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P O L Y T E C H . N U

Tornado TV TAB display unit: Low Voltage Power Supply (LVPS)

general description
The display unit is powered by 115 VAC 400 Hz three phase voltage and the low voltage power supply (LVPS) converts the input energy to low voltage signals for the device. The LPVS produces the following voltages:
+5 VDC (for logic chips)
+15 VDC
-15 VDC
+100 VDC (for the CRT)
-100 VDC (for the CRT)
+6,3 VDC (for the CRT filament/heater)

The power supply module is shown below. It's made of a heavy duty aluminium case with one female type 37 pin two row sub-d connector.



type plate

Manufacturer: Marconi Avionics
Manufacturer project: KO 656
Project: MRCA TV TAB DU (Multi Role Combat Aiplane Televesion Tabular Display Unit)
Panavia spec number: SP P970 152
Item: LVPS (Low Voltage Power Supply)
Part number: V22.498.76



input power
The low voltage power supply is fed with a three phase 115 VAC 400 Hz power signal including a neutral line. 400 Hz is common in aircrafts since power transformers are much smaller than 'usual' 50/60 Hz transformers. A rather high voltage of 115 Volts results in thinner wires compares to lower voltages. This results in less thick wires in the wire harness of an aircraft. Some avionics uses 400 Hz as a reference for position resolvers for example. The MRCA TV TAB DU does not need a reference frequency, so the input voltage is rectified to a single line voltage. This is very good news since it's rather hard to generate a three phase 115 VAC 400 Hz signal. So I think the device can be powered with a single line DC voltage of approximately 134,5 VDC. Remind that the hour counter at the rear panel doesn't work since that needs 400 Hz for normal operation. But I have to validate this assumption yet..

voltage conversion be written...

filement current sensing
At pins 9 and 10 is a 6,3 VDC voltage available for the crt tube heater/filament. There's a 0,1 Ohms resistor installed in the 6,3 VDC and connected to pins 11 and 12. Both pins 11 and 12 are connected to wires of the wire harness, so it's likely the current of the crt tube filament is monitored. I assume that there's some error message somewhere is the current drops to zero thus indicating a failing crt tube filament and therefore losing the display picture. The 6,3 VDC voltage flows trough the 0,1 Ohms resistor resulting in a (minor) voltage drop. I measured and calculated the voltages to be expected.

The filament resistance in cold state is 5,69 Ohms. This results in the following information:
voltage: 6,3 VDC
resistance: 5,69 Ohms
current: 1,11 A
power: 6,99 W
voltage drop over 0,1 resistor: 111 mV

The filament current in steady state 'hot' condition is 268 mA. This results in the following information:
voltage: 6,3 VDC
resistance: 1,69 Ohms
current: 268 mA
power: 1,69 W
voltage drop over 0,1 resistor: 26,8 mV

A (crt) tube filament has in cold state a higher resistance resulting in a higher startup power and therefore larger voltage drop over the 0,1 Ohms resistor. The voltage drop over the 0,1 resistor is therefore likely 111 mV. After the filament is warmed up the current drops to approximately 268 mA. This results in a voltage drop over the 0,1 resistor of 26,8 mV. Under normal operating conditions the voltage on pins 11 and 12 are approximately 26,8 mV. If the voltage drops (far) below this voltage the filament is probably broken and it's likely an error message is generated somewhere.

There are several test pints on the connector. There are wires connected to strategic places in the power supply for diagnose purposes. This makes it possible to diagnose the working of the power supply without having to open the device. Some diagnose wires are connected to the connector, but not wired externally. Some diagnose wires are connected to the wire harness. I expect that some diagnose signals are monitored live for diagnose purposes. Since these signals are not that relevant for me, I didn't reverse engineer these diagnose/test points any further.