Friday, September 5, 2014

Designing a Brew Chamber Controller (Part 7)

I've been busy with another project and work, so here is a quick update on parts research, which I will update as I actually place my parts order.

Most small chest freezers operate in the 5 Amp to 7 Amp range with a start up surge of 2 to 3 times the operating range, so we need to find a switch that will handle the surge current for 10 seconds then handle twice the operating current. This keeps the heat generated by the switch low and extends the life of the switch.  The switch also has to be able to be turned on by the 3.3VDC rail of the Beaglebone Black (BBB).

The heat source I am using will be a 40W terrarium heat lamp in a paint can.  Double 40W is 80W and the amperage at 120V is 0.66A, rounded up to 1A. So we'll need a switch that will handle 1A for the heater. This switch will also need to be operated by 3.3VDC from the BBB.

Since we need to be able to operated the switches with the BBB 3.3VDC rail and handle the surge/operating current of the freezer/heater, we need to find a device that will meet those specifications.

Solid State Relays (SSR) fit that bill.  They can operate on a control voltage as low as 2VDC (look for the "must turn on voltage") and handle current up to 100 amps or more.  These can be expensive, so we need to size it accordingly so we operate it at a comfortable level to extend the life of the relay yet keep the price down as much as possible.

If you don't mind spending the extra money then get 2 of the 10A SSRs. That way if the freezer relay does go bad you can swap the heater relay over for a quick fix while ordering a new relay.  I can't see that happening any time soon if you size the SSR correctly though.

One other thing to look for in your research is operating temperature and whether the relay needs a heat-sink to maintain safe operating temperatures under normal operating conditions.  The heat-sink should be aluminum and could be the box/container used to house the project. The aluminum box needs to be connected to the safety ground wire in case there is a short in one of the relays that produces voltage on the heat-sink.

Controller parts research:

Solid State Relay for Freezer:
  • 3V min DC input (will be switched with BBB 3.3V rail supply)
  • 10A output rating, will need to handle 20A for 10 seconds of start up current
  • 208VAC rated, will run at 110-120VAC 
  • Can be found for $30 - $40 (Under $10 on eBay from China)
Solid State Relay for Heater:
  • 3V min DC input (will be switched with BBB 3.3V rail supply)
  • Should handle switching switching a 40W light bulb. Double for safety 80W 0.66A
  • 208VAC rated, will run at 110-120VAC
  • Can be found for less than $20 (Under $10 on eBay from China)

[edit 14SEP2014]
Alternatively you can get one of these: if you want something that is plug and play.  It has a 3V to 12V control voltage range so it should work with the BBB. By the time you buy all the parts separately you will spend more than the cost of this device.

Misc Parts:
  • 110VAC 15Amp 2 outlet receptacle - 1 wired to the 10A relay the other wired to the 1A relay
  • 3 strand 15Amp electrical wire to wire everything together
  • 2 - 110V 15Amp plugs
  • New 1 gallon paint can with lid
  • Light bulb socket to mount to paint can lid
  • Aluminum enclosure to house the project


  1. Hi, thanks for tihis article. Very interesting stuff for me. Do you have any schema of this project and if so, can you publish it? Thanks again!

  2. This project is being designed as I go and no schematic has been designed as of yet. There are some simple schematics in the 3rd post on the series to light LEDs etc.