Variable Power Supply Multi Sim

Thread Starter

amano001

Joined Sep 7, 2012
35
I rebuilt the circuit in MS 12 and it seems to be more on point at least. But still the output of the rectifier is being affected by the larger capacitor and I can't figure out why that is. Always seems to put out 32 volts with the 1000uf cap but with the .1uf cap it puts out the correct 21v.

Here is my math by the way

full wave rect

2Vpeak/3.14

vpeak = 24v * sqrt(2) = 34v

2*34/3.14 = 21v approx.

I just want to know if this is the programs fault or I am missing some math somewhere that has to deal with large capacitors in this circuit.
I will test my real circuit tomorrow to confirm what is going on for sure.



Thanks guys.
 

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bertus

Joined Apr 5, 2008
22,270
Hello,

The peak voltage is 24 * sqr(2) ≈ 24 * 1.414 = 33.9 Volts.
I do not know why you want to divide it with 3.14, wich is wrong.

Bertus
 

Audioguru

Joined Dec 20, 2007
11,248
The large capacitor correctly smooths and charges to the peak voltage minus two diode voltage drops (+32.4VDC).

The tiny capacitor allows the rectified voltage to wildly swing up and down so the average voltage is only +21VDC. The regulator will not work if its input voltage drops below about +8.4V which will occur when it has a load and the tiny filter capacitor.
 

Thread Starter

amano001

Joined Sep 7, 2012
35
Thanks everyone I think I finally understand everything.

Why do you say it will not work below 8.4 volts though? THe LM 317 only needs I think 1.3v to run?
 

Audioguru

Joined Dec 20, 2007
11,248
Why do you say it will not work below 8.4 volts though? The LM 317 only needs I think 1.3v to run?
You want the regulated output voltage to be 6.9V. The datasheet shows a graph of its typical "dropout voltage" which is 1.5V when its load current is 20mA and is higher when the load current is higher. 6.9V + 1.5V= 8.4V.

The dropout voltage is the amount of voltage that the input must be higher than the output. The LM317 is not a generator.
 

Thread Starter

amano001

Joined Sep 7, 2012
35
Hmm I am back to being confused about the 6.9 volts.
The datasheet says it can regulate from 1.25v to 37v. What if I wanted my regulated output to be say 5v? Would that not mean

1.5v + 5v = 6.5v now on the input?

Thanks.

PS. Bertus that unit12 link is awesome.
 
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