Pico Relay Question

Hi Everyone,
I’m considering using one of these Relay Modules with a Raspberry Pi Pico.

There is some debate in the product thread regarding if it will work consistently with a 3.3v control signal. Has that debate been settled or is the answer still “Maybe”?
If so is there an alternative product available that is known to work consistently at 3.3v?

My project runs on batteries and I want to use the relay to switch off a connection to save power.
I can’t see how to tell how much power this relay device will draw if any while the contacts are in the Open / Off state.

Basically I want to completely disconnect the voltage divider discussed in this thread when I’m not measuring the voltage.

Currently the device will run for about 2 weeks before flattening a 12v 7Ah battery.
I have the sleep mode stuff working well and can measure the draw on the battery from the pico during sleep and wake phases and it is extremely low.

Now I suspect the resistors in the voltage divider are the killer and I want to disconnect it using a relay.

Thanks
David

Hi David
There has been a lot of conversation about using these relays (5V) at 3.3V, a fair bit by me.
Mainly because the relay is being used well outside its manufacturers specification for the “must operate” voltage level.

I personally have bowed out of this conversation and have recently left it up to the person to make up their own minds with the intention of completely ignoring any complaints or requests for help if any of these units fail to operate at a later date.

These would also operate much slower I think at the lower voltage. I have not got one to measure the operate time at this voltage and don’t feel like purchasing one just to find out.

So will leave it up to you to make up your own mind here. I think you have been around this Forum long enough to do that.

By the way, some of the prebuilt modules on the Core web site are fitted with 3V relays. Why don’t you consider one of these. Alternatively some can isolate the relay from the 3.3V supply and can have 5V for the relay operating voltage. Unfortunately I think the particular one you have linked needs the relay to operate at the same voltage as the logic. I seem to remember someone reverse engineering one and coming up with a circuit. Some manufacturers provide a circuit on their Wiki page but not this one.
Cheers Bob

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Hi Bob,
Well that answers my first question. I have enough problems without working outside specs :cowboy_hat_face:.

I did a search but didn’t find any 3.3v products on cores site.
Have I missed them?
David

Hi David

My thinking exactly. I think there have been a couple of people that reported their 5V relays did not work at 3.3V. One description described it as being a half hearted click sometimes. One problem seems to be that the way the circuit is arranged with an “indicator” LED the LED will work but that does not meant that the relay has worked so you get a positive indication with a negative result, Nothing.

There are some there. If you blow up the pics until you can read the printing on the relay you can find them. BUT I think there is one where the relay is switched by a Mosfet. This is supposed to switch at 3.3V but if you look at the appropriate switching graph for the Mosfet it seems very iffy and borderline, I personally would not trust it.

There are others switched by a transistor which may switch at a 3.3V operating signal and you could apply 5V to “VDD” and switch with 3.3V. That might be better if the transistor switches but I don’t know and am probably not going to find out.

I am afraid I am a bit old school and not a big fan of this 3.3V. I know that these days it is used extensively but I still find drawbacks. Like it is useful in a lot of cases to use a diode for various things. But at about 0.7V forward voltage drop you don’t get to use too many at 3.3V before you run out of voltage. Sure there are schottkys but they are not drastically better. There are also pretty low voltage drop diodes too but with some trade offs like lower reverse voltage, high reverse leakage current and for the higher frequency use, greater stray capacitance. A couple of modules I note use a transistor switch for the relay. When you consider the 200 or 300mV drop across that and think of this has to be subtracted from what is available for the relay the 3.3V looks very sick indeed.

You can use a Mosfet as a diode but as I see it by the time you factor in the Mosfet, a couple of resistors and arrange the circuit to use it where is the advantage. You have to watch out the Mosfet body diode is not going to interfere when you do this.

I think I have said enough. I think when experimenting the use of 5V just allows for that little bit of head room that is handy to have sometimes.
Cheers Bob

How about this MOSFET Module?
[ CE09733 ] MOSFET Power Switch Module

At V_{GS} = 3.3\text{V} the MOSFET (AOD4184) will pass at least 5A.

Disadvantages are that it is not galvanically isolated, and is generally only suitable for low-side switching.

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Hi Michael,
I did spot this product but don’t understand them enough to know how they work.(yet).
Now there are two expressions I don’t understand :cowboy_hat_face:

galvanically isolated, and low-side switching.

I found a relay on a different site rated at 3.3v so I bought a couple of them to play with and only cost a couple of bucks.
As I mentioned in my original post all I want to do is disconnect the resistors that provide the “voltage divider” so they don’t trickle away battery power when no monitoring is going on, which is 99.99% of the time.
One thing I can’t get out of the relay spec sheets is do they consume any power when they are not activated or in the “normal” position. I’d expect not but not sure.
I notice some have an LED for each relay position and diodes in there that I assume stops things feeding back.
Thanks
David

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Hi David,
Not sure if it helps, but I used that particular relay with a Pico and it worked fine. It was all powered by a 12V plug pack with a buck converter ( https://core-electronics.com.au/dc-dc-multi-output-buck-converter-33v5v9v12v.html ) feeding the Pico VSYS with 3.3V.

With the 3.3V also on the VCC of the relay and the Pico signal at about the same, the relay then switched 12V through to a small pump.

Cheers, Dave

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Hi David

NO.

I think one diode is to stop any reverse voltage on the LED. They don’t like that and the reverse breakdown voltage is quite low.
Another one directly across the relay coil is the “Flyback” diode and soaks up the reverse voltage the coil generates when the field collapses at switch off. This can be quite high and damage other components like transistors or mosfets
Cheers Bob

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Hi David

It probably did.

My statement concerns the 3.3V being way outside the manufacturers spec for “must operate” voltage level. There is always a concern (to me anyway) that some might be borderline and not work so I think it is a bit misleading to imply that the unit as sold WILL work at 3.3V.
The other concern is the possible slower operation which might shorten life considerably if excessive arcing occurs when switching.
Cheers Bob

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I did purchase a relay from a different site that was rated to operate at 3.3v so that will be the simplest solution for my limited electronics skills.
I’ll post again if anything interesting happens :cowboy_hat_face:
Thanks
David

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