# Power Supply with additional current pull

#### Shantanu Jagtap

Joined Nov 19, 2016
30
Hey everyone. I am kind of new to making physical electronic circuits. Currently I am trying to build a simple power supply.
Input = 230VAC 50HZ
Load 1: 12V 12mA
Load 2: 12V 1.2A
Load 3: 3.3V 80mA.

Now I designed a circuit using multisim using a LM7812CT regulator that has a rating of 12Vout 1A. Now I am confused. If the rating of this regulator is 1A, how is it able to power 1.292A loads all connected together? Is it possible that it will run in the simulation but will not work when i actually build it?

Also the step down transformer I am using is 230V to 15V step down with 1A rating. Maybe i am unaware of some concept?

Also, If i add a relay with rating 12V 1A, will i need to increase the transformer to a higher rating? maybe a 15V 3A? But then again if the loads together pull 2.3A from a regulator that has a rating of 1A, won't the regulator burn out?

These questions may seem very stupid, but i am starting out new into building electronics and I am very keen to learn and make circuits. Any help or advise is appreciated.

Thank you.

#### R!f@@

Joined Apr 2, 2009
9,708
Welcome to AAC.

Simulation does not prove a real life situation.
The 7812 cannot supply even 1A without a good heatsink. You have to consider the power dissipation. A simulation shows you what is actually been supplied to the load.
To get 1.2A you might need to consider a 5A regulator or connecting 2pc of 7812 in parallel.

A relay does not draw 1 A. It is the contact capacity ratings.

A 15V 1A transformer cannot supply 1A DC. It will be less than that if you are using a bridge rectifier.
If you are using a center tap transformer than 1A AC is equal to 1A DC. But the voltage will be less.

#### R!f@@

Joined Apr 2, 2009
9,708
The sum of current for the 3 loads you specified will be 1.3A Approx.
So to get atleast 12V DC out you need around 18V DC input to the 7812.

The transformer for 18V at 1.3A DC will be;
1 . 13VAC at 2.1A . A secondary of 0-13VAC at 2.1A to a FULLWAVE BRIDGE Capacitor Input Load will give you 18V @ 1.3A DC
2. 26VAC (CT) at 1.3A . A Secondary of 13-0-13 VAC at 1.3A to a FULLWAVE Capacitor Input Load will give you 18V at 1.3A DC

Look at the pdf.

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#### Papabravo

Joined Feb 24, 2006
13,459
It might be worth mentioning that if you put a load on a power supply that it cannot handle, it will adjust it's output in such a way that the output power will always be less than the input power. Sometimes it will be a great deal less.

#### MrChips

Joined Oct 2, 2009
20,862
It might be worth mentioning that if you put a load on a power supply that it cannot handle, it will adjust it's output in such a way that the output power will always be less than the input power. Sometimes it will be a great deal less.
And more likely it will go into self protect mode, i.e. it will shut down and give you no output.

#### #12

Joined Nov 30, 2010
18,210
The basic problem here is that simulators are not a substitute for human engineering skills.
You are missing the idea that transformers have power limitations which seem distorted when using them for DC circuits. When you rectify the AC voltage, you get the peak voltage on the filter capacitors. A transformer that says, "12V 1A" will produce 12 watts in AC, but when rectified, the voltage on the capacitors is now about 16 volts, and 12 watts divided by 16 volts is only 3/4 of an amp. (Actually, it's worse than that. We have charts and drawings to show you the limitations.)

Most of your confusion comes from believing the simulator. What you're really doing is noticing the false results and wondering if it's your fault. Simulators can be excellent tools, but they aren't design engineers. They have inherent limitations and learning how to use them well is another challenge. You can get very good at using simulators, but, right now, you're still learning.

#### Papabravo

Joined Feb 24, 2006
13,459
And more likely it will go into self protect mode, i.e. it will shut down and give you no output.
They can do that, or they can blow a fuse, or they can reduce the voltage. You won't really know until it happens what an unfamiliar supply will do. The datsheet could tell you of course, but how many people keep power supply datasheets? Oh...wait, new designs don't have datasheets yet. Maybe they don't even have design requirements...shudder.

#12

#### Shantanu Jagtap

Joined Nov 19, 2016
30
Thank You for All the help guys!!! I really really appreciate it. I have gone through all the advises and made TWO changes in my supply. They are
1. Instead of LM7812 CT regulator I am now using LT1084t-12 regulator which is a 12V 5A regulator.
2. The transformer I am using is now a 0-15V with current rating of around 7 to 10A. (I am still searching for one since I need a SMD package)

Do you guys think this configuration will be able to supply 12V to 3 loads with a total maximum current pull of 1.3A?

The bridge rectifier I plan to use is the MDB6S with a forward voltage Vf = 0.935V @ 1A...and...1.165V @ 5A.

#### Shantanu Jagtap

Joined Nov 19, 2016
30
Also, I might probably need to add another load of 12V 1.2A. So, now the total current pull will be maximum of 2.3A. You guys think the above mentioned components will be able to provide that much current?

#### R!f@@

Joined Apr 2, 2009
9,708
2. The transformer I am using is now a 0-15V with current rating of around 7 to 10A. (I am still searching for one since I need a SMD package)
Where can you get a SMD mains Power tranformer of around 105 - 150 VA.????

With a 0-15V @ 7-10A
21VDC through a Bridge rectifier.
A. With a 7A secondary rating you will get 4 Amps DC continues.
B. With a 10A secondary rating you will get 6 Amps DC continues.

You can regulated any voltage to a max of 16VDC out. ( 5V for regulator drop out )
You can put any number of load as long as the Tx Maximum current does not exceed ( as specified in A and B, above ).

Plus you will need around 6800uf to 10000uf 35V smoothing cap. Bigger the better.

Choose the bridge rectifier that can handle double the load current.

#### Shantanu Jagtap

Joined Nov 19, 2016
30
Where can you get a SMD mains Power tranformer of around 105 - 150 VA.????

With a 0-15V @ 7-10A
21VDC through a Bridge rectifier.
A. With a 7A secondary rating you will get 4 Amps DC continues.
B. With a 10A secondary rating you will get 6 Amps DC continues.

You can regulated any voltage to a max of 16VDC out. ( 5V for regulator drop out )
You can put any number of load as long as the Tx Maximum current does not exceed ( as specified in A and B, above ).

Plus you will need around 6800uf to 10000uf 35V smoothing cap. Bigger the better.

Choose the bridge rectifier that can handle double the load current.

Great Help. Thank you very much. Now, my last stupid Question. Is only ONE 10000uF 35V smoothing capacitor enough? Most of the power supply circuits I have seen have at least 4 capacitors. I am assuming they do the function of smoothing and prevent surges i suppose.

And by "can handle double the load current", you are referring to the "Absolute Maximum Peak Forward Surge Current" right?
In the datasheet for my bridge rectifier, the
Absolute Maximum Peak Forward Surge Current = 30A
and
Average Rectified Forward Current = 1A

My load pulls 2.4A MAX.

Last edited:

#### R!f@@

Joined Apr 2, 2009
9,708
For a power supply lotta capacitors are really not necessary if the transformer is rating is high.
Lot of Caps are used in Audio amps where bass produces lot of surges on the supply.
Typically for a PSU..capacitor is chosen like 1500uf per Amp (DC, not AC). So for like 10Amps, it will be 10*1500 = 15000uf. And that is for good ones.

You need to decide what will be your maximum loading capacity of the PSU and then get the correct cap, bridge and Tx.

If your max load is 2.4A then a Bridge rated at 5A continues will be adequate enough. Even 3 amps is OK but that is cutting quite closely. A 3A bridge putting 2.4A will heat up and for continues use it is not suggested.
AND if you plan on putting high capacitance smoothing caps you need to go for higher current bridges to keep bridge safe while the cap is charging. If not you will need a surge current limiter to ease the load on the bridge during power on.

If your max load is 2.4A, then a TX of secondary current should be atleast 4A AC, with a bridge of 5A continues ( not surge ) min for a 4700uf smoothing cap. These value should good for you.

PS..Surge is not continues rating. And you can use Multiple smaller caps to get higher values which is better than a single large one. Which I did for my PSU

#### Shantanu Jagtap

Joined Nov 19, 2016
30
For a power supply lotta capacitors are really not necessary if the transformer is rating is high.
Lot of Caps are used in Audio amps where bass produces lot of surges on the supply.
Typically for a PSU..capacitor is chosen like 1500uf per Amp (DC, not AC). So for like 10Amps, it will be 10*1500 = 15000uf. And that is for good ones.

You need to decide what will be your maximum loading capacity of the PSU and then get the correct cap, bridge and Tx.

If your max load is 2.4A then a Bridge rated at 5A continues will be adequate enough. Even 3 amps is OK but that is cutting quite closely. A 3A bridge putting 2.4A will heat up and for continues use it is not suggested.
AND if you plan on putting high capacitance smoothing caps you need to go for higher current bridges to keep bridge safe while the cap is charging. If not you will need a surge current limiter to ease the load on the bridge during power on.

If your max load is 2.4A, then a TX of secondary current should be atleast 4A AC, with a bridge of 5A continues ( not surge ) min for a 4700uf smoothing cap. These value should good for you.

PS..Surge is not continues rating. And you can use Multiple smaller caps to get higher values which is better than a single large one. Which I did for my PSU

My Loading will not exceed even 2.5...MAXIMUM 3A it will not even touch for sure! So, according to what you said...3*1500 = 4500uF cap is good enough. correct?

#### R!f@@

Joined Apr 2, 2009
9,708
4500uf not standard value.
4700uf is a standard value.

#### Shantanu Jagtap

Joined Nov 19, 2016
30
Great. Got it. I searched around, and obviously, you were right. No SMD transformers for my application. So, I am thinking of now making a SMPS. Which will help me reduce the size of the transformer greatly. But is a SMPS ideal to provide to a 12V approximately 3A load???

#### R!f@@

Joined Apr 2, 2009
9,708
Wow!
You hardly understand the concept of linear supply and it's requirement in building one and now you want to make an SMPS.
Good luck.

#12