Basic DIY Linear Power Supply Question

Thread Starter

ryno555

Joined Mar 24, 2024
3
I am getting into the hobby, and I have built the the attached homemade linear power supply.

I am trying to run a DC fan with this, under no load I am measuring around 17 VDC on the output, and when powering the fan voltage drops to around 10VDC. Fan is drawing ~150mA average with peaks of ~300mA. I replaced the fan with a resistor that would draw the same amount of current and I am experiencing this same voltage drop. Lower value resistors drawing less current do not show as severe voltage drop.

What are my options for increasing the amount of current that my linear supply can provide without getting the voltage drop I am currently.
Does it need a much larger output cap?
Thanks for the help.IMG_2077.jpg
 

MrChips

Joined Oct 2, 2009
30,986
Welcome to AAC!

The defining parameter is the power rating of the transformer.
What is the output voltage and current capacity as written on the transformer?
 

Thread Starter

ryno555

Joined Mar 24, 2024
3
Thanks
Welcome to AAC!

The defining parameter is the power rating of the transformer.
What is the output voltage and current capacity as written on the transformer?
Thanks for the reply! I believe it is a 2W transformer, V out is 9VAC. My assumption was that the xfmr would be able to drive current until it melted (heat being the limiting factor) and during my tests temperature of the xfmr seemed fine. but like I said I'm new.
 

MrChips

Joined Oct 2, 2009
30,986
Let us assume for the time being that that information is correct.
2W @ 10VDC would translate to 0.2A.

But it is not that simple.
9VAC is about 12.6VDC peak. The available current is about 120mA.
Basically, the transformer is not capable to supply the power needed.
 

MrAl

Joined Jun 17, 2014
11,587
I am getting into the hobby, and I have built the the attached homemade linear power supply.

I am trying to run a DC fan with this, under no load I am measuring around 17 VDC on the output, and when powering the fan voltage drops to around 10VDC. Fan is drawing ~150mA average with peaks of ~300mA. I replaced the fan with a resistor that would draw the same amount of current and I am experiencing this same voltage drop. Lower value resistors drawing less current do not show as severe voltage drop.

What are my options for increasing the amount of current that my linear supply can provide without getting the voltage drop I am currently.
Does it need a much larger output cap?
Thanks for the help.View attachment 318351
Hi,

I have to agree with all of the previous posts about 100 percent, but also have to add a little that might help.

First, to test the transformer, load that with resistors first without anything else connected to it and see what the voltage drops to with a couple resistors of various sizes drawing maybe 200ma and 300ma or something like that, and no load. Measure both the input AC voltage and the output AC voltage for EACH test, and what you used to measure these voltages with (True RMS or a more regular meter).

Then, repeat the same tests with the same resistors by connecting them to the output after you connect the rectifiers and cap back into the circuit.

These tests will tell us a lot about what is going on.

Now about rectifiers and filter caps...
With a bridge rectifier and filter cap, the cap charges up during a small time during each half cycle when there is no inductance present. The current during this time could be much higher than the average current draw from the cap by the load. That's normal. The thing is, since the current is limited, the average cap voltage will be at some level that may not be the maximum obtainable. By adding another cap in parallel to the original cap, you can raise the average DC value. This does put more of a strain on the rectifier diodes though and so it may be necessary to increase the size of the diodes also. It also depends on what the transformer can take that's why we need to know what the transformer can do also.

If the rating really is just 2 watts, that's not very good. If the fan draws 300ma at 12 volts, that's 3.6 watts (12 times 0.300 is 3.6). That's almost twice the transformer rating so running this fan with that transformer it may never be able to work correctly. If the fan draws 200ma at 12 volts, that's 2.4 watts, you may be able to sneak by with that although it may be hard to get the average voltage high enough even with more caps. Going by the transformer rating and the fan voltage, the max current would be 167ma, which may or may not be enough. If the voltage drops, the fan speed will decease although it should still run. There is the possibility that the fan speed may decrease too much though and thus make it ineffective.

There are a lot of DC wall warts available online in a lot of different places and are not too expensive. Amazon is one such place. You could get a 12v, 1amp DC wall wart and that should be able to run the fan without any hassle, and it would probably even be regulated so the fan speed would not change as the input line voltage changed.
I also realize you may want to do this for the hobby value, and in that case you could just get a higher power transformer, one that could put out 1 amp and that would work pretty well. The diodes in the bridge rectifier should be rated for 1 amp or better. The series 1N400x works pretty well, like the 1N4004 for example.
You should also be aware that the diodes drop some voltage also, and for a bridge rectifier that could be 1.5 volts.
 

ElectricSpidey

Joined Dec 2, 2017
2,839
My only comment is...the output voltage will droop from any unregulated supply when you attach a load.

Ok...another comment...calling that a "linear supply" is a stretch as there is no regulation.
 

Thread Starter

ryno555

Joined Mar 24, 2024
3
Hi,

I have to agree with all of the previous posts about 100 percent, but also have to add a little that might help.

First, to test the transformer, load that with resistors first without anything else connected to it and see what the voltage drops to with a couple resistors of various sizes drawing maybe 200ma and 300ma or something like that, and no load. Measure both the input AC voltage and the output AC voltage for EACH test, and what you used to measure these voltages with (True RMS or a more regular meter).

Then, repeat the same tests with the same resistors by connecting them to the output after you connect the rectifiers and cap back into the circuit.

These tests will tell us a lot about what is going on.

Now about rectifiers and filter caps...
With a bridge rectifier and filter cap, the cap charges up during a small time during each half cycle when there is no inductance present. The current during this time could be much higher than the average current draw from the cap by the load. That's normal. The thing is, since the current is limited, the average cap voltage will be at some level that may not be the maximum obtainable. By adding another cap in parallel to the original cap, you can raise the average DC value. This does put more of a strain on the rectifier diodes though and so it may be necessary to increase the size of the diodes also. It also depends on what the transformer can take that's why we need to know what the transformer can do also.

If the rating really is just 2 watts, that's not very good. If the fan draws 300ma at 12 volts, that's 3.6 watts (12 times 0.300 is 3.6). That's almost twice the transformer rating so running this fan with that transformer it may never be able to work correctly. If the fan draws 200ma at 12 volts, that's 2.4 watts, you may be able to sneak by with that although it may be hard to get the average voltage high enough even with more caps. Going by the transformer rating and the fan voltage, the max current would be 167ma, which may or may not be enough. If the voltage drops, the fan speed will decease although it should still run. There is the possibility that the fan speed may decrease too much though and thus make it ineffective.

There are a lot of DC wall warts available online in a lot of different places and are not too expensive. Amazon is one such place. You could get a 12v, 1amp DC wall wart and that should be able to run the fan without any hassle, and it would probably even be regulated so the fan speed would not change as the input line voltage changed.
I also realize you may want to do this for the hobby value, and in that case you could just get a higher power transformer, one that could put out 1 amp and that would work pretty well. The diodes in the bridge rectifier should be rated for 1 amp or better. The series 1N400x works pretty well, like the 1N4004 for example.
You should also be aware that the diodes drop some voltage also, and for a bridge rectifier that could be 1.5 volts.
Testing the transformer the voltage drop problem still exists, and gets worse as more current is drawn by smaller resistors, so looks like I need a bigger xfmr. Thanks
 

MrAl

Joined Jun 17, 2014
11,587
Testing the transformer the voltage drop problem still exists, and gets worse as more current is drawn by smaller resistors, so looks like I need a bigger xfmr. Thanks
Oh sorry to hear that, but then maybe you can use that smaller transformer for something else in the future.
 
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