Hello all.
I am building a few power supplies using a single voltage regulator for each power supply. I will be using a 5 Volt, 9 Volt, and 12 Volt Output Power Supplies using the TO220 Voltage Regulators. The output amps will range between 1 to 2 Amps.
Is there a formula I should use when choosing capacitors (and if a resistor is required) for the output supply caps?
Should I use an CRC output layout for the supply cap, or would one cap do?
Will I need a bleeder resistor for the cap(s), and if so, what (formula) should I use when I select a bleeder resistor?
Do I put the bleeder resistor right on the caps terminals?
Thanks much all!!!
I am building a few power supplies using a single voltage regulator for each power supply. I will be using a 5 Volt, 9 Volt, and 12 Volt Output Power Supplies using the TO220 Voltage Regulators. The output amps will range between 1 to 2 Amps.
Is there a formula I should use when choosing capacitors (and if a resistor is required) for the output supply caps?
Should I use an CRC output layout for the supply cap, or would one cap do?
Will I need a bleeder resistor for the cap(s), and if so, what (formula) should I use when I select a bleeder resistor?
Do I put the bleeder resistor right on the caps terminals?
Thanks much all!!!
Some of the better LM78XX only need a small 0.1uF cap right at the output, like Fairchild's. Others may need 10uF there.
It's also a good idea to add a similar small local cap next to the regulator input as well.
The large cap at the regulator input is sized by the current and ripple. Roughly, at least 1000uF to 2200uF (or more)
per load amp is typical. You can calculate the minimum input capacitor value by requiring that the input voltage
not decay more than say 5% between rectifier pulses (17mS for 60Hz) at the maximum load current.
A reverse connected 1N4005 diode between the input and output of the regulator can be used for discharge of a larger cap.
Thanks much for the advice. What are some of the better LM78xx regulators that I should look for when using a 7805, 7875, 7809 or 7812 voltage regulator.
I was thinking LM78sxx series regulators since they can handle up to 2 amp input.
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I was thinking LM78sxx series regulators since they can handle up to 2 amp input.
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Last edited:
@dbx01
Hi, have you thought about using LM338? They a 5A max rated, do not require a pass transistor and are variable.... depending on what you require of your power supplies (if you need them all at once) you could cut down significantly on your parts count.
The formula for reservoir cap is:
C = (I*T)/V
Where C is capacitance, I is current, T is Hz (rectified is 2* mains), and V is acceptable voltage ripple.
Other than that, follow the data sheet of your chosen part.
Hi, have you thought about using LM338? They a 5A max rated, do not require a pass transistor and are variable.... depending on what you require of your power supplies (if you need them all at once) you could cut down significantly on your parts count.
The formula for reservoir cap is:
C = (I*T)/V
Where C is capacitance, I is current, T is Hz (rectified is 2* mains), and V is acceptable voltage ripple.
Other than that, follow the data sheet of your chosen part.
T is 1/Hz. Usually either 10ms or 8.333ms.
Designing by formula is better than designing by ear, but understanding will take you further.
Designing by formula is better than designing by ear, but understanding will take you further.
Fairchild are good, but make sure any regulator has adequate heat sinking under full operation.
Thanks much to everyone for your responses.
I had not thought of using an LM338 variable regulator. Wouldn't a dedicated voltage regulator put out constant voltage better then a variable voltage regulator? Would the ripple voltage be better on a constant voltage regulator?
I don't mind using a dedicated voltage regulator with supply caps if it means better performance.
I am putting together a purchase from Digikey for various parts. I do have heatsinks as part of the order. Thanks for the advice.
Also, I do not have an oscilloscope to measure ripple voltage. I have a DMM and an LCR meter. Any advice on how to find out about voltage ripple to smooth it out?
I had not thought of using an LM338 variable regulator. Wouldn't a dedicated voltage regulator put out constant voltage better then a variable voltage regulator? Would the ripple voltage be better on a constant voltage regulator?
I don't mind using a dedicated voltage regulator with supply caps if it means better performance.
I am putting together a purchase from Digikey for various parts. I do have heatsinks as part of the order. Thanks for the advice.
Also, I do not have an oscilloscope to measure ripple voltage. I have a DMM and an LCR meter. Any advice on how to find out about voltage ripple to smooth it out?
Without a scope, you'll have to size the input cap by calculation and measure the average input voltage with your DMM
at the maximum current load. Then you can subtract half the calculated ripple p-p voltage from the measured input voltage.
The result should be larger than the minimum needed for regulation, which is usually about 2V more than the output voltage.
Accuracy will depend on your meter, some average differently than others ( worse), true RMS meters are better.
Lm317. Newer better design. App notes from TI http://www.ti.com/lit/ds/symlink/lm117.pdf probably the most extensive.
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