# Bridge Rectifier LTspice

Joined Sep 22, 2020
11
Good day ,
I have to design a Linear Power Supply using LTspice.
The power supply is to be driven by the mains wall outlet. A multi-output transformer with a 1 A secondary current rating i is to be used in the power supply design ‒ its secondary voltages are 3, 6, 9, 12, 15 and 18 Vrms . The power supply must use a bridge rectifier constructed from four discrete diodes. The LPS design is to have three outlets : ±5 V DC at 200 mA, ±9 V DC at 300 mA and ±12 V DC at 400 mA. Furthermore, the LPS is to have a proper DC ground .

However , I have managed to simulate the multi-output transformer using inductors and I have obtained the correct secondary voltages . I am just not sure how to input it into a bridge rectifier to obtain 3 DC voltages. Can anyone help me please ? Do I use one rectifier or 6 different rectifiers ?

#### ericgibbs

Joined Jan 29, 2010
10,760
Are you working with a local 230Vrms Mains source, if so check what LTSpice expects as a voltage entry for V1....

Please post your best attempt at creating a bridge rectifier from individual diodes, we can then help.

E

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#### Alec_t

Joined Sep 17, 2013
11,637
Welcome to AAC!
The LPS design is to have three outlets : ±5 V DC at 200 mA, ±9 V DC at 300 mA and ±12 V DC at 400 mA.
The pedant in me says I expect you mean +5VDC, +9VDC and +12VDC? (The ± sign would normally indicate a dual-polarity supply, e.g. +5VDC and -5VDC.)
A multi-output transformer with a 1 A secondary current rating i is to be used in the power supply design ‒ its secondary voltages are 3, 6, 9, 12, 15 and 18 Vrms
Does the transformer have independent secondary windings for the different voltages, or is there just one multi-tapped secondary winding?

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#### ci139

Joined Jul 11, 2016
1,696
? Do you want 1 common GND for your 3 OUTP DC voltages or an isolated one for each so you could "tile" the outputs at will ?

Joined Sep 22, 2020
11
This is my sample bridge rectifier , I am just not sure how I will input all 7 inductors into it?
I figured that I need to use one bridge rectifier connected to a to a center-tapped transformer to obtain both +ve and -ve output dc voltages and then generate the largest DC output required. Then the second DC output is derived from the first one, and the third is derived from the second output.

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#### ericgibbs

Joined Jan 29, 2010
10,760
hi t,
Check where you have load connected.??

Where is the transformer.?
E

Update:
You posted
I figured that I need to use one bridge rectifier connected to a to a center-tapped transformer to obtain both +ve and -ve output dc voltages and then generate the largest DC output required. Then the second DC output is derived from the first one, and the third is derived from the second output.

No!...

The question states:
A multi-output transformer with a 1 A secondary current rating i is to be used

#### Alec_t

Joined Sep 17, 2013
11,637
Your bridge circuit is incorrect. Check the current through any diode in your posted asc file !

#### ericgibbs

Joined Jan 29, 2010
10,760
As this is homework we have to limit the help.
Look at this option as a starting point. 25Vpk, 18Vrms & +/-Vdc

Post back what you think the next step will be.

E

Update: replaced incorrect docs!

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Joined Sep 22, 2020
11
When I connect my inductors as you did in your example , I get this error : ltspice coupling to shorted out (ignored ) inductor.
What am I doing wrong?

#### ericgibbs

Joined Jan 29, 2010
10,760
hi,
Add a low resistance to the V1 voltage Source, say Rser =0.1R or so.
E

Post your asc file, when you have a query.

#### Alec_t

Joined Sep 17, 2013
11,637
The nv rail in the post #8 circuit has a positive voltage. Is that intentional?

#### ericgibbs

Joined Jan 29, 2010
10,760
hi alec.
I Posted the wrong drawing duh!!!

Now replaced.....

Thanks alec.
E

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#### DickCappels

Joined Aug 21, 2008
6,580
The inductances look very low for 50 Hz operation. It looks like your magnetizing current on the primary will be about 7,300 amps RMS.

#### ci139

Joined Jul 11, 2016
1,696
the transformer with the transfer ratio 1:1 behaves quite normally at 50Hz having 1H inductance for it's coils (the value may be a bit low for the primary of medium-low power transformer)

Joined Sep 22, 2020
11
If I need a 400mA output current and 12V output voltage , what is the best voltage regulator to use ?

#### ericgibbs

Joined Jan 29, 2010
10,760
hi,
You could use a 7812 linear reg, what is your input voltage.?
E

Joined Sep 22, 2020
11
hi,
You could use a 7812 linear reg, what is your input voltage.?
E
Thank you , 23 V.

#### ericgibbs

Joined Jan 29, 2010
10,760
hi,
This is the datasheet for the 7812.

With a 23V supply and a 0.4Amp load current, you will drop 11V across the 7812 at 0.4A, so thats 11V * 0.4A = 4.4Wattts,

Without an heatsink you could allow a 2Watt dissipation, but with 4.4W you must use a suitable heatsink.

E

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#### bertus

Joined Apr 5, 2008
20,750
Hello,

With the standard 7812, you will have to dissipate (23 - 12) Volts X 0.4 Amp = 4.4 Watts.
A heatsink will be needed.
Can you use a switching regulator?
Then you could have a look at the RECOM voltage regulators.

Bertus

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