Last updated: 4 February 2002

Circuit Polarity Protection

Sent:	Saturday, January 5, 2002 10:32:26
From: (Richard Seymour)
>I was wondering if there isn't some kind of gizmo or circuit that can be
>added to the power cable that would inhibit power going to the ETX if
>the polarity is wrong.

And the answer is: Yes.  It's called a "diode" (die-ode) or "rectifier"
(two names for the same thing).  Dirt cheap ($0.10?).
What they do is allow current to pass in only one direction.
If you put one in series as a part of your power cord you will -totally-
protect yourself from reversed battery connections.

  (battery +)-----|>|------o  ETX connector middle pin

That -|>|- in the middle is the diode.  You only need one, in one wire.

The drawing symbol for a diode is an arrow facing a vertical bar.

Some of the "homebuilt" powercords drawn on Mike's site show them.

Buy one with at least a 1 amp current rating, or 2 amps for an LX90.
Any votage rating above 20 volts (110 volts is fine).

Unlike a fuse, they are -not- destroyed protecting your telescope.
When you get the wires back into the proper order, the diode will
still be happily functioning.

They do cause the -loss- of about 1 volt as the current passes 
through them, so you'll be dropping your effective battery voltage
from 12v to 11v.  The price one pays for safety.

have fun
From: (Fernando Pertuz)

Thanks for your quick reply. It gave me an idea,
I'll try it and if it works I'll make it public.

Thanks & Happy 2002

Sent:	Monday, January 7, 2002 06:55:14
From: (Tom Kulaga)
Using a bridge rectifier is a quick and elegant way to ensure that your
scope is not fried by crossing the cables.  This is going to be my next
mod to my ETX-60, after I add a focus extension.

Mike has the following Tech Tip under Helpful Information:

Alternative Power Supply (3/15/00)

And this:
Sent:	Monday, January 7, 2002 09:09:50
From: (Shekhar Athavale)
You can use a simple diode bridge with DC output terminals connected to
the scope and your external power supply to the AC input terminals.This
way you will always be assured of right polarity.If you need a circuit
diagram, please let me know. I can mail it to you. Regards
From: (Shekhar Athavale)
I am enclosing herewith a schematic of diode bridge for polarity protection. 
The schematic uses 4 nos. of 1N4003 diodes. But you can use any no. upto 
1N4007. Alternatively you may look for a ready made diode bridge 25 volts , 
1Amp. capacity. I suppose these should be available from any electronic 
component shop such as Fry's or Radio Shack. Do not forget to add  that 100 
K resistance.Have tension free viewing with your ETX
polarity protection

And more:

Subject:	Response to Polarity Woes.
Sent:	Wednesday, January 9, 2002 06:03:52
From: (David Tindall)
I am a television technician and know of a very simple way to prevent
reverse polarity damaging the electronics in any telescope base unit.
All that is required is a device called a Bridge Rectifier, which
contains 4 diodes. The Bridge Rectifier needs to have a voltage rating
of 50volts @ 2amps. A higher rating is ok, but the device will be
physically bigger. The device is connected in series between the power
supply and the telescope.
A typical bridge will have four terminals marked as follows:
~ + ~ -   That's: AC. Positive. AC. Negative.
You connect the output from the power supply (or battery) to the two ~
AC terminals, it doesn't matter which way they are connected.
Then, connnect the + Positive terminal to the Positive wire of the lead
running to the 'scope.
The - Negative terminal connects to the Negative lead.
No matter which way the power is connected to the circuit, the polarity
will always be correct at the telescope.
David Tindall, Queensland, Australia.
polarity protection

Subject:	Polarity Woes
Sent:	Sunday, February 3, 2002 12:50:08
From: (RB)
Raphael BERTHE from France here. An answer for the General Feedback page

I saw the Fernando Pertuz ( question about a gizmo
for protection against polarity reversing of external power supply.

As an electrical engineer :-), I will use a device like that :

Power (+)----------|______|-----o-----------O To center Jack
                    Fuse        |
Battery                        _|_                To ETX
                               /_\ D1
Power (-)-----------------------o-----------O To Outer Jack

In normal use, the diode D1, in reverse, don't let the current pass. But
if the polarity on the battery side is reversed, the diode is polarized,
and the current is drawn into it. As the internal resistance of a
polarized diode is low, the current is high, and the fuse must blow !

The blow-out current of fuse must be high enough for a ETX normal
consumption, and lower than the power supply short circuit current. I
have no idea of an ETX+Autostar power current draw, but I think that a
250mA fuse should be a good choice. The lower the fuse current, the
safer it is! I think it is better to have to change a blowed fuse in
normal use for one with a little greater blow-out current, than to let
the diode or the battery fries...

For the diode, any standard diode with a reasonable power standing
should be fine. I think that 1N400x series (1N4001, 1N4002, etc...) are
suitable. The lower the threshold voltage is, the better it is. The
Radio shack stores can provide the components for few bucks, and with a
little electrical and soldering skills, that's do it !

DISCLAIMER : I haven't tested yet this scheme in a real inversion error,
with a real ETX on the jack side ! If your ETX fries, despite of this
wiring, you are on your own !

Voil, I hope this is clear enough.

Mike, I will be pleased if you can correct some of my huge English
mistakes, if any, before publishing it, I will be less ashamed... Thanks!

Thanks you again for your great site !

Clear Skies,


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