Electronics > Beginners
Simple filter for a DC/DC-converter in an automotive application
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acrophobic:
Hi! A few years ago I installed a USB-port in my Volvo for charging phones. I used a Traco TMR 6-1211 to supply the power. However when I start the car, the phone stops charging for a second, so I think the voltage drops below the converters minimum input voltage the moment when the starter motor tries to crank the engine. That the phone stops charging briefly is obviously not a big problem, but I do have a similar converter supplying a Bluetooth adaptor and it's annoying to connect the phone again if you just turn off the engine for a few minutes (like when waiting for someone). About a year ago (or was it two) the converter failed, so I'm going to replace it and I wonder if I can add a simple filter consisting of suitable capacitor after i diode (to stop the current rushing from the cap towards the starter). Is a diode in series a good approach and how big a capacitor would I need? The capacitor would only have to supply the converter for a very short period as I once disabled the ignition and measured the voltage while cranking the engine and it was well above the minimum voltage for the converter, so it must be the split second when the starter goes from still to turning that's the problem.
Oh, and if you would like to see my phone charging project, it's here: https://acrophobic.jalbum.net/AUX-adapter_Volvo_S60/

Kind regards
max_torque:
Where did you take the power from?  Quite a few low priority power feeds are deliberately interrupted when cranking in order to protect against voltage excursions during cranking and to ensure the starter motor gets as much power as is available.  Typically, the classic round "cigarette lighter" socket is one of those that is generally interrupted! (to ensure any consumer loads added are not drawing power from the battery during cranking)
acrophobic:
It does feed from the cigarette lighter circuit, which originally is interrupted when you crank the engine. I have however modified that by moving a pin in the fusebox to a rail that feeds directly from the battery. https://www.jagrullar.se/forum/viewtopic.php?f=19&t=96106
So it does have constant power:)
Anyway, would it be a good idea to use a diode in series and then a suitable cap before the converter. If I try to google such a configuration (diode and cap), I only find information on half wave rectifiers..
A follow up question after reading another thread on electronics in an automotive setting, where it is warned about rather harsh voltage spikes in such an environment. Wouldn't such spikes be largely absorbed by all loads in a car (lighting etc.) and could those spikes damage my poor DC/DC-converter and is there a simple but efficient protection circuit?
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