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5000watt510
| | Posted on Sunday, December 21, 2003 - 3:47 pm: | 
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Hello, I'm rebuilding a 35 amp power supply and was wondering how much difference i would notice swapping the 2n3055 trans. with the 2n3771 trans. Also is there a supplier where i can get them at a good price. The people at the local Rat Shack are pretty stupid and i don't wanna have to deal with them again!!! Thanks for any help.. cef 246 5000watt510 |
Tech8541
| | Posted on Monday, December 22, 2003 - 9:54 pm: |
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if it is current regulated, you will not notice any difference in the power supply. it may run a little cooler and will be harder to kill.
you can get those trans from any part supplier. the ecg equivalent is nte/ecg181. i don't recommend using those unless the power supply has 'ballasting' reistors to keep things balanced. if you don't know about the ballasting part, don't use the 181's. |
Bullet
| | Posted on Tuesday, December 23, 2003 - 12:30 am: |
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rf parts is were i got my 3771's |
Znut
| | Posted on Tuesday, December 23, 2003 - 11:14 am: |
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You can't beat the price from Mouser on those 3055's made by SGS/Thompson (they also make the wildly popular SD1446's found in many amps.)
At least then you know the PS will operate well and won't worry about flooding those 3771's. |
2600
| | Posted on Friday, December 26, 2003 - 1:26 am: |
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Uh, guys the 3771 is rated for 30 Amps. The 3055 is a rugged transistor, but has only half the current rating, 15 Amps.
If the original design calls for 3055s, they should do just fine. The 3771 is a lot more rugged, in practice.
The only difference you will see is how long it lasts under stress. If you treat it well, there will be no difference to tell at all.
The higher rating of the 3771 won't magically increase the rating of the REST of a power supply. Think of a power supply as a water hose. Making only part of it wider won't get you any more gallons per minute to flow through it. It's only as wide as the narrowest part.
73 |
Bigbob
| | Posted on Sunday, December 28, 2003 - 9:38 am: |
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YOU gotta disable the current limmiter then put in a transformer twice as big then your new transistors will work,you may even be able to rework the current limmiter,but then you could probably buy a bigger p.s. with a WARRANTY in case your rig goes POOF for the same price.Bigbob P.S. you would need twice as much filter capacitor,too,getting pricy aint it! |
Znut
| | Posted on Sunday, December 28, 2003 - 6:07 pm: |
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I'm with Bigbob. Save your trouble and money and use the really inexpensive exact replacements. I did so with mine and it has worked fine. I nail it all the time with more draw than it's rated for. What originally killed it was static from a nearby lightning strike! |
Bigbob
| | Posted on Wednesday, January 07, 2004 - 9:46 pm: |
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Ya know,I dicovered something,you gotta look at the watt rating more than the amp rating,check to see what the draw down voltage is,then take that no.and divide it into the watts and you get the true amps out,very enlightening.Say your ps idles at 27 volts input,then with a load it drops to 23 volts,115 watts divided by 23 volts equals 5 amps,my supply draws down to 28 volts from 32,it has 9 150 watt pnp pass trans. for a total of 48 amps,now these are 15 amp transistors but at 10 volts(mj2955),the 2n3055 will pass 15 amps at 7.6 volts,ohms law: amps x volts =watts(part of it anyway).I have a pyrimid 10 amp ps with 2 2n3055 trans.it just happens to idle at 27 volts and produces 10 amps at 23 volts(input)13.8 volts(output)it is 8 amps continuous,10 amps intermittent,if input voltage drops to 22 volts the overload protector kicks in,my homebrew ps was originally set up as a 20 amp with 5 trans.but I needed more for a bigger amp,just so happened I got a transformer free from a ham friend,perfect added 4 more trans.and removed the current limmiter,the mains are fused with a 5 amp fuse,just right.Someday soon I want to put in a 60 amp battery charger transformer,it has 8 gauge secondary windings,and idles at 28 volts,GAWD,I could put 10 more trans. on and really have a light dimmer,those are rated at 200 amp boost.Bigbob |
Tech8541
| | Posted on Wednesday, January 07, 2004 - 11:51 pm: |
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bigbob, you may want to take this back to the drawing board. the watt rating is how many watts the device can dissipate as heat, not how many watts it can put out. this heat comes from the voltage drop across the device. like the 23v input and 14v output. the power drop across this device is in the form of heat.
you have to take this into consideration when building a power supply. lets take the 2n3055 for example. it can dissipate 115w and pass 15amps of collector current. the 115w is 115w no matter what, but the 15amps is not always the case. lets take your 23v in and 14v out example. that means 9v is being dropped across the 3055. so 115w/9v = 12.8amps. this means that with a 9v drop across the device, it can only pass 12.8amps without surpassing its max dissapation(115w).
the 2n3771 is rated at 150w dissipation and 30amps of collector current. many people look at the 3055 and 3771 and think the 3771 can pass twice the current. lets see. once again 23 in and 14 out. that is 9v dropped across the device. it can dissipate 150w, so 150w/9v = 16.7amps out before the heat dissipation is passed. that isn't twice the amperage of the 3055, but it is higher. the smaller increase in dissipation (115w to 150w) is what offsets the larger increase in collector current.
the only way to pass more current through these devices would be to lower the input to output voltage ratio. like lowering the input to 20v instead of 23v. then the voltage drop isn't as high, which means the heat dissipation will not be as high. |
Bigbob
| | Posted on Thursday, January 08, 2004 - 7:35 pm: |
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OR just add a whole bunch more transistors,spread out the load so to speak,thats why I'll sub. NTE 30s for the mj2955s,300 watts as compared to 150 watts,so it'll be a 180 instead of a 90,but actually it'll be more like a 90 istead of a 45,you see I haven't forgotten about the heat dissipation factor,I'm not stupid,man.Bigbob |
Tech8541
| | Posted on Thursday, January 08, 2004 - 9:07 pm: |
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bigbob, in one of your post that is not showing now it said that not all of the power dropped across the device would be heat or else there would be no power. YES all the power -DROPPED- across the device is heat. that is why the term dropped is used. the rest, what is not dropped or turned to heat, goes on to power whatever is external of the power supply.
no one is saying that you are stupid. your reasoning sounds correct, even to me, but it isn't correct. i'm just trying to show you why.
if you don't feel that i am calling you stupid, i would like to explain to you how there -IS- power left after the power dropped in the device. its your call. |
Bigbob
| | Posted on Friday, January 09, 2004 - 6:33 am: |
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O k I made some assumptions according to one anecdotal experience,But,I have a transformer,120 in 33 out,47000 mfd filter,40 amp bridge rect.,317t,and 9 mj2955 pnp pass transistors,originally 5 pass trans. but it would not power the boomer 400 amp,I had 5 15 amp trans.;75 amps,but a light bulb came on in my head and I looked at the watts,regardless of what formula I used,right or wrong,I came to the same conclusion as you,you have to look at total device dissipation and not just amps,you must have ohms law handy,just to get into the ballpark,so I added 4 more transistors,and voila problem solved,I don't care about the minutia, all I care is what works,and it works now ,and I really don't want an explanation,just save it for your college students,they'll appreciate it,thank you .Bigbob |
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