2 questions - equivalent transistors and “creating” a 0v rail - Page 1
Forum section: electronics / beginners

author: Dan123456
author_rank: Regular Contributor
timestamp: September 30, 2023, 04:37:26 am
content: Hey allFirstly, apologies if the 2 questions in one post format is frowned upon here! I figured it was better then spamming new threads and as the old saying goes “it’s better to ask for forgiveness rather than for permission”My first question is, is there a real quick and dirty way of working out if a transistor will work in a circuit for hobby situations?I was searching online and found some fairly useful pages that list about 10 things you should compare and use a transistor with the same or “slightly higher” values. That makes perfect sense for professional work / stuff you want to actually put to work but can that be dumbed down even further for just hobby messing around (I.e. what are the most critical values and what is “slightly higher” - I.e. like max 20%)?Specifically, I am looking to just build a little crapy FM receiver to feed into some audio amps and all the diagrams I have found seem to have obsolete / hard to find transistors listed. On the latest schematic asks for a few BF324’s. Using alltransistors.com to run an equivalency search brings up a whole bunch but the closest ones all seem to be obsolete / hard to come by too!My second question is regarding creating (or more specifically grounding) a 0v rail. I had the idea of creating a variable positive and negative power supply using my 12VAC wall power supply (thank you so much again to everyone who recommended that to me previously! It has been a blast to play with!), a bridge rectifier, a LM317T and a LM337T.However, after looking for schematics, I saw they were all connecting the 0v rail to the AC earth. As far as I understand, my AC power brick only has 2 prongs so doesn’t have an earth.My question is, do I have to connect that 0v rail to earth by running a cable to the earth of a PowerPoint (or more realistically, via the earth pin on my proper lab bench power supply) or is there another way I can do it?Please feel free to tell me I’m an idiot on this oneI tried researching it but came across a whole bunched of new terms from google such as “earth ground” vs “signal ground” vs “digital ground” vs “analog ground” and left more confused than when I started :pIt very well could be the case that I am just not ready for this project yetThanks so much in advance once again all

author: MrAl
author_rank: Super Contributor
timestamp: September 30, 2023, 10:50:56 am
content: Hi,This is probably easy to figure out but you should post a schematic to make sure everyone understands exactly what is needed. There are sometimes subtle differences that need to be addressed and without a schematic these differences may not be understood properly. A simple schematic clears it all up.

author: Dan123456
author_rank: Regular Contributor
timestamp: September 30, 2023, 11:46:16 am
content: Quote from: MrAl on September 30, 2023, 10:50:56 amSorry! That’s a fair point!For the FM radio / receiver, I have 2 versions on hand. The one I was talking about uses the BF324’s and the other uses BF494/495 transistors. Pretty much I am wondering if pretty much any old transistor would “work” in their place (even just poorly - provided you aren’t using a NPN in place of a PNP or something wildly different like that)? Or if you really, really do need to match all the transistors values as closely as humanly possible otherwise the circuit isn’t likely to work at all (and if this is the case, how far “off” can you usually go before you are likely to run into major issues?)?As for the adjustable positive and negative power supply, I was thinking something close to the 3rd picture but using adjustable voltage regs rather than 12V ones. Issue is, it looks like the 0V goes back to the din socket to go to earth via the wall where as I will be using a 2.1mm Jack from my 12VAC wall wort power supply that doesn’t have a ground pin that plugs into the wall (only active and neutral on the plug that goes into the wall).Hope that helps and thanks so much once again

author: Manul
author_rank: Super Contributor
timestamp: September 30, 2023, 01:01:29 pm
content: I suppose first pass of choosing transistors is looking at maximum voltage, current, power. Very dumb, very obvious. Next pass is looking at more specific parameters like Beta, transition frequency, maybe capacitances, noise figure. Generally you want transition frequency to be well above working frequency, otherwise you will not get good performance if any. Beta might be important, but depends on the circuit. Most circuits if well designed should be fairly tolerant to (sensible) transistor substitutions, so don't be afraid to experiment.Regarding the 3rd picture, the 0V is the middle tap of AC transformer secondary, not mains earth. Of course, you may choose to connect that secondary middle tap to mains earth if you wish, but it's not a functional necessity. In one case you get mains earth referenced supply, in another - isolated floating supply. But in any case middle tap is essential for this kind of balanced rectifier which rectifies into positive and negative rails.

author: Ian.M
author_rank: Super Contributor
timestamp: September 30, 2023, 01:35:10 pm
content: For the BF324, look atBF824- Its SMD (SOT23), but other than that is near-identical. How did I find it? Well, put key parameters into a distributer's parametric search (e.g. Mouser), starting with fTand Vce, selecting a range from the desired value to about double it. Also select 'In Stock' and see what pops out. Add parameters one at a time in order of decreasing importance, to reduce the no. of results. When the list gets down to a reasonable number of items (<50), sort by price ascending then start looking for the best match, checking datasheets as distributor parametric search databases aren't entirely trustworthy.As Manul just said, the 0V on the DIN socket probably didn't go back to mains PE 'earth' ground. I'd also bet it was fed by a center tapped transformer and the center tap is '0V'. Each half of the secondary gives 11.5V RMS. The total voltage across the ends would be 23V RMS.I gather you have a 'wallwart' transformer with a single 12V RMS output, so you cant use the balanced bridge rectifier circuit. Instead you need matching positive and negative half-wave rectifiers. In the above circuit delete D3,D4, increase the 1000uF capacitors to 2200uF, and connect your transformer in place of pins 2,3 of the DIN socket. You have to double the capacitors because they are fed with pulses of current at half the frequency the full-wave circuit feeds them at, so need more capacitance for the same amplitude ripple.

author: Dan123456
author_rank: Regular Contributor
timestamp: September 30, 2023, 02:34:09 pm
content: Awesome thanks allIan.M, Thanks for the half bridge idea mate! I am still new to the world of AC and got confused when I remember there is no nice easy positive and negative to work withYeah I tried that in Digikey (I haven’t given Mouser a crack yet! Will have to keep them in mind!) but was filtering out SMD packagesMy hands shake like I’ve been on a week long bender at the best of times so am still just buying little SMD practice boards at this point to try and get my skill up with that stuff before I try and make anything using those bloody tiny little thingsIs there an issue with going significantly above the rating? I.e. I sometime see some that look good but are 50V+ higher on the Vcb value or what have you.Sounds like lower values are a no go (which is pretty much what I expected) but is there an issue with going too much higher?Like let’s just say for arguments sake I was to find a transistor and all the values were double that of the original. Would that be a good option or is it better to find something closer to the original (and if so why? I’m guessing twice as “good” usually means twice the price but would that be the only downside?)?Thanks so much again everyone

author: Ian.M
author_rank: Super Contributor
timestamp: September 30, 2023, 02:52:45 pm
content: Lower values may or may not be a no-go. See the specific circuit for what the transistor is actually being run at (do the math!), then allow generous margins on voltage, current and power. E.g. your BF324 circuit uses a 5V supply, so 15V Vceo transistors would probably work. However, the closer the ratings are to the one you are trying to match, the more similar its likely to be, so the odds of matching other parameters NOT available in the parametric search improve, resulting in less digging through datasheets before you 'get lucky'. Pricing is all over the place, and often depends on popularity rather than performance! e.g. a high sales volume 'jellybean' part may be both cheaper and higher spec. than a niche market legacy part.

author: MrAl
author_rank: Super Contributor
timestamp: September 30, 2023, 05:32:54 pm
content: Quote from: Dan123456 on September 30, 2023, 11:46:16 amHello again,Very good, and as you see you get some good answers now.To add a little...Yes, that 0v point is the center tap of a center tapped secondary of the transformer. That allows you to use full wave rectification for both positive and negative supplies.The half wave suggestion is good, but you will need to double all your capacitor values to get the same degree of ripple filtering. Not too big of a deal though i think.There are other tricks for getting a zero voltage output for plus and minus supplies, but they are a little more involved. That would allow you to get plus and minus with full wave rectification, but not sure the extra circuitry would be worth it. For low current though it can be very easy.If you can get hold of a center tapped transformer you can use the original power supply drawing, something to think about.