Regulated power supply questions

[merlinb]

Merlinb, thanks for your reply. I didn't know this happened, why is this? Is it always half the current?

No it's not always half, but half is a good rule of thumb.

If you want to get technical it depends on the type of rectifier, size of the reservoir capacitor (or choke if it's a choke-input power supply), and the ratio of the load impedance (Rl) to the transformer's internal source impedance (Rs). The maths is frighteningly complicated because a rectifier is a non-linear circuit.

But the end result is that for power supplies of 'typical' i.e. practical design, the AC current in the transformer will be between 1.5 and 2 times the DC current, so we use 2 times to err on the side of caution.

[juan_felt]

Hi,

Again, thanks for all the replies. I'm still a bit confused with the design. I know what to do with the circuit, but the 9v/18v switching part I'm a bit lost.

My idea is to do something similar to the T-Rex Fuel Tank Chameleon, but without so many different outputs (maybe 2 or 3). How do you think they do the switching? For what I've seen, they use dip switches with resistors, so I guess that they change the Adjustment lug in the LM317. What I don't understand is why they state that in 9v and 12v they have an output of 300mA, and in 18v 150mA.

If I wanted to accomplish those voltages, with those amperes, what size transformer should I buy?

Here are some pictures of the power supply.

http://1.bp.blogspot.com/-Meg5dR6Luro/UlO_eJc_9aI/AAAAAAAARJw/Kxofri-lCes/s1600/Guitar+work117.jpg

http://1.bp.blogspot.com/-TF2HM_0Q8mc/UlO_dhejE8I/AAAAAAAARJs/zCUeUHZKCEM/s1600/Guitar+work116+copy.jpg

http://1.bp.blogspot.com/-kfUxwtoZfgQ/UlO_ch-5RUI/AAAAAAAARJg/gqd-Vd7K9sM/s1600/Guitar+work115.jpg

Again, thanks!

[merlinb]

What I don't understand is why they state that in 9v and 12v they have an output of 300mA, and in 18v 150mA.

At first I though it might switching in a voltage doubler to acheive 18V, which would halve the available current. However, there are only five big caps (one for each regulator), so I suspect that's not the case. Maybe if you try to suck the full 300mA it woudl pull the voltage down too far, causing the regulator to drop out?

If I wanted to accomplish those voltages, with those amperes, what size transformer should I buy?

Keeping it as simple as possible:
Take your target output voltage, e.g. 9Vdc;
Add 3 volts to this (regulator dropout voltage);
Add another 1V to allow for ripple;
Add another 1.4V to allow for rectifier drop;
Multiply by 1.05 to allow for mains brown out;
Divide by 1.35 to convert from peak to RMS. This is your nominal transformer voltage.

Take your target DC output current;
Multiply by 2. This is your nominal transformer current.

(You'll need to select the nearest off-the-shelf values, obviously. You don't find many 11.2V transformers in the catalogues!)

[juan_felt]

Thanks a lot Merlinb,

Your reply is very clear, I'll start designing the power supply with those calculations. I'll post how it ends up working!!

Cheers!