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.
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
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
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
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
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
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
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
Thanks
David
As you know the total series resistance of the voltage divider resistors, you can calculate the current (and therefore power) the divider draws.
If it’s too much, you could increase the values (10X, 100X, etc).
EDIT SLOPPY WRITTING:
The total series resistance is the sum of the voltage divider resistors (unless you are using any in parallel), you can calculate the current (and power) the divider draws.
If the current/power is too much, you could increase all the values by the same amount.
10X, 100X, etc makes it easier to stick with resistor series (E12, E24, etc).
Well I could calculate that if I had ability
Currently the draw is too much as it flattens the battery I’m trying to monitor.
Adding a relay will rule it out completely.
I’m just waiting for the relay to arrive.
Thanks for your input.
David
You probably want to have 10k (10,000) Omhs or more.
So for 12V, 12/10k = 0.0012A (1.2mA).
P=I*V, so 12 *0.0012 = 0.0144W (14.4mW).
Say the battery has a usable capacity of 20Ah, it can draw 1A for 20 hours, 20A for 1 hour, 2A for 10hrs, 10A for 2hrs, etc.
In the above case (1.2mA), the battery should last 20/1.2mA (20/0.0012) = 16,667 hours
I mention usable because a lead-acid battery has a usable capacity of approx. 20% of its rated capacity (without doing long-term damage to it)
.
