Early acorn board output sink current

[Oldboy79]

Hello.
I live in China and ordering anything from outside China is difficult especially the USA in the last year. I purchased my Acorn kit in November 2017 when there was no 8 relay module available, I know there is a kit now but I will not be ordering it for the above reasons. There are are many 8 relay modules
in China of different types but before I can select one I need to know the safe max sink current the Acorn board outputs can support. I assume the outputs are open collector type at 24 volts? Once I have the info I can purchase an appropriate unit here or make up my own from discrete components(last resort) I have attached a photo of a board that may be suitable. I would really appreciate an answer, thanks.

[Muzzer]

Centroid presumably publish a spec for the open collector outputs but if your board is like mine, the output drivers are Toshiba TBD62083 series transistor arrays https://toshiba.semicon-storage.com/inf ... D62083AFWG.

These are simple open drain (FET) devices with no overcurrent or overtemperature protection, so rely on sensible loading. The FP package (wide SO-18) seems to be limited to around 1W at 25C, depending how many outputs are on and what those currents are. That is derated with temperature by about 200mW at 50C ambient, so permissible dissipation is perhaps ~750mW total.

It might be easier to work out the worst case dissipation if you know the maximum number of relays you plan to drive at any one time, what the coil resistance is and the max supply voltage.

As these are MOSFET outputs, the output is resistive with an on resistance of about 2 Ohms at 25C, so you can scale the voltage drop proportional with current or simply use P = I squared x R to estimate the dissipation in an output once you know the load current. A hot device will have a higher resistance, perhaps 50% higher. Taking the on resistance as 3 Ohms should give you a pretty good idea of what things will look lime when up at temperature.

Those relays look like 1600 Ohms for the 24V version https://www.generationrobots.com/media/JQC-3FF-v1.pdf, so at 24V, the current will be about 15mA. The dissipation in each output of the IC will be about 1mW, which is trivial. Unless I've completely misread the datasheets, you could run all 8 relays all day in a hot ambient and the IC wouldn't break a sweat.

[martyscncgarage]

Your relay module needs to be NPN not PNP. That is voltage is fed to the board either +5VDC or +24VDC and the -relay outputs are run to the Acorn board. Acorn SINKs the voltage to turn on the relay. (Low level trigger) I can not tell from your picture what you have there. (The board may be selectable, you would need LOW level trigger)

[Richards]

Here's a screen shot of the Acorn User's Manual for a V.3 board. It lists the current of the Open Collector Outputs at 10mA. Typical voltage is 24VDC.

[Muzzer]

I see the relay board in the photo seems to have optoisolators on the inputs. I wasn't expecting that, as it means there would be 2 levels of isolation between the Acorn outputs and the switched load. So the load on the Acorn with these boards is the opto current, rather than the relay coil current. They say "less than 5mA". Looking at the component values it looks closer to 3mA from a 5V supply.

https://protosupplies.com/product/relay ... isolation/
Same result though - 5mA load is trivial for the Acorn outputs.

[Richards]

Measure the current drawn by the board. Check that the suppression diode circuit with a 'scope.

I have a six-port NPN relay board, bought on EBay, the same board that was popular before Centroid released it's 8-port relay board. I used the six-port board to turn on the high pressure nozzles on my high-pressure aeroponic/hydroponic system using a CLICK PLC as the timer. The CLICK worked just like the outputs on a V3 Acorn. The common on the NPN relay control circuit was connected to +24VDC and individual relay control circuit was pulled low by the CLICK (sinking circuit). (The CLICK turns on the nozzles for 3 seconds every minute to spray the roots of the produce with a fine mist of nutrients.) It worked fine for about two months and then it stopped. The relay board burned up the CLICK output connected to that relay. Because the water/mister valve uses 24VAC, I was able to substitute an SSR on another output. That seems to work properly.

What I'm trying to say is that just because the spec paper from China says that a device only draws X-amount of current, doesn't necessarily mean that the device actually only draws that much current. Whether the relay circuit pulled too much current or whether the suppression diode failed, I'll never know. That relay board was tossed out before it could ruin the vegetables in my system.

[Oldboy79]

Thanks to Muzzer, Marty and Mike Richards for your quick and detailed replies. The relay board pictured does have optoisolators and by removing the jumper I can use my 5v psu for the isolators only, and the 24 volts for the relays only , an even better solution. The board is activated by a negative signal. Now I can get on with my life again, thanks everyone.

[Muzzer]

Good to hear. Where are you in China? I worked in and out of China for many years, most recently 3 years in Kunshan until 2018 with suppliers and customers in Shanghai, Suzhou, Ningbo, Shenzhen, Changchun, Baoding, Beijing etc. Hot, sticky, noisy and smelly - I was getting too old for all that.

Struck by how little our Chinese engineers actually did outside of work - and how limited their knowledge was relating to their jobs. Working on engine and electronic components without even a clue how a car or engine worked.

[tblough]

I did four years in Suzhou and noticed the same thing. Their schools teach memorization, not applied thinking.