Last updated: 10 April 2003
|
|
Last updated: 10 April 2003 |
|
Power Supplies
Subject: A warning of sorts
Sent: Wednesday, April 9, 2003 08:13:38
From: "Cass Alexander" (cassx@bigpond.com)
Thank you for your warnings items.
Earlier today, I posted the following reply to the LXD55 group regarding
a grab bag of problems which could be faced by anyone considering using
a mains to 12 volt DC converter with their LXD55 (or any other motorised
scope for that matter). There is a real sting at the end of the tale
(pardon the pun)!
--- In LXD55telescopes@yahoogroups.com, "zackar32" wrote:
>Have any of you built a power supply for the LXD55, converting household
>110v AC to the needed 12v Dc?, and if you have, where did you get the
>parts, Radio Shack? Cuz it seems to me that $80 for a power adapter is
>ripping someone off, don't you think?
I agree wholeheartedly with the battery solution raised in other
responses. Except in very special circumstances, a 12 volt Sealed
Lead-Acid Battery with 7 amp hour (or greater) capacity really is the
ONLY solution Here's why:
If you DO go with a mains converter, you'd have to make sure that it
provides WELL REGULATED 12V DC supply. Basic plug packs (even some of
the higher current units) consist of a transformer, a couple of diodes
and an electrolytic capacitor (around 470 ~ 1000 microfarad). There are
several problems with these ...
a) The 12V DC is NOMINAL : what this means is that if the converter is
rated to supply (say) 1.5 Amps, then under circumstances when 1.5Amps is
actually being drained out of the device, the output voltage will be 12
volts. When the LXD55 is in fast slew mode, I seem to remember that its'
current drain is 1.5 amps. However, if you're drawing significantly less
than the rated current drain (such as in siderial tracking) the output
voltage is likely to shoot up to a value which is significantly higher
than 12V. If the LXD55 telescope has no over-voltage protection (and I
simply don't know whether it has or not), then it's a case of bye-bye
electronics & so-long motors.
b) Even if the LXD55 has over-voltage protection, chances are that the
on-board regulation components are selected to work with voltages up to
about 13.5V, the sort of voltage which may be sourced from a 12V auto
battery. If the LXD55 electronics are forced to reduce a significantly
over-voltage mains converter output (say 16.5 volts - I seen quite a few
of those) then the heat which must be dissipated within the telescope's
internal regulation system may rise significantly : the more current you
draw, the greater the heat to be dissipated .... except, of course, that
the higher current has probably dropped the converter output voltage so
we're back to where we started.
c) Finally, there is the lack of hum filtration, the job that the
electrolytic capacitor is supposed to do. The output voltage may be
fairly smooth (little residual mains ripple) when the current draw is
low. Go to fast slew, and the ripple will jump higher than a pole
vaulter on steroids. High residual ripple levels from such a supply
would play havoc with any unprotected electronic components, and I'd
hate to think how the motors might respond.
SO ....
Let's now assume that you've found a regulated converter. If it's a
"continuous" regulator (i.e., the same components as for the unregulated
converter, PLUS a voltage regulator, a higher capacity smoothing
capictor, one or two resistors to set the voltage (maybe) and another
cap or two to prevent the regulator from going into uncontrolled
oscillations, then you've got two residual problems.
To handle the 1.5 amp fast slew current and guarantee minute levels of
ripple for the electronics when the scope motors are under maximum load,
you'd need a converter rated for three amps current draw. Units like
that often RUN HOT and their transformers are HEAVY.
"Ah", you say, "the man at Radio Shack recommended a switchmode
converter".
I'll bet he did!
The prices of such converters have dropped significantly, and although I
paid 150 Australian dollars for a 4 amp unit to run my LXD55 at a
reputable Astronomical Supplier's establishment, an identical unit was
advertised in a local electronics market's catalog for $49.95, so it
obviously pays to shop around.
There can be niggling "electronic noise" problems from switchmode
converters, which work by generating very high frequency oscillations
within their high voltage circuits, and are then able to use a
comparatively tiny transformer to step the "switched" high voltage
oscillations down to the nominal low output voltage which you'll be
using. Switchmode supplies can supply remarkable amounts of
well-regulated current (lots of ripple-free amps to play with).
However, their circuitry uses a very high speed solid-state switching
device (transistor, etc.) and coil, and can generate some fearful
high-frequency interference to sensitive electronic circuitry if not
properly shielded.
To be fair, on the one occasion that I used my $150 converter on my
brand new LXD55, both the converter and the scope appeared to work well.
Certainly the scope's primary fast slewing user-feedback mechanism
worked flawlessly .... lights came on in the house of every nerd-hating
neighbour within our block, its occupants no doubt shaken from their
slumbers by the seismic cacophony of my little scope doing it's thing!
Ah, revenge is sweet!
Several hours later, I discovered the real reason why I should have
invested in a battery pack instead of that overpriced box of goodies.
After my fast-slewing tests had been successfully completed ;), I
unplugged the converter from the telescope, then applied mild pressure
on both the converter plug and the female end of the extension cables
which led from the mains outlet in the house to the open garden area
where the telescope had been setup.
That was when the mains safety cutout switch fired, saving me from a
potentially fatal shock (Australia uses 240 volts, double the North
American potential).
Both cables were wet, very wet, and - as if to add insult to injury -
careful examination of the converter revealed the following piece of
advice in small lettering affixed to it's underside:
"INDOOR USE ONLY : NOT APPROVED FOR USE OUT OF DOORS"
So, the moral of the story is that unless you've got the LXD55 in a
permanent observatory setup with a floor that's electrically safe (the
exception noted at the start of this reply), don't risk running a
Mains-to-DC converter of any kind out of doors.
If you ignore the shocking potential of these ubiquitous devices in
pursuit of the stars, you may suddenly cease looking at heavenly objects
and become one yourself!
ADDITIONAL COMMENTS:
After my converter "experiences", I now use a triple 7.2 AH Gell Cell
case-mounted setup. Each of the batteries is protected by a 3A circuit
breaker. Polarised connectors on the case wall couple the batteries to
the various facilities on my LDX55 setup as follows:
- Telescope : Primary Battery and switched access
to reserve power from the Secondary Battery
- Ancillary Computer: Secondary Battery
- GPS Time Source: Secondary Battery
- Dew Controller: Tertiary Battery
- Tripod Lighting: Tertiary Battery
- Field Setup/Pulldown Lights: Tertiary Battery
I am currently upgrading the case by adding a high-efficiency solar
charging capability for sequential maintenance of the charge on all
three batteries for use both while on field trips and when the scope
isn't used for a few days.
Six dollar circuit breakers (or equivalent fuses) and polarised
connectors (to prevent inadvertent reversal of power supply polarity)
are both items which people often overlook in their enthusiasm to get a
new scope up & running.
Given that even a 7.2 AH gel cell can source more than 50 amps for a
short period, a fuse or circuit breaker is as important for fire
prevention as it is for normal circuit protection.
Anyway, if you feel that any or all of these comments are worthwhile,
please feel free to add them to your website.
Best wishes,
Cass Alexander
Return to the top of this page.
Go back to the Warnings Page.
Go back to my LXD Home Page.