TOKELLY posted:The rectifier changes the track voltage which is 18 VAC to 18 VDC.
The alternating current voltage is typically measured as an RMS value, that is, using a root mean square that "averages" the voltage of positive and negative swings. A rectifier converts AC voltage to DC, but that is a combination of positive and negative peaks that ends being 1.414 times greater than the AC RMS value. That means that 18VAC ends up being nearly 25.5 volts DC out of the rectifier. That increase means that one must carefully choose a big enough resistor to restrict that current flowing to the LEDs at the higher voltage.
I was fooling around with some more sparky stuff the other day and remembered this post with the focus on the highlighted sentence in red above.
I had another of the rectifiers I had used in the PE LED strip circuit so I decided to test the statement to learn something. I used an 8B American Flyer transformer and tested the output voltage for the accessory post and base post. It measures 16.7 volts.....AC.
I hooked leads from those posts to the two center posts on this rectifier which is the one I used in the conversions.
I then tested the DC output on the two outer legs noting the polarity.
Well it tested at 15.3 VDC........not a higher value of 23.6 VDC as would be expected from the multiplier shown above. My buddy , an electrical engineer, whom I call "Professor Electron" told me that while the RMS value could result in a higher DC output than the AC input in theory the test shows that not to be the case in reality. The voltage drop was due to the action of the rectifier's diodes. I think he said that the average diode drops the voltage about 0.7 volts. This rectifier is a full wave bridge rectifier with four(?) internal diodes that work in pairs (?). Each pair drops the voltage 0.7 volts. So it drops 1.4 volts in total. Too deep for me.
No big whup. I did learn though to always verify all voltages by direct testing to avoid any surprises. Since my circuit sends the rectified voltage into a 9VDC regulator it didn't really matter in the end...the lights get 9VDC and it stays constant.
I find all this stuff fascinating, theory and all, but to me what counts is "Does it work?" None of this is meant to start an in-depth discussion about electricity and all the associated formulas and theories. It's just an observation.
Mark
