Its cool man. Rick Ehrenberg wrote up a bit on how to replace the old power supply for the dash lights, gas gauge & such. Its a good read, and might work for some folks. I'm not sure if 1.5 amps at 5 V, a mere 7.5W is enough juice for the gas gauge AND one's instrument panel lights, so I figured a more robust 5V supply would be better. IFF you like USB gadgets, then you REALLY owe it to your self to scale up to at least 10A at 5V. THAT should supply ALL your low voltage desires.
The voltage limiter doesn't power any of the instrument lights. Those are still connected to your full system voltage, just through the dimmer switch, which is a simple potentiometer (variable resistor). The voltage limiter is just to power the gauge movements and sensors: gas gauge and oil pressure if equipped.
The problem with a 7805, as has been noted, is that it can fail SHORTED if it overheats. That would put full system voltage through your gauges and fry them. That happened to me with a circuit I was experimenting with once, and burned out an expensive microcontroller chip I was programming.
I would recommend not combining the gauge voltage limiter circuit with any other function like charging USB devices. It might seem like a cool idea, but not worth the risk of plugging in a device and which affects your gauges. USB power supplies are cheap and small. Wire-in a separate one and hide it elsewhere in the dash close to where the USB plug will be.
While I could build a voltage regulator circuit myself, I like the RT-Eng replacement regulators. They are a plug-and-play direct replacement if your original voltage limiter is the external plug-in style and not built into one of the gauges. They also have built-in current-limiting in the event of a short circuit.
As an electronics engineer (retired), I am concerned about noise being produced by a converter. With all of the electronics that we now use, many of them are succeptable to noise caused by switching power supplies and / or ‘chopping’ power supplies.
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The 12 VDC power form running around a vehicle is inherently dirty, as the alternator, and relays switching on and off create noise on the 12 VDC ‘bus’.
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Now a filament type incandescent light bulb that is used to illuminate your dash instruments is very robust and forgiving), but dimmable LEDs and more importantly, electronics such as GPS receivers, radios and other modern conveniences that are in our present vehicles and ‘toys’ are and much of them are designed and manufactured in China (read: JUNK) and extremely susceptible to input power quality - in fact, they expect 5 VDC to be rock solid as if they are connected to Hoover dam.
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I worked on military instrumentation systems and a decent DC-DC converter for 28 VDC to 5 VDC was sharp (1000+ US$) as aircraft 28 VDC is horribly dirty.
I am going to do a little research on how I might come up with something that is off the shelf that doesn’t cost 1k$+.
IMO the best things you can do to eliminate noise in the C-body electrical system are to replace the electromechanical voltage regulator (the one which controls your alternator output) with an electronic one, replace the points ignition with solid state, and make sure you're running carbon-core spark plug wires and ensure the insulation on them is good so they don't "leak" EMI. This should cause a significant improvement in electrical noise at the source.
Beyond that I wouldn't over-think the noise filtering. I think that, when retrofitting electronics in an old car, it's easiest to consider the noise filtering requirements at the point of use, based on the type of load.
LED replacement bulbs will not be damaged by ripple on the power supply, and you're unlikely to see any flickering from them at the frequencies you're talking about. Any plug-in devices for automotive use such as GPS or aftermarket stereos should be designed to accommodate the variable conditions of automotive power supply and have adequate noise filters built into them already. GPS units have a battery built-in, which acts like a large capacitor, and their own internal power supplies with noise filtering.
If audible noise comes through on the stereo/amp speaker output, there are passive LC filters available to install inline on the power wires, or you can certainly design one. An equally important consideration IMO would be to route high-impedance audio signal wires to ensure they don't pick up any radiated EMI from surrounding wires/devices, and ensure that all high-impedance audio wires are shielded and not simple twisted pairs. (I'm talking about signal wires between stereo head unit and external amp, not between the amp and speakers.)
The original voltage limiter powering C-body gauges was very crude, and the electro-mechanical sensors and instrument movements were designed to handle a very noisy environment. They will not be affected by transient voltage spikes from relays or high-frequency ripple. Using a more stable voltage regulator, even a noisy switch-mode one, will be a drastic improvement.