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Originally Posted by DerAlte * What we see on the HU is the perceived power-down because we see the display going off and coming back on again. Actually the HU resets itself without interrupting the BAT line (if it did, the settings will be lost, including the time setting). When the ignition key is in OFF position, the HU goes to sleep, but is still powered (that's how the red LED can flash after the front panel is removed). Reset is just a short 'sleep'.
* In the headlamp circuit, the relay is turned off for the duration the key is in the 'Start engine' position. The headlamp circuit has an ECU today. In older cars they wouldn't go off, one could see the headlamps dimming when the car was started (voltage drop, as you correctly explained) |
Actually, the logic for the headlamp and the HU is similar. Let me explain.
It is a common misconception that the (Yellow) always on wire is used to maintain memory (settings, clock, FM stations etc) and the HU physically runs off the (red) ignition on 12V, ignition off 0V wire.
In effect the red accessory supply merely triggers the HU to operate or not. Except for the circuits that need power always on to keep the memory, every other circuit shuts down using the accessory on and off. But the true load of the HU remains on the (yellow) always on wire. A simple testament to this fact is that the yellow wire is always of a better gauge than the red wire.
And that is the same logic(not method ofcourse) by which a headlamp relay/solenoid works. While the switch controls the headlamp state, the actual load of the lamp is transferred directly to the battery.
and that's what I was saying earlier
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Originally Posted by Sam Kapasi The voltage will automatically translate as a sharp drop in the +Vcc of the (already) running HU.....
.....Far easier to shut down the Vacc for those few seconds, get the car fired and restore Vacc. |
Where Vcc is the battery voltage(yellow) and Vacc is the accessory wire (red)
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Originally Posted by DerAlte * Power antenna is sub-system of HU, like HU is sub-system of the vehicle. So everytime HU resets, PA goes down and then up again. |
A power antenna funtions perfectly well without a HU. It is a simple circuit, similar to the 3 wire logic of a headlamp solenoid and a HU and an amplifier and many other devices in the automobile.
Earth, Always on and trigger.
The power antenna can be opened simply by providing 12V (low current) to the trigger terminal. You don't actually need an HU for this. The minute the 12V is 0V, the antenna retracts. When the HU is on, the blue wire (often the same one that switches power amps on and off) triggers the antenna.
My point was, in a working HU, if the Vacc is switched off, the load is immediately gone. (The 12V running the memory circuit is less than a trickle) At the same time, since the trigger wire is now 0V, all power amps (if any) immediately go off, and their load is immediately gone too. In an "on" headlamp, if the Vacc is switched off, the relay/solenoid clicks back into the Normally_Open state and the load is immediately gone.
However...
A completely open power antenna has no load on the battery, but if the Vacc is switched off, the load increases for a few seconds while the antenna retracts itself. Instead of the load reducing (like the other gadgets) the power antenna actually draws current (to retract itself) while you are cranking.
AND I'd like to add that the power antenna actually draws more current during the opening and retraction motion than a standard HU does.