Hello Everyone,

I have a 1965 Barracuda and I think the 5V voltage limiter for my gasoline and water temperature gauges is on its way out. There more about this device on this page:
http://www.allpar.com/history/mopar/electrical2.html

On my car, the voltage limiter is contained within the gasoline gauge and I think it would be easy to bypass it with fiber washer on the 12V gauge terminal. The Allpar article recommends replacing the mechanical limiter with a 7805 regulator. I checked the resistances in each gauge and both have 20 ohms. Full-scale on each of the sensors is 10 ohms. Each leg of the parallel circuit is 30 ohms so I calculate the total 5V draw from the instrument regulator would be 0.33 amps. I believe a 7805 regulator would have to dissipate up to 3.1 W of heat to run my gauges.

Building a instrument regulator based on a 7805 device would be straightforward enough but I thought I would see if there were any better way of doing this. From reading the posts here and on other sites, I've come across the following recommendations

• use a cheap USB 12V car charger
  
• use an RTE Limiter
• use a DE-SW050 switching regulator

I'm leaning more towards using the DE-SW050 but I would like to know if there are any other options I could look at. Any suggestions?

 

7805 regulator is cheap.if heating problem use one with TO-3 package. i did this in my maruti esteem to get a 5V. no problem , place away from ignition section.

 

Thanks for the advice. I'm new to electronics so there is much I don't know.

There are heat sinks specifically made for the 7805 so it is not clear to me why using the TO-3 package would be better than the TO-220.

Why should the regulator be placed away from the ignition section? I was planning to locate it somewhere on the back of the instrument panel, near the gauges.

 

You can get rid of 3 watts of heat in a 7805 chip with a TO-220 package, so you don't need the TO-3 package.
The regulator should be placed away from the ignition (spark plug wires) because they can cause all kinds of noise. Fortunately, your dashboard is already "away from" the spark plug wires and ignition coil.
There is another caution. When you turn off the headlights, a thing called an unloading spike happens and it can be as high as 60 volts. Therefore, I tend toward the RTE device, assuming it is built to survive in a car. The LM7805 wasn't designed to survive in a car and neither was the DE-SW050. I could make them survivable, but why? It's already available.

 

Using 7805 to replace the mechanical voltage limiter seems to be a fairly common upgrade and the 7805 I had in mind is rated for a maximum of 35V. The data sheet recommends using a 0.33 μF decoupling capacitor on the input power pin. Would that capacitor be too small to absorb such a voltage spike?

 

I'm not the expert on this but I can do some math.
If a 90 amp alternator was out of control for a thousandth of a second, it would require 4500 uf to limit the increase to an additional 20 volts. So, no. 0.33 uf is not in the ballpark.
There are chips that end with "H" and some of them are good to 60 volts. LM317H I think.

If not that, I would try to use up some of the voltage with a resistor before the regulator.
A third of an amp? Lets use up 4 volts with a 12 ohm resistor rated for 3 watts or better. That will take some of the heat off your regulator.
Follow that with a capacitor to ground, like maybe 1000 uf at a 50 volt rating for 49 cents.
Then throw in a zener diode rated for 1 watt, 25 volts in parallel with the capacitor.
That gives you 3 layers of protection for the chip.
Then do the 0.33 or whatever the manufacturer recommends for the chip.

Other people here might give you better advice. I'm just using my experience to say what I would try if I couldn't get a chip with a fat safety rating on the voltage limit.

http://www.mouser.com/Passive-Compo...tors/_/N-75hqt?P=1z0wrjwZ1yx4avv&Ns=Pricing|0

 

Just curiosity more than anything, but why not keep original equipment?

 

Thanks for the advice. I'm new to electronics so there is much I don't know.

There are heat sinks specifically made for the 7805 so it is not clear to me why using the TO-3 package would be better than the TO-220.

Why should the regulator be placed away from the ignition section? I was planning to locate it somewhere on the back of the instrument panel, near the gauges.

TO 3 package have high ampere rating, try 7805 if any problem proceed further.

 

TO 3 package have high ampere rating, try 7805 if any problem proceed further.

You don't need a "high ampere rating" for 1/3 of an amp.