dual rail dc power supply

p l

You may wish to review your drawing and figures.

Firstly you have drawn a centre tapped transformer that has two 55v secondaries (or a 110v with centre tap).

You have then connected the centre tap to earth BUT you have connected one side of the secondary to Neutral, which is connected to ground for 230v supply arrangements (definately in NZ and Australia), so you now have one side of the secondary shorted.!

Assumming you disconnected the centre tap, it is NOT possible to get 0-240v AC from the 110v AC on the secondary.

The best you can hope for is 0 to 110v.

Rectified this will give 154v DC.

You would be much closer to achieving your idea if you put the variac on the primary side (230v) and left the centre tap to earth.

Connect the secondaries to the AC inputs of the bridge rectifer, and put your capacitors between the +ve and earth and the -ve and earth.

This should give you a 0 to 77v +/- rail either side of earth.

I suggest you google 'center tap dual supply' and view images

The filtering capactance required is based on the load, and as a general rule of thumb, 1000uF per Amp.

Just because your components are rated at 5KVA, (BTW that is a VERY large Variac) doesn't mean the load draws that much.

I'm curious what the function of this is.

Australia is even more strict than NZ, so 'experimenting' with mains is frowned upon.

Mark

Thanks to make me aware of my errors in the drawing

I was hoping I could configure it in this order for the following reason:

The step down transformer is used for safe isolation and provides 110V for the VARIAC

It can be cheaply obtained from the UK, as those transformers are mandatory on UK building sides.

The VARIAC is from the US and according to the specs it has a 110 V  input with 0-250V output.

1000uF per Amp (20A) would mean  20000uF which is a dangerously huge cap.

Anyway my idea was to test AC appliances to run on DC

….. any ideas to accomplish this with other means?

Pasqual

Hi Pasqual,

There are ways to get a high voltage DC supply, but at  20A is not easy.

With that type of implementation, you will need a capacitor (or multiple) of that order. Also, the ripple current could be huge (tens of amps if not more),

so the capacitor will need to be physically large.

If the appliance is intended for AC, anything with a motor won't run by the way (it will just get hot).

By the way, such a supply would almost certainly kill in the event of accidentally touching the wires. The isolation will not help in this respect.

Pasqual

The step down transformer is used for safe isolation

You need to remove any connections to earth or neutral in order to achieve isolation.

That means that either phase or neutral has no voltage relative to ground/earth.

The VARIAC is from the US and according to the specs it has a 110 V  input with 0-250V output.

Not possible with a true variac.

Do you have a link.?

Anyway my idea was to test AC appliances to run on DC

Are you electrically certified ?

Why would a device that is designed to run on AC need to work on DC.?

Especially in your design where you have a + and - rail, and since it is running at twice the maximum AC load for a 230 appliance (10Amps)

I am becoming increasing worried that this is not a good idea.

If its a research project, I suggest you refer back to your tutor as its bordering on Dangerous.

All manufactured devices sold in NZ and Aust need to have the voltage and current requirements afixed as a permanent label.

They cannot be certified/tested for use without that being intact and visible to a user.

If your idea is to run them on xxx DC, then you will either need to get the 'appliance' (assumming it has a plug) tested by a recognised and appoved laboratory and the new marking attached.

If the device/appliance has a mains transformer then expect a bang and flash when you connect it, and hopefully no smoke.

FYI many of the new multi voltage (23/110) appliance simply rectify the mains and use a DC-DC converter to supply the various voltages.

Mark

Shabaz

Believe it or not Universal motors are designed to run on AC or DC.

Not particularily common, expecially at 230v.

I'm more concerned with the lack of understanding around mains and also why the +/- rail requirement.

Mark

Very interesting! Just been reading the wikipedia page on it. I'd not heard of it before.

Hello p l,

Welcome to the Community, we hope you are enjoying it. Thank you Mark and Shabaz for your assistance! 

Jamie