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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets. |
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3rd Dec 2020, 8:03 am | #41 | ||
Dekatron
Join Date: Jul 2009
Location: Sleaford, Lincs. UK.
Posts: 7,657
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Re: 2N3055 series pass questions.
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Not very technical but needs must when you have 3 tins full of old HS's and no temp device. Thanks Mark & Resto73, I will see if I can get a temp device in the future. Quote:
What puzzles me is how different devices in the same package can differ in current when they're all made more or less the same. What's even more mind boggling is the thiness of the junction wires going to a die, how the hell are they supposed to handle 25A in some cases, they're thinner than a fuse element. I asked about this once and it was explained by the length of bonding wires, that may be, just looks wrong. I also get that some Q's have bigger die's and thicker metal bases, still it's bl**dy amazing what Q's can do for their size. I will definitely look at using SMPSU's and or buck convertors in the future Peter, my only reservation is their complexity. If a linear PSU develops a fault it's easy to fix, not so with a switching device. most folk seem to treat them as a single part, if they stop working, bin them and get another. This presents two problems - 1) if I fit one as part of an amp and it stops working after a year the chances of me finding the same SMPSU again is slim. 2) I hate this approach, as Aldous Huxley said "mending is better than ending" One last observation, a PSU supplying heaters has a high inrush current,for this application I measured a peak of 5A +. I fitted a thermistor at one point, but the one I used, the only type I had got so hot it unsoldered itself. Am I right in thinking a linear supply can handle this better than a SMPSU? Does it present any significant problem to a PSU using a regulator IC on it's own, I'm thinking in terms of a reg IC's protection circuit. Andy.
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3rd Dec 2020, 9:49 am | #42 |
Dekatron
Join Date: Jan 2004
Location: Newcastle upon Tyne, Tyne & Wear, UK.
Posts: 8,194
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Re: 2N3055 series pass questions.
Hi Andy, emitter resistors go between each emitter and load on series pass devices. Typically 0.1R
Also there were quite often base sharing resistors from the base drive to each individual base, perhaps 10R or so Ed |
3rd Dec 2020, 11:36 am | #43 |
Pentode
Join Date: Nov 2020
Location: Bristol, UK.
Posts: 132
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Re: 2N3055 series pass questions.
A word of caution when choosing switch mode controllers. Many have short circuit protection which shuts down the output if there is an excessive current demand, and require the equipment to be switched off and back on again to re-start. Now a valve heater has a very low resistance when cold and draws a surge current many times its normal operating current when power is first applied causing the aforementioned shut-down. I found this to my cost when building my valve tester. The solution is simply to choose a smps which has adjustable current limiting as well as variable voltage output. They are surprisingly cheap at around £3
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3rd Dec 2020, 12:05 pm | #44 |
Nonode
Join Date: Jan 2009
Location: Bristol, UK.
Posts: 2,061
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Re: 2N3055 series pass questions.
Hi Andy, Ed's right, 0.1R should be fine in your situation. For more general cases you want the voltage drop across the emitter resistor to swamp variation in Vbe in the transistors. Usually a few hundred mV.
As an approximation, the Vbe voltage varies ... about -2mV per C, so going down as the temperature goes up (-ve coefficient) about +60mV per decade (x10) of current increase The pretty horrific consequence is a +30C change in temperature would be equivalent to 10x the current. A series pair of transistors (no current sharing resistors) can result in 1 being 30C hotter and carrying 10x the current of the other! (then 60C hotter and 100x the current). Smoke will result dc |
3rd Dec 2020, 12:54 pm | #45 | |||||
Dekatron
Join Date: Jul 2007
Location: Evesham, Worcestershire, UK.
Posts: 4,244
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Re: 2N3055 series pass questions.
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The second attachment shows the single pass transistor in the Thurlby PL310 PSU, and the 4 pass transistors in the 2A PL320. The current sharing resistors are 1 ohm each. They're arguably a bit higher than needed, but only dropping half a volt each, so no harm done... Quote:
The die in a BC548 - a 100mA small-signal transistor - is 0.35mm square, whereas a 2N3055 die measures 2.7mm square. And that's fairly modest for a power transistor - the MJ15003 is much bigger again, as the photos on Rod Elliot's article show. Obviously, you need a certain amount of area to handle the current, and the bigger the area, the lower the thermal resistance between the die and case. I'm not an expert here, but honestly wouldn't be surprised if I was told that in some cases the die area was dictated by the thermal requirements as much as - if not more than - the current-handling requirements. Quote:
Last time I needed a step-up boost converter I started looking at buying some ICs to do the job, but admit that I did just buy some of those modules. IIRC, I bought 5 for less than just one IC would have cost - which is crazy, but the reality at the time. Obviously for home projects that's fine - not really practical for production. Quote:
Personally, I've never been comfortable about relying on the over-current or over-temperature protection built in to linear regulator ICs. They are generally very reliable, but the failures I've encountered have been those that have run too hot or have been overloaded. It seems that some designers are happy to trust what the datasheets say, but I've always preferred to be more cautious. Earlier I mentioned that the regulator IC dies can't run as hot as a simple transistor - only 125 degrees C for many. Contrast that with 200 degrees for a 2N3055. Also, the thermal resistance from junction to case is a massive 4 degrees per watt (though some sources say 2.3C/W) for the LM317K. Contrast that with the 2N3055 at 1.52C/W. For all of these reasons, I would always prefer a transistor over a beefy (and expensive) regulator IC. Hope this is of interest, Mark |
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5th Dec 2020, 12:33 am | #46 | |
Octode
Join Date: May 2007
Location: Liss, Hampshire, UK.
Posts: 1,873
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Re: 2N3055 series pass questions.
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Parts that I used for a design over 10 years ago are still available. The vast majority of the DC-DC convertors that I used were made by Traco with a few made by Vicor - not the cheapest but they seemed very reliable. It may also be worth saying that some of the Vicor supplies are designed to be used in current limiting mode. They reduce their output voltage if the current goes above a certain threshold and restore the voltage when the current reduces. |
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5th Dec 2020, 1:38 pm | #47 |
Octode
Join Date: Dec 2008
Location: Wincanton, Somerset, UK.
Posts: 1,780
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Re: 2N3055 series pass questions.
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5th Dec 2020, 1:41 pm | #48 | |
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Re: 2N3055 series pass questions.
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5th Dec 2020, 2:39 pm | #49 |
Octode
Join Date: Dec 2008
Location: Wincanton, Somerset, UK.
Posts: 1,780
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Re: 2N3055 series pass questions.
Well, the problem with 'yer traditional capacitor, she don't pass DC.
And them four diodes, they's only pass DC! J |
6th Dec 2020, 12:09 am | #50 |
Dekatron
Join Date: Feb 2007
Location: Leominster, Herefordshire, UK.
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Re: 2N3055 series pass questions.
They can pass a fair amount of AC too- the ripple current in the smoothing cap isn't DC.....
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6th Dec 2020, 10:05 am | #51 |
Dekatron
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Location: Sleaford, Lincs. UK.
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Re: 2N3055 series pass questions.
Thanks Mark, I didn't think to look there.
Andy.
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6th Dec 2020, 11:00 am | #52 | |
Octode
Join Date: Dec 2008
Location: Wincanton, Somerset, UK.
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Re: 2N3055 series pass questions.
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