Question about wiring TO-3 transistors - Page 1
Forum section: electronics / projects

author: liquibyte
author_rank: Frequent Contributor
timestamp: February 25, 2014, 03:56:52 pm
content: I'm working on a power supply and have a question about hooking up the TO-3 transistors in the attached schematic.I've got the transistors mounted at the back of the case on a big heatsink outside of the case and being cooled by two 80mm fans. There are actually four transistors mounted there because I'm building two of these into a single case.My question is when hooking up the shared TO-3 power transistors mounted at the back of the case what is the best way to wire them. Given the current requirements can you wire the common points on the transistors to each other and use a single wire to the board or should you wire each pin to the board seperately? I'm using solid core 18 awg wire if that's a consideration and there will be 3 amps flowing to the output. The transformers are theseVPS28-4600connected in series for 28V @ 4.6A and the transistors are 2N3055's.

author: pickle9000
author_rank: Super Contributor
timestamp: February 25, 2014, 04:14:21 pm
content: Normally you would wire each leg by itself. Mainly it's to simplify the manufacturing and repair process. It eliminates the need for multiple lengths and gauges of wire. It's easier for the assembler to follow.

author: sync
author_rank: Frequent Contributor
timestamp: February 25, 2014, 04:40:57 pm
content: Quote from: liquibyte on February 25, 2014, 03:56:52 pmIt doesn't matter. But I would use stranded wire. It's easier to use and more reliable.QuoteSome math. It specified 28V@4.6A, +25% at no load and with +10% mains variation you get 38.5Vrms max. Which is about 54V after rectification with no load. The op amps don't like this. They are specified at 44V absolute maximum.

author: liquibyte
author_rank: Frequent Contributor
timestamp: February 25, 2014, 05:03:35 pm
content: Quote from: sync on February 25, 2014, 04:40:57 pmNow I'm a bit worried. The design has been built by others and has proved to work. One of the people on the forum I got this from has actually build a surface mount version that works. The only difference that I had to make was the current output of the transformer. The original called for a 28V 4.3A transformer but I couldn't find one with those specs off the shelf so I went with 4.6A at the same voltage.Edit: forgot to add the parts list.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 25, 2014, 06:31:28 pm
content: Offhand, I'd put in a small series resistor (a couple ohms I think) for each base terminal, to reduce the chances of parasitic oscillation. One possible mode is the two in parallel, one base goes up while the other goes down, their base capacitances resonating with the wiring inductances. This would occur in the low MHz, so if you happen to get relabel 2N3055s that are actually useful (Vce(sat) well below spec, and hFE and fT well above), you could get oscillation. Another option is a capacitor from base to collector, at the transistor, which has the effect of lowering fT; probably 100-1000pF would be typical.Anyway, 18AWG is more than enough, I'd be fine with 22AWG. And your transformer's ratings are a fine choice.Tim

author: liquibyte
author_rank: Frequent Contributor
timestamp: February 25, 2014, 06:59:53 pm
content: Quote from: T3sl4co1l on February 25, 2014, 06:31:28 pmI did look up my wiring choices for current rating and realized that 22AWG would be enough but I was trying to be safe with the margin.Are you saying to do both or one or the other? I have some low ohm 1/4 watt stuff (smallest around 27 ohms off the top of my head without looking) but would have to buy anything in a larger wattage range. I also have some pF rated tants around here somewhere as well and may have some film types from salvage but would have to look for those as well. The transistors I have are ON Semi's from Digikey and I have no way to test them other than using them in circuit as I haven't built a transistor tester yet but it's on my todo list.

author: SeanB
author_rank: Super Contributor
timestamp: February 25, 2014, 07:08:00 pm
content: If it is a 2N3055 just wire it in and do not bother with the resistors. Never found one yet that would oscillate unless you have 5m of wire on each leadout and wound it in a giant air cored inductor. 20cm of wire is about equal to a short with any reputable 3055, they are going to struggle at anything over 100kHz at currents over 5A. RF power devices though will need it, and definitely any MOSFET. If it has a Ft over 50MHz then it will oscillate with long enough leads, but not for a dead slow 3055 which barely is able to handle low power audio.

author: liquibyte
author_rank: Frequent Contributor
timestamp: February 25, 2014, 07:24:37 pm
content: The current will be limited to 3A tops although there will definitely be more than 20cm of wire length per pin to where these connect to the board. Putting two of these in a case required a largish case and I'm actually starting to run out of room. I've managed two transformers for the independent supplies plus a 12-0-12 RS special for the instrumentation and fans. The circuit is known to work but I have never seen an in depth explaination of how well or what issues have cropped up using it as a bench supply. I wish I had more testing equipment to do a proper review once I get the thing up and running but I'm limited to a multimeter and my new (old) Conar 255 which doesn't have a probe yet.

author: SeanB
author_rank: Super Contributor
timestamp: February 25, 2014, 07:35:09 pm
content: Less than 1m of wire is still no issue, the 2N3055 is barely able to do 20kHz for audio at 3A, so it will work. If you are worried just slip a ferrite bead over the base pin before soldering the wire to it. if no ferrite bead then a short sleeve has the same effect in a power supply. At 10MHz it will be different, but at DC they are very stable.I used to use them as dummy loads by soldering 5m of wire to the 3 leads, twisting them together tightly and dunking the device in a glass of cold water then use them to boil the water by using a resistance box to give base current. 15A and boiling water and they never oscillated ever.

author: liquibyte
author_rank: Frequent Contributor
timestamp: February 25, 2014, 07:55:49 pm
content: I'll build it and see how things go. I've managed to get the AC side of things worked out and have the back plate and heatsink drilled and put together. I've yet to put the components on the board and am going to wait until I'm sure things are working well enough to do the front panel. I'll be building my own displays for this so I'm trying to take my time and make sure things are working as expected as I go so if I have to change something I don't have to rework too much. It's an "on the fly" build but I have a bit of prior experience with a company I used to work for so things are shaping up nicely. I'm finding that you can never have enough stock on hand for little things that you never thought you'd use until you need it.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 25, 2014, 08:39:05 pm
content: Quote from: SeanB on February 25, 2014, 07:08:00 pmNo such thing. The originals are shit, new/relabels are liars. The important part of my post:Quote from: T3sl4co1l on February 25, 2014, 06:31:28 pmI have at least one Motorola "2N3055" (dated 1990) that's probably an out of spec MJ15010 or something like that. It hasexcellent switching performancein a latching drive circuit I tested. hFE = 5, Ic = 2A, Vce(sat) = 0.6V (sharp turn-on, not sloppy), t_f = 180ns. Four times faster than a (probably authentic) TIP31C in the same circuit (which also saturates slowly). This is not at all a linear fT test, but based on similar devices, I would guess it's at least several MHz (at typical conditions), possibly peaking (at optimal Vce and Ic) to 10, maaaybe even 20MHz. This is, in no possible way, a typical 2N3055!Very old parts, like 2N3055, 2N2222, 2N2907, etc. should be avoided if at all possible. The specs are minimums only. There is no guarantee you won't get something vastlyin excess ofspec. I've also heard horror stories of circuits failing EMC because of 2N2222s singing UHF because their actual fT was over 800MHz or something like that. (A good lesson to use parts suitably spec'd for your application, not over or under, and without unbounded ranges.)For TO-3s, I wouldn't recommend anything less than a MJ15010, and those are almost as old as the 2N3055 itself. There are even better sort-of-new types, still very common in audio amps and such. For TO-92s, I use 2N3904/6 myself, except when I need a little more current, where I use 4401/3, or something else more special-purpose.2N4401 is essentially identical to a typical 2N2222 (well, PN2222), but with guaranteed bounded parameters, and many more characteristic graphs. Ironically, 2N4401 is a better drop-in replacement for 2N2222 than 2N2222 is!QuoteActually, now I'm curious... why "definitely" any MOSFET?I can barely get anything out of an IRF510 above 5 or 10MHz. There's no fT spec for a MOSFET, but the same rules apply, and it's very difficult to make a (general purpose) MOSFET run fast. Lots of Crss and R_in. The ones you have to be careful with, the slick-snot RF MOSFETs, are fabricated quite differently from GP types. Something like an IRF510 (or a bit more apples-to-apples, maybe an... IRFP150..?) will most likely oscillate with shorter lead lengths than a "top quality" (so to speak) 2N3055, but I don't see how it could by orders of magnitude.Tim

author: sync
author_rank: Frequent Contributor
timestamp: February 25, 2014, 08:49:11 pm
content: Quote from: liquibyte on February 25, 2014, 05:03:35 pmA rule of thump is that the amperage of the transformer should be 1.8 times the DC output of a power supply (when using a bridge rectifier). In your case 4.8A (1.8*3A). Your 4.6A transformer should be ok.Regarding the voltage. Your transformer has an output of 28V@4.6A with 115V input. AFAIK the mains voltage in the USA can be between 114 to 126V (+/?5%). At 126V the transformer will output 29.4V@4.6A. Also the transformer will have a higher output voltage at no load. From the data sheet: "Voltage Regulation: 25% TYP @ full load to no load". So at no load the voltage can be +25% higher than the nominal Voltage. With 126V line voltage and +25% at no load it can output ~37VRMS. Which is ~51VDCafter rectifying (37V*1.4). (Sorry, In my first calculation I used +10% for the line voltage and got ~54VDC.) Of course this a worst case calculation. In reality it could be better.But even at the 115V line voltage and more realistic +11% at no load you will get about 43VDC. Which is too high for the op amps. Will it work? Probably. Would I trust it? No.

author: liquibyte
author_rank: Frequent Contributor
timestamp: February 25, 2014, 09:52:57 pm
content: I get 122.4 out of the wall typically. I just did a power test of one of the transformers and I get 35.5 VAC. After rectification I get 28.5-28.6 VDC (it's fluctuating a bit). I don't have much else wired up yet so I can't do any further measurements but this seems correct to me for an unloaded circuit before and after rectification. I'm using theseGBPC15005FS bridge rectifiersbut the datasheet specifies only a 1.1 V drop per bridge and not diode.

author: sync
author_rank: Frequent Contributor
timestamp: February 25, 2014, 10:45:22 pm
content: I you measure the DC voltage with the filter cap (C1)? 35.5VAC is about 50VDC after rectification with a filter cap.

author: liquibyte
author_rank: Frequent Contributor
timestamp: February 25, 2014, 11:39:37 pm
content: Quote from: sync on February 25, 2014, 10:45:22 pmLike I said, others have built this using basically the same parts I'm using and they report a working power supply so something must be pulling the voltage to a tolerable level. I'll attach a modified schematic showing the parts values. Excuse the weirdness of the layout, I just modified it in place but the values are usually close to the part.

author: sync
author_rank: Frequent Contributor
timestamp: February 26, 2014, 01:04:04 am
content: There is nothing which limits the op amp supply voltage to acceptable limits. Build the power supply and measure it. You can add a limiting circuit afterwards.

author: liquibyte
author_rank: Frequent Contributor
timestamp: February 26, 2014, 05:21:45 pm
content: Quote from: sync on February 26, 2014, 01:04:04 amSo I hooked up the filter cap and measured again. From the transformer I still get 35.5 VAC, after rectification I'm also still getting 28.5-28.6 VDC and after smoothing with a 15,000uF 63V cap I get 45.1 VDC. I'm thinking that should be tolerable and why everyone is reporting their circuit works.

author: sync
author_rank: Frequent Contributor
timestamp: February 26, 2014, 06:07:16 pm
content: The TLE2141 has an absolute maximum supply voltage of 44V. This means it shouldneverhigher than that. Good design practice is to stay lower than the absolute maximum ratings.Will it work with 45V in practice? Probably.Will it last? Maybe not.I had designed a circuit using a TLC272 op amp. It has an absolute maximum supply voltage of 18V. Unfortunately sometimes it got up to 20V. Does it worked? Yes, for a few weeks. Then the op amps died. After several dead op amps I found the design flaw and used another op amp with higher ratings. But there are also cases where op amps worked for 20 years with to high supply voltages.Just build the power supply and see what's happen. Worst case is the op amp fail and the power supply outputs as much voltage it can (>30V) with no current limit. A lot of magic smoke.

author: liquibyte
author_rank: Frequent Contributor
timestamp: February 26, 2014, 08:21:41 pm
content: Have any suggestions on how to drop a volt or two before the op amps? A big diode or voltage divider perhaps? Maybe a higher rated op amp comparable to the TLE2141?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 26, 2014, 10:27:20 pm
content: The first amp already has some voltage drop (though it could probably use more), the second could probably stand it too, but the last one has to be rail to rail. Otherwise, you need a discrete gain stage to drive the output follower. Which wouldn't hurt, but it's more stuff to do. If you can find a chip with 80-100V ratings without being too boutique-ish, that'll be the easier solution.Tim

author: liquibyte
author_rank: Frequent Contributor
timestamp: February 26, 2014, 10:37:48 pm
content: Quote from: T3sl4co1l on February 26, 2014, 10:27:20 pmI was looking at OPA604AP @ 48V and OPA552PA @ 60V but reading op amp datasheets has given me a headache so I'm not sure if I'm even close with these two choices. I did talk to a guy on the other forum that said he has one of these built with a home made transformer @ 60V and it's been working for several years. I've been doing steady measurements on the transformers and am getting a consistent 44.6 V out of it after rectification and smoothing. I'm going to go ahead with the build and see how it goes. Worse case scenario, I'll have to replace the op amps but I'll have them socketed so it will be easy to swap out.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 27, 2014, 06:59:31 am
content: Going that close, I'd like to see an MOV or TVS across the rails...... but that's not going to do very much, because you need almost a factor of two range above the rating of one of those devices anyway (e.g., a 12V TVS breaks down at 15V, and clamps a spike at about 18V, so your circuit needs to tolerate 24V just in case).Doesn't look like Digikey offers much in that range...$5, and only 60V:http://www.digikey.com/product-detail/en/OPA551PA/OPA551PA-ND/295612also available in double (OPA552).$7, 100V, a little slow but it'll do the job:http://www.digikey.com/product-detail/en/OPA454AIDDA/296-22997-5-ND/1770310everything else is power amps (TO-220-5, etc.), or those specialty hybrid modules ($$$!).So yeah, a low voltage approach with discrete gain is looking good...Tim

author: dannyf
author_rank: Super Contributor
timestamp: February 27, 2014, 12:21:29 pm
content: QuoteTwo ways to solve that:1) you can float the power supply for the opamps, to the output for example. This will greatly reduce the requirement for Vcc-Vee, or R2R output. I did that in one of the avr-controlled power supply - the principle is the same.2) you may also look into audio opamps, or even power opamps, to use as the error amp. They tend to go pretty high in output voltage as well as current capabilities - potentially saving you the driver.

author: megajocke
author_rank: Regular Contributor
timestamp: February 27, 2014, 02:59:47 pm
content: Why not regulate the opamp dupply to 35-40 V or so? If the output of the circuit is 0-30 V then you don't need more. If you can make this regulator low-dropout, it wouldn't increase the required input voltage by much.

author: liquibyte
author_rank: Frequent Contributor
timestamp: February 27, 2014, 04:26:25 pm
content: I was talking to the guy that did the redesign on the circuit from the original and he said to "Connect the + supply pin of all the opamps to the 10V zener diode, and feel if it gets hot". I'm still several weeks away from doing the actual circuits themselves so I can't really test this possibility right now.My main concern at the moment was getting the case and the AC side of things done right so that I didn't have any wicked surprises later on while using this. I've got quite a bit of cash tied up in this thing so I was trying to keep any additional costs as low to nil as possible. I still don't have the design of the displays done and I'm fairly sure that I don't have everything I need for those yet although I have the chips and displays themselves.