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Is it feasible to use voltage regulators in series?

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bigal_scorpio

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Hi to all,

I am making a project that requires 2 different voltages. The first to power strings of LEDs needs to be 30v at about 1000mA. The second (the actual control circuit) needs 5v at 200mA.

To make the circuit "bomb proof" I wondered if I should run the 30v supply through an LM7812 or LM7818 first before it reaches the LM7805 that will power the micro etc.

Would this be the best way to avoid stressing the LM7805 or is there a better way?

Thanks Al

PS Eric, my man flu seems to be gone now. Well the missus has got me back on dishes detail so it must have! hehe
 
Yes & NO

The LM7805 will add a 200mA load to the LM7818. Many of the LM78xx are only rated to 1 amp.
Running the LM7805 from a lower supply sound good to me!!!
Most of the LM78xx are only rated to 35 volts on the input side. Watch out!

For the top regulator look at National's LM317HV. I think it is rated at 57 volts not the normal 37. (HV) Some versions are raed at 1.5 amp. The voltage can be set to any voltage by two resistors!
 
Hi to all,

I am making a project that requires 2 different voltages. The first to power strings of LEDs needs to be 30v at about 1000mA. The second (the actual control circuit) needs 5v at 200mA.

To make the circuit "bomb proof" I wondered if I should run the 30v supply through an LM7812 or LM7818 first before it reaches the LM7805 that will power the micro etc.

Would this be the best way to avoid stressing the LM7805 or is there a better way?

Thanks Al

PS Eric, my man flu seems to be gone now. Well the missus has got me back on dishes detail so it must have! hehe
You can run regs in series. You have a power dissipation issue: with 30V in to the 5V reg, it has 25V across it and 0.2A makes about 5W power disspation. You will need a TO-220 device and a pretty good heatsink (thermal resistance less than about 8C/W).
 
Yes & NO

The LM7805 will add a 200mA load to the LM7818. Many of the LM78xx are only rated to 1 amp.
Running the LM7805 from a lower supply sound good to me!!!
Most of the LM78xx are only rated to 35 volts on the input side. Watch out!

For the top regulator look at National's LM317HV. I think it is rated at 57 volts not the normal 37. (HV) Some versions are rated at 1.5 amp. !
Be careful: look at the data sheet. You only get the rated current when the input-to-output voltage differential is low (like 5V - 10V). Current drops quickly as the differential voltage increases, because it is the built in safe operating area protection for the power transistor. There is a curve on the data sheet showing this.
 
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Another way to reduce the regulator power dissipation is to put a power resistor in series with the 5V regulator. Select its value and power rating to have a voltage drop no more than required to still keep the regulator operating (typically 2-3V above the output voltage) at the maximum anticipated output current. For example for a 30V supply and 5V output at 200mA maximum, the maximum resistor value would be (30-3-5)/0.2A = 110 ohms with a power dissipation of 4.4W. You should use a least a 7W resistor.

Edit: With 110 ohms in series with the 5V regulator, the maximum regulator power dissipation will occur when the voltage drop across the resistor is 1/2 the total 25V drop or 12.5V. At the point the current is 114mA and the regulator and resistor power dissipation are 1.42W each.
 
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Where is the main power coming from? 35V? 40V?
Can the LEDs be changed so they run at 12V? or 6V and use a lower supply voltage?
You must have a string of the 3Watt LEDs.
Control logic that uses 200mA? Must be a power hungry beast.
Please tell us more about the LEDs. I have an idea!
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The LM2841 will take 30volts down to 5 volts and wast .3 watts in heat.
The LM7805 will take 30volts down to 5 volts and wast 5 watts. (200mA)
With this much power I would use two switching power supplys.
 
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