Help with Distribution Board components choice

Thread Starter

timur.hal

Joined Jan 29, 2022
19
Hello All,

I am designing a distribution board for devices.
The main concept is that I have a power supply (AC to DC - there are 5 different DC voltages at the output) and different devices connected to those voltage rails (in some voltage rails there are multiple devices)

I need to have a distribution board that will switch those devices only when the signal from a computer is applied (signal will be a GPIO, basically High or Low), I have attached a brief block-diagram to get an idea of what I am looking for

Most of the Devices are running at 0.5-2A and only 2 of them running at 10A (24V)

To do that I thought to use either Relays (simple to implement, but a bit bulky) or a P-channel Mosfets (as a Mosfet should be treated as a High-side switch and to use NMOS a driver should be used, which I don't want to do or NMOS would be beneficial to use?)

What is the best device to this application? Advantage and disadvantages ?

Thanks in advance
 

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KeithWalker

Joined Jul 10, 2017
2,403
Either relays or MOSFETs would do the job. Making the best choice will depend on the characteristics of each of your loads and what space you have available.
 

Thread Starter

timur.hal

Joined Jan 29, 2022
19
Either relays or MOSFETs would do the job. Making the best choice will depend on the characteristics of each of your loads and what space you have available.
Hello Keith,

As I said load will be from 0.5-2A and the maximum for 3 devices 10A (loads will be mainly stepper motors, sensors, evaluation boards and for high power LED drivers). In terms of space there are no limits, but obviously I want to make a small as possible

I wanted to know which one is more reliable and beneficial to use. I know that for the Relays there is a lifespan for how many switches it can make (because it is a mechanical switch). Some sort of this limitations and drawbacks would be interesting to know before the design
 

ScottWang

Joined Aug 23, 2012
7,243
GPIO→ ULN2003 → PMOSFET
Why add the ULN2003, because the GPIO initial status is low, so when you output low then the PMOSFET will be turned OFF, and when you output high then the PMOSFET will be turned ON.

Edit :
Don't forget to add the pull high resistor for each Vg of PMOSFET and the Vg=12V when the Vin=24V.
This is just a rough thought.
 

Thread Starter

timur.hal

Joined Jan 29, 2022
19
GPIO→ ULN2003 → PMOSFET
Why add the ULN2003, because the GPIO initial status is low, so when you output low then the PMOSFET will be turned OFF, and when you output high then the PMOSFET will be turned ON.
Hello Scott,

I believe this is what I want, to be initially OFF and turn ON only when needed.

GPIO will go to an optocoupler and it will drive the PMOS (I haven't included entire circuitry, it is not the topic for discussion for now)

My main concern is which device is better to use for load switching. PMOS, relays or NMOS
 

Thread Starter

timur.hal

Joined Jan 29, 2022
19
I would look into SSRs, solid state relays like the one below. They come in sizes from 3 amps to 20 amps and only require 3 volts at 15 ma to activate.
View attachment 259080
https://www.jameco.com/Jameco/Products/ProdDS/2218126.pdf
I thought about SSRs as well, but they seems to have higher ON resistance, therefore higher voltage drop, but it is essentially the same as using optocoupler and Mosfets

It is a good alternative. Do you know if there are any other drawbacks of using them?
 

Thread Starter

timur.hal

Joined Jan 29, 2022
19
No.
I see from the data sheet that the 10 amp version has only a 100mv volt drop at 10 amps.
Sorry I've been looking at another SSRs for lower current rating, but this one seems to have low voltage drop. They are £30 each and I would need around 20, so it a bit costly to use them
 

AnalogKid

Joined Aug 1, 2013
10,096
My main concern is which device is better to use for load switching. PMOS, relays or NMOS
I vote solid state. The question is - will any of your devices get upset if they have no GND connection while other devices do (through an n-channel switch)? With p-channel switches, all devices are hard-connected to GND all the time. This might be required if an unpowered device's signal input is touchy. The down side is driving a 24 V switch with a 3.3 V GPIO pin.

If you use n-channel switches and switch the GND connection to each device, the driver circuits become way more simple. The down side is that signal inputs and outputs could float up to whatever the device's Vcc is when the GND connection is unswitched.

If GND-switching might be an issue, then a ULN2003 is a great level translator for this application.

ak
 

MaxHeadRoom

Joined Jul 18, 2013
25,469
You can also get the Opto22 series of I/O boards, the control is either 5vdc or 24vdc. Also supplied as boards of 4, 6, 8, 16, modules.
Status indicator LED and outputs are fused.
Often come up on ebay cheap.
They provide input OR output DC, AC, etc.
I have rafts of them surplus.

1643477039118.png
 

Thread Starter

timur.hal

Joined Jan 29, 2022
19
I vote solid state. The question is - will any of your devices get upset if they have no GND connection while other devices do (through an n-channel switch)? With p-channel switches, all devices are hard-connected to GND all the time. This might be required if an unpowered device's signal input is touchy. The down side is driving a 24 V switch with a 3.3 V GPIO pin.

If you use n-channel switches and switch the GND connection to each device, the driver circuits become way more simple. The down side is that signal inputs and outputs could float up to whatever the device's Vcc is when the GND connection is unswitched.

If GND-switching might be an issue, then a ULN2003 is a great level translator for this application.

ak
All devices on a single voltage rail operate independently, so there are no signals from one device into another.
GPIO signal will go to the optocoupler and optocoupler will drive a PMOS, so it should be fine

For the NMOS to switch ground, do you mean something as shown on the figure below (just an example from internet)?

Can you explain please how ULN2003 will help if GND-switching is a problem?
1643750318278.png
Thanks
 
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