Help with schematic needed!! - Page 1
Forum section: electronics / beginners

author: deltaman
author_rank: Newbie
timestamp: March 07, 2013, 09:04:06 pm
content: I am very new electronics and trying to learn it hands-on way.I have hit a wallas I think the following schematic needs some tweaks to workor even if it works its not very efficient.The full article link :http://www.instructables.com/id/Arduino-controlled-light-dimmer-The-circuit/CS004LJHDYZBB20The schematic is attached with the post.I have a newbie question, if somebody could answer that . In the schematic the 240 v DC is directly fed to the 4n25 opto isolator, my question is isn't this voltage too much for the 4n25 opto - isolator? I checked the datasheet for the 4n25 and the Max forward voltage is 1.5 v and also the Max current allowed is 60mA. Shouldn't we be using some current limiting resistor and also bring down the voltage to the allowed Max .Also are there any alternatives to this?One of the options could be use a step-down transformer to bring down the voltage , please reply with ur suggestions and comments.And also its feels great to a part of EEVBlog and I admire the Crazy Aussie bloke.Cheers MateThanks in advance

author: Kremmen
author_rank: Super Contributor
timestamp: March 07, 2013, 09:19:15 pm
content: Actually, the opto diode won't be subjected to full voltage. There are the two 30k resistors that limit current / drop the voltage. Actually the opto diode is only forward biased after the rectifier bridge. The voltage over the forward biased diode is only a volt or so. The rest is dropped over the resistors. The diode current will be only 4 mA RMS which is OK.

author: MikeK
author_rank: Super Contributor
timestamp: March 07, 2013, 11:11:10 pm
content: deltaman, you might be confusing voltage and current. "Max forward voltage" refers the the maximum voltage drop that you can expect to see by the diode. The voltage drop is a function of the current going through it, it doesn't care about the applied voltage.You can figure out the current by taking the input voltage, 240V, subtracting the diode voltage drops (1 for the opto, 4 for the bridge) and dividing by the resistance (2x 30K resistors). Like Kremmen said, it's about 4mA, well under the limit.

author: deltaman
author_rank: Newbie
timestamp: March 08, 2013, 04:24:14 pm
content: Thanks a lot for your response MikeK and Kremmen, I am happy to be a part of this wonderful community !!Hope to learn a lot from you guys in the coming days !!

author: MikeK
author_rank: Super Contributor
timestamp: March 08, 2013, 05:40:57 pm
content: I should have said 2 diode drops for the bridge, not 4, since only half of the bridge is ever conducting at one time.

author: deltaman
author_rank: Newbie
timestamp: March 08, 2013, 06:59:20 pm
content: Quote from: MikeK on March 08, 2013, 05:40:57 pmOn the same note the Bridge rectifier could be replaced with 4 diodes connected in a particular fashion, am i correct?Also is there any advantage of using a Bridge rectifier, instead of 4 diodes apart from the package size?Also I am thinking of buying an oscilloscope, would it be of any use for a beginner ?

author: Kremmen
author_rank: Super Contributor
timestamp: March 08, 2013, 08:06:43 pm
content: Yes that is right. 4 diodes is exactly what you have inside a bridge component. The metal packaged bridges are more efficient for cooling, other than that it is mainly size and pinout.

author: firewalker
author_rank: Super Contributor
timestamp: March 08, 2013, 09:32:17 pm
content: Also the diodes inside the bridge are most probably identical (in on die perhaps?).Alexander.

author: Kremmen
author_rank: Super Contributor
timestamp: March 08, 2013, 09:43:13 pm
content: Quote from: deltaman on March 08, 2013, 06:59:20 pmSure, if you build projects where one is useful. OK, that was maybe a bit circular logic, but if you have any ambitions at all and projects where the circuits are other than essentially DC, then yes absolutely. It doesn't have to be a top of the line, a pre-owned older but functional one would not break the piggy bank, yet let you learn all the basics and not-so-basics of scoping a circuit.