Cap Value

Lightfoot

What should the value of C1 be? I got this from the LM338K datasheet but it does not say.



Thanks

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Well well, I'm seeing things, three of them.

-Stanley Blystone

PJAllen

Go with anything 1 to 47uF.· Just make sure its WVDC rating is > the output voltage.· [noparse][[/noparse]This cap is kind of undone if the circuit following has a cap across its input.· Don't get really hung-up on this, it's not so bloody critical as all that.]

Lightfoot

I have a 3300uf 35v filtering the input, so it is not needed?

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Well well, I'm seeing things, three of them.

-Stanley Blystone

PJAllen

Three of Them said...
I have a 3300uf 35v filtering the input, so it is not needed?

You asked about C1 in the schematic that you posted -- and C1 is on the Output.·
A smoothing capacitor IS totally needed for the Input, but it isn't shown on your schematic.· 3300uF is likely fine; if·with higher current demand you find that your input ripple increases significantly then you'll benefit from more capacitance.
(Are we gonna fish or cut bait here?)

Lightfoot

Sorry I did not make myself more clear, i'm talking about C1, its not necessary if I have the input cap, right? Also, how do I find out the amount of capacitance I need in accordence to the current?

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Well well, I'm seeing things, three of them.

-Stanley Blystone

PJAllen

Here is the scoop on C1, the output capacitor, and it comes right from out of the Datasheet, Application Notes, External Capacitance (emphases mine):

Although the LM138 (LM338) is stable with no output capacitors, like any feedback circuit, certain values of external capacitance can cause excessive ringing.· This occurs with values between 500 - 5000pF.· A 1uF solid tantalum (or 25uF aluminum electrolytic) on the output swamps this effect and insures stability.

Three of Them said...
i'm talking about C1, its not necessary if I have the input cap, right?

Not exactly -- C1 has nothing to do with the input (filtering/smoothing/stand-up) cap.

Three of Them said...
Also, how do I find out the amount of capacitance I need in accordence to the current?

If you're asking about the input (filtering/smoothing/stand-up) cap... use a scope or an AC voltmeter to monitor ripple vs. output current.· If the ripple is significant, such that·it affects regulation, then add more capacitance (1000uF.)·
You don't want to go hog wild with the capacitors, because a large, uncharged cap makes for a big current spike at power-up (turn-on), which, over time,·could be hard on your transformer, bridge rectifier, and fuse.

Lightfoot

This is the circuit I am currently using. The volt meter reads 12.01 volts for DC, the result I want. I do not have a scope so I am blind.



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Well well, I'm seeing things, three of them.

-Stanley Blystone

PJAllen

I think that you're good to go.· I'd figure C1 is close enough.

As I replied previously, you can (also) use an AC voltmeter (a DMM set to "ACV") to measure ripple on the input, or anywhere else, if that is an over-arching concern.· [noparse][[/noparse]N.B. -- low-frequency AC, 60Hz, 120Hz; not spikes or high-freq.]

Lives aren't at stake -- Relax.

Lightfoot

I'm not familiar with the AC voltage trick, if there is no ripple, should the AC read 0 volts?

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Well well, I'm seeing things, three of them.

-Stanley Blystone

PJAllen

It's no trick, mate.· You're measuring AC voltage riding on DC.· Yes, practically speaking, if the ACV = "0.00" then it's clean.