10 replies to this topic

[Coding Smackdown]

I'm in the process of building an Electric Homebrewing system and I am planning on using the Netduino as the process controller. This means I'll need to use the outputs from the Netduino to turn on and off 120V and 240V AC devices, mostly motors and heating elements based on temperature readings. I've built the control interface and have the temperature monitoring circuit and code working just fine, now I'm ready to build out the power switching circuits and add the code to control the power to the devices. I've purchased some nice T9AS5D22-12 30 Amp 240V AC board mount relays, but was wondering if this was overkill. I'm needing to power one 120V 15 AMP Pump and one 5500W 240V electric heating element. Could I get away with some heavy duty power switching transistors or would it be safer to stick with the relays? I've looked at the BrewTroller boards which are built for the Arduino, but can't quite figure out what they are using to perform the switching. Any ideas what would work best? Thanks

[Magpie]

Definitely a relay for the motor. High current inductive loads will cause problems on a lot a solid state switches. If it's high power and not being turned on and off more than once every few seconds then relays are nice. They give isolation as well as simplicity. For the element you may as well use a relay unless you want to do some really fancy control. By fancy control I mean, If you want to be more accurate you could rectify the mains and run dc into the heater via a fet. Then you could use pwm from the netduino to give more accurate heat control. Just be careful about over heating the element, as you will more average power through the element by converting it to DC and running it at 100% duty cycle. A third option for the heater is a Solid State Relay, they use less holding current so are good for minimising power when holding on light loads. For your case I wouldn't bother.

[Mario Vernari]

Definitely a relay for the motor.

High current inductive loads will cause problems on a lot a solid state switches.

Any inductive load could cause problems on relays as well, such as sparks and consequently contact "glueing".

However, I agree that the relays are the easiest solution. BTW, I seriously consider the SSR (=solid state relay), which basically are a triac and a opto-coupler.

Absolutely I won't consider rectifying the mains: complex, useless and wastes energy without any valid reason.
Cheers

[Magpie]

complex, useless and wastes energy without any valid reason.

Ok I'll admit it's not a realistic alternative, but it's in the spirit of brewing your own beer with a 15 amp pump and a 5kw heating element.
And I'm sure it would be a learning journey too.

[Mario Vernari]

Ok I'll admit it's not a realistic alternative, but it's in the spirit of brewing your own beer with a 15 amp pump and a 5kw heating element.
And I'm sure it would be a learning journey too.

Oh, that's for sure!
Cheers

[Coding Smackdown]

Magpie, You are right about it being in the spirit of brewing your own beer. If I'm too damn stubborn to buy my beer off a shelf at the bottle store, why would I not make my own components. I appreciate the good advise. I do not plan on cycling the pump on and off, it will be on for 60 to 90 minutes when I recirculate the mash and then on for 20 to 30 minutes when I use it to push cold water to cool the wort. However I believe the heating element will be cycled on and off quite a bit as I try to maintain a constant temperature between two thermocouples during the mash which lasts for 60 to 90 minutes. During the boil the heating element will be on all the time which could go as long as a couple of hours.

[Paul Newton]

I have come accross mains extension leads in the past that have a relay built in that is operated by 12 or 5V. I would favour something like that because all the dangerous voltages have been encapsulated and isolated making it less likely to end up with a trip to casualty! Sorry - but I don't have a recent link to hand, so I can't give specifics. Paul

[Fred]

I switch mains lighting from my Netduino and went with relays. Obviously you'll need a transistor and flyback diode to protect the Netduino. This is standard relay driving - a quick Google should show you a diagram. I had problems with flourescent light welding the relay contacts and used a varistor across the relay contacts to prevent this. It's fairly similar to how you protect the Netduino from the relay kickback, but protects the relay contacts from arcing caused by the flourescent as it's switched off.

[Magpie]

You could just start with the relays and see how they go. Here's a link for the schematic for driving the relay http://www.arduino.c...ning/relays.pdf I think you can get away with the relays, even for heat control because of the high Specific heat of water. In other words the temperature of a large vat of half cooked beer isn't going to change that fast. Your relays seem to have a mechanical lifespan of 10 million switches ,so you can switch fairly quickly for a long time before your relays die. You may as well start with this because you already have the relays. If you did need finer control and wanted to go the way of the Fet one day, then you would need a rectifier and a few decent bjts and a low rds n channel enhancement mosfet. + resistors. Basically you are putting a light dimmer on your heater. There is a fair bit more to learn. But it is actually nearly as efficient, and gives very fine responsive control. In some ways it is a bit safer too because to power is rectified to DC, and putting mains power AC near water is always a bit unsafe. I looked at the spec sheet it said that the relay contacts were only 20A, not 30A, for the pump I think ideally you would use an more oversized relay to help cope with the inductive load, 20A may not be enough. Also When wiring up the contacts to the relays use at least 1mm2 wire, dont use pcb tracks.

[Coding Smackdown]

Magpie, I actually had drawn up my original circuit based on that exact diagram, I guess I got a little carried away thinking that maybe I was being too cautious. But looking at the specs on the Brew Trollers, their relays are only handling 10A at 240V, so my 30A at 240V should be more than safe enough. Now I just have to spend a fortune on water tight connectors and all of the rest of the hardware. The money I spend on my toys is outrageous!

[Magpie]

To quote George Best, the soccer player.

I spent a lot of money on booze, birds and fast cars. The rest I just squandered.