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  A Budget Implementation of Battery Backup on an MTR2000 system
This page was developed by Mike Morris WA6ILQ from an email and attachments sent to Repeater-Builder by Eric Lemmon WB6FLY
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To: Repeater-Builder
From: Eric Lemmon WB6FLY
Date: 22-April-2007

I have finally gotten around to formalizing my battery backup circuit 
for the low power VHF or UHF MTR2000 station.  It has not been tested 
on the 100 watt VHF or UHF stations (which require 28 VDC for the PA 
deck) simply because I didn't have an isolated source of 28 VDC... yet 
I now have an Astron LS-35M power supply on order from AES, so I'll have 
the 28 volt design soon.

The 250 watt DC power supply that is common to all 30 and 40 watt 
MTR2000 stations has a dual PowerPole-type of DC input connector at 
the rear.  The connector is normally covered witha metal plate to 
discourage its use by clueless owners.

Since the MTR2000 station is designed to work with the very expensive 
Argus Battery Reverting Charger, I theorized that it must be okay to 
connect a battery to this connector without harming any components 
within the power supply.  Sure enough, the schematic reveals that 
there is an internal Schottky diode for isolation just upstream of the 
PowerPole connector.  All lower-voltage regulators are downstream of 
the PowerPole connector.  So far, so good!

Unfortunately, one cannot simply hook a 12 volt battery to the PowerPole
connector.  The good part is that when an AC power failure occurs, the
station will continue to operate on the battery without missing a beat. 
The station control module senses the loss of AC power and automatically
reduces the RF output power to a lower value (which is determined by one 
of the softpot settings within the RSS).  The RSS has an option that 
causes a periodic beep to be heard over the air when the station is on 
battery power. This feature is intended to remind the users to be brief.

The bad part is that when AC power returns, the station power supply 
will likely be overloaded when it tries to feed the transmitter while 
simultaneously the battery is sucking up several amperes of charging 
current.  It will probably go into self-protect mode, and shut itself 
down.  The solution to this problem is very simple:  Limit the charging 
current with a resistor!

I determined that the MTR2000 power supply could afford to supply no more
than two amperes of charging current if it was simultaneously supplying 
the power amplifier.  Assuming that the MTR2000 will automatically shut 
down if battery voltage drops to about 10 VDC, I figured that the resistor 
should be sized to limit current to two amperes when the worst-case disparity 
was four volts.  Ohm's Law says that would be two ohms.  Although a maximum 
of eight watts might be dissipated by this resistor, I wanted to use a 
heavy-duty power resistor in an aluminum case that could be panel-mounted.  
It happens that Vishay-Dale makes a type RH-50 resistor in a 2.0 ohm 1% value 
rated at 50 watts.  Mouser Electronics carries this resistor as Catalog Number 
71-RH50-2.0, for $4.26 (as of April 2007). Overkill, but not too expensive.

Referring to the attached schematic diagram, a heavy-duty relay with a 120 VAC 
coil rated for continuous duty is used to switch the resistor in and out
of the circuit.  The relay supply cord should be plugged into the same outlet 
or circuit as the repeater's power cord.  Under normal conditions, the relay 
is energized, and the battery is float-charged through the current limiting 
resistor.  When AC power fails, the relay de-energizes and connects the 
battery directly to the station.  Works like a champ!

73, Eric Lemmon WB6FLY

Clicking the schematic below will display a larger image oriented for printing.

Here's the same circuit modified for the 100w models that use 28 volts.

Note from WA6ILQ:
The relay shown in the diagram has to carry the full operational current of the MTR, so select a good quality continuous-duty rated relay with appropriately sized (i.e. 20 amp) contacts. Mount the resistor in a location with good airflow.

The MTR2000 has a very versatile internal controller, however if you have yours wired to an external controller (i.e. a Link, Scom, CAT, etc) the repeater controller will have to be powered from the same battery to keep the repeater on the air. An extra pole on the relay could be used to trigger a digital input on the controller to tell it when AC mains power is present. The actual connection of the relay contacts and the programming is dependent on the brand of the controller, but the most common implementation is that when power fails the contacts switch a a digital input pin to ground, and the programming senses that transition and changes the courtesy beep... perhaps from a single "blip" to the morse letter "B". Just educate your users that when the system is in power-fail mode to keep transmissions short to conserve the battery.

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Original text (the white background and the image) © Copyright 2007 Eric Lemmon WB6FLY
Hand coded HTML © Copyright 2007 by Mike Morris WA6ILQ
This page created 22-Apr-2007

This web page, this web site, the information presented in and on its pages and in these modifications and conversions is © Copyrighted 1995 and (date of last update) by Kevin Custer W3KKC and multiple originating authors. All Rights Reserved, including that of paper and web publication elsewhere.