LIBRA-UL-R repair on the Phobos Gate Opener System


LIBRA-UL-R controller

After more than two years of operation, the controller stopped working of the Phobos BT L system at the end of 2016.  I wrote the company via the website multiple times, but received no response.  From inspection, it was clear that the R23 power resistor (above the bridge heatsink in the previous image) was burned up.  I could not tell the original value, so I had few options.

Analyzing the schematic
Prior to removing the board, I checked the power supply voltage at the white terminals at the transformer, and measured 33Vac (labelled as 25V on the sticker).  Taking the board to my workbench, I was able to power up the board with a 24Vac transformer and could then measure voltages and trace the schematic.

photo of board
Photo of the solder side of the board.  The main driver for the relays is U20 in the middle of the photo (pin 1 is top-right).

I only traced the Power/IO section of the board.  This is the section most likely to fail and besides, it would be nearly impossible to find a replacement for U6, the main chip of the logic section.  

Power Supply Section
Referring to the schematic, we start at the bottom of the page for the power section.  Incoming AC power is first filtered by C1 and RV1 and then applied to JP9 terminals #11+12 as the Aux output, and also D1 for full wave rectification.  This creates the B+ bus, which is used to power the motor.  At an input power of 28Vac, B+ will measure about 26Vdc.  D20 isolates B+ from the C+ bus, which is filtered by the C3 2200uF electrolytic capacitor.  Due to the filtering, this bus will measure higher, or around 36V in my case.  After that, this bus powers the red LED (LD1), and a 28V zener preregulator formed by R30 and D24.  This in turn feeds the 5V regulator.

Motor Control Section
I will focus first focus on how Motor2 is powered.  Current for this is switched by K2 and K3.  When these relay coils are unpowered, the motor is shorted, causing the motor to act as a brake.  Throwing K2 (power from U20-15) will cause B+ application to terminal #4 on .  If then Q4 is also turned on (via MTR_DISABLE from U6), current will flow through the motor and use R4 as the current sense shunt.  The opposite occurs when K3 is thrown, and the motor runs in the opposite direction.  The circuitry inside the motor and its limit switch is a guess, but I am pretty certain this is correct.  It causes the switch output to go to B+ when that switch is closed, which pulls down the open collector output to U6.

However an interesting and initially puzzling action occurs before the motor runs.  That is when the remote button is pushed.  The first resulting action is a 30 msec pulse low on the U20-12 pin (Sensor Check).  Via Q22, R23 and D25, this applies C+ to both terminals of the motor and Q4 (neither K2+K3 are actuated yet).  Initially, Q4 is OFF, but in the middle of the 30 msec pulse, MTR_DISABLE goes low for 10 msec, and that causes about half an amp of current to flow in Q4.  This results in about 50mV across R4 for 10 msec, which is routed to the op-amp at U21.  My best guess is that this 30 msec pulse is a sensor check. It allows the U6 controller to tell if the system is up against a limit switch, and if the current sensor is working.  As an experiment, I shorted the base-emitter junction of Q20, causing the lack of the 50mV pulse.  The result was that the motor move was immediately aborted, and none of the relays actuated.  This matches what occurs with my original controller with the blown R23 and Q22.  

When Q22 is ON, the instantaneous power on R23 is 17W.  This resistor is rated for 1W per my inspection.  It is unusual for these power resistors to fail shorted (they overheat and open), so my guess is that Q22 failed shorted in the original controller.

Actuation of Motor1 is similar, except power to it is not applied until about 920mseconds after Motor2 runs.  This delay allows proper phasing of the gates.  The Flasher output is composed by diode ORing all four motor output terminals with diodes D27 through D30, and applied to #9.  Current from this feed is returned through steering diodes D31+D32.

The interface from the logic section to the Power/IO section is nine logic lines.  Two of them are the MTR_DISABLE lines mentioned above.  The remaining seven get buffered by U20.  Momentarily jumpering the lowered numbered pins to 5V causes a relay to throw and a motor action to occur.  These pins and functions are:
  1. Motor 2 at +25V (Measured at JP9-3 to JP9-4)
  2. Motor 2 at -25V
  3. Motor 1 at +25V (Measured at JP9-6 to JP9-7)
  4. Motor 1 at -25V
  5. Enable Sensor Check (active low)
  6. Small relay K8 (probably light output on JP9)
  7. Small relay K4 (probably "Safe" output on JP9)
One thing that you can do is jumper these to 5V one at a time and see what power output does not work.  Note that these logic control lines into the ULN2003 all have a capacitor to return allowing you to find them easily.

Per the technical manual, the max working current to the motor is 3.5 Amps, and 1.5 Amps nominal.  So a good dummy load is about 20 ohms.  In the case of my board, repair was not possible so I purchased a new one for $211 (link below).  With the new (working) controller in hand, I could better trace the schematic and could now read the value of R23.

If you have a bad Libra, you can send it to me for repair.  Just send me an email for my shipping address.

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(c) 2017 Edward Cheung, all rights reserved.