Just 2 Questions about a Schematic Online - Page 1
Forum section: electronics / projects

author: kolbep
author_rank: Frequent Contributor
timestamp: December 06, 2014, 06:03:32 am
content: Hi All.I am trying to build this DMX/Analogue Phase Controlled Dimmer found online (the link is below the schematic).I have just got some questions :1) R2, The Current limiter / Pullup for teh BC547 Transistor in the Zero Cross Detection :The schematic has it as a 4.7 (Which I presume is R),Now the way I see it is that, when the transistor is activated, it can allow 5v/4.7R = 1.06A to flow through the transistor (and also draw that much from the regulator).Shouldn't it be a higher value. If so, will 4k7 work?2) The capacitors on the regulators outputs are 0.1uF (which corresponds with the datasheet),but there is no capacitors on the input of the Regulators. Shouldn't there be a bulk capacitance (100uf or so) before the 7812 regulator?3) What decoupling capacitors!! (I will stick some in, just in case)Lets hope this one works...http://www.epanorama.net/digipack/

author: miceuz
author_rank: Frequent Contributor
timestamp: December 06, 2014, 07:11:25 am
content: The value for R2 is clearly an error. And you are right about adding some bigger electrolytic caps and caps for decoupling. No other stuff I could spot to be wrong.

author: mij59
author_rank: Frequent Contributor
timestamp: December 06, 2014, 08:17:39 am
content: Hi,You can leave out D27, it will never turn on, if you worry about negative going spikes use a 1N4148.The 12 VAC input voltage is critical for the 7812 regulator, its needs at least 14,5 V to regulate.

author: void_error
author_rank: Frequent Contributor
timestamp: December 06, 2014, 09:19:45 am
content: It's definately missing some filter caps... 100-470uF (ballpark figure, no idea about the current drawn by the circuit) before the 7812 and 100nF ceramic caps across everything that's IC power supply pins.The B1-D13 configuration is quite wasteful, I would have personally gone with B1 positive tied directly to the 7812 & filter cap, disconnect R3 and use two 1N4148s as half of a bridge rectifier (AC1-R3, AC2-R3)connected to R3 (AC1-diode1-R3, AC2-diode2-R3). It's an extra diode but you won't have an extra 0.5-0.7V dropped across D13 making sure the 7812 is within its dropout voltage limits.

author: kolbep
author_rank: Frequent Contributor
timestamp: December 06, 2014, 08:02:07 pm
content: Well, that sucks.something went wrong with the 7812. Overheated like crazy. Was giving the full 17v out.replaced it. The load measured was 20mA. Now something on the zc circuit is shorted....

author: Gall
author_rank: Frequent Contributor
timestamp: December 07, 2014, 11:35:38 am
content: The whole zero cross detector is a crap. The base voltage of the transistor is limited incorrectly. It would work, but D27 does not limit the base voltage at all.The correct circuit would have an additional resistor divider between D27 and Q8.

author: rob77
author_rank: Super Contributor
timestamp: December 07, 2014, 01:42:46 pm
content: Quote from: kolbep on December 06, 2014, 08:02:07 pmi bet it started to oscillate and killed itselfplace the filter caps between the kathode of D13 and input of 7812 , also put a 100nF ceramic to ground there.if you place the missing filter cap before D13 (anode of D13), then the zero cross detection will not work. the zener D27 is pointless - just remove it.

author: dannyf
author_rank: Super Contributor
timestamp: December 07, 2014, 02:22:50 pm
content: QuoteDepends. This kind of arrangement is common among zero cross detectors - they will not work with bulk capacitors.The place to place a bulk capacitor is between D13 and IC2's input.

author: Richard Crowley
author_rank: Super Contributor
timestamp: December 07, 2014, 03:55:35 pm
content: Clearly, the main, bulk power filter capacitor is completely missing.I would put at least 500uF @ 20V, with the + side connected to the node between D13 and IC2, and, of course - to gnd.This looks like a faulty "transcription" of the original circuit diagram.Either done by someone with no electronics experience, or not proof-read before publication.The circuit description explicitly mentions power filtering, but it is not present in the circuit.I generally expect better of Epanorama, but he has so much stuff out there, it is probably hard to do proper quality-control anymore.

author: mij59
author_rank: Frequent Contributor
timestamp: December 08, 2014, 03:56:51 am
content: If your looking for a better zero crossing detectorhttp://www.dextrel.net/diyzerocrosser.htm

author: kolbep
author_rank: Frequent Contributor
timestamp: December 08, 2014, 03:57:05 am
content: Now that sucks.the circuit was working fine, 100%.But then something happened. Now my pic is toasty (and shorting out the power rails) and my rs485 converter has died...this was when I tried reprogramming (yes I do have the diode and resistor between mclr and +ve....maybe I plugged the icsp header in backwards....now to order another pic (as well as the extra triacs I need)...

author: kolbep
author_rank: Frequent Contributor
timestamp: December 15, 2014, 06:16:07 am
content: Well, The circuit on Epanorama.net worked great.Built up a 8 Channel dimmer using it.Used 6 Channels to run Halogen lighting for a Playschool concert on Saturday.2 Channels were each running 1Kw of QI Halogen lighting. The other 4 channels were each running 500w.There were a few glitches (but that is probably because I was driving it through 2x Runs of Microphone Cable (about 40m Total).All this was running off a 2.5mm^2 Cable and Decent rubber plugtop (plugged into a MES socket. Needless to say the socket is not the best. It was nice and toasty several hours later when the concert was over.I made up an OpenDMX dongle by butchering a car diagnoistics interface that had the FT232RL chip in it. I just cut the tracks in it (that go to the pic in the interface, and then deadbugged a MAX485 chip on it to provide the DMX signal.Next steps is to get (or build) a better interface.Thanks for all your help and suggestions....P