Reverse Engineering Techniques and Project - Page 1
Forum section: electronics / projects

author: jpc6204
author_rank: Contributor
timestamp: April 15, 2012, 09:28:13 pm
content: I've been following the EEVblog videos and forums for some time now and, before I make my first post, I just wanna say many thanks to all of those who have contributed to this rich collection of information.I have this little black box (literally, see pictures) with 7 wires coming out of it. I thought that I could learn quite a bit by opening it up, tracing out the PCB (see attached schematic), trying to figure out what it is, and then reverse engineering it such that I can make my own. This may seem pointless to some, but for me it's a good learning experience.I think this is a 24V power supply with a relay that can be turned on and off via a signal line. I'll be curious to see what more experienced folks here think. when I hook it up to AC 120V I get 24V on the output red and black wires.I have several questions:1. What tips do you have for someone doing a reverse engineering project like this?2. Are there specific techniques used for determining what some of these surface mount chips are?3. What do you think Q1 and Q2 are?4. What electrical measurement should I make to determine what Q1 and Q2 are?I believe that in order to get the resistor measurements, I will need to de-solder them from the board before I measure them. Luckily the caps are labeled so figuring them out is simple. I'm assuming the transformer puts out under 50 V on the secondaries (the caps are rated for 50 V) and I will be able to use a 1N4001 diode bridge configuration (these diodes are rated for 50 V reverse bias). I'm pretty confident Q3 is a LM317L, as I used the label to ID the part and the data sheet corresponds with how the this is being used on the board. My basic difficulty is determining what Q1 and Q2 are. Transistors possibly? I have no idea what I would buy from Digikey to mimic these. Any ideas?Jeff...

author: Spikee
author_rank: Frequent Contributor
timestamp: April 15, 2012, 09:33:56 pm
content: Here as an electrician Q usually means (high power) relay.Don't know if it will help you ...

author: jpc6204
author_rank: Contributor
timestamp: April 15, 2012, 10:00:29 pm
content: Here is an attempt to make the schematic upright.Thanks for the response Spikee. Q1 and Q2 are chips on the PCB that are too small to be relays.Jeff...

author: dcel
author_rank: Regular Contributor
timestamp: April 15, 2012, 10:07:54 pm
content: "Q#" on a pcb ususlly denotes a transistor. Although the type of transistor is anyones guess. Are there any numbers on the transistors?Chris

author: Spikee
author_rank: Frequent Contributor
timestamp: April 15, 2012, 11:07:56 pm
content: There are dip sized relayseven smd ones at digikey :O

author: jpc6204
author_rank: Contributor
timestamp: April 15, 2012, 11:18:27 pm
content: Quote from: dcel on April 15, 2012, 10:07:54 pmon each chip "1BW" is printed in the center and then, towards the edge still on the front surface, "19" is printed in very small font. The surface mount chip measures about 1 x 2.75 mm and has three leads.Jeff...

author: amspire
author_rank: Super Contributor
timestamp: April 16, 2012, 12:17:01 am
content: Q3 definitely looks like a LM317L. Pins 2,3,6 and 7 are all the output (shorted together). The circuit makes more sense if you show pins 2 and 3 connected to pins 5 and 6. Pins 5 and 8 are not connected, so this board just connects them to the output for convenience.Q1 and Q2 are general purpose transistors like 2N2222. Q2 is limiting the output current (emitter on neg rail, base via resistor to neg output, collector connected to the LM317 adjust input. Basically when the current reaches a point when the voltage across r23/r25 reaches about 0.65V, Q2 starts to turn on pulling the sense input of the LM317 lower. The output diode is interesting. It does two things, It stops a voltage on the output terminals getting back into the supply, particularly is the supply is off. Maybe more importantly, it adds a diode drop between the regulator output and the output pins. When the circuit is in current limit, you get another diode drop across R23/R25 so that means if there is a dead short on the output, the voltage regulator output will be at about 1.3 V, which means it is still regulating. So the output current means the current limiting will still work with a short on the output. Without the diode, the current limiting would not work properly with a short.The output diode does mean the regulation on the output is not great, so the designer was not bothered if the 24V output varied by a few hundred millivolts.It is almost like a battery charger, but I would expect to see a different voltage. Perhaps a mains supply for equipment that normally runs of a 24V battery.Q1 is connected with the emitter to the negative rail, and the collector to the relay coil. Once you have connected the LM317 output pins in your diagramOverall, you have pretty much got the circuit correct.Richard.

author: jpc6204
author_rank: Contributor
timestamp: April 17, 2012, 02:56:10 am
content: Thanks Richard. I've made the changes and have started breadboarding. You're right about the output diode - pretty clever!Jeff...

author: pickle9000
author_rank: Super Contributor
timestamp: April 17, 2012, 04:17:26 am
content: Although not much use in this case many Wellon device programmers have basic chip identification for 4000 and 74 series logic (not to mention roms and micros). You can put in your own pattern detection as well for unsupported chips. I find it very useful. It will give false positives (5 chips instead of one) but really narrows down the choices. It's also technically possible to fry a chip, delicate op amps in particular. But in my case I usually have multiple boards to work with. they are very old industrial controllers from companies that no longer exist. It's very satisfying to bring old stuff back from the dead and put it back in service.The programmers run from 100-600 us dollars and even the 100 dollar one can do the chip ident stuff. Just make sure you look in the manual before ordering to make sure it has the functionality. Also it is a very nice device programmer, constantly updated software. The software is made in Canada (at least for now) and is very good....mike