Cheapo Rigol dp832 equivalent power supply project

[BroderB]

Hi everyone! I, just like many of you out there, have embarked on a mission to create my own power supply.
I want it to be a triple output power supply with modest current and voltage capabilities (3-5A& 30V MAXIMUM)
I want every output to have VC as well as CC, and I want both voltage and current to be displayed simultaneously.
Now, what I've found to be the easiest and cheapest way to meet these criteria might make some of you laugh, but stay with me.
I'm opting to buy three of these 50V 5A buck-converters with built in panelmeters, CC, VC and so on from Banggood: http://www.banggood.com/DP50V5A-Buck-Adjustable-DC-Power-Supply-Module-With-Integrated-Voltmeter-Ammeter-Color-Display-p-1050061.html
To supply these with power I´m planning to use a big ole' 230V to 42V laminated iron core transformer (rated at 200VA but it is at least as big and heavy as a 750W microwave transformer, so is this just a conservative number?) , followed by a rectifier and a suitable smoothing capacitor. My intention is to then hook all of the buck-converters up in parallell with each other to the output of the rectifier and cap.
For rectifying I'm thinking of this here overkill rectifier: http://www.banggood.com/KBPC5010-1000-Volt-Bridge-Rectifier-Metal-Case-1000V-Diode-Bridge-p-87234.html
I've done some simple calculations and come to the conclusion that I need a capacitance of at least 12 000uF in order for the rectified peak voltage to never go below 55V, but that sounds like a lot of capacitance to me. If i can, I'm going to attach a photo of my calculations after this post.

The plan is to then put all of this in som sort of enclosure, with binding posts connected to each buck-converter, and possibly even a mains earth-post, just in case I'll need it.
My main question is if outputs from the buck-converters will be at floating potentials with reference to each other? (So that I can hook them up in series and create both positive and negative voltages for powering op-amps etc.)
Lastly, I would like to make it known that I know, that it's a pretty bold statement to call this crude model a Rigol dp832 equivalent, mainly because of the accuracy uncertainty regarding the cheap buck-converters. In my opinion however, these buck converters will do the job just fine, since I'm mostly interested in being able to adjust current and voltage in small increments, and in case I need a REALLY precise reading I can just hook up my DMM.
Since I already have the stated transformer, this project would only cost me somewhere in the ballpark of around 100-120$, do you think it would be worth the investment?
All inputs and replies are appreciated!

[nowlan]

You might be interested in this vidoe.

[crazyguy]

do you means connect all the input of the buck converters in parallel and connect all the output in parallel ?

I wonder that connect three independent buck converter output together is a good idea.

[MosherIV]

My main question is if outputs from the buck-converters will be at floating potentials with reference to each other? (So that I can hook them up in series and create both positive and negative voltages for powering op-amps etc.)

NO, I do not think you can because the buck converters use a common GND/0V between the input and output (but you need to check the circuit diagram for the 'bangood adjustable voltage/current modules' that you quoted).

What do you think will happen when you connect the positive of one to the ground of another when they all share the same ground?
 


Please stop calling the Current Limit - CC (Constant Current).
A CC PSU is a very different thing to a CV PSU. The Chinese miss-label their products.

To do what you want, you would need 1 transformer per buck/CV/CL device and keep each of them separate (floating). This will allow you to join them together.

[rstofer]

I've done some simple calculations and come to the conclusion that I need a capacitance of at least 12 000uF in order for the rectified peak voltage to never go below 55V, but that sounds like a lot of capacitance to me. If i can, I'm going to attach a photo of my calculations after this post.

You have to consider 'conduction angle' the amount of time (in degrees) the rectifiers actually conduct during each cycle.  As the capacitor voltages rises to somewhere close to the rectifier peak voltage, the rectifier conducts for shorter and shorter periods.  Nevertheless, the entire power required of the circuit needs to be supplied in an increasingly shorter period.  The rectifier gets overloaded.

There is always a desire to filter to pure DC but if that happens, the rectifier just can't handle the load.

[BroderB]

NO, I do not think you can because the buck converters use a common GND/0V between the input and output (but you need to check the circuit diagram for the 'bangood adjustable voltage/current modules' that you quoted).

What do you think will happen when you connect the positive of one to the ground of another when they all share the same ground?
 


Please stop calling the Current Limit - CC (Constant Current).
A CC PSU is a very different thing to a CV PSU. The Chinese miss-label their products.

To do what you want, you would need 1 transformer per buck/CV/CL device and keep each of them separate (floating). This will allow you to join them together.

1. It seems so obvious now that you wrote it, I should have studied the topology of buck converters a little better before!  I actually have two of the transformers, so I guess i could use both of them and use one of them for two of the buck converters and use the second one for the third buck converter....
2. Right again (concerning the 'CC'), I didn't write what I meant... 

[BroderB]

I've done some simple calculations and come to the conclusion that I need a capacitance of at least 12 000uF in order for the rectified peak voltage to never go below 55V, but that sounds like a lot of capacitance to me. If i can, I'm going to attach a photo of my calculations after this post.

You have to consider 'conduction angle' the amount of time (in degrees) the rectifiers actually conduct during each cycle.  As the capacitor voltages rises to somewhere close to the rectifier peak voltage, the rectifier conducts for shorter and shorter periods.  Nevertheless, the entire power required of the circuit needs to be supplied in an increasingly shorter period.  The rectifier gets overloaded.

There is always a desire to filter to pure DC but if that happens, the rectifier just can't handle the load.

Thanks for the input! I understand what you mean, except for how the rectifier would get overloaded. I'm thinking that the diodes simply wouldn't conduct any electricity so long as the voltage form the transformer is lower than the voltage stored in the capacitor + 2,4V (,each cycle)? What you're suggesting is that the voltage supplied form the rectifier and cap simply never would be as high as 55V under load?

[rstofer]

I've done some simple calculations and come to the conclusion that I need a capacitance of at least 12 000uF in order for the rectified peak voltage to never go below 55V, but that sounds like a lot of capacitance to me. If i can, I'm going to attach a photo of my calculations after this post.

You have to consider 'conduction angle' the amount of time (in degrees) the rectifiers actually conduct during each cycle.  As the capacitor voltages rises to somewhere close to the rectifier peak voltage, the rectifier conducts for shorter and shorter periods.  Nevertheless, the entire power required of the circuit needs to be supplied in an increasingly shorter period.  The rectifier gets overloaded.

There is always a desire to filter to pure DC but if that happens, the rectifier just can't handle the load.

Thanks for the input! I understand what you mean, except for how the rectifier would get overloaded. I'm thinking that the diodes simply wouldn't conduct any electricity so long as the voltage form the transformer is lower than the voltage stored in the capacitor + 2,4V (,each cycle)? What you're suggesting is that the voltage supplied form the rectifier and cap simply never would be as high as 55V under load?

If you had perfect capacitors and no load, the capacitors would charge to peak voltage and the rectifiers would never conduct again.  As you apply load, the capacitor voltage drains away during the time the rectifiers are not conducting.  At light load, the rectifiers don't conduct very long at all.  But we begin to see a bit of ripple.  With heavier load, the voltage sags more and the rectifier conducts longer but we get more ripple.  Then we increase capacitance to reduce ripple, the rectifier conducts less but still has to provide all the power the load requires.  If we let the capacitance approach infinity, the conduction angle approaches zero and in that near zero width pulse, the rectifier has to provide the entire load.  There are limits on peak repetitive current and certainly limits on peak non-repetitive current.

Just think about the little area under the sine wave and above the capacitor voltage and realize that all of the power required by the load has to be provided in that small area.  The capacitor may smooth things out but, over time, it doesn't provide any power.  That has to come from the rectifier.

There have been papers written on the subject of conduction angle but I'll be darned if I can find a good one.  Here is an LTSpice simulation of the input to a regulator.  Note the rectifier current waveforms.  You can see that the diode is conducting over a high percentage of the half cycle and the ripple voltage shows it.  But the low point of the ripple is higher than the drop-out of the regulator so all is good.  If more capacitance is added, the ripple will be reduced but the current spikes will be narrower.

If you use LTSpice, it might be worth modeling the circuit just to play with capacitance while observing conduction angle.

http://electronics.stackexchange.com/questions/27881/ripple-current-in-a-linear-power-supply-transformer

[MosherIV]

Something else to bear in mind - switchers are noisier than linear PSUs.

What kind of stuff will the PSU be used for?

Micro controller, LED work should be fine.
Audio is also ok because the switching freq is well above audio freq.

Where the switchers are a problem is op-amp or any low voltage signal analogue stuff.

Those modules look like a very easy way to put together a decent PSU for a beginner.
I look forward to hearing about your progress.
Try and post some information on ripple noise and load regulation please?

[omgfire]

Since I already have the stated transformer, this project would only cost me somewhere in the ballpark of around 100-120$, do you think it would be worth the investment?

Doesn't sound much cheaper than a stack of CPS3205 http://www.aliexpress.com/wholesale?SearchText=CPS-3205