Author Topic: Bistable relay and 3.3v ldo for diy thermostat  (Read 360 times)

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Offline Clickau

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Bistable relay and 3.3v ldo for diy thermostat
« on: March 10, 2019, 02:28:58 pm »
Hello, 2 questions:
So I'm trying to build a smart thermostat for my home using an ESP-12E board. I have most things laid down, but I'm not sure what relay to use. I need a bistable relay, as this thermostat needs to be powered from 3xAA batteries for at least a few months, so it has to only draw current when switching. I looked at the RSM850B, but I don't really understand the datasheet. Will this relay work with the 3.3v microcontroller? How much current will it draw when switching? And how do I actually use it? If it's not good for my purpose, could you give me a suggestion? I live in Romania, so it has to available from a website like tme.eu, or another which ships here.
Also, I need a regulator to convert the voltage from the batteries to around 3.3v. Is the MIC5209-3.3YS a good choice? It's an LDO, it has a low queiscent current, and I think the 500 mA it provides will be enough for my application, but I'm still a bit paranoid.
Any help will be appreciated. Thanks.
 

Offline mariush

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Re: Bistable relay and 3.3v ldo for diy thermostat
« Reply #1 on: March 10, 2019, 02:59:22 pm »
It depends on the version of rsm850b you pick, see table at bottom of page 1 of datasheet

Coil code 1003
Rated voltage 3v
Coil resistance 90 ohm
Coil operating range : 2.25v ... 4.5v max  - in theory the relay will work as long as the voltage is higher than 2.25v and lower than 4.5v
Power consumption 100mW

Voltage = Current x Resistance 

3.3v  = Current x 90 ohm -> Current = 3.3v / 90 ohm = 0.0366 A  (37mA)  for at least 3 seconds (the switching time)

The ESP12 datasheet says average operating current 80mA , so I'd say reserve twice as much for this. Reserve 50mA for the relay ... so I'd say a minimum of 250mA for the regulator

As for that MIC5209 regulator, the dropout voltage could be a problem but unlikely

If you're using 3 rechargeable batteries your input voltage would be 3 x 1.2v = ~ 3.6v

Datasheet says dropout voltage is 165..300mV drop at 150mA, so you'd get 3.3v, but at close to 500mA it says dropout voltage is between 350..500mV, so your voltage may drop below 3.3v
The ESP12 should still work (datasheet says 3v min), the relay should still work with a voltage a bit below 3.3v 
 

Offline Twoflower

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Re: Bistable relay and 3.3v ldo for diy thermostat
« Reply #2 on: March 10, 2019, 03:11:12 pm »
For the LDO calculation you shouldn't use the average power consumption but the max consumption. According to the datasheet The ESP8266 can draw up to 170mA alone (WiFi enabled and active). And the LDO has to provide still a stable supply for the CPU not to fail.

On the other side I wouldn't power the relais through the LDO. But direct from the batteries. In the LDO has plenty of headroom for the required FALSH, sensor, ...
 

Offline Clickau

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Re: Bistable relay and 3.3v ldo for diy thermostat
« Reply #3 on: March 10, 2019, 04:02:34 pm »
3.3v  = Current x 90 ohm -> Current = 3.3v / 90 ohm = 0.0366 A  (37mA)  for at least 3 seconds (the switching time)
Did you mean 3 ms, because that's what it says the operating time should be? Thank you for your reply, I didn't even see the table :-// .
For the LDO calculation you shouldn't use the average power consumption but the max consumption. According to the datasheet The ESP8266 can draw up to 170mA alone (WiFi enabled and active). And the LDO has to provide still a stable supply for the CPU not to fail.

On the other side I wouldn't power the relais through the LDO. But direct from the batteries. In the LDO has plenty of headroom for the required FALSH, sensor, ...
Ok, but then how should I drive the relay? Looking at the drawing of the pins, I understand i should switch polarities to reset? How do I do that? Should i look for a double coil relay?
 

Offline Twoflower

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Re: Bistable relay and 3.3v ldo for diy thermostat
« Reply #4 on: March 10, 2019, 04:28:14 pm »
To be honest, I haven't checked your relay. Right for a single latching coil you need a H-Bridge to change the polarity; there are chips for that but I'm not sure if they're available for low power/low voltage usage. A dual coil latching relay might be the EC2-3TNU from KEMET (check the contact specs if they're matching your requirements).
 

Offline kripton2035

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Re: Bistable relay and 3.3v ldo for diy thermostat
« Reply #5 on: March 10, 2019, 04:41:41 pm »
some ideas :




Offline David Hess

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Re: Bistable relay and 3.3v ldo for diy thermostat
« Reply #6 on: March 10, 2019, 06:11:41 pm »
If it is using battery power, then why does it need to be isolated?

DC isolated charge pumping to drive MOSFETs configured as a solid state relay can be used to replace latching relays when low power is required.
 

Offline Clickau

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Re: Bistable relay and 3.3v ldo for diy thermostat
« Reply #7 on: March 12, 2019, 02:39:31 pm »
Ok thanks for the useful answers. I don't want to use mosfet's because I want it to work with other heating systems, if I ever change mine. I couldn't find the EC2-3TNU on a site that shipps to Romania, but I found the ZETTLER AZ850P2-3, which I think is similar. But, since it draws up to 140mW, so almost 50mA at 3V, I needed a mosfet to drive it, so I also bought the RFP12N10L. Now waiting for it to ship so I can start working on it. I might update again afterwards.
Thanks again for the help.
 


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