[sebhc] H17-3 Power supply

Lee Hart leeahart at earthlink.net
Sun Aug 6 00:30:23 CDT 2006

Mark Garlanger wrote:
> I just received an H17-3 drive case and drives... there is NO diode
> bridge on the power supply board. There is the spot to put 4 diodes,
> but two of them are empty and the other two (very top and bottom)
> just have a wire there...

The original H17 case held two floppy drives. These could safely be
powered by the four original Heath #57-42 (3A1 3amp 100v) diodes.

Later, Heath came out with a modification to put *three* floppy drives
in the H17 case. To make this work, they had to beef up the power supply
accordingly. They replaced these 3amp diodes with a bridge rectifier
mounted on the chassis for better cooling. It sounds like your H17 has
this modification.

> the local Fry's are 5A, 100V - I'm assuming that the higher amps
> just mean they can handle more and can be used in place of the 3A
> ones. Let me know if that is the wrong assumption. Also, Fry's
> had a square little 'diode bridge'... rated 8A, 100V...

The ratings on diodes, like most semiconductors, are absolute maximums
under ideal conditions. So 3 amps is the absolute maxiumum continuous
current the diode can carry with infinite cooling. The actual cooling is
far from perfect, so the *actual* current has to be considerably less
for it to survive.

In a bridge circuit, each diode conducts half the time (one half-cycle).
So, with a 3 amp DC load current, each diode is really carrying 3 amps
for half the time, or 1.5 amps average current. Thus is is running at
half its rated current. That's why they survive.

A bridge rectifier with all 4 diodes packed into the same case is less
effective at getting rid of heat -- each diode heats the others. So they
need higher-rated diodes to survive. An "8 amp" rated bridge is actually
good for no more than 4 amps continuous due to cooling limitations.
Ring the bells that you can ring
Forget the perfect offering
There is a crack in everything
That's how the light gets in
	-- Leonard Cohen
Lee A. Hart   814 8th Ave N   Sartell MN 56377  
leeahart_ at _earthlink.net

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