# Is it better to buy a bench PSU, or build one?

#### Joe Stavitsky

Joined Apr 5, 2020
15
Building a PSU is one of the few "academic" projects I'm willing to consider. I don't see it taking me too long, and I can probably make something nice to look at. However, I may consider buying one at the $300-$500 pricepoint if I'm convinced that there are major advantages. Thoughts?

Joe

#### SamR

Joined Mar 19, 2019
3,405
Depends on what you want and need... I have 2 tubs full of various AC and DC wall warts. 2 bench supplies as shown below which are great for starting out. Plus a bunch of XFMRs and both homemade and kits power supplies for special projects. Also Isolation Transformers and Variac. Fixed DC voltage switch mode power supplies of various wattages. Etc. $300-400 will buy you a lot of power supply. As you will hear around here, You can never have enough power supplies. https://i.ebayimg.com/images/g/wIEAAOSwNmVf2PXr/s-l400.jpg$85 including shipping for US delivery. I have 2 of these. Good quality linear power supply. Easily adjustable, accurate, and has current limiting. Good for most DC work. Can be used floating or earth grounded.

As to buy or build... You cannot build one as nice as most like the one above for what they sell for. Everyone at some time or another should build some kind of power supply with a XFMR, rectifier, smoothing filters, regulators, diode output protection, +/- adjustable voltage, and current limiting as a learning exercise.

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#### Joe Stavitsky

Joined Apr 5, 2020
15
"What I want" is something of moderate power but high accuracy, precision and reliability, suitable for light repair, electrodeposition, and introductory experiments.

Thanks again

Joe

#### Joe Stavitsky

Joined Apr 5, 2020
15
Thanks for the welcome . First off what do you mean by "a large scale"? Second, why do I need a separate unit for AC? Can't I just connect an oscillator?

#### SamR

Joined Mar 19, 2019
3,405
Well I wouldn't try and use a bench PSU to electroplate a bike frame...

For starting out in electronics the one shown is great and at an excellent price. Reliable, accurate, and quiet both audible fan noise-wise and electronic noise-wise since it is a linear PSU and not a switch-mode PSU. Current limiting goes down to 1mA and voltage to hundredths. I have only rarely gone over 20VDC and usually not much more than 200mA building experimental circuits. So that power supply serves most of my needs. When you get into Op Amps and dual + and - rails, then you will need more. So I built some PSUs just for that and still use the bench PSU to power the rest of the circuit.

The only need for AC is to power some electronics that have a built-in power supply. Hence Isolation XFMR for shock protection and Variac to slowly ramp up AC on old XFMRs in radios so they won't give off the magic smoke.

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#### Joe Stavitsky

Joined Apr 5, 2020
15
Gotcha, thanks again

#### crutschow

Joined Mar 14, 2008
27,188

#### MrChips

Joined Oct 2, 2009
23,515
"What I want" is something of moderate power but high accuracy, precision and reliability, suitable for light repair, electrodeposition, and introductory experiments.
Sorry, none of those words have any meaning until you specify a number.
For example, moderate power could mean 10W for one person and 100W to another.

#### Joe Stavitsky

Joined Apr 5, 2020
15
@SamR, I just got this thing, it's sweeeeeeeet. The digitizer control is off the chain. I haven't tested it with a bench dmm yet, but given the enclosed test report if there's any problem I'll assume the dmm is to blame.

#### Ian0

Joined Aug 7, 2020
2,189
If you fancy a bit of audio work, then you'll need a dual supply with positive and negative outputs.
If you make your own power supply, the most important feature is the overload protection. You can make a fantastically well regulated, low-noise DC power supply, but if it blows up the first time (of many) that you short it out, you have wasted your time!
I'd also wager good money that at some point you'll use it for charging batteries, so make sure it can before you buy one.

#### MrChips

Joined Oct 2, 2009
23,515
@SamR, I just got this thing, it's sweeeeeeeet. The digitizer control is off the chain. I haven't tested it with a bench dmm yet, but given the enclosed test report if there's any problem I'll assume the dmm is to blame.
Just keep in mind that the digital readout of V and I on the PSU are not control settings. These are simply inexpensive voltmeters to give an indication of output V and I.

Another thing you have to learn is how to set up constant V and constant I. Ask if you don't know how.

#### SamR

Joined Mar 19, 2019
3,405
The digital V & I metering is very accurate but your DMM is the ultimate reference standard. I usually set the current limit to 50mA and if the V on the meter goes below what I set it at I increase the current limit as needed for what I'm working on.

#### Joe Stavitsky

Joined Apr 5, 2020
15
Another thing you have to learn is how to set up constant V and constant I

#### Joe Stavitsky

Joined Apr 5, 2020
15
you'll need a dual supply with positive and negative outputs.
You mean 4 outputs total? Or 2?

#### MrChips

Joined Oct 2, 2009
23,515
Lab bench power supply units (PSU) are usually designed and sold as constant voltage and constant current PSU.
You can set the voltage and current. You cannot run the PSU at both V and I set-points because V and I are not independent. V and I are governed by Ohm's Law:

I = V/R

For a given R, the current I will be determined by V and R.

V = voltage in units of volt
I = A = current in units of amp

Instead of thinking of V and I as set-points, think of them as limits. You set the max V and the max I that the PSU will allow. One of the two limits will be reached. If the V limit is reached then the PSU will be in constant voltage mode. If the I limit is reached then the PSU will be in constant current mode.

To set the limits, remove all loads to the PSU. Turn the I control knob above the minimum position; half way or fully clockwise is fine. Turn the V control knob until you reach the desired voltage output as indicated on the V display. Ignore what you see on the A display (which should be indicating 0A). Now turn the I control knob counter-clockwise to the lowest position.

Connect a jumper wire between the + and - output banana jacks, i.e. short circuit the output.
Turn the I control knob until you reach the desired maximum current limit as indicated on the A display. Ignore what you see on the V display (which should be indicating close to 0V, since you have shorted the output).

#### MrChips

Joined Oct 2, 2009
23,515
You mean 4 outputs total? Or 2?

If you have a PSU similar to this one, this is a single output PSU. Output is available from the +ve and -ve terminals (banana jacks). GND is EARTH ground for safety.

You have one supply voltage, not two.
This is called a floating power supply and can be turned into a single positive or single negative supply.
In order to create a dual power supply for opamp work you will need two PSU, one for +ve voltage and another for -ve voltage.

You can create a dual (bipolar) power supply from the single PSU by creating a pseudo 0V reference by using two resistors or using a virtual ground circuit (using opamps or specialty chips).

#### Joe Stavitsky

Joined Apr 5, 2020
15
View attachment 230264

If you have a PSU similar to this one, this is a single output PSU. Output is available from the +ve and -ve terminals (banana jacks). GND is EARTH ground for safety.

You have one supply voltage, not two.
This is called a floating power supply and can be turned into a single positive or single negative supply.
In order to create a dual power supply for opamp work you will need two PSU, one for +ve voltage and another for -ve voltage.

You can create a dual (bipolar) power supply from the single PSU by creating a pseudo 0V reference by using two resistors or using a virtual ground circuit (using opamps or specialty chips).
I hate to be stereotypical but I feel like Keanu in that first Matrix movie. MOOOOORE.

#### SamR

Joined Mar 19, 2019
3,405
On that PSU push the voltage or current button to set the decimal place of the value and push and hold the button again to Lock overvoltage or overcurrent. It cannot do both fixed V and I. I only use the OCP (over current protection). It's in the manual. Using the + and - jacks you have floating relative voltage and with + and GND jacks it is relative to earth ground (rarely used). You can have + or - voltage but not both without another PSU.