Tantalum capacitors

Siri

I plan to build a regulated 5V 3-4 Amp power supply.

110 ac --> 12VAC --> Bridge rectifier --> capacitor ---> Voltage Regulator ---> capacitor ---> 5 Vdc.

I have a Voltage regulator ( LM323K) and a 12VAC transformer rated 4 Amps. I am planning to use Tantalum capacitors.While I was looking at circuits I noted most used Aluminum foil capacitor values 3000UF.
Tantalum capacitors of that value are very expensive and hard to find.I also read that Tantalum capacitors can are 10 times more active hence require only 1/10 th the value of Aluminum electrolytic caps.
Then I saw the product literature of LM323K regulator which shows 0.1UF solid Tantaum and a 1UF solid tantalum used in a power supply circuit.(which is attached).

Please help me with this decision.
Thank you.

Siri

Franklin

I'd go with what the manufacturer specifies. They would know best. You might need/want some additional caps in the part of the circuit that does the rectifing to reduce ripple but those would be in addition to the ones around the regulator. I'd also try to stay closer to 5v with the input power to minimize heat in the regulator.

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- Stephen

pwillard

Why Tantalum?

Ok, here's the scoop.· 1st thing to consider---> Size.· Using a pellet of Tantalum powder versus a wrapped foil liquid electrolytic (with a need to stay wet being imperative) allows for a slightly smaller physical "size" for the same value capacitance compared to foil type.·

Tantalums also seem to have a longer lifespan... that is unless you install them backwards and the slight POP you might hear when a normal electrolytic dies that way·is replaced by a nastier BANG explosion from the tantalum reversed. (So don't install them with the wrong orientation).

Tantalums might "behave" different, but if the designed circuit says use a 10uF, use a 10uF... regardless if it's Tantalum or Foil Electrolytic.· Again... 10uF is 10uF!!!· That's what counts. The material used may only change the physical size or accuracy of being the labeled value (AKA Tolerance).

Adding capacitors around the input and output·leads if a 3-pin regulator are tools to snub oscillations and spikes that are not handled by the large Filter Capacitors used in the input side (Unregulated side) of the power supply.· In the drawing you showed, the large supply filter capacitors are not shown, but they are assumed to be a part of the·+Vin·supplied by the*DC* source being used.

So, you still NEED the 1000 to 3000 uF filter capacitor (Aluminium foil eletrolytic to keep price down) after the bridge rectifier.· In additin, you need the snubbers (1uF on the input pin and 0.1uF on the output).· Using tantalum was recommended, not required.

110 ac --> 12VAC --> Bridge rectifier --> 1000uF to 3000uF capacitor--> [noparse][[/noparse]parallel with]--> 1uF capacitor·---> Voltage Regulator ---> 0.1uF capacitor ---> 5 Vdc.

Make more sense?

Siri

Thank you very much for the replies.

Siri

Chris Savage

This thread is being moved from the BASIC Stamps Forum·to the Sandbox Forum.

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Chris Savage
Parallax Tech Support

Dave Hein

Siri,

A 12VAC signal has a peak volatage of about 17 volts. If you use a very large capacitor to produce very little ripple you will have to regualte 17 volts down to 5, which is a 12 volt drop. At 4 amps this would be 48 watts! That's a lot of heat to dissipate.

The regulator you're planning on using can disipate up to 30 watts, and it handles up to 3 amps. The minimum input voltage is 7.5 volts, so you could use a capacitor that will maintain a minimum voltage of 7.5 volta at 3 amps. The purpose of the large input capacitor is to provide power during the time that the input voltage drops to zero volts. For 60 cycle power this time period will be less than 1/120 of a second.

The size of the capacitor is equal to the current times the time diviided by the voltage drop (C = I*T/V). In this case, I = 3, T = 1/120 and V = 17 - 7.5 = 9.5. Therefore, your capacitor should be around 2,600 uF. You transformer may produce less than 12VAC at 3 amps, so you may need to increase the capacitance to compensate for this.

With the input volatage swinging from 17 to 9.5 volts every cycle the regulator will not have to dissipate as much power. On the average is will have to dissipate 20 to 25 watts at 3 amps. This is still a lot of heat and you will need to use a heat sink to enusre that the regulator does not get too hot.

Dave