# Building of a lab bench power supply

#### Gigi666

Joined Aug 29, 2022
1
The full bridge rectifier
A step down transformer is used to step down the main 220 VAC to 28VAC
14N005 diodes is used to for the rectification, the output DC voltage is 24V ,
with a 470uF capacitor 25V , is used at the output of the full bridge.

to be continued, as the project is still in progress

#### Ya’akov

Joined Jan 27, 2019
9,237
The full bridge rectifier
A step down transformer is used to step down the main 220 VAC to 28VAC
14N005 diodes is used to for the rectification, the output DC voltage is 24V ,
with a 470uF capacitor 25V , is used at the output of the full bridge.

to be continued, as the project is still in progress
Welcome to AAC, do you have a question?

#### Jon Chandler

Joined Jun 12, 2008
1,073
Not quite......

28VAC when rectified will be

28VAC when filtered = 28 × sqrt(2) – 2Vf = 38VDC.

The filter cap will be charged to the peak voltage of the AC sine wave, minus 2 diode voltage drops from a bridge rectifier.

#### MrSalts

Joined Apr 2, 2020
2,767
Not quite......

28VAC when rectified will be

28VAC when filtered = 28 × sqrt(2) – 2Vf = 38VDC.

The filter cap will be charged to the peak voltage of the AC sine wave, minus 2 diode voltage drops from a bridge rectifier.
Right. But the 470uF smoothing caps are a bit small for most power supplies unless the load will be in the milliamp range.
You could put 5 to 10 of those in parallel or get a bigger one 2200 to 4700uF. Look for caps with low ESR (equivalent series resistance).

#### BobTPH

Joined Jun 5, 2013
9,174
So far, you are nowhere close to having a lab bench supply.

#### MrSalts

Joined Apr 2, 2020
2,767
Not by your definition of Laboratory Power Supply but I don't think you've seen his laboratory or his specifications (unless I missed it).
It's called iterative development.
Let him build his first iteration and we can help him make it adjustable, and remove ripple, and limit current as it evolves if and when he needs any bells or whistles.

#### LowQCab

Joined Nov 6, 2012
4,217
Unless this is strictly for very small Circuits,
1-Amp Diodes are not going to cut it.

What's the maximum Current and Voltage that You expect from this Power-Supply ?
What is the "VA" Rating of your Transformer ?
Does it need to be a "Split-Supply" ?, ( plus & minus Voltages ).
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#### MrSalts

Joined Apr 2, 2020
2,767
Unless this is strictly for very small Circuits,
1-Amp Diodes are not going to cut it.

What's the maximum Current and Voltage that You expect from this Power-Supply ?
What is the "VA" Rating of your Transformer ?
Does it need to be a "Split-Supply" ?, ( plus & minus Voltages ).
.
.
.
I've been using a 0.75 amp Hewlett Packard power supply for 35-years. It really depends on what he is doing.

#### atferrari

Joined Jan 6, 2004
4,784
And once this saga ends (if at all), he will realize how convenient would be to add two fixed outputs (+5V and +3.3V).

#### Martin_R

Joined Aug 28, 2019
137
470uF caps rated at 25v used at 38v input is going to produce big bangs. Need caps rated at 50volts.

#### MrSalts

Joined Apr 2, 2020
2,767
470uF caps rated at 25v used at 38v input is going to produce big bangs. Need caps rated at 50volts.
If he has a pocket full of 25v capacitors, he can put pairs in series and then put 10 to 20 of those pairs in parallel.

#### Martin_R

Joined Aug 28, 2019
137
If he has a pocket full of 25v capacitors, he can put pairs in series and then put 10 to 20 of those pairs in parallel.
True, as long as there are balance resistors to equal the voltages out. Personally I'd rather have capacitors with the correct rating, will take up less board space and reduce the chance of accidentally inserting them the wrong way round.

#### dl324

Joined Mar 30, 2015
17,010
Welcome to AAC!
14N005 diodes is used to for the rectification, the output DC voltage is 24V ,
I assume you mean something more like 1N4005.
the output DC voltage is 24V ,
with a 470uF capacitor 25V , is used at the output of the full bridge.
Normally we'd derate the caps more than that, but that's probably okay in this case because they're formed at a voltage higher than the rated voltage.

After you've built your first power supply, you'll probably want to buy a 30V @3-10A switching supply. They cost under \$75 if you shop around.

I've got this one on eBay on my watch list:
Adjustable DC Power Supply Variable,30V 10A Switching Regulated Digital Display | eBay

#### MrSalts

Joined Apr 2, 2020
2,767
I think building a power supply and understanding the limitations of what you've built is a great leaning experience. Ya just learn so much from mistakes, optimistic assumptions and bad estimates.

From enclosure design, enclosure sizing, knob layout, heat management, and then you can discuss the circuit and bells and whistles to make it do what you need (and what you want).

How much will someone learn from a purchased power supply... and a purchased amplifier,... and purchased speakers... and purchased, well, everything. I think he'll learn like my brother. He'll have a whole garage full of tools and no projects to work in because he just purchased the muscle car of his dreams instead of built the muscle car of his dreams.

Of course you can always buy better than you make but, then you have no idea how it works, no interest in figuring out what if fails and you've trained yourself to buy what you want instead of build what you want.