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M1BYT > TECH 17.09.03 18:41l 138 Lines 6024 Bytes #999 (0) @ WW
BID : 3C1858M1BYT
Read: DB0FHN GUEST OE7FMI
Subj: Re: Power supply fan mod?
Path: DB0FHN<DB0RGB<OK0PPL<DB0RES<ON0BEL<SP7MGD<WB0TAX<N7FSP<GB7FCR
Sent: 030917/1552Z @:GB7FCR.#16.GBR.EU #:15326 [Blackpool] FBB-7.03a $:3C1858M1
From: M1BYT@GB7FCR.#16.GBR.EU
To : TECH@WW
M1BYT Opined with considerable skill:-
> Hi,
>
> A couple of years or so ago, I sent out details of a modification I made
> to a PSU which enabled the fan to run all the time at low speed, rather
> than cutting in solely under control of a thermal switch. It helped keep
> the PSU quieter, in as much as the fan would cut in at full speed much
> less frequently.
A few of you asked for this modification to be resent, however I do not
have a copy of my original text to resubmit it. I have therefore pulled
apart a couple of the modified PSU's to reverse engineer what I did...
This modification applies to a popular PSU, which is sold under a
variety of brand names, with ratings of 15A, 20A, 25A and 30A (It may
apply to others in the same range). Two of the names I have seen are
'Manson' and 'Lowe'.
Looking at the front panel, there is an ammeter on the right and a
voltmeter on the left, with an output voltage pot in the top RH corner,
which drops into a 'notch' at 13.8v Two large current terminal and two
pairs of lighter current ones. Power switch in the bottom RH corner,
power LED and an over current LED. Not all versions had the same
options, some lacked the meters, others lacked a cooling fan.
A small fan is fitted on the rear case panel for cooling, which runs
triggered from a thermal switch on the regulator heatsink. This is very
noisy, when it does need to cut in at full speed and this modification
enables it run at a tickover all of the time, which delays the need for
the fan to cut in at full speed and might even prevent the need. By
keeping the PSU cooler, it also prolongs its reliability and longevity.
The fan control circuit is needlessly complex, involving a large thermal
switch, an op-amp comparator, a current boosting transistor then finally
a switching transistor. All that just to turn the fan on ;-)
The modification....
Componenents 1N4001 22K (?) Switch (plus a bit of wire)
___ _/
+----------->|----+----|___|----+-----o/ o----------+
A B C D
Remove the top U shaped cover completely. Below the fan, held to the
bottm casing a TO??? transistors heatsink, should be a small pcb (solder
side up). This is the switching transistor and PCB for the fan.
Looking from the front of the unit, at the dusty inside part of the fan,
connect point B to the LH solder connection nearest to you, on that
small fan control PCB. Connect point D to the RH solder connection
nearest you. Point C is just a joint which should be covered with a
little sleeving.
Now for the difficult to find connection point.... Remove the two screws
in the front feet, which allow a little freedom of movement of the front
panel. Turn the whole unit around, so you are looking in from the back
with it the usual way up. Look at the solder side of the big square PCB,
the ammeter is soldered onto this PCB with two large flat tabs which fit
through slots in the PCB. A little way down and to the right of these
tabs, is a thin lonely wire soldered in place. This is the attachment
point for point A.
In one example this lonely wire was red, in another it was green, but
just to confirm the location.... It should be almost dead centre of this
PCB V and H plus level with the an IC, which is slightly to the right of
the point you are looking for. It is not easy to get your fingers in,
but it is worth struggling rather than the much longer job of getting
the front panel completely off.
Going back to the circuit, point A to the diode needs to be a long
length of wire (10" ?). point C to the switch and from the switch to D,
can be about 4", which enables the switch to be fitted on the back
panel. The switch is of course optional, if you do not fit one C should
be linked to D. Or to put it another way, fit the resistor directly
between B and D on the small fan control PCB. I used a single hole
fixing toggle type switch.
Now for a little experimentation with the value of the resistor....
The additonal circuit works from an unregulated source, which means that
the speed of the fan will decrease as the load on the PSU increases. The
value of the resistor may require a little adjustment, so that the fan
has enough voltage so it starts up reliably and runs even under full
loading of the PSU.... Yet you don't want it running so fast that it
becomes noisy. The compromise is yours, or it could be replaced with a
25k pot.
What it does.....
The fan should now run at a very much reduced speed all of the time the
PSU is turned on. If this small amount of air flow proves at times to be
not enough, the thermal switch should cut in and run the fan at full
speed exactly as it did before.
There...
I take no responsibility for any ensuing damage you may do to your PSU
and I cannot guarantee that my description is absolutely accurate. I
also cannot guarantee your PSU is identical to mine. I have done my best
to reverse engineer what I did. Please consider it an experimenters
circuit and report any success or errors you find with respect to my
suggested modification.
If asked, I may be able to email digital photos of the relevant points I
have described above.
ÉÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍ»
º 73 from Harry, M1BYT @ GB7FCR.#16.GBR.EU º
ÌÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍ͹
º International Museums Weekend 19th+20th June 2004 º
º www.ukradioamateur.org/imw/index.html º
ÌÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍ͹
º The ARC's FL/RAE courses + Newcomers Intro to º
º to amateur radio:- http://www.ukradioamateur.org/ º
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º QTH: Garforth, W. Yorks.. º Timed 15:48 º
º harry.m1byt@tiscali.co.uk º 17 Sep 2003 º
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