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LM7805 simulation

neptune

Member
Hello,
Can anybody help me run this simulation of LM7805.
it is suppose to give 5V of clean output.

i have to charge 5V battery from it.

Thanx
 

alec_t

Well-Known Member
Most Helpful Member
LTSpice has the LT1021-5 voltage ref model. Can't you use that?
 

neptune

Member
LTSpice has the LT1021-5 voltage ref model. Can't you use that?
i dont have LT1021-5 in my LTspice but have LT1021A + i already brought a chip LM7805
 

ericgibbs

Well-Known Member
Most Helpful Member
i dont have LT1021-5 in my LTspice but have LT1021A + i already brought a chip LM7805
hi,
IIRC you are a member of the Yahoo LTS user group.???

Its in their Files/Lib

EDIT:
The LTS asc circuit is an earlier circuit, for REF only.
 

Attachments

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neptune

Member
i have downloaded 78xx Series.zip (4.5 KB) and only 4 linear regulators are given in that including LM7805 but no LT1021-5

is my circuit a problem ?
 

ericgibbs

Well-Known Member
Most Helpful Member
i have downloaded 78xx Series.zip (4.5 KB) and only 4 linear regulators are given in that including LM7805 but no LT1021-5

is my circuit a problem ?
You said you had bought a LM7805, so why do you want the model for a LT1021-5.??

With reference to your 5V reg asc, part of the bridge is shorted out with the grounds also there is no output capacitor
 
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ericgibbs

Well-Known Member
Most Helpful Member
hi,
The problems with your 5V reg are:
1. You had a 6Vpk AC input to the bridge.
2. The 1N4148 are only signal diodes
3. Incorrect and missing capacitors.
4. Part of the bridge shorted out via the 'ground's

LTSpice is only a simulator, you have to design the circuit correctly for it to give you the required performance.

Run this edited version of your circuit.

BTW: in your TRAN, you have a Run time of 1 Sec set, with a STEP interval of 5 Secs!!!

I did tell you about this previously.
 
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neptune

Member
BTW: in your TRAN, you have a Run time of 1 Sec set, with a STEP interval of 5 Secs!!!
my mistake , that got left out !

-you have used 9V supply, but my Transformer output is 6V .. what should i do ?
-what is the use of capacitors at the output of LM7805 ? i thought LM7805 gave clean DC output voltage
 

ericgibbs

Well-Known Member
Most Helpful Member
my mistake , that got left out !

-you have used 9V supply, but my Transformer output is 6V .. what should i do ?
-what is the use of capacitors at the output of LM7805 ? i thought LM7805 gave clean DC output voltage
hi,
A 6V RMS transformer, will have a peak of 8.4Vpk, after the rectifier it will loose two forward diode voltage drops, say 2*0.7v = 1.4V.

So the Vcap smoothing voltage will have a maximum of 8.4V - 1.4V = 7V.

For reliable operation a 7805 requires at least 2V higher input than output, so a 6Vrms transformer is unsuitable for a 7805, because as soon as you draw current from Vcap the input to the 7805 will below the required input voltage.


The 100nF caps are recommended to ensure that the 7805 will not oscillate, the 100uF on the output acts a 'fast' current source for the load, its standard practice.
 
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ericgibbs

Well-Known Member
Most Helpful Member
hi,
This is the circuit with a 6V rms transformer, note the high ripple with just a 100mA load current.


BTW: LTSpice expects the specified sine wave voltage source to be the Vpeak of the Sine wave input, not the RMS value.... so 6Vrms == 8.4Vpk..

Likewise for a mains supply of 230Vrms, the Vsource would be 230*1.414 ie: 325Vpk
 
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Willen

Well-Known Member
hi,
The problems with your 5V reg are:
1. You had a 6Vpk AC input to the bridge.
2. The 1N4148 are only signal diodes
3. Incorrect and missing capacitors.
4. Part of the bridge shorted out via the 'ground's

LTSpice is only a simulator, you have to design the circuit correctly for it to give you the required performance.

Run this edited version of your circuit.

BTW: in your TRAN, you have a Run time of 1 Sec set, with a STEP interval of 5 Secs!!!

I did tell you about this previously.
For why this 1k Resistor in parallel at output of 7805? Please
 

Adich

New Member
I know this is an old post, and this might be a long shot, but I can see that you have uploaded a circuit that I could use, but I can't download it. Do you still have the circuit?

Adich
 

eTech

Active Member
hi,
Try this zip.
E
That worked..thanks...

But the 1N4006 model was also missing, so I used this:

*SRC=1N4006;DI_1N4006;Diodes;Si; 800V 1.00A 3.00us Diodes, Inc. diode
.MODEL 1N4006 D ( IS=76.9p RS=42.0m BV=800 IBV=5.00u CJO=26.5p M=0.333 N=1.45 TT=4.32u )

thx

eT
 

Adich

New Member
Thanks guys, that was the exact one I was looking for, and all my questions have already been answered I see, thanks ;)
 

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