Design issues (Rant)

[teemuk]

While I like designing circuits and seeing my designs come to life I'm really not that enthusiastic about the process of actually building something. ...Well, etching and soldering is actually kind of fun but unfortunately those are usually minor parts of the building process when compared to stuff like mechanical/layout design or hunting for parts. These are such nuisances that they almost make me hate the whole hobby. While the following is sort of a "rant" I hope you will find its content helpful.

At the moment, I'm in the middle of building this amplifier that uses TIP142 and TIP147 (complementary) output devices. These devices used to be pretty "standard choice" few years ago but unfortunately they are not anymore and most component stores seem to have run out of them. Fortunately, they usually can be substituted with TIP141 and TIP146 that are almost identical transistors except for their lower Vce.

So here I am at this component store specifying that I will buy either TIP142 and TIP147 or TIP141 and TIP146 since they are complements. Well, they're run out of TIP142 and 147, which I pretty much could expect since that's been the case for more than half a year. Can I order? No. Fortunately this is no problem since they have TIP141 and 146 in stock. So, the clerk hands me these transistors and when I specifically ask if they are indeed TIP141 and TIP146 (and therefore complementary) I get an answer that they are. I trust the clerk's word and don't bother to check it out.

The plot thickens: I get home, open up the package and find out that they were TIP141 and TIP147 instead! Damn! I don't have use for them alone in the amp project since they are not complements to each other! Obviously the clerk wasn't aware of this - or simply did not care. Anyway, I decide that it was my bad (should have checked it out in the store) and since these things cost nearly nothing and will definitely have future use it's not a big issue – I just have to find few TIP142 or TIP146 transistors as well to get that complementary pair.

So, the next day I'm in this other component store. No TIP142 or TIP146 in there either. Luckily I can order through them – except that I find out that these transistors are no longer in production. Or actually they are, but in another case style that is not specified by their datasheets so I'm afraid to buy them. (I'll get to that later). Long story short, I'm fortunately able to order TIP146 in the correct case and everything looks all right.

(People, don't misunderstand me: The local component stores are doing wonderful job. Unfortunately they can't sell stuff that they don't have - or keep stuff that gives them practically no profit in stock. They are also as helpless with the apparent component variation as anyone else).

So, anyway, what's my gripe about these different case styles? This: When you have a design made for specific power dissipation and PC board you can't just go on and substitute the transistors with ones that have another case style. First of all, the transistor in the alternative case may have different power dissipation or the case may have different pinout or distance between the leads and the mounting hole. This means that the substitute transistor is pretty much unusable.

For example, TIP142/147 are different than 142/147T, which I learned by the hard way few years ago. T comes in TO-218 (I guess - I'll get to this later anyway) while the other one comes in TO-220 (I guess). The other package is significantly smaller and has a power dissipation ratio of only 80W (versus 120W stated by the datasheet of TIP14x). So, it's not a proper substitute in most cases. Unfortunately, there is no standard for the alphabet letter code following the device name so you'll have to figure out what differences are in A, B, C, G, GF, T and et cetera. They also mean different things in different transistors - and I guess in the worst case even among different manufacturers. Have fun browsing through dozens of datasheets. You'll should also pretty much memorize the differences and ALWAYS specify the correct type when you order or shop something – otherwise you could get anything. It's foolish to think that transistors would have some standard form – although that is what most people generally expect.

Another gripe I have is about these TO-218, TO-220 and SOT-93 cases. These seem to make no sense! Run a few searches and you'll find out that they seem to mean the exact same thing – except one of the cases is actually much smaller than the other and naturally has a different power dissipation figure. Unfortunately there's no way to make sense which one of them it is because all sources depicting those cases are conflicting – even datasheets. Run a Google image search with those package names and you'll see what I mean. The only way to know for sure of what you are getting is only at the point when you have the part in your hands. Clerks at electronics stores are also lost with this issue.

It doesn't stop there: Just today I found out that even the specific case styles are not standard: The TIP142/147 and TIP141/146 come in seemingly identical case, except that the TIP142/147 has about 1 mm longer leads and about 1mm longer distance from the bottom of that plastic case to the center of the mounting hole. Everything else is identical. While the difference may seem small, it is actually a very big issue in mechanical layout: If you design based on the "larger" case it may be that the smaller case does not fit to the same layout (the shorter leads simply do not reach to the other side of the PCB). If you design based on the "smaller" case but use the transistors that have the bigger case you'll have to bend leads of them. This means that the solder joints become subject to breaking under thermal stress (the leads should be left sort of "spring-like" so that they have room to "live" when the component warms up or cools down). So basically, you'll have to make unique design for each case style, which means you have less choice of substitutes. It's pretty pathetic considering the fact that the package is supposed to be "standard". You also need to be aware of this issue in the first place.

...Which is another gripe of mine: The datasheet typically shows these physical dimensions but am I the only one who thinks those images are extremely confusing? Usually they depict few possible measures for each dimension coupled with some approximate tolerance value (e.g. 160 – 230). Very helpful indeed... Those are usually in some obscure unit format as well, so you can't really make up whether the dimensions are stated in mm, mil, thou, cm or inches. Comparing different datasheets in this respect is also pretty much impossible. I like to refer to this unit as "FY" because its only function seems to be acting as a nuisance. I don't care to state what the abbreviation means but I'm sure you can figure it out. I never had any help of these pictures but perhaps some people know stuff that I don't.

And it doesn't stop to semiconductors... There are also similar issues with potentiometer and jack bushing diameters that are always different as well. Not to mention the "mounting depth" (if you want to mount these on board and attach the bushing to the chassis). And the issue with all those varying packages of those potentiometers you have! Damn annoying! I don't even want to go to issues with various jack switches... It also seems that today I can't get these insulated jacks anywhere either.

I really truly hate this part of building! If I had to design commercial stuff I'd probably go crazy in a week.  :grr

Do you guys have similar experiences? ...Practical stuff you would like to make people aware of?

[noobiePT]

hey, im still new at this diy stuff, so dont have much experience, but i already realized how different the parts can be! just some days ago i changed the pots of my guitar(im changing the whole electronics) and one pot was the size of the one i had before, other was bigger! i basicaly had to grab a drill and drill the hole bigger lol

other thing is buying in a fisicaly store... im never gonna go there anymore, i went there with the parts i wanted in a paper, especifying what part, value, voltage rate, they went and give me the stuff but didnt even care about the voltage rates, just gave what they had, and i ended up having caps rated at 10v.. and the circuit i was building was suppose to have 12v, i ended powering it with a 9v battery

for me its internet stores, ok it takes alot of time to reach home, but at least it comes what you choose