PDA

View Full Version : Great article! Question


newkie
07/03/2002, 06:40 PM
Jon, wow your Aquarium Electrical Systems article was fantastic. I learned something and was particularly interested in your low water top off system. I have designed something similar: http://www.livinglava.com/diy_water_level_alarm.htm as well as a float switch: http://www.livinglava.com/diy_float_switch.htm.

What I really liked about your design is the use of a Full Wave Bridge Rectifier. That is much cleaner than using an additional transformer laying around the house and putting that bulky thing in the case. I would love to update my designs but I don't fully understand how it works.

http://server108.hypermart.net/nelniech/nelniech/br8d.jpg
The above is sold as Full Wave Bridge 2AMP 100PRV for $.39. From the image is appears the middle two pins are for AC. Does it make a difference which goes to the hot wire? What does 100PRV stand for... about 100volts? So if I want 12volts DC, how do I calculate what resistance I use? I'm familiar with V=IR but the result sounds wrong. Would you mind giving me rectifier 101?

Snailman
07/03/2002, 10:22 PM
I believe PRV is Peak Reverse Voltage. This means that it is not rated for line voltage. Two of the pins are for the AC line and it does not matter which wire (hot or neutral) goes to which one because they both have back to back diodes on them. The resistance depends on the voltage adn current the solid state relay needs to fire. Most of these relays run 3 - 30 VDC so it is better to keep the system closer to the 3 VDC end so the voltage on the float switch is low. Yes I have seen your designs before and I like you am not wild about the wall warts. HTH

newkie
07/08/2002, 12:21 PM
So this particular example wouldn't work? Can you point me to where I can order ones that will? What ratings should it have? How can I make sure its rated for line voltage? Can you point me to the source and model?

And again, how do I calculate the resistance to get the correct DC voltage? (The relays in my design trigger at 12vDC.)

Thanks!

Snailman
07/10/2002, 08:27 PM
The suppliers in the article adn in [DIY] as well as Radio Shack should carry a number of full wave bridges that are rated for 120 VAC. You are right about E = I * R so R = E / I and I = E / R so the voltage will be around 140 VDC and the relay input circuit draws x ma so R = 140 / x. HTH