Interrupting a Larger Power Supply (how can i do this?)

HobbyCoder

Hello, I am a hobby coder and had a good experience with the basic stamp. I am now about to purchase a Propeller Start Kit.

I would like to know if (and how) it is possible to use the propeller to interrupt an electrical path.

In my case the propeller board would act as a power interruption unit for some devices that are downstream, so you would have the electricity some how passing through the grasp of the propeller then when I needed to I could cut off the power so the devices that are 'downstream' of the propeller would lose their electricity supply.

Consider the amount of electricity that of an average (american) household.

I considered using solenoids but could find very little info on solenoids other than solenoids used in water valves.


Does any one have information pertaining to this?

(as an extra question, would it be possible to measure the amount of electricity that is passing through?)

Mike Green

What you want is to use a relay (look up the term in the Wikipedia). This is a combination of a solenoid and an electrical switch. The Propeller is fairly limited in what it can control directly and most relays require more current to operate than the Propeller can supply. Typically, the Propeller controls a transistor and the transistor operates the relay. There's a Nuts and Volts Column on the subject in the context of the Stamps, but the same idea applies to the Propeller (www.parallax.com/Portals/0/Downloads/docs/cols/nv/vol1/col/nv6.pdf). There are solid state relays (SSRs) that can be controlled directly by a Propeller. Usually these have an internal LED that the Propeller controls and a light sensitive switch, usually a thyristor, that controls the power. Crydom is one manufacturer. Look at their website for ideas.

You can measure the current in a circuit by putting a small resistor in series and measuring the voltage across the resistor. Exactly how depends on the voltages and currents involved.

HobbyCoder

Ok, now for another question. I have drawn a rough diagram of my intended setup. Which (http://www.crydom.com/products/productFamilyList.aspx?idProductLine=2) should I buy?

Mike Green

If you are controlling two LEDs, you don't need relays as shown. Before you go any further, how about describing in some detail just what you want to control (AC or DC, voltage, current)? It's impossible to advise you further without some details.

HobbyCoder

The purpose of controlling the LEDs is to demonstrate a larger scale application. Right now my long-term goal for this is to be a model of a propeller based energy management system that would measure what uses the most electricity in the house and when it uses that electricity; also to be able to kill power to any part of the house's internal 'grid' during certain times of day (to save energy). The LEDs are examples, I may in the end demonstrate it working with other various electronics. The propeller would manage a screen that would let the user set these things and learn the facts about the electricity. I believe that is a sufficient explanation for the existence of the relays. Does anyone know of which relays I would need for a 9-volt battery's electricity passing through the system? (The goal of this is to be used as my science fair project, but I need help knowing which relays to get as I don't know that much about relays and am learning about electronics with this project. So help is greatly appreciated.) Also, the reason for using the propeller is so that most everything can be managed in its own thread (video out, keyboard in, power cut off management, power measurement, etc); without need for using crude methods of basic stamp type multitasking.

Post Edited (HobbyCoder) : 11/24/2008 3:26:08 AM GMT

Mike Green

The choice of relay is very different if you're controlling an LED or a standard US home power circuit (120VAC at up to 15A) or an appliance circuit like for an electric stove or drier (240VAC at 30A). Perhaps you should consider the X-10 home power control system. There are all sorts of modules that can either be plugged into a wall socket to control a lamp or can be permanently wired into a junction box to replace a light switch or outlet including an appliance outlet. These have a uniform control mechanism and there's a computer interface that can be controlled by a Stamp (see the XOUT statement chapter in the Stamp Manual). There are also safe methods to measure the AC power drawn by an appliance using what's called a clamp-on ammeter

See: www.x10.com/automation/x10_pl513.htm (This is the transmit-only version which works fine with the Stamps)

And: www.x10.com/products/x10_ck11a_ci.htm (This is the new version of the TW-513 for transmit and receive. I'm not sure if it works with the Stamps)

Some people have been experimenting with Propeller and the CK11A, but I don't remember how far they've gotten with it.

Post Edited (Mike Green) : 11/24/2008 3:44:06 AM GMT

HobbyCoder

So... which relay would be appropriate for this demonstration (if I were to follow my given diagram)? (I don't need a finished product or that type of thing) I need to build my own from scratch.

Mike Green

Just to make sure, you want to control two relays with a Propeller and those relays each need to be capable of handling the current for one standard LED running off a 9V battery?

Mike Green

Here's the page from the DigiKey catalog for some relays. The one mentioned (their Z622-ND) will work directly off a Propeller I/O pin and can switch up to about 30VDC at 0.5A. You need the diode. A 1N914 will work or any small silicon switching or power diode will also work. The drawing shows the basic circuit for each relay. The advantage of reed relays is they're small and low powered. This one will work with 3V across the coil and only 12mA of coil current. The Propeller can only furnish about 3V and only up to about 30mA with limits on the total current for the Propeller. If you want a higher current or higher voltage relay coil, you'll need to use some kind of switching transistor (see: www.parallax.com/Portals/0/Downloads/docs/cols/nv/vol1/col/nv6.pdf).