5v regulator
hacksign23
Posts: 15
Another silly question :P
so. I have a 5v regulator in a to-92 package and...
1. what's the point of having a capacitor?
i know a capacitor 'stores' energy and can expel this energy quickly, but other than that, i have no idea what its use is. I have the propeller educational kit, and it has a capacitor hooked up to the 5v regulated voltage.
so what exactly is the capacitor's role on supplying energy?
also,
can the 5v regulator in a to-92 package withstand the current of a 9v battery?
thanks.
oh, would i need anything else other than a 5v regulator to make a 5v board from a 9v power supply?
thanks.
so. I have a 5v regulator in a to-92 package and...
1. what's the point of having a capacitor?
i know a capacitor 'stores' energy and can expel this energy quickly, but other than that, i have no idea what its use is. I have the propeller educational kit, and it has a capacitor hooked up to the 5v regulated voltage.
so what exactly is the capacitor's role on supplying energy?
also,
can the 5v regulator in a to-92 package withstand the current of a 9v battery?
thanks.
oh, would i need anything else other than a 5v regulator to make a 5v board from a 9v power supply?
thanks.
Comments
the primary purpose of the capacitor in a power supply is to "Smooth out" the output. As current demands of your circuit vary, the power supply may not react as quickly. The capacitor gives the power supply some time to react. (More experienced people here will probably have a better answer).
As for the 9v Battery, I think you mean to ask, can the 5v regulator withstand the Voltage of the 9v battery. All regulators have a maximum voltage that they can take in while providing the expected output voltage. They will also have a minimum voltage above the target output that you must supply in order for the regulator to maintain a regulated output. The only way to know these two limits and if 9v is withing the limits is to look at the spec sheet. Learning how to read a components spec sheet is an important part of learning to work with electronics.
Having said that, I would be VERY surprised if 9v was too high (or too low) for a 5v regulator.
It can indeed act as a storage depot. The current demands of logic circuitry can vary widely and change very quickly (nanoseconds). An output capacitor on the regulator can smooth out these changes and supply current for a few microseconds until the regulator can react.
Voltage regulators contain comparators and amplifiers and these sometimes need external capacitors to create stability, so the amplifier doesn't oscillate or over correct for changing current demands
The input capacitor on a regulator provides a short-term source of power for the regulator to operate. If the regulator is some distance from its power source, perhaps a foot or several feet, the inductance of the wiring can cause a delay in the delivery of power if there's a sudden surge of demand. The input capacitor can take care of that for a few microseconds.
2) Most regulators can handle input voltages up to 20 or 30V (their datasheets give the exact voltage limits). The issue is one of power dissipation. With a 5V regulator, every Volt of input above 5 has to be dissipated as heat with the amount of power involved depending on the current. A 5V regulator operating at 9V input will have to dissipate 9V - 5V = 4V x output current. If the regulator is handling 100mA, that's 400mW of heat. If the regulator is handling 1A, that's 4W of heat. A TO-92 package might be able to handle 400mW, but it will get very hot and the regulator may shut down due to over-temperature. You can add a small heatsink, but this package isn't normally designed for use with an external heatsink and it won't help a lot. A TO-220 package can handle several Watts and you can attach a large external heatsink to it with good heat dissipation.
ohoh, and the 5v 7805 regulator i have doesn't really regulate...
the 9v battery i have has 7.8 v, and the 5v regulator outputs 6.9 volts @_@.
after putting 3 5v regulators in series (kind of), i got about 5.5 volts. ._.
any help?
and if i have a 5v supply, how many uf's should i have for the capacitor?
thanks (:
edit: what kind of heatsink would i put on the chip ._. ?
and my 5v to-220 i got from the pe kit says it's giving 6.5 v ._.
The datasheet for the 7805 shows sample circuits and gives the recommended capacitor sizes. For the 7805, the typical input capacitor is 0.33uF. This can be ceramic or mylar with a voltage rating greater than what you have for the highest input voltage. A minimum output capacitor is 0.1uF. This is usually ceramic rated at 6V to 15V or so. For most uses, I'd add a 10uF to 100uF electrolytic capacitor in parallel with the 0.1uF capacitor. The electrolytic capacitor could be rated at anything over 6V.
Your heatsink needs depends on the amount of current and on the difference between the input and output voltages. For a TO-220 package with power dissipation in the hundreds of mW range, you may not need a heatsink or maybe just bolting the tab of the package to the printed circuit board would be enough (with a thin layer of heatsink compound between the tab and board). RadioShack sells several styles of heatsink for TO-220 packages. Have a look at them. They're probably good for a Watt or two.
Make sure you have your regulator connected properly. Again, check the datasheet for specifics.
As for the size of capacitor. With aluminum electrolytic, generally speaking, the larger the better. Higher capacity aluminum electrolytic usually have lower ESR (resistance) and it rarely hurts to have too much power stored. You will have to weigh cost, size, and need. An easy value is 1000µF. This is usually more than sufficient on input and output. A small decouple cap is a good idea too on the input (0.1 - 0.47µF ceramic or the like).
>>after putting 3 5v regulators in series (kind of), i got about 5.5 volts
that is probably just because of the voltage drop through whatever circuitry you are putting it through. Can you send us a picture or diagram of what you are doing? Regulators with the right components are supposed to regulate. Multiple regulators are not the solution.
Make sure you aren't looking at the wiring diagram backward, sometimes datasheets will display the pins from the bottom rather than the top.
in the title, it said
78L05 L78L05 7805 5V Voltage regulator TO-92
i'm pretty sure that pin 1 is voltage in, pin 2 is ground, pin 3 is regulated v. flippping it made it come out with 0v.
is my favourite for data sheets ( there is datasheetcatalog also, google will find it)
That is a bit confusing as it lists a 7805 as a TO92 which I don't think it is.
A TO220 package (with the big bit of metal at the back) is (looking at the writing) in, ground, out.
A T092 package is (looking at the writing) out, ground, in.
Why did they flip them around? I don't know.
Probably because they have stock in PCB fab houses
7805 Examples:
=============================
7805 (TO-220) PACKAGE
5V Regulator : 1.5A Max
http://www.sparkfun.com/datasheets/Components/LM7805.pdf
========================================================
78L05 = TO-92
Maximum Input Voltage 35V
Output Voltage 5V
Maximum Output Current 0.14A(Typ)
http://www.jameco.com/webapp/wcs/stores/servlet/Product_10001_10001_51182_-1
The picture shows the 7805 in a T0-220 package and the TO-92 package
Don't count on that - it's not true of all TO-220 package voltage regulators. For instance, take a look at the LM1117:
http://www.national.com/ds/LM/LM1117.pdf
I agree that it would be very nice if there were a consistent standard for this - I can see I'm not the only one who has gotten burned with a PCB of my own design turning out to be wrong because I didn't pay enough attention to voltage regulator pinouts.
Also not all 7805 regulators are in TO-220 packages. There ARE TO-92 7805s (and other 78xx series regulators):
http://www.national.com/mpf/LM/LM78L05.html#Overview
How does our OP test out a (possibly unknown) regulator?
ohoh, and the 5v 7805 regulator i have doesn't really regulate...
I wonder, if you guessed one pin was ground (probably the middle one), and fed the input volts via a 1k resistor, and measured the output, would that resistor provide sufficient protection?