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G4EBT > TECH 24.09.03 16:16l 97 Lines 3435 Bytes #999 (0) @ WW
BID : D71558G4EBT
Read: DB0FHN GUEST OE7FMI
Subj: Re: Pwer Supply Fan Mod
Path: DB0FHN<DB0RGB<OK0PPL<DB0RES<ON0AR<GB7FCR
Sent: 030924/1039Z @:GB7FCR.#16.GBR.EU #:16414 [Blackpool] FBB-7.03a $:D71558G4
From: G4EBT@GB7FCR.#16.GBR.EU
To : TECH@WW
Not wishing to labour the point, I would again sound a note of caution
concerning messing with the cooling arrangements on these PSUs. As
previously explained, the heatsinking is very poor, and the heatsink
is shielded from the fan by the transformer, further exacerbating the
problem.
It should be evident to anyone who takes the lid off, that these PSU's are
very much built down to a price rather than up to a standard. It has to be
admitted that there are many thousands in use all over the world that have
given trouble free service. Certainly five 2N3055's are up to the task,
but it is puzzling why in several I've come across, all five 2N3055's have
(thankfully) gone open circuit. Had they gone short circuit from emitter
to collector, that would have plonked some 25 V across any equipment
connected to the PSU.
The equipment might stand that, but it isn't a risk I'd be willing to
take. It may be that many - perhaps most, amateurs operate these PSU's at
well below their claimed output of "25A continuous", and instead, run
them at perhaps no more than 5 Amps at say a 50% duty cycle, as would
probably apply if using them on packet. (Say "50/50 TX/RX for a few
minutes an hour).
The Manson EP925 has the following component count, excluding the
transformer, meters and so on:
44 R's
23 C's
7 Transistors
13 Diodes
1 Bridge
2 IC's
90 total.
Of those, the following are in the fan control circuitry:
0.5 IC (shared with PSU)
1 Transistor
6 R's
2 C's
2 diodes
1 Thermostat
13 total.
The proposed fan speed modification introduces another 12 components,
taking the total component count up to 102. Of that total, 25% will then
be in the fan control circuitry, to control a fan that does little to cool
the heatsink in the first place.
I'm not quite sure what it is that users hope to achieve by that. I can
only quote from an excellent article by G4KUB and G4KUC, Stephen and John
Goodier,0 in the now defunct Amateur Radio Magazine back in 1989 when
specifying the requirements for a 13.8V 20 A PSU for amateur use:
Quote:
Protection Circuits:
"Any PSU that provides power to an expensive piece of equipment needs to
have a certain amount of protection built in. The first of these is an
over-voltage sensor, which can be based around the MC3423 over-voltage
crowbar protector".
"If you consider what happens when a short circuit develops between the
emitter and collector of any pass transistors, then you understand why
such a circuit is necessary. If this fault does develop, then all of the
unregulated supply (about 25 Volts) will appear on the output terminals".
End quote.
It does seem ironic that whereas such an over-voltage protection circuit
would use say a thryristor, four R's, a preset pot, and a cap - 7
components in total, the Manson EP925 (and similar variants) which offer
no over-voltage protection, already uses 13 components in the fan control
circuitry, and some people seem intent on adding another dozen.
For my money, and outboard crowbar circuit would be a must, unless I was
just using the PSU to float charge a battery or run some fairy lights at
Christmas.
Happy homebrewing!
73 - David, G4EBT @ GB7FCR
Eddystone User Group Member
G-QRP Club Member No: 1339
QTH: Cottingham, East Yorkshire.
david@crofters89.freeserve.co.uk
Message timed: 11:35 on 2003-Sep-24
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