Gauges Dont Work On 5.89 V

35ragtop

35ragtop

Senior Member
Hi fellas, re my 1938 pickup.(12 volt)
I decided to upgrade my voltage reducer from ceramic to electronic. After reading loads of threads on here I bought the L7806 voltage reducers and added the 1uf capacitors as I had read. The trouble is they reduce the voltage from 12.4 to 5.89, but even with one on each gauge (oil and fuel) there doesn't seem to be enough volts to drive them. I bought 4 of these little units and they all test the same. Anyone had any experience with these?
The gauges worked fine with the other system but I didn't want to ruin them with over voltage.
They do make a 7808 model which supposedly puts out 8 volts, maybe I should get those.
Any suggestions?
Dave
 
Excess voltage will result in inaccurate gauge displays and can harm the heating coils in the gauges and sensors. You can buy Runtz reducers, diodes which don't get hot, for each gauge (12v to 6v) for $3.00 each on-line. They have been around for a long time and work fine. i.e. autorewire.com and others sell them.
 
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I have been running a pair of the L7806CV for years and they work great on my fuel gauge and oil pressure gauge. Are you sure yours are wired and grounded correctly?

PS: I mounted mine on the back of the gauge cluster. Lots of wires so I used a terminal block.
 

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The gauges don´t care if you run 6 or 8 v. what you probably got is a cheaper version with less current capability.
They where running at 7.4v in the car with generator charging.
Go for the version that handles 1.5A and use some kind of heatsink or the thermal safety in them will shut them down.
I run them and no issues so far.
 
Your LM7806 is a +6v regulator. And 6v is what you got.

If you want +7.0 v, use a LM7807 which is a +7v regulator. Available on ebay for $1.50

These will give 1 amp at 7.0 volts as long as you input 10v or more. If it gets hot, provide a metalic heat sink.




 
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The 6-volt systems are regulated at 7 volts+ when the engine is running but they still work at 6-volts. The instrument voltage regulators used after 1956 would work and keep the 12 to 14.5 voltage more constant.

Switching transistor regulators (runtz) can be made in the home shop but Randy Rundle's are easier and they don't cost all that much. Getting the transistors and the capacitors/resistors & soldering all that crap together is a waist of my time. They are polarity sensitive since they swap polarity around. They are set up to work with 12-volt negative ground systems.

Just to Add: Randy has instructions for these on his site with troubleshooting info.
 
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Thanks for all that info,
The Runtz ones look to be the way to go.
I liked the idea of building my own hence the L7806 plus capacitors. I guess its doing its job , but I do need more voltage. I agree with flatheadmurre.
Dave
 
Modifying a LM7806 voltage regulator

Modifing a fixed regulator with 2 resistors is poor practice as the internal circuitry of the regulator can become unstable in that environment.

If you are taken with the idea of adding 2 resistors to achieve a particular output voltage, best standard engineering practice says you should use an adjustable regulator, LM317, which is specifically designed for that purpose.
 
Adding a diode in the groundreference of a fixed regulator works fine.
The dropping voltage of the diode is equal to the amount the output is raised.
Don´t need to be a zener any cheap diode will work...if you need more then a few volts use another regulator.
You don´t practie this in electronics due to the noise the diode create making the regulator less good but for this aplication just about anything works.
Agree that the resistor splitting is not the way to go...i can give several reasons if anyone is interested in getting bored...
 
Voltage is the product of current times resistance (Ohm's Law). Unless you can calculate the total current for the gauge set, you are better to obtain a voltage adjuster for each gauge. In old cars, the voltage level in the charging system is irregular, making a "home brew" resistive circuit chancey. Too much may damage the gauge, too little will make the readings inaccurate.
 
With no gauge attached do you get exactly 6v out of them? I have used the runtz drops several times with one on each gauge with no issues. Are they getting a good ground? Those are rated up to 1.5a and would think that should be fine.
 
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