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Old 28th Nov 2020, 3:42 pm   #21
Lucien Nunes
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Default Re: 2N3055 series pass questions.

If you are short of space, how about this: 9-36V input, 6.5V 2A output, size of a sugar cube, minimal heat dissipation:
Traco 6.5V 2A buck regulator at Mouser
You get an extra 0.2V to overcome wiring drop if needed, or you can lose it in a resistor or filtering if you need any. I use lots of these little Traco regulators, they have been excellent; One thing to avoid is stressing the leadouts too much, they can detach internally. Definitely recommend a crowbar with any buck regulator with this wide disparity between input and output. It can be very simple and slow because the load has a long thermal time constant.

TBH I would never consider dumping 40W of heat to regulate 2A of heaters; that's enough wastage to power a modest complete listening system!
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Old 28th Nov 2020, 3:49 pm   #22
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Default Re: 2N3055 series pass questions.

Quote:
Originally Posted by Radio Wrangler View Post

If it's about as stable as the mains voltage, you could just use a big resistor.
That's an idea, one I'll experiment with

The problem I have is that I don't want to go making too many changes. The valves, two 6H6N (750mA) and 6AU6 (300mA) are wired in parallel, i can't change that. The tfmr winding is tricky to change due to space restrictions, the same applies to space within the chassis. I'm kinda stuck with making samll changes to the existing circuit. It really could do with a complete re-think, redesign.

Thanks Lucien, that looks the job.

You've all given me plenty to cogitate on, I'll have a think about all your suggestions and a bit of an experiment this afternoon and get back to you all.


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Old 28th Nov 2020, 4:02 pm   #23
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Default Re: 2N3055 series pass questions.

Had a play in LTSpice and the thing below gives 6.4V RMS.
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Old 28th Nov 2020, 4:04 pm   #24
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Default Re: 2N3055 series pass questions.

I bet there is some clever way of putting the cap after the rectifier so it doesn't need to be unpolarised!
 
Old 28th Nov 2020, 4:31 pm   #25
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Default Re: 2N3055 series pass questions.

Quote:
Originally Posted by Lucien Nunes View Post
If you are short of space, how about this: 9-36V input, 6.5V 2A output, size of a sugar cube, minimal heat dissipation:
Traco 6.5V 2A buck regulator at Mouser
Those Traco DC-DC converters are excellent, I used one recently (not the same family) to get a 1W, 9v supply from USB (5V). Very little noise on the output and run cool despite being tiny.
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Old 28th Nov 2020, 4:34 pm   #26
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Default Re: 2N3055 series pass questions.

I'm pretty sure I got one from RS or Farnell a while back, to replace a discrete sub-regulator on an SMPSU that had vapourised itself beyond repair. It enabled an economic repair from something I'd otherwise have to scrap.
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Old 28th Nov 2020, 5:48 pm   #27
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Default Re: 2N3055 series pass questions.

Hi a few ideas.
1, AC feed the valve via a Diode and a 7 ohm resistor rated at 25 Watts.
2, A buck converter will give the output you need.
3, Remove turns from the secondary of the transformer.
4, See if there is a 300ma "P" series valve that'll do.

What valve are you intending to use?
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Old 28th Nov 2020, 5:51 pm   #28
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Default Re: 2N3055 series pass questions.

surely those three valves in series with a suitable R across the 300mA one turns the whole thing into simplicity using existing setup?
Or am I missing something too obvious?
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Old 28th Nov 2020, 8:21 pm   #29
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Default Re: 2N3055 series pass questions.

Hi.
I'd agree with Les here. I missed the post with the valve line up. A balancing resistor and wiring the valves in series would certainly reduce the dissipation in the 2N3055 down to about 7.5 Watts.

From what I can gather though is heater current variations between 6H6N valves, between 680ma & 750ma.
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Old 28th Nov 2020, 11:44 pm   #30
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Default Re: 2N3055 series pass questions.

Quote:
Originally Posted by Lucien Nunes View Post
If you are short of space, how about this: 9-36V input, 6.5V 2A output, size of a sugar cube, minimal heat dissipation:
Traco 6.5V 2A buck regulator at Mouser
You get an extra 0.2V to overcome wiring drop if needed, or you can lose it in a resistor or filtering if you need any. I use lots of these little Traco regulators, they have been excellent;
I've also used the slightly smaller TSR1 version of that a few times and never had any problems with them. They're a great alternative to the 78xx series when you want to waste as little power as possible.
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Old 29th Nov 2020, 10:22 am   #31
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Default Re: 2N3055 series pass questions.

Quote:
surely those three valves in series with a suitable R across the 300mA one turns the whole thing into simplicity using existing setup?
Another good idea Les, thanks.

Thanks too to Tevor et al.

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Old 30th Nov 2020, 10:49 am   #32
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Default Re: 2N3055 series pass questions.

Ok, valves in series don't work, as I two different valves the 6AU6 glows like a lightbulb dropping most of the voltage across it.

I have been messing with pre-regulation as follows, I need a 12v supply for a couple of fans,so my idea is/was to use a variation on this circuit - http://www.rason.org/Projects/vreg/vreg.htm but with two TIP3055's in parallel, the only problem is it slams shut with a full 28v at SW on,if I backthe IP voltage off to 15v IP,then up it to 28v it works ok.

The idea then being to use the existing LM317/one TIP3055 6.3v reg after this. As mentioned though this is less than ideal or efficient.

Looking at the piggy bank regarding the buck convertors Lucien linked to,but if not my gut feeling is to re-wind the tfmr, the thought of hours of work dis-mantelling the tfmr then re-winding it doesn't appeal, though it is the best solution and most efficient.

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Old 30th Nov 2020, 12:47 pm   #33
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Default Re: 2N3055 series pass questions.

I would become tempted to rewind an old CRT screen power supply.
The simple ones use a common 8 pin chip and 6 pin opto. There are loads of data sheets on line.
The opto driver on the isolated side monitors the heater supply for the CRT where the capacitor has a hard life and brings the set in for repair before more damaging faults occur.
The cores are quite hard to get apart but can be done.
You would need to take off all the secondary windings noting the number of turns on the heater winding.
Rewind the heater with hefty wire and add a few more turns and another diode for the fan.
It should easily go into the space the iron transformer took up with screening.
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Old 1st Dec 2020, 11:26 am   #34
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Default Re: 2N3055 series pass questions.

"Ok, valves in series don't work, as I two different valves the 6AU6 glows like a lightbulb dropping most of the voltage across it."


Did you remember to add a 14 ohm 3W or greater resistor in parallel with the 6AU6's heater to pass the extra 450mA? Without it, the 6AU6 heater will glow rather brightly.
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Old 2nd Dec 2020, 8:11 am   #35
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Default Re: 2N3055 series pass questions.

Quote:
Did you remember to add a 14 ohm 3W or greater resistor in parallel with the 6AU6's heater
No, that'll be why it was doing a good imitation of a Xmas tree light : )

In the end I ripped the tfmr out, pulled off 5 windings and undid hours of work then rewound it for 10v ish to give 14v, this gives DC without ripple and an extra 2v dropout for a 12v 7812 to drive a couple of fans. One TIP3055 copes ok with 7v x 2A = 14w diss. A lot better than the 40w I had previously.

I haven't disregarded all your suggestions, they're all logged away in a notebook for future reference.

One last Q, the TIP3055 is rated for 90w, the 2N3055 in TO3 package 100W; when designing a series pass element how much dissipation is about right? Obviously not 100%. I know it's about temp junc etc but without a Flir thermo camera or HS with a spec sheet I wondered if there's a rule of thumb?

Andy.
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Old 2nd Dec 2020, 9:58 am   #36
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Default Re: 2N3055 series pass questions.

Hi Andy,

It can be worth buying a digital temp gauge. I have one with a simple trigger which instantly displays the surface temp of power semiconductors and helped me to see that I needed a bigger heat spreader for a bridge rectifier.

As you know acceptable power dissipation in watts goes down as temp goes up, so you’re in the dark if you don’t know how good your heatsink is or what temp the power semiconductor is cooking at.

The upper limit temp of 150 degrees C is breathtaking but only makes sense if you know the huge derating that goes with it. Conversely , it’s good to have a sense of what 40 degrees feels like to a finger, or 60 degrees.

It’s been quite helpful to get my head around the complex heat characteristics of heatsink s by placing one in an oven and measuring all around it ... not what I expected at all.
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Old 2nd Dec 2020, 10:23 am   #37
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Default Re: 2N3055 series pass questions.

I would limit Ic to 5A or less for a single 3055 as a series regulator. If you wish to use a lower number of devices try 2N3772 (3771,3773) which have double the current rating.

However if you're using a regulator driver e.g. 78S15, you might be better off using
a Darlington device. I had this problem a while back and found I had some BDX67s in the spares box, problem solved.
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Old 2nd Dec 2020, 12:01 pm   #38
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Default Re: 2N3055 series pass questions.

Quote:
Originally Posted by Diabolical Artificer View Post
One last Q, the TIP3055 is rated for 90w, the 2N3055 in TO3 package 100W; when designing a series pass element how much dissipation is about right? Obviously not 100%. I know it's about temp junc etc but without a Flir thermo camera or HS with a spec sheet I wondered if there's a rule of thumb?

Andy.
The maximum power dissipation of a transistor depends on case temperature.

A 2N3055 can only dissipate its rated power of 115W if you hold the case at 25 degrees C. If you have a case temperature of 100 degrees, then the maximum allowed dissipation is around 70 watts. To see that, look at the graph on page 1 of this datasheet: https://www.onsemi.com/pub/Collateral/2N3055-D.PDF

This applies to all power devices, but not all give a graph. For example, the MJ15003 datasheet states the maximum of "250W at 25 degrees, de-rate above 25 at 1.43W/C". Meaning if you run the case at 45 degrees, the maximum power rating is 250 minus 20 times 1.43, or 221W.

This is because the junction can't exceed a certain temperature (200 degrees for both these transistors), but remember that there is thermal resistance between the case and junction. This means that for a given case temperature, the junction will be hotter.

The thermal resistance between junction and case is 1.52 degrees C per watt for the 2N3055 and 0.7 C/W for the MJ15003 - the reason for the lower resistance for the MJ15003 is because the die is much larger, so there is more contact area to transfer the heat. Some cool pictures of the MJ1500x transistors and some fakes here: https://sound-au.com/fake/counterfeit-p1.htm

So when you measure the temperature of the case, you have to know the thermal resistance from junction to case to calculate the actual die temperature. But follow the datasheet guidelines described above to stay safely within the limit.

Basically, those datasheet guidelines are a simple way to ensure the maximum junction temperature is not exceeded. To prove that, let's do a quick sample calculation:

If we ask your 2N3055 is to dissipate 40 watts, and the thermal resistance is 1.52 degrees C per watt, that tells you that the temperature difference either side of that thermal resistance is a whopping 60.8 degrees C. At a junction temperature of 200C, the case will be at 139 degrees C.

Looking to the graph, I'd say that 139 degrees C does indeed imply a maximum power rating of 40W. So there you go - it's that simple. It's all about the thermal resistance from junction to case
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Old 2nd Dec 2020, 1:36 pm   #39
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Default Re: 2N3055 series pass questions.

Marks explanation is spot on. Once you have the operating conditions double check the datasheet to see if you are still inside S.O.A with respect to secondary breakdown, given the volts dropped C-E and the collector current.

Bit of a puzzle why your original attempt at current sharing with emitter resistors didn't work ...

dc
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Old 2nd Dec 2020, 9:39 pm   #40
Peter F4VSA
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Default Re: 2N3055 series pass questions.

Please find my thoughts on the problem. From a reliability standpoint dissipating a lot of heat in a small space is never a good idea and if a fan is used this is likely to be a big failure point and will be noisy. The ebay switcher regulators are normally good and can be adjusted to 6.3v if needed. Maybe get a 3a version for £3.26, free post. If worried about noise run it at a higher voltage followed by a linear regulator for the last few volts. There will still be some radiated rf from the switcher though, depends on the application.

This has worked for me in the past for audio applications

Cheers Peter
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