Transistor schematic symbol query - Page 1
Forum section: electronics / beginners

author: seb1982
author_rank: Regular Contributor
timestamp: February 10, 2013, 03:50:44 am
content: Hi guys and gals!I stumbled on Dave's YouTube channel a few days ago - what a find! I'm just getting back into electronics after not doing any since school, so I'm very rusty at the moment!I'm about to make a variable bench power supply as my first project, and I was looking at a few circuit diagrams for ideas. I found one which contained a transistor with a symbol I hadn't seen before. The transistor is an SK100, and the symbol consists of the normal "k" in a circle, but the circle has multiple lines radiating out around it, like round a cartoon light bulb.I was just wondering what that symbol meant as regards the transistor, and why it's different to a normal pnp transistor.The circuit diagram is athttp://www.wiringcircuit.com/powersupply/4_Output_Stage__5V_6V_9V_12V__Stabilized_DC_Power_Supply_1361.htmlif it helps, and the transistor is T2 in the bottom right hand corner.Thanks a lot!Simon*Edited for unforgivable spelling!

author: Moshly
author_rank: Regular Contributor
timestamp: February 10, 2013, 04:04:23 am
content: It is an unusual symbol.first thought was maybe a photo-transistor but after analyzing the function, it will be carrying a bit of current so the radiating lines must be a heatsink.

author: David_AVD
author_rank: Super Contributor
timestamp: February 10, 2013, 04:12:55 am
content: My guess is that the radiating lines represent that it's on a heatsink. (radiating heat)

author: free_electron
author_rank: Super Contributor
timestamp: February 10, 2013, 05:42:39 am
content: Heatsink

author: Bored@Work
author_rank: Super Contributor
timestamp: February 10, 2013, 05:46:29 am
content: http://www.progressive-concepts.com/info/item.php?id=211

author: c4757p
author_rank: Super Contributor
timestamp: February 10, 2013, 06:05:46 am
content: Ugh... that diagram sets off my million OCD-ish pet peeves all in one...The transformer is X1 (not T1), the transistors are T (not Q), labeling is inconsistent (they say "D1-D3 = 1N4007" at the bottom, do not label D1 and D2 individually, butdolabel D3), the LEDs are "LED" (not just D), IC not U, everything but the voltmeter has a reference, every LED is labelled differently (two are labeled by color, one by purpose, two are white on black, one is black on white), the bicolor LED symbol looks like a TVS diode with funny marks above it, there is a heat sink but they do not say what kind, "O/P" is a stupid abbreviation for "output" when they could just write "output", C3 and C4 have messed-up symbols, a pinout is given for BC548 but not SK100, blah blah blah... Not to mention the design problems: SK100 is only rated for 500mA yet they fused it at 1A, theyfused a power supply at 1A(seriously, it would be so easy to accidentally blow that fuse), it appears to have no overcurrent protection - just a damn buzzer - but a crude overcurrent circuit would be just as easy to implement as the buzzer, they appear to do nothing to compensate for the voltage lost in D3 and T2, they are using a crapload of individual voltage regulators when they could have just used one adjustable reg and switchable set resistors, etc... In Dave's voice: "WHY?!"Sorry, I'm feeling picky today.Edit: Oh, and the biggest WTF: they take a 18VAC-output transformer through a rectifier straight to the output labelled "18V". Um... that's not 18V. That's 25Vnominal(well, actually about 22-23V with the couple of diode drops and T2), subject to mains variation and some pretty nasty ripple under load.

author: David_AVD
author_rank: Super Contributor
timestamp: February 10, 2013, 07:34:58 am
content: Quote from: c4757p on February 10, 2013, 06:05:46 amMy guess is that resizing of the diagram image has removed part of those two capacitor symbols. Have seen it happen before.

author: c4757p
author_rank: Super Contributor
timestamp: February 10, 2013, 07:37:55 am
content: Ha! You must have read pretty much that whole rant then - I'm so sorry to put you through that!

author: vk3yedotcom
author_rank: Frequent Contributor
timestamp: February 10, 2013, 07:52:57 am
content: Quote from: seb1982 on February 10, 2013, 03:50:44 amSimon - for a no-frills regulated variable power supply I'd suggest an LM317 circuit.Along the lines ofhttp://www.eleccircuit.com/lm317-linear-power-supply-regulator-selector-15v3v45v5v6v9v-15a/orhttp://www.electronicecircuits.com/electronic-circuits/lm-317-power-supply

author: ciccio
author_rank: Frequent Contributor
timestamp: February 10, 2013, 08:10:46 am
content: Quote from: David_AVD on February 10, 2013, 04:12:55 amI agree. Itmustbe on a an heatsink.Anyway, the schematic is a little confusing, because of "volubile labeling".Best regards

author: David_AVD
author_rank: Super Contributor
timestamp: February 10, 2013, 10:04:07 am
content: Quote from: c4757p on February 10, 2013, 07:37:55 amYeah, I was amused (and a little saddened) that you were so picky about that circuit.

author: vk6zgo
author_rank: Super Contributor
timestamp: February 10, 2013, 12:36:19 pm
content: Quote from: Moshly on February 10, 2013, 04:04:23 amYes,very common symbol for "heatsink" on both Brit & US circuit drawings of the '70s & '80s.

author: lewis
author_rank: Frequent Contributor
timestamp: February 10, 2013, 03:35:46 pm
content: Quote from: c4757p on February 10, 2013, 06:05:46 amAND - there are no decoupling caps on the 9V, 6V and 5V regulators. Dave's voice: 'Bloody dickheads!'

author: c4757p
author_rank: Super Contributor
timestamp: February 10, 2013, 03:57:41 pm
content: For what it's worth, some 78xx datasheets claim the output capacitor is optional.ON Semi has this gem in theirs: "COis not needed for stability; however, it does improve transient response. Values of less than 0.1 uF could cause instability." Er... what? So... the capacitor isn't needed for stability, but not having enough capacitance could cause instability? Is this a datasheet or a riddle?

author: seb1982
author_rank: Regular Contributor
timestamp: February 11, 2013, 07:09:22 pm
content: Brilliant - thanks for all the replies. As Dave would say, ... I like it!I'm now taking a good look at all the (much better) circuits you've kindly linked to. Looks like you saved me a lot of wasted time - thanks ever so much!Simon