tr volts/amps
hello pitot tube & others,
to add to lahso 's post. for TR1 & TR2, you read DC bus voltage/amps. for TR3, you read actual volt/amps of the device. so, when either TR1 or TR2 fails, by selecting volts/amps, you will still read DC bus1 or DC bus2 volt/amps. on the otherhand, if TR3 fails you will read 0volt/0amps. for this reason, after start, we always select TR3 on the DC meter. there is also on B737NG, a TR amber caution light, with a different logic on ground or airborne. on ground the caution light will illuminate for any TR failure, whilst airborne either TR1 or TR2+TR3 must fail for the light to illuminate. so, to answer your question, the different values come because the measured pickup points are different & going back to basic electricity : Ohm's law states that E= IxR. E= volts, I= amps, R= resistence(ohm). E being roughly constant (28volt), then, if TR3 amps are half the values of TR2 or TR2, then resistence of TR3 must be twice the value of resistence of DC bus 1 or 2.
hope this helps.
kind regards,
bm