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my car 12v constant and switched live drops volts

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maximus999

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Hi, i am new to this site so be friendly as i am not as intelligent as others lol. I have bought a dvd/cd unit and wired it upto my car, but it is keep losing its memory on the unit. I have tried reversing the 12v live and 12v switched wires around and it still loses memory, I have also conected the constant live to the battery with an in line fuse and it is still losing memory. I have tested a little further with my mulit-meter on the direct line from the battery and the cars 12v live and switched wires and here is my results. On all 3 wires i have permanent 12 volts or over when the car is not cranked over. When i crank the car over they all drop to around 10v approx for a second or two until the car starts and they go back to 12v when running, I beleive i am losing the memory feed when i crank it over. Is there anything on the market what will hold the voltage whilst i crank the car over? here are the unit specs: Supply voltage 10-15v, current consumption 4~15a (this i am not to sure on! any pointers on this is much appreciated).
Just off topic i have an electronic boost gauge which calibrates itself when i turn the car on to the 1 position, when i crank the car over it loses power and goes through the calibration test again, is this due to the same fault as my car dvd unit as the power for the boost gauge? Thanks in advance for any replies Ian
 
It's almost certain that the voltage is briefly dropping far below 10V. A voltmeter won't be fast enough to measure it. An easy fix might well be a new battery.

If you want to support the permanent supply, it's probably quite complicated. The problem is that most modern ICE units take most of their power from the permanent supply, and the ignition supply is just a signal to tell the unit to turn on. If you add a capacitor it would have to be very large, and you would need a diode to stop current flowing back into the starter motor during cranking. The entire current for the DVD/CD unit would have to go through the diode.
 
The addition of a diode and a capacitor in the keepalive input to your player should take care of it. The current drawn by a keepalive input is usually just 1uA or so. A 33uF capacitor charged to ~12 will hold up the voltage on this pin for a couple of seconds while you are cranking...
 

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Thanks for the replies guys, i have been advised that it might be my battery at fault or the fact that when i had it replaced they didnt put a strong one on, is there any sence in this saying, could i get a stronger cranking amp battery what wont drop as many volts when i crank my car over Thanks again Ian
 
If the battery is strong enough to crank the engine it should not lose memory in a modern radio.

These symptoms can come from corroded connections in the battery circuit. A strong candidate is the strap(s) that attach the negative (-) of the battery to the car's frame and to the engine block. If either of these is corroded (or if the installer forgot to tighten them) they can develop a voltage that subtracts from that necessary for the memory. Sometimes the path is hidden, where there is a strap from (-) to the frame and then from the frame to the engine, or from the (-) to the engine and then to the frame.

Problems with these connections can also lead to poor charging and will cause a good battery to be replaced prematurely.

On much older cars, when the bonding strap from engine to frame became corroded, the speedometer/odometer/tachometer cables carried the current (and then those instruments failed too).
 
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