Hi this will be my first every project using a micro controller and after thoughts and maybe recommendation on hardware.
The project is controlling two 24v coils @ 100% ED 1.29A and 1 horn/buzzer also 24v no more than 2A.Was looking at running it with a Raspberry Pi Pico WH.
Regarding the coils these are used to control a hydraulic valve which is used to steer a boat via a joystick. Would a mosfet be the ideal way to switch these loads, I thought this because over a day the coils will energised hundreds of time a day so after reliability. To keep it simple for me I thought a Mosfet module would be idea. just after any recommendation for a module with integrated flywheel diode to achieve this?
Regarding the horn will i need a different module, one with a pull down resistor instead? or the same used for the coils be fine.
My end goal is to connect the pico via bluetooth to a computer and have a designated keystroke turn on/off the modules.I currently use a xbox controller as a mouse to control the Plotting/chart software on the boat and thought why not the steering as well.But for the time been just start playing around and learn some simple code to control a load.
1 Like
Hi John
Mosfets will certainly handle that sort of current.
Could be a bit hard to find. The diode would normally be connected across the coil and to be included in a module the coil supply would have to be connected to the board which is not always the case. Mostly the supply would be connected directly to the load and the low side of the load connects to the Mosfet drain. ie; one connection to the drain and source to Ground. But his is not a real concern as the inclusion of the diode is not too difficult.
You are obviously looking for high reliability which looks like a mission critical set up. My personal concern is your intended use of what is primarily hobby or experimenter type components. Now I am not going into discussion on the merits or otherwise of RPi reliability and will leave that up to you and others. Suffice to say that I personally would look for something a bit more robust for an application such as this. If this is just an experiment or proof of concept exercise then go for it. If something more long term and permanent I would have to think about it a bit.
Whatever you use you will need a fail safe fallback system where you can over-ride the electronic wizardry and steer the boat manually. I think this would be a must.
The same switching set up would be fine. A low side Mosfet switch.
Cheers Bob
1 Like
Thank you for you advise, this is a bit of proof of concept. I Understand your concern about the application, all helms still have manual steering and a power switch to turn off power to the electric steering in place. Most of the time when the electric steering is used is in open ocean miles from anything around us.
1 Like
Hi John
Like the container ship recently which had the whole ocean to sail in but despite all the electronic safety gizmos on board still managed to run into a tanker which was moored with catastrophic results.
OK that was very extreme and fortunately does not happen too often but it will be very interesting when the results of the investigation are released. Something very drastically wrong here.
Back to your query. The actual switching of your loads (coil, horn etc) is actually pretty simple as below
The flywheel diode should be a schottky type with enough current capacity to swallow the reverse pulse which could be more than the coil operating current.
The Mosfet has to be driven with a high enough voltage to turn the Mosfet fully ON with enough current to charge the Mosfet Gate capacitor at switch on.
Then you have to provide the signal to do all this.
All this can be the tricky part.
Start thinking
Cheers Bob
3 Likes
Hi John
Had a bit of a look around for a suitable low side Mosfet switch but are a bit hard to find. Especially on the Core site. I searched for something like "opto coupled Mosfet switch and the first page of results was LED addressable strips. What that has to do with any of the words in my search I don’t know. Maybe Core does.
I specified “opto coupled” as this has a better chance of working reliably at 3.3V source/gate voltage. I believe this to be pretty iffy if driving directly from 3.3V source. If you look at the source/drain resistance v source /gate voltage 3.3V seems very marginal where at 5V the Mosfet is almost fully ON. Personally I would not use 3.3V with these Mosfets but that is me being conservative. I don’t apologise for that, just like things to work properly.
Most of this type of board has a relay on board with various configurations.
Core have one I think possibly of their own design but next to no information on it and no schematic that I can find. It has 2 Mosfets on it but one of them could connected as the flyback diode as there are terminals for “supply” and “load”. Due to the lack of info I won’t comment on the use of this one. SKU CE09732.
However Core have another high side switch that I reckon will work. It makes little difference in your application whether the switching is high or low side. This unit is opto coupled and quoted operation is down to 3.3Vand is also quite happy at 5V + so your input selection is flexible. It also has all the components (except the flywheel diode which has to be across the load) you need and all you need to do is connect and go. (in theory that is). SKU DFR0457.
Quick sketch below. Have a look, the choice is yours…
Cheers Bob
2 Likes
Hi Bob, Thank you for all your information from here i can start designing and an ordering some parts much appreciated. Great community
2 Likes
