12VDC to 5VDC Regulator

Thread Starter

tracecom

Joined Apr 16, 2010
3,944
I think this question is within the TOS, but if not, I apologize in advance.

I want to build a LM7805 regulator to be plugged into the cigarette lighter jack on my truck. Do I need any special protection for the circuit as a result of it being used in a vehicle? In addition to the standard .33 μF on the input and .1 μF ceramic caps on the output of the 7805, what capacitors should I use?

Thanks.
 

KJ6EAD

Joined Apr 30, 2011
1,569
I'm going to sidestep your question and suggest you use an old cell phone charger that you can get for as little as $1 at a thrift store. They are mostly well regulated, efficient DC-DC buck converters based on the LM34063A or similar IC and already have the 5V output you want.
 

SgtWookie

Joined Jul 17, 2007
22,220
Neither the 78xx nor the 117/317 are suitable for an automotive environment. The spikes in an automotive electrical system would kill them.
 

Thread Starter

tracecom

Joined Apr 16, 2010
3,944
I had considered a cell phone charger, but have always been somewhat leery of them (especially the dollar store variety) because of the unknowns (to me) regarding voltage output stability and the possibility of damage to connected equipment. I assumed that because they were made just for battery charging, the regulation and/or filtering might be poor. Apparently, I am wrong; can I use the output "as-is" to power a 5 V audio amp circuit with no additional conditioning?

Thanks.
 

KJ6EAD

Joined Apr 30, 2011
1,569
Your concerns are valid but the chargers I was referring to are high quality devices made for major cell phone brands such as Motorola, Nokia and LG. They're available cheaply only because they're second hand. The load regulation is usually quite good and since most oscillate at 20kHz or higher, any unfiltered ripple should be inaudible. You can always add another capacitor to help with filtering and transient response if you like. I usually look for models that can be disassembled by unscrewing the ferrule on the tip or removing screws which makes them easy to modify or recable.
 

Thread Starter

tracecom

Joined Apr 16, 2010
3,944
The maximum supply current for my audio amp is 1.3 Arms. That is quite a bit above the 600 mA rating of most car chargers. I don't plan to run the amp at full output, but I still wonder if the car charger will supply enough power for the audio to be heard in my truck.
 

KJ6EAD

Joined Apr 30, 2011
1,569
The maximum supply current for my audio amp is 1.3 Arms. That is quite a bit above the 600 mA rating of most car chargers. I don't plan to run the amp at full output, but I still wonder if the car charger will supply enough power for the audio to be heard in my truck.

If that amperage is a peak rating then the charger might be adequate with a big capacitor on the 5V output to handle the transients. You also can parallel the outputs from two or more of the charger circuits.
 
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