>-----Original Message-----
>From: Russell McMahon [mailto:apptech@PARADISE.NET.NZ]
>Sent: 25 June 2004 13:18
>To: PICLIST@MITVMA.MIT.EDU
>Subject: Re: [EE:] Current sharing resistors
>
>
>> Without going down the whole reverse engineering, desgn calculation
>> path, does someone have a simple, rule of thumb value for emitter
>> resistors to promote current sharing in paralleled bipolar
>transistors
>> ?
>>
>> Now that I actually need 5A out of my 5A bench supply (Wonix YW-
>> APS305 from Dick Smith for those who may also have one), I find that
>> they have 2 2N3055's in parallel (all three leads) and
>invariably, one
>> of them cooks and dies. Pity they put thermal sensing on the other
>> one.
>
>
>I hope that's not the DSE Q1760, 30v, 5A supply that I just
>bought :-) CLAIMS to do 5A "at all selectable output voltages".
>
>A bipolar transistor has ABOUT 0.6v Vbe when driven and more
>like 0.7V on a power device under heavy drive. If you cause
>0.1v emitter drop at full current that would start to haven a
>significant affect and 0.2v more so.
>
>So if it's 5A/2 = 2.5A each then R = 0.2/2.5 = 0r8 per
>transistor should be about right. Either 0r68 or 1r0 are
>probably available at DSE. Power = 2.5 x 0.2 = 0.5w so a 1w
>would do and 1 5w cheapo wire wound would run cool. .
>
>1r0 5w    R1602        in stock everywhere
>0r47        R1601
>
>No 0r68s

Think you'd better recheck the calculations! :o)

0.2/2.5 = 0.08 Ohms

A 1R0 resistor would be dissipating 6.25 Watts!

Regards




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