A noob question and Introduction - Page 1
Forum section: electronics / beginners

author: DrRunCMD
author_rank: Newbie
timestamp: September 26, 2019, 10:53:49 pm
content: Hello the EEVBLOG forum!I'm Chris Aka DrRunCMD (name on youtube).About me:Off the bat, I am from the UK, unemployed on sick due to diabetes complications etc. Live at home with parents, have kids and am nearly approaching 40! I am not an engineer or pro, I do small repairs and troubleshoot by logical thinking and some knowledge of electronics! I own a crap ton of circuit boards from TV power supplies to XBOX slim 360 boards. All used for spare parts or replacement boards.I mainly do repairs of electronic consumer goods from TVs to PC's Servers etc for friends and family. Not as a paid job.This DC LOAD project was inspiring to me, (thanks Dave), when he discussed the project on youtube. This was and is my first project where I designed the PCB and layout using EasyEDA online from scratch. I looked at the basics of how the DC LOAD circuit works which is simple, and began testing my own with variations of components. I remember having an issue the first time I built the basic circuit. I had a scope back then, the Rigol DS1054Z again was purchased after watching Dave's review lol. I only looked at ringing at the fet gate at turn on and switching anomalies etc. Then I had to sell the scope. Fell on bad times.Im also a PC geek, server/network buff and do some old BASIC programming from time to time learnt from the ZX Spectrum. I own also a BBC Micro, very, very good working condition and I think is a relic and a treasure to own. Not thinking of selling it either. Depends on the price I suppose!I own an EEVBLOG BM235 multimeter, and I love it! Can you buy the cables for the meter separately? I have lost mine at a previous job, now use crap ones!I have a noob question..I have made from scratch 2 DC LOAD circuits back when Dave was discussing as a project. The newest video I saw no #1247 was about signal propagation made me think of this project again. When driving two matched MOSFETs as a pair for power load/power-sharing, I wanted to ask, would the traces from the op-amp driving the fets have to be equal in length to each resistor then from resistor equal length to MOSFET?. The PCB's are 6x4cm, gold plated through-hole. I'm a noob, I told you. ( EXAMPLE ATTACHED IN PIC ).Simply put: For driving MOSFETs with a control device (op-amp) for example, would gate control traces need to be at an equal matched length?Not sure if this is signal propagation, integrity or even impedance, so my apologies if I'm wrong.It just got me thinking about after watching the video.Cheeky mention:If anyone here has a personal, not using scope and would want to kindly donate one, I would be interested. Im in the UK.Thanks, guys!DrRunCMD

author: dazz
author_rank: Frequent Contributor
timestamp: September 26, 2019, 11:19:16 pm
content: I don't think trace length will matter in that application, if I got Dave right, it only matters for clock lines running in the MHz range. But I'm a complete noob too, so take that with a pinch of salt as I might be missing somethingHope things get better for you in the near future, Chris

author: magic
author_rank: Super Contributor
timestamp: September 27, 2019, 06:19:19 am
content: Quote from: DrRunCMD on September 26, 2019, 10:53:49 pmPropagation speed is some 20~30cm/ns. 2cm mismatch is 0.1ns difference. Not a problem for a circuit with a few MHz bandwidth.Now, if you mismatch them by 2km... that could make a difference

author: MosherIV
author_rank: Super Contributor
timestamp: September 27, 2019, 06:48:02 am
content: Hi, welcome to the forum DrRunCmd.QuoteNot quite true. It can make a difference when running signals in parallel. That is why you sometimes see pcb traces in wiggly lines instead of straight lines, to make the trace length the same length.As magic says, will make little difference in this application.

author: AVGresponding
author_rank: Super Contributor
timestamp: September 27, 2019, 01:13:23 pm
content: Still, even if it isn't critical, it's good practice, and looks prettier!

author: DrRunCMD
author_rank: Newbie
timestamp: November 23, 2019, 10:44:42 pm
content: Just an update on my DC load project.I decided to go back to this project as I enjoyed and learned a lot from it.I have cheated a little, by purchasing this circuit on ebay:As is, it is rated for 75W. So I thought I'd test it with a Linear IXYS X class power fet (x1). I think it is the IXFT26N100XHV. Rated for maximum 850 pd(W) and at 1kv voltage. A little overkill but seeing as I was spending money, I'd do it properly.The circuit has nichrome current shunts (x2) which I was worried about at first. My previous dc load had used a huge power resistor with its own heat sink originally!Heatsink choice for the FET, I bought a Fischer Electric 150cm square forced air heat sink with fan rated for 500w. Works well. And the Nichrome shunts are warm at a constant 250W power draw!!!!Fet does get toasty after 5 mins of constant power draw lol. Have a meter measuring V, W and PD using small gauge wires attached to the device under test for a bit of accuracy on the current measuring!I never have used nichrome current shunts before, is this application suitable or not recommended??They have not blown yet!Thanks guys!

author: DrRunCMD
author_rank: Newbie
timestamp: November 23, 2019, 11:02:01 pm
content: *UPDATE*Purchased the DC LOAD from banggood for under £8.00 so, if anyone would be interested in making their own high power DC load, then, this would be a great start:https://www.banggood.com/DC-1V-100V-Constant-Current-Source-Electronic-Load-Board-75W-0-10A-Power-Tester-p-1148591.html?cur_warehouse=CNAs supplied can draw 75W! I can't find the schematic for it. I will post back if I do find one.The picture here shows only 1 current shunt. Not sure if there is a different version of this board or if it now has been updated and has x2 of them?Thanks.

author: magic
author_rank: Super Contributor
timestamp: November 24, 2019, 06:32:52 am
content: The watt ratings are bogus (calculated for rather unrealistic conditions, that is). You need to start looking at thermal resistances instead.