Heating element / Arduino amperage draw

[Mannelito]

With great power comes great responsibility - or so they say.
Therefore I'd like nothing more than not messing things up.

I'm currently looking for heating elements and my situation is that I'm connecting my RIMS to a central 10A fuse. The pump is calculated to draw about 2A when on, leaving 8A (or 10A when pump is inactive) for heating and powering the arduino.

The first part of the question is that if I order a 1800W heating element (230v 8A), and connect it to the main power line in parallel with the pump and arduino, will the heating element always only draw 8A at most or will it get flooded with 10A when the pump turns off?

In similar fashion, if I hook up a AC/DC Switching PSU and connect the DC out port directly to the Arduino, which only needs about 0.5A-0.9A at most, do I need to put in a resistor first or will the Arduino only draw what is needed?


If I'm being unclear or left things out, let me know and I'll try to fill the blanks.


//Manne

 

[IslandLizard]

It's simple, each device will draw whatever current it can, to the max. It doesn't get flooded if the circuit has more amperage available.

Current drawn is based on a device's "internal resistance" and the voltage supplied (Ohm's law).

If, at any time, all your devices plugged into a circuit draw more current combined than the circuit fuse or breaker is rated for, the fuse will blow or the breaker will interrupt the circuit, protecting your wiring.

The same is true for your PSU and the Arduino. The Arduino pulls 0.5-0.9 amps (I) at 5V which is 2.5-4.5 Watt of power (P). P=V*I. The PSU in turn will draw this power from your mains circuit, however it does this at 230V, which equates to 4.5W/230V = 0.02 Amps + whatever the PSU draws to operate it's own circuitry.

Another edit:
There are several voltages coming from your (adapted) PSU. 3.3V, 5V, 12V, and -12V. Make sure to use the right one (5V). Do not use a resistor to drop higher voltage to your Arduino.

A word of caution:
If you're not familiar with these principles, and working with electricity in general, you should consider leaving the construction of your electric system to someone who is savvy.

 

[Mannelito]

I'll heed your advice. After my tinkering is done I'm having an electrician overlook the finished result and correct errors. Once the heating elements are in place I'll post a build thread to see general comments, feedback and suggestions are always interesting.

Getting back to the topic, after adjusting DC current draw the equation should look like this then:

Heating element (8A) + DC Pump ((12v*2A)/230) + Arduino ((5v*1A)/230) = 8+0.1+0.02 = 8.21A current draw in total on the 10A breaker?

Meaning that I could bump up the heating element to 9A and still be in the green?

 

[IslandLizard]

You're picking up fast!
And you applied the formulas correctly.
The PC PSU will draw a bit more to operate it's own circuitry than just what's needed for the Arduino, but nothing drastic.

So yes, you're fine current wise with that setup. It is better not run the circuit at its max capacity for long times, keeping a bit of a reserve is always good.
The easiest way to calculate power needs is by calculating your max supplyable power: 230V * 10A = 2300 Watt, in your case. Then add up all your devices' power needs (easy summation). And usually you don't want to exceed 80-90% of the supply, depending on the circuitry.

You say heating elements? Are you thinking of more than one?

On a side-note, is your DC pump powered by its own wall wart/transformer? Or are you planning to use the PC PSU for that too?

 

[Mannelito]

Ah, no, there is only one heating element, but I'm building 3 RIMS, one for me and two for friends, so in total there are 3 elements, but only one per controller box and circuit.


I have the mains coming into the controller box, the heating element is connected in parallel with a 60W 12v 5A DC switching PSU. A 12V to 5V step down converter is connected to the PSU in parallel with the pump (which is connected directly to the PSU).

The pump is hooked up to a SSR which should block back EMF, or so Í've read.

Do you see any problems with having the arduino and pump hooked up to the same PSU?

 

[IslandLizard]

I see no problems with Arduino and pump on same PS. The Arduino's contains its own voltage regulator IIRC.

Now running the (DC) pump of an SSR does not sound right, but I'm no expert on that end. Can an SSR regulate DC and feed the pump the power shape it needs? Maybe with the use of an integrator (large capacitor).

Sound like a great RIMS build. I'm looking for one in the near future too.

 

[Mannelito]

I have no clue, never saw anything about that, perhaps someone can comment on that.

As far as I figured, going with a Fotek 40DD (DC-DC) SSR would be enough. Hopefully I'm not wrong, I'm going to be scavenging for info about that now...

 

[IslandLizard]

How are you regulating the temp in the RIMS or mash tun? With a PID/SSR combo or a manual on/off switch?

 

[Mannelito]

I'm going with an Arduino that sends a logic high to the element SSR when needed. The temp is measured with a DS18B20. Both SSRs accept DC control signals. The pump SSR is probably going to be on for the entire brewday as it is a vessel in vessel RIMS, no PWM.

 

[IslandLizard]

What advantage does the Arduino give you over a PID? Run advanced mash step-up schedules?

You can put a restrictor valve on the output port of the pump to control flow.
What is a vessel in vessel RIMS?

 

[Mannelito]

PID-wise, not much perhaps, both can monitor temp,time and connect to SSR. But as the arduino works as a smaller computer you have the possibility to log data, save brew programs, calculate necessary pre-heat times, remote brewing and whatnot. You can check out BrewPi for more information, they have a similar system with more comprehensive descriptions.

Doing it on my own gives me the possibility to tailor my own menus, pre-config settings, work flow and other parts too.

The vessel in vessel is simply but a BIAB RIMS but with a pot instead of a bag. I got the inspiration from BrauBushka and modified it slightly to fit my needs.

 

[srice]

I have no clue, never saw anything about that, perhaps someone can comment on that.

As far as I figured, going with a Fotek 40DD (DC-DC) SSR would be enough. Hopefully I'm not wrong, I'm going to be scavenging for info about that now...

The pump will run just fine off that relay. It isn't the cheapest method of running the pump, but it will work. If using an Arduino, I would have picked something like this - http://www.ebay.com/itm/DC-5V-Relay...509?pt=LH_DefaultDomain_0&hash=item27e4a0a3e5

 

[Mannelito]

The pump will run just fine off that relay. It isn't the cheapest method of running the pump, but it will work. If using an Arduino, I would have picked something like this - http://www.ebay.com/itm/DC-5V-Relay...509?pt=LH_DefaultDomain_0&hash=item27e4a0a3e5

But it clicks and even though no-one will see it, the SSRs are more symmetrical now. Vanity and stupidity costs money .