldown resistor would meet this requirement. If you don’t need to go below about 3V, nothing exotic would be needed.

Bob
 

Harald Kapp

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This here comparator starts from 2 V and has a built-in reference as well as adjustable hysteresis.

Search online for "undervoltage lockout circuit" to disable the comparator for voltages lower than 2 V.
 
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Michael Berg

May 21, 2017
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THanks for the tip on the LTC1540. Still not quite sure how I go about applying this for the type of scenario I'm looking for. The input voltage charges a cap up to 2v at which point the uvlo would kick in. The circuit would begin operating but two problems immediately present themselves: 1) This being a cap, any drain will immediately push the voltage down below 2v, causing the uvlo to disconnect. And then reconnect. And disconnect. I've had this going on before with more crude 12v battery uvlo's, which would sit there and click on and off at high frequencies.

Second problem is the actual wiring up of the LTC1540. When presented with the 2v (and rising) source (assuming we fix the uvlo oscillation problem above) the 1540 should open at some higher voltage (10v for example) and then stay open until the uvlo basically kicks in. It was not immediately clear from any of the example applications which one might be applicable or modifiable to be used in that way.

-Michael
 
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