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Author Topic: Useful part  (Read 3484 times)
lyndon
1988 MC-9 DDC 6V92TA Fuller T-11605D
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« Reply #30 on: December 12, 2007, 10:46:42 PM »

Tim, would not a small zener diode be the best way to go? I do not think there would be any lost energy like with a resistor.

That is how I dropped the 12 volts to 6 volts for my 46 ford instrument panel. No heat sink or anything.


Richard

Richard, wouldn't a zener diode limit the voltage by conducting whenever the voltage exceeds 6 volts, or always, in the case of 12 V applied? Lost energy either way, no?

I assume a zener is more certain than a resistor to maintain the desired voltage under current fluctuations, though, so it would have that advantage.

Don
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Don
1988 MC-9
Tim Strommen
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« Reply #31 on: December 14, 2007, 03:55:10 PM »

Tim, would not a small zener diode be the best way to go? I do not think there would be any lost energy like with a resistor.

That is how I dropped the 12 volts to 6 volts for my 46 ford instrument panel. No heat sink or anything.


Richard

I wouldn't recommend it for anything other than a low power load (less than a watt).  The zener will dissapate anything over the specified reverse bias voltage (so for a 12Volt part with a 28.8 volt system voltage, the zener will dissapate 16.8 volts)

If the series load is 5Watts at 12volts (5W / 12V = 0.417A), the zener will need to dissapate: 16.8V x 0.417A = 7Watts.

If you recall the V-Reg approach, the power disapated by the V-reg was also 7Watts, so you're not really saving anything here - the drawback is finding a zener which can support that power (the limit for common zeners is 5Watts).

In the end, a zener isn't as suitable for voltage regulation of power loads as a task-designed power supply or voltage regulator.  Zeners are good for low-power voltage references (less than 5mA of current) for larger voltage regulation devices.


Cheers!

-Tim
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Fremont, CA
1984 Gillig Phantom 40/102
DD 6V92TA (MUI, 275HP) - Allison HT740
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DrivingMissLazy
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« Reply #32 on: December 14, 2007, 04:41:46 PM »

Tim, would not a small zener diode be the best way to go? I do not think there would be any lost energy like with a resistor.

That is how I dropped the 12 volts to 6 volts for my 46 ford instrument panel. No heat sink or anything.


Richard

I wouldn't recommend it for anything other than a low power load (less than a watt).  The zener will dissapate anything over the specified reverse bias voltage (so for a 12Volt part with a 28.8 volt system voltage, the zener will dissapate 16.8 volts)

If the series load is 5Watts at 12volts (5W / 12V = 0.417A), the zener will need to dissapate: 16.8V x 0.417A = 7Watts.

If you recall the V-Reg approach, the power dissipated by the V-reg was also 7Watts, so you're not really saving anything here - the drawback is finding a zener which can support that power (the limit for common zeners is 5Watts).

In the end, a zener isn't as suitable for voltage regulation of power loads as a task-designed power supply or voltage regulator.  Zeners are good for low-power voltage references (less than 5mA of current) for larger voltage regulation devices.


Cheers!

-Tim


Guess I misunderstood how a zener works. I thought it only turned on when the voltage input exceeded the voltage rating of the zener and the only power dissipated in the zoner was that caused by its internal impedance.

Regardless it worked great for the dash lights on my 46.

Richard
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