Aluminium capacitors in linear power supply

[Tioleco]

Can I use aluminum capacitors in linear voltage regulators such as the 78xx series?

[Krankshaft]

Re: Aluminium capacitors in linear power supply

For filtering they are the only cost effective solution one large capacitance cap before the regulator and a smaller cap on the output.

[Trinite]

Re: Aluminium capacitors in linear power supply

I'm working on an audio system right now that uses 78xx/79xx series regulators, and it makes generous use of aluminum capacitors.

[Tioleco]

Re: Aluminium capacitors in linear power supply

Sorry , I made a mistake , I meant solid aluminum capacitors ( polymer ) in linear power supply.

[Slakky]

Re: Aluminium capacitors in linear power supply

They don't really care what you use. Not much benefit unless you plan on flirting with the temperature rating.

[Scenic]

Re: Aluminium capacitors in linear power supply

.. or you have too much money on your hands
polys are expensive & overkill for linear powersupplies..

[Khron]

Re: Aluminium capacitors in linear power supply

I've read about certain linear regs oscillating when having too-low ESR caps on the output.

Oh, i've also found some "official documents": HERE

[Slakky]

Re: Aluminium capacitors in linear power supply

That only pertains to LDOs, which LM78xx/79xx are not.

[kaboom]

Re: Aluminium capacitors in linear power supply

Don't forget a .1-.47u ceramic before and after the 78xx/79xx. I always use 'em for my dual op-amp supplies. If you use large caps after the regs, use 400x diodes to keep the caps from backfeeding thru the ICs when power is removed.

-Paul

[mariushm]

Re: Aluminium capacitors in linear power supply

That's bullshit. Less ESR doesn't make the system faster and they're not used in motherboards to gain high speed processing.

Back to the topic, 78xx regulators actually don't need any capacitor on the output side but it doesn't hurt to use one, usually 100-220uF is enough. It makes no difference if the capacitor is the polymer kind, but you're just wasting money, these regulators don't need (don't make use of) the advantages of polymer capacitors.

It is important though to use a 0.1 to 0.47 uF capacitor on the regulator's input pins. The datasheets say it's needed only if the power comes from more than a few centimeters away, but it's better to have one all the time. It must be ceramic or tantalum, not electrolytic or polymer.

Last, keep in mind that these voltage regulators manage to give you a lower voltage by getting rid of the difference in the form of heat. You'll need a heatsink if you use the power supply for more than powering a few leds or a microcontroller like Arduino or something like that.

Without anything connected to the tab on the regulator, they can remain stable dissipating about 0.5-1 watts.
If you take for example the 7805, the 5v regulator, it has a voltage drop of about 1-1.5v so that means you need to give him at least 6.5v in order to give you stable 5v, but 7-7.5v is recommended. I'll use 7.5v

A few leds, a microcontroller, a few opams in an audio amplifier.. that uses up to about 100mA or 0.1A, so we can calculate how much power is dissipated as heat in the regulator:

(Voltage Input - Voltage Output ) x Current = watts wasted as heat

So with 7.5v input, we have (7.5v - 5v ) x 0.1A = 0.25 watts
The IC will get hot but will manage without a heatsink.

If you were however to power, let's say, a small headphones audio aplifier which needs up to 1A, then the dissipated power will be up to (7.5v-5v) x 1 = 2.5 watts, which needs a good heatsink.

For such requirements, it's really recommended to go to LDOs which only need a few mv above the output voltage to regulate properly.
For example, MIC2941 only needs 0.35v above 5v to work properly, so you can safely feed it about 5.5v and get 5v out, and the heat dissipated would only be (5.5v - 5v) x 1a = 0.5 watts