High power transistor - Page 1
Forum section: electronics / beginners

author: davelectronic
author_rank: Frequent Contributor
timestamp: November 13, 2016, 11:22:03 am
content: I was looking for a pnp transistor with more current capabilities than an MJ2955. I've found a 2N5684 and from what i can see it would be a higher current transistor, my intended use was for a pass element in a voltage regulator circuit. I was wandering if anyone can confirm this 2N5684 pnp transistor as a higher power alternative to the MJ2955 transistor. Thanks for reading any advice much appreciated.

author: 2N3055
author_rank: Super Contributor
timestamp: November 13, 2016, 12:31:18 pm
content: They look fine.. What currents do you expect.. These guys need serious driving currents, low hFE... Also make sure you check SOA, as in linear series regulators most of the time it's power that kills you...

author: davelectronic
author_rank: Frequent Contributor
timestamp: November 13, 2016, 12:48:27 pm
content: Thanks for your reply. Would they be suitable as a higher current alternative in these circuits below. Thanks for your help.

author: Kleinstein
author_rank: Super Contributor
timestamp: November 13, 2016, 01:05:47 pm
content: The TIP36 is a little more powerful than the TIP2955, but not much.The circuit with an normal regulator driving external PNPs cause quite some voltage drop, and is not very stable against oscillations. So for a high power regulator I would prefer a different circuit.

author: davelectronic
author_rank: Frequent Contributor
timestamp: November 13, 2016, 01:15:54 pm
content: I have tried it with a mj2955 and it worked well. I went on to try it with a high power darlington, and that did oscillate. I was looking for a higher current handling pnp transistor than the mj2955. But its not so easy to find one, that's why i asked about the use of the 2N5684 pnp transistor, and if it would function in these circuits.

author: Cliff Matthews
author_rank: Unknown
timestamp: November 13, 2016, 01:38:02 pm
content: FWIW, look for a design that isn't so much of a room heater.. Maybe try £54 and get some quiet adjustable efficiency:If you sell the huge transformer, rectifier and filter caps it will need, you can save double on electricity cost and buy 4 of these:http://www.ebay.co.uk/itm/400-Watt-9-15-Volts-DC-36-Amp-HAM-CB-Power-Supply-12-Volt-Real-MegaWatt-/180777043955

author: davelectronic
author_rank: Frequent Contributor
timestamp: November 13, 2016, 01:44:24 pm
content: Thanks, I've done this with 1200 watt server power supplys. Its building this linear circuits I'm interested in. The TIP36 looks promising, although a transistor that had a collector current of 50 Amps would be better. If i could find a suitable component. Preferably in a TO3 package. Do you think the 2N5684 is a viable option ?

author: Cliff Matthews
author_rank: Unknown
timestamp: November 13, 2016, 02:00:24 pm
content: Dropping 20V at 50a is serious biz.. and 2N5684 is 300w. Unless you expect a lot of brownouts, even dropping down to an 18V transformer would help. Is the final objective learning and/or to swap the reg to an LM317? Is that the reason for 24V?

author: 2N3055
author_rank: Super Contributor
timestamp: November 13, 2016, 02:48:20 pm
content: Quote from: davelectronic on November 13, 2016, 12:48:27 pmSearch internet for HAM PSU schematics 12-13.2 V 20-30APlenty of them, those based on old LM723 are actually quite good.. You will get all kind of practical explanations that go with it too..This was the topic few days ago.. Although making a simple PSU is very easy, making good, stable, high power linear PSU is not simple. Plenty of details to go wrong...

author: David Hess
author_rank: Super Contributor
timestamp: November 13, 2016, 06:31:12 pm
content: For a linear power supply this large, I would also want foldback current limiting which is common in 723 designs. Some improvement can be made in this design by adding diodes to compensate for transistor Vbe allowing the integrated regulator to better protect the transistors as shown below from the 1980 National Semiconductor Voltage Regulator Handbook. If the integrated regulator is mounted to the same heat sink and the thermal resistances are matched, then it can also protect the transistors thermally.In high power designs, one advantage of using a greater number of lower power transistors is that heat is spread over a larger area effectively lowering thermal resistance over using fewer higher power transistors. I usually end up selecting parts based on power and thermal characteristics rather than current capability which also means that they operate at a higher part of the hfe curve.I have never had a problem with stability when PNP boosting an integrated 3 terminal regulator.

author: SeanB
author_rank: Super Contributor
timestamp: November 13, 2016, 06:35:07 pm
content: Simplest is just to add more TO247 PNP transistors and emitter resistors in parallel, as the drop in gain with increased current is going to really ruin your day. That way you run each individual transistor at a safe SOA point ( lower current, the 15A current is as a saturated switch with gain of 2in most cases, not what you want as a voltage regulator) with lower current, and they will also be able to dissipate the heat properly, and as a bonus the gain will be a lit higher per device. giving a better performance.TO247 over TO3, as the heatsink space is smaller, you only need one hole and insulator ( unless you use a ISOTOP version in which case you only need the hole and thermal compound), they have the same thermal resistance junction to heatsink ( surprise, but comes from not having a steel case but Kovar instead) and are a lot cheaper.6 for a 30A supply is barely able, giving each a 5A share, but better is to drop it to around 3A using 8, and then for higher current use 8 per 30A block, though there will need to be another transistor then as a driver, your 78xx regulator is only going to be able to supply 0.7A or so of drive at most, even flat out.Over 20A I would personally recommend using a different topology, using a 723 regulator instead, as you can take this and scale it to a 100A power supply and have it still be stable, and still have current limiting that works. You then also add a power transformer that has multiple voltage taps on it, so you can select a range to minimise differential somehow.

author: davelectronic
author_rank: Frequent Contributor
timestamp: November 13, 2016, 06:56:05 pm
content: Thanks for the replys. To many to quote..Here in the uk brown outs are very rare, if you've ever seen photonicinduction on youtube you're understand why. I don't endorse his stunts though. I'm all for the 78xx and the Lm 317 and similar regulators. The 24 Volts is to high on the input, i plan to close that gap, where each transistor shares 5 of the 30 Amps in the big circuit, that's to much for me from each mj2955.This is the main reason i asked about the viability of the larger transistor, or one that can carry more current. I would add input and output protection to that big circuit. I have built it with 4 x TIP2955 transistors, but at 10 Amps (powering a linear amplifier ) It ran rather hot for my liking. And that was air cooled. I'm after a transistor, preferably TO3 package that's pnp that will give far greater head room in that big 6 x transistor circuit. So does anyone know if my finding that larger current capabilities transistor above will do the job ? Or if not, do you know a transistor with greater collector current and power rating that will. Thank again all.

author: SeanB
author_rank: Super Contributor
timestamp: November 13, 2016, 07:10:24 pm
content: There is such a transistor, but the price is such that using 50 regular ones will be cheaper, and you will still need a 20kg heatsink on it. You are still limited by input output differential voltage times current which is heat in watts, and that has to be dissipated somehow, and the 2 ways are convection cooling with a great big finned heatsink, or a slightly smaller one with forced cooling.If your unregulated input is 20VDC, your output is 12VDC and you are wanting the full 10A then you will have to dissipate 80W, and this will need a big chunk of metal to keep the transistor junction temperatures below 70C for best reliability, or you will have long term failures with then going short circuit and applying 20V to the radio, most of which really do not like having 20V shoved into the power connection instead of 13V8.

author: davelectronic
author_rank: Frequent Contributor
timestamp: November 13, 2016, 07:22:30 pm
content: I'm using a secondary voltage of 15 to 18 volts AC. I've just had a wander round again and come across the mj4502 ! Its collector current is 30 Amps. Can anyone tell me if that's a viable transistor in place of the mj2955, it again looks promising. I will get round to an LM723 based psu circuit, but first want to try this minimalist circuit above. Its attraction is low components count and cost. I can get the mj4502 reasonably cheap, is that one a viable unit ?

author: SeanB
author_rank: Super Contributor
timestamp: November 13, 2016, 07:43:22 pm
content: Not going to work, unless you can find a heatsink under 0.5C/W to mount it on, and your regulator would not drive it anyway. At that spec for the supply you have the gain for a "typical' device is 5, and thus your supply ability will be 5A.You will still need at least 4 in parallel on the same 0.5C/W heatsink to get to the 10A mark, and it will still need to dissipate 110W of power in the pass element. Still a room heater, nothing aside from a 300x300x40mm slab of heatsink, RS 490-7040 will do, or RS 692-0629 with a fan.

author: davelectronic
author_rank: Frequent Contributor
timestamp: November 13, 2016, 07:55:55 pm
content: I have used a 50 Amp darlington, but it was unstable and oscillated. Its gain was 1000,the mj 4502 has a gain of 100 maximum. Would it not work in place of an mj2955.

author: SeanB
author_rank: Super Contributor
timestamp: November 13, 2016, 07:58:17 pm
content: It will work in place, but you will still need to keep it at a case temperature under 70C to keep the transistor alive.

author: 2N3055
author_rank: Super Contributor
timestamp: November 13, 2016, 08:00:19 pm
content: Like Sean and everybody else say, max Pd of transistor is not MaxIc * MaxUce... It is defined by thermal design, or choice of transistor+whole cooling solution, that includes ambient temperature (and humidity, even above the sea height ...)Ptot in datasheets are defined as keeping case of transistor on a heatsink that has const temp of 25C, and no thermal resistance whatsoever.. Also they keep the junction at max. temp allowed (sometimes at 175C)... I real life you can count on 1/5 of that figure...For starters look at this :http://www.designworldonline.com/how-to-select-a-suitable-heat-sink/#_http://www.swarthmore.edu/NatSci/echeeve1/Class/e12Code/HEAT-NOTE.pdfhttp://nvlpubs.nist.gov/nistpubs/Legacy/SP/nbsspecialpublication400-14.pdfAnd plenty more..

author: davelectronic
author_rank: Frequent Contributor
timestamp: November 13, 2016, 08:02:40 pm
content: Ok thanks, looking at the data sheet again the mj4502 has a minimum gain of 25 and a maximum of 100, other parameters don't look a million miles of the mj2955 specs. Other than a greater collector current and a higher power unit.

author: davelectronic
author_rank: Frequent Contributor
timestamp: November 13, 2016, 08:09:33 pm
content: If i take the boost regulator circuit above..Would the mj4502 a single transistor supply current up to a maximum of 3 to 5 Amps on a suitable heatsink ? Its gain is 25 minimum and 100 maximum. Would the mj4502 work as a single current boost for a 78xx or lm317 circuit.

author: Cliff Matthews
author_rank: Unknown
timestamp: November 13, 2016, 08:43:24 pm
content: With the 24v transformer, you were dropping 20v (~32v to 12v) so 100W looks like it'll be toasty warm..I see SOA is OK at Vce 20v is 9 amps, but the Hfe graph above it has me confused.. Would that read .33 x Hfe (at temp)?(guess that's why we're learning here..)

author: 2N3055
author_rank: Super Contributor
timestamp: November 13, 2016, 08:46:21 pm
content: Quote from: davelectronic on November 13, 2016, 08:09:33 pmAs I said, in this case you go in opposite direction... you calculate max power dissipated, divide that by power per transistor (35W, very good reserve, 50W possible ) and than see..Funny thing is that since thermal resistance of transistor is summed with that of heatsink.. Putting two transistors in parallel will allow you to make heatsink smaller, even in cases where on transistor would be good enough..Sometimes that is cheaper, heatsinks are expensive... That is reason where I saw 13.2V/30-40A PSU with 8 2N3055 in parallel.. 4 would have been quite enough in theory, 8 gave greater thermal reserve and slightly smaller heatsink.. Not to mention that it made PSU practically indestructible...

author: davelectronic
author_rank: Frequent Contributor
timestamp: November 13, 2016, 09:02:52 pm
content: this is not "how far can a push a transistor" its how much more head room can i put between what i need and the transistors maximum output. So for example, if i need 15 Amps continuous, its handy to know i've got a maximum of 30 Amps.The mj2955 does the job well, but i'm looking for a transistor that has less voltage sag, is rated to dissipate more power (not to max it out) but for greater head room and more efficiency.when i tried a high power darlington, when it wasn't oscillating, its efficiency was good, it ran cooler, no voltage sag under load etc. But it did oscillate periodically. That was a 50 Amp collector darlington, so all i'm after is better head room for the same current use. The two i've found are the 2N5684 pnp and the MJ4502 pnp transistor. all i'm looking or, is are they viable options for better head room than the MJ2955.

author: David Hess
author_rank: Super Contributor
timestamp: November 13, 2016, 09:06:30 pm
content: The current rating in this case is less important that the power rating and heat sinking. Once the later are taken care of, the total current rating will be plenty.Instead of looking at the current ratings, consider the physical layout and heat sinking requirements. More than 4 TO-3 transistors on a single heat sink will be difficult. 2 TO-3 transistors on two separate heat sinks may be more convenient. In the past when doing high power designs for cheap, I started with the heat sink I had available and worked from there to get the best performance out of it. For what it is worth, the cylinder head from a Volkswagen bug as seen below has good power handling capability.I have used PNP Darlingtons in this circuit without problems but it takes more care in design to prevent oscillation. The Sziklai pair shown in the application note I posted solves the gain problem and allows for NPN output transistors but adds complexity; that was more important in the past before good and inexpensive PNP power transistors were available.The PNP MJ4502 (and NPN MJ802) is a fine choice and probably what I would use but only because I have a drawer full of them.Consider what happens with an output short. A more complex design which includes foldback current limiting will not self destruct with an extended output short. To achieve the same without foldback current limiting requires a design capable of handling higher power.

author: 2N3055
author_rank: Super Contributor
timestamp: November 13, 2016, 09:09:22 pm
content: I hate to state the obvious with 8 transistors in parallel you have 8x the reserve... No single transistor can do that, and as I said, thermally, there is NO single transistor that can dissipate as much as a few in a parallel...Best case to junction numbers I've seen was maybe 0.5W/K, with few TO220 transistor in parallel you can do better than that...

author: Cliff Matthews
author_rank: Unknown
timestamp: November 13, 2016, 09:14:01 pm
content: Quote from: davelectronic on November 13, 2016, 09:02:52 pmThese are the issues I want to know as well. That upper graph shows the Hfe sag way down (after 5 amps especially), but it regains when the heat goes back up. Wrapping my head around all the parameters is difficult at my age.

author: Cliff Matthews
author_rank: Unknown
timestamp: November 13, 2016, 09:15:50 pm
content: Quote from: 2N3055 on November 13, 2016, 09:09:22 pmI think the OP wants simplicity in the circuit, without all the ballast resistors etc..

author: davelectronic
author_rank: Frequent Contributor
timestamp: November 13, 2016, 09:25:45 pm
content: I'm fine with ballast resistors, i'm after efficiency. This is going to be a taste it and see thing i think. Thanks for everyone's help and input.

author: David Hess
author_rank: Super Contributor
timestamp: November 13, 2016, 09:36:19 pm
content: Quote from: davelectronic on November 13, 2016, 09:25:45 pmIf it is a linear regulator, then its efficiency is completely defined by its input and output specifications. The power difference has to be dissipated somewhere and the temperature rise of a given amount of air flow is completely defined by the power.

author: 2N3055
author_rank: Super Contributor
timestamp: November 13, 2016, 10:04:16 pm
content: Quote from: Cliff Matthews on November 13, 2016, 09:15:50 pmWell I'm sorry to be party pooper but at 30-40 A there is no simple and reliable in the same box.. Actually paralleling is as simple as it gets..For instance, since you run individual transistors at lower currents, you will have less of a HFE nonlinearity too..Of course, it's engineering, everything has pro et contra... That is why I suggested designs that already work well...That is simpler than inventing something from the scratch..Best way to do this is with a preregulator, or if there is no need for super quiet source, to go switcher route..And that is even less simple..

author: davelectronic
author_rank: Frequent Contributor
timestamp: November 13, 2016, 10:30:00 pm
content: Earlier successful attempts with TIP2955.'s and 7812 regulator. The 4 x on a cpu heatsink got up to 72°C with 50% duty cycle use at 10 Amps, just over. I will try these two other transistors, first as a single, then in parallel. If they don't oscillate i'm hopeful for better efficiency.

author: davelectronic
author_rank: Frequent Contributor
timestamp: November 13, 2016, 10:37:19 pm
content: I don't want a single transistor to take the place of x4. I'm looking for like for like improvement and efficiency. So replace 1x mj2955 for mjxx and 4x mj2955's for 4x mjxx where xx is the unknown at this stage. See maybe what i'm getting at !

author: davelectronic
author_rank: Frequent Contributor
timestamp: November 13, 2016, 10:40:34 pm
content: Quote from: 2N3055 on November 13, 2016, 10:04:16 pmTo step outside the box and try unknown scenarios.. And switching would present issues, in some circuits. Only because i want it for powering RF equipment.

author: 2N3055
author_rank: Super Contributor
timestamp: November 13, 2016, 10:44:50 pm
content: In that case, experiment away and have lots of fun doing it...At 30-40A eye protection is recommended...

author: davelectronic
author_rank: Frequent Contributor
timestamp: November 13, 2016, 10:53:18 pm
content: Quote from: 2N3055 on November 13, 2016, 10:44:50 pmCheers buddy

author: Seekonk
author_rank: Super Contributor
timestamp: November 13, 2016, 10:59:32 pm
content: Safety in numbers. Even if one component is 40A, the SOA of that device is just about the same as a lower current device in a linear use.I had a military project and the presentation by the Air Force had a bullet....Radio Shack Compatibility. I always liked that. There was nothing in it from Radio Shack. It just meant that it used all off the shelf stuff. Don't like using transistors that are exotic.

author: David Hess
author_rank: Super Contributor
timestamp: November 13, 2016, 11:22:22 pm
content: Take a look at the configuration of the larger Astron linear supplies.The 37 continuous amp RS-50A uses 8 (!) x 2N3771 (30A 150W) transistors in parallel divided onto two different heat sinks with 4 transistors each. The 57 continuous amp RS-70A does the same thing but includes forced air cooling.And that is *with* foldback current limiting and a transformer secondary optimized to supply the optimum unregulated DC voltage. At 37 amps, the input filter capacitance of 70k microfarads allows about 4 volts of input ripple so the unregulated peak voltage is about 22 volts DC; I can measure it if someone is really interested but I think this is a close enough estimate. My guess is that power dissipation is then about 250 watts lost for 500 watts out or 66% efficient.

author: David Hess
author_rank: Super Contributor
timestamp: November 13, 2016, 11:26:18 pm
content: Quote from: 2N3055 on November 13, 2016, 10:44:50 pmOne advantage of the metal can TO-3 package is that the destruction is contained. Plastic packages tend to explode producing shrapnel which sticks in things like the ceiling, my face, etc.

author: davelectronic
author_rank: Frequent Contributor
timestamp: November 13, 2016, 11:28:40 pm
content: I'm quietly looking for useful old new stock to use. Another reason i look for dated parts. Herein the uk the lm317k is expensive, i found a few for cheap, linear technology manufacturer, £13 for x10 bargain i'd say. And that's it really,good hardy cheap usable components, mostly semiconductors. I got bitten by cheap Chinese fakes, so just use a handful of trusted sellers I'm ok with.

author: David Hess
author_rank: Super Contributor
timestamp: November 13, 2016, 11:36:38 pm
content: Also do not forget the LM350 which is a 3 amp LM317. The LM338 is the 5 amp version. All are or were available in TO-220 *and* TO-3 packages. You might run across either cheap.At some point I picked up a tray of TO-3 packaged and a tube of TO-220 packaged LM337s which are the negative 1.5 amp version of the LM317. They are more finicky about stability but can use an external NPN instead of PNP pass transistor.

author: Cliff Matthews
author_rank: Unknown
timestamp: November 13, 2016, 11:55:22 pm
content: ** Not High-jacking thread, but in #20, I was asking for light reading the top MJ4502 graph. Is this a percentage of rated typical hFE?

author: 2N3055
author_rank: Super Contributor
timestamp: November 14, 2016, 12:20:30 am
content: Quote from: Cliff Matthews on November 13, 2016, 11:55:22 pmYes that is correct..Typical rated hFE is specified at certain operating point, this is spread over whole range..And it also shows the spread over temperature...

author: davelectronic
author_rank: Frequent Contributor
timestamp: November 14, 2016, 12:41:23 pm
content: Not wanting to start another thread...As its the same type of topic.Does anyone know how i can stop a single mj11015 darlington power transistor from oscillating intermittently. This is in a 7812 circuit, and the above trasistor is the pass element. The hfe is high at 1k. Any thoughts on this ? Because if i can cure the oscillations, it would be a rock solid transistor with no voltage sag under load.

author: Cliff Matthews
author_rank: Unknown
timestamp: November 14, 2016, 12:58:58 pm
content: You mention oscillations often, so is it measured on a scope? At what percentage of load at what freq?Is this not just a case of the 7812 cutting out because the pass device is not carrying enough of the load?

author: CJay
author_rank: Super Contributor
timestamp: November 14, 2016, 01:15:41 pm
content: It's essentially the simple truck dropper circuit, crude and not particularly reliable but it does work and can be useful, you gain thermal protection if the pass transistors are thermally coupled to the 7812 for instance.Not sure what you mean by 'efficiency' though, no matter which transistor(s) you use they will always have to 'waste' the same amount of power as heat so there's no efficiency gain by using a different transistor.Definitely investigate the LM723, there are a ton of very useable designs out there, PW Marchwood is a good one for radio work and is not prone to bursting into oscillation (as being discussed on another thread here, the 78xx series can have all sorts of issues)(And I think we've discussed this on another forum, TM1?)

author: David Hess
author_rank: Super Contributor
timestamp: November 14, 2016, 07:14:39 pm
content: Quote from: davelectronic on November 14, 2016, 12:41:23 pmMake sure you are bypassing enough current around the Darlington for the 7812 with an external base-emitter shunt resistor. Add a base series resistor.I might implement the circuit as shown in the National Semiconductor application note but with 2 diodes in series to compensate the 2*Vbe of the Darlington.

author: davelectronic
author_rank: Frequent Contributor
timestamp: November 14, 2016, 07:37:46 pm
content: The circuit with the mj11015 was put together in simulation on another forum by other members,this was after i built it to fault find. Several members did this in simulation and it showed no issues. And one member was building it, to establish the problem, he hit on the same problem. Eventually he emailed a friend that worked for linear technology, but i heard no more after that. So thought I'd throw the question out here. The only thing i could work out was maybe it was that transistors massive gain of 1K that might have been the issue. It was always when loaded the voltage jumped about a bit, but only a volt or so up then back down to normal. It didn't do it continously. Unfortunately i don't have the use of a scope yet. I know your going to tell me it might not be oscillations, but the general feeling in the other forum and by multiple members thought it was. Thanks for the replys and help from everyone in this thread.PS. Just a high to a CB good buddy CJay (-;

author: dmills
author_rank: Super Contributor
timestamp: November 15, 2016, 05:11:32 pm
content: And as for efficiency, it is a LINEAR REG, to a first order (Vin - Vout) * Iout, maybe plus 10% if your devices have really low beta.For me, I would never try to find a higher power device in that class, parallel devices with small current sharing resistors is a much better way to go (Thermally, and in terms of effective SOA), but then I probably would not be trying a linear reg at that current level anyway.Regards, Dan.

author: David Hess
author_rank: Super Contributor
timestamp: November 15, 2016, 05:40:39 pm
content: Quote from: davelectronic on November 14, 2016, 07:37:46 pmThere is enough variation in transistors and regulators that a simulation would likely miss the marginal cases.I only used a Darlington for this once and since it was just a quick hack and worked, I did not bother to do stability analysis. I was also very conservative about the external base-emitter shunt resistor and external emitter resistor to control open loop gain.

author: davelectronic
author_rank: Frequent Contributor
timestamp: November 16, 2016, 01:10:11 pm
content: I'm going to exaust all options with viable transistors, then move on to the LM723 as a circuit option for a linear psu. Finally end up building a permanent units for HF radio use.

author: CJay
author_rank: Super Contributor
timestamp: November 16, 2016, 02:11:52 pm
content: Honestly, I would skip experimenting with transistors and 78xx regulators, just go straight to the LM723 and experiment with the transistors there, it offers so much more and is RF quiet/immune.Grab a copy of the PW Marchwood PSU articles, there are many versions kicking around the 'net and I think Warrington Amateur Radio CLub hosts a known good version on their website.

author: davelectronic
author_rank: Frequent Contributor
timestamp: November 16, 2016, 07:30:55 pm
content: Cheers Cjay ! I will certainly have a look, I've been over it once, but that was a while back. I came across a blinding enclosure that would be ace for such a project.

author: David Hess
author_rank: Super Contributor
timestamp: November 16, 2016, 08:27:38 pm
content: Quote from: CJay on November 16, 2016, 02:11:52 pmI might not simply because the 723 is more complex to use.When I hacked together a quick 10+ amp power supply for a mobile VHF/UHF transceiver, I used an LM337, 2N3055, and an operational amplifier to control the variable speed cooling fan.

author: davelectronic
author_rank: Frequent Contributor
timestamp: November 16, 2016, 09:54:38 pm
content: 12 Volt 20 Amp Power Supplyhttp://www.zen22142.zen.co.uk/Circuits/Power/1220.htmI'm not done with the 3 terminal regulator yet, repairing a 3 to 5 Amp Altai ebay buy with the LM317 and a MJ2955 first.But what do you make of the circuit in the link above ? Its been a good source before now. Thoughts anyone !

author: Seekonk
author_rank: Super Contributor
timestamp: November 16, 2016, 10:14:44 pm
content: If your transformer is a little shy, people often supply the driver with a little booster supply. That is preferable to extra heat in the output transistors. A 5V wall wart sitting on top would work well.

author: 2N3055
author_rank: Super Contributor
timestamp: November 16, 2016, 10:46:30 pm
content: Well that is one of those 723 designs with paralleled transistors we all spoke about...Seems OK on a first look, also there a many more like this, with some better protection circuits..I would definitely try to find one with foldback current limiter, when so much watts are available...Also, some people find it nice to have overvoltage (crowbar) circuit, to protect equipment if PSU blows transistors and apply full rectified voltage to the output...http://www.americanradiohistory.com/Archive-Practical/Wireless/80s/PW%201983%2006.pdfhttp://www.americanradiohistory.com/Archive-Practical/Wireless/80s/PW%201983%2007.pdfTake a peek here, it is a very good article that goes into practical design, cooling and other...

author: David Hess
author_rank: Super Contributor
timestamp: November 16, 2016, 10:57:46 pm
content: The Astron 723 designs which I linked earlier are built that way. They include a bias supply for the 723 so it can drive the output transistors with a minimum of dropout, an output crowbar circuit to protect the load in case of failure, and foldback current limiting.I accidentally had a grounded coaxial cable touch the exposed case of the drive transistor on my big Astron which melted one lead of the bias supply winding where it came out of the transformer. To fix it, I added a small power transformer to supply the bias to the 723.

author: davelectronic
author_rank: Frequent Contributor
timestamp: November 16, 2016, 11:01:36 pm
content: I'm not sure why you'd want to use a plug pack for power, you get any additional voltage from the source. The schematic i posted reference to was a concept for the main circuit, any additional protection is easy to add on. Such as current limiting and over voltage protection. I'd opt to put these on a separate board. So i guess the circuit in the link i posted as a concept is a viable one ? Unfortunately i never download pdf files I've not sourced myself, for security reasons.

author: 2N3055
author_rank: Super Contributor
timestamp: November 16, 2016, 11:21:03 pm
content: Okeyyyy.......http://www.americanradiohistory.com/Practical_Wireless_Magazine.htmJune and July of 83, Marchwood power supply

author: davelectronic
author_rank: Frequent Contributor
timestamp: November 16, 2016, 11:54:15 pm
content: Thanks, but not sure where it is on that page. Thanks for posting anyway, I probably could find the Marchwood project if i hunted it down. I often look all over the net for psu and other circuits. Its a win to find working examples.

author: David Hess
author_rank: Super Contributor
timestamp: November 17, 2016, 12:32:18 am
content: Quote from: davelectronic on November 16, 2016, 11:54:15 pmThe design shows how complicated a 723 can get although it includes several safety features.

author: davelectronic
author_rank: Frequent Contributor
timestamp: November 17, 2016, 01:37:58 am
content: Thanks, but i never download unknown pdf files, sorry, as a Unix /Linux user you learn a lot.Back on track, I'm not phased by complexity by the Marchwood project. I can visualise lay out and schematics very well, complex math formula can be my weak spot, although i usually get there in the end.Cost is a factor, and some physical ability problems as well at the moment )-: more the hardware work involved, although ABS can make life easier in some areas. Just thinking lately, after nearly 12 years as a hobbyist I've never bread boarded a project, maybe its time i did.

author: davelectronic
author_rank: Frequent Contributor
timestamp: November 17, 2016, 06:50:43 pm
content: Build A 10 Amp 13.8 Volt Power Supplyhttp://www.rason.org/Projects/powsupply/powsupply.htm(Sharefrom CM Browser)This one and its associated protection circuit looks good, a variation on similar circuits. Any negatives to this one ? I know current is a little lower, but this looks like not to much of a monster to breadboard for proof of concept, thoughts anyone ?

author: davelectronic
author_rank: Frequent Contributor
timestamp: November 17, 2016, 06:52:49 pm
content: Link don't work.

author: Kleinstein
author_rank: Super Contributor
timestamp: November 17, 2016, 08:01:41 pm
content: The circuit has a few points to improve:1) The compensation cap looks quite small, so C5 might need to be larger (e.g. 500 pF-1nF) to make the circuit stable.2) There is no need to use an 2N3055 for Q1, a smaller (e.g. TO220) transistor should also do it.3) For lower noise a capacitor from pin5 to GND can be added.4) 5 A for each of the 3055 is already quite a lot of current, if no foldback current limiting is used.So either add a 3rd power transistor or fold back current limiting (resistor from pin 2 to GND).5) The drop out voltage is relatively high, similar to the 7812+PNP+NPN. I might be worth using separate diodes and filter cap for the regulator supply. This could save same 2 V on the transformer or about 1/3 the filter caps, as the regulator supply would have less ripple and thus higher min voltage.6) The output capacitor likely does not need to be that large. 100 µF electrolytic + 100 nF ceramic should be OK7) There should be a fuse at the transformer, at least one.

author: davelectronic
author_rank: Frequent Contributor
timestamp: November 17, 2016, 08:49:52 pm
content: Thank you for the tips, yes they all make sense to me. I did think the driver for the 2x 2N3055's was a bit hefty. And any saving on voltage drops is really welcomed. Thanks again for looking the circuit over.

author: davelectronic
author_rank: Frequent Contributor
timestamp: November 27, 2016, 10:28:18 pm
content: Its been a few days now... I finally built the circuit for restoring this old Alti 3 - 5 Amp 13.8 Volt linear psu. It was an ebay buy that someone had been happy with the side cutters and more.Using an LM317K and an MJ4502 power transistor, it running again. I've set it up at 12.70 Volts with fixed value resistors. Its where i like to power radio gear. A test unmounted outside the case (for tesing) I'm getting a shade under 12.70 Volts.Tested with 20 watt load, then 40 watt load, the transformer is only rated at the above maximum current. With a 40 watt load the regulator was steady at 45°C and the power transistor was steady at 52°C. I measured the voltage across the base resistor ( 10R 0.5 watt ) and the ballast resistor ( 7 watts 1.0R ) These where 180mV for the base resistor, and 0.95 Volts for the ballast resistor.It run with the 40 watt load for a couple of hours very steady, so just have to mount it in the case now. I sould have taken some more measurements, base emitter voltage, and base collector etc. I'll do that when its wired in the case. One positive was the MJ4502 ran much cooler than the same circuit with an MJ2955. Might try a few other high power transitors in other circuits. I will be happy with a 20 Amp psu built from scratch using the LM317K, then i will move on to the LM723. With the above my voltage dropped 0.3 Volts with that 40 watt load.

author: 2N3055
author_rank: Super Contributor
timestamp: November 27, 2016, 11:12:40 pm
content: Quote from: davelectronic on November 27, 2016, 10:28:18 pmWell everything else equal, it should... MJ4502 has junction to case thermal resistance of 0,875 °K/W , while MJ2955 has about 1.5 °K/W... so at 40W that should be approx 20-25 °C less inside transistor...

author: Cliff Matthews
author_rank: Unknown
timestamp: November 28, 2016, 02:24:10 am
content: Dumb question: I thought a "ballast resistor" is only named as such if >2 parallel pass elements use low value emitter resistors to "balance" away from single element thermal runaway and possible cascade failure. Is this not the reason we employ a nautical term?

author: davelectronic
author_rank: Frequent Contributor
timestamp: November 28, 2016, 02:32:09 am
content: Yes I'm sure it is, in more than one its a ballast resistor, as a single i presume it prevents thermal runaway of the transistor. So not strictly ballast, as a single resistor.

author: David Hess
author_rank: Super Contributor
timestamp: November 28, 2016, 02:50:25 am
content: Quote from: 2N3055 on November 27, 2016, 11:12:40 pmWhat temperature was davelectronic measuring? The larger MJ4502 will have lower junction temperature but the external temperatures should have been the same.

author: davelectronic
author_rank: Frequent Contributor
timestamp: November 28, 2016, 10:22:38 am
content: I was measuring to side of the TO3 package, as close to the junction as i could get. This is what i was after from the start of this thread. Whilst i knew the same power would be dissipated, i wanted a higher power device to run cooler, so a larger overhead margine in power terms.

author: davelectronic
author_rank: Frequent Contributor
timestamp: November 28, 2016, 10:28:41 am
content: I've got a square bridge rectifier bolted to the same heat sink as the regulator, not perfectly ideal. The regulator would be much cooler on its own. But the trade off from heat there is not bad really. This supply won't see more than 3 Amps from my intended use anyway. As long as it can handle that continuous, and say a bit over at 50% duty cycle, then I'm happy with that.

author: David Hess
author_rank: Super Contributor
timestamp: November 28, 2016, 02:39:59 pm
content: Quote from: davelectronic on November 28, 2016, 10:22:38 amThat is only going return an unspecified Tcase which will vary depending on both the temperature measurement and the thermal resistance between the case and heat sink. The transistor with the larger die will have a lower Tjunction for a given Tcase but that is not what you measured.Typically Tjunction is measured via the change in Vbe at a given current and this works very well but the since Tjunction has a time constant of milliseconds, the measurement needs to be made while power is applied or immediately after power is removed.That is an awful lot of work for a single project so another way I have done it in the past is to measure Tcase as specified by the manufacturer which usually involves drilling a small hole in the heat sink and then estimate Tjunction from the power level and specified junction to case thermal resistance which is sort of what you did.

author: davelectronic
author_rank: Frequent Contributor
timestamp: November 28, 2016, 07:25:12 pm
content: That's a step to far for me, i will take my chances on what i measured, i know its far from perfect, but it will do for me. If i have over specified the components for a project (like this one) I'm less likley to run into trouble. Thanks for the info. Things in my world move slow....I've still got to get it cased up, and put in service.

author: David Hess
author_rank: Super Contributor
timestamp: November 28, 2016, 08:08:56 pm
content: Quote from: davelectronic on November 28, 2016, 07:25:12 pmI actually agree and was just pointing how a better measurement could be made. Just measuring the case temperature at a convenient spot even if it is not optimal will be good enough; I would probably use the center top of the TO-3 package which should follow the base temperature pretty closely but the same temperature should be returned for both transistors unless the mountings are not equivalent which is a pretty big factor.Assuming both transistors are mounted well thermally, the MJ4502 will have more operating margin than the MJ2955 but the MJ2955 may be more than adequate.