Should I go off the Bridge or off the regulated supply?

[lesabb]

Hi all, I've been learning and trying different PSU configurations. Check the following schematics. I got both to work fine on boards (allelujah!) but I have a couple questions:

1. I'd like to know if one is preferable over the other. It is a 48V transfo from which I need 48VDC and 24VDC. This will power audio preamp
2. Why is the voltage reading ~51V at the output of the 24V if it's unloaded?

[wraper]

Both are not good because will dissipate a huge amount of heat if there is any significant load. Also maximum input voltage of 7824 is only 40V. Using 50V rated capacitors on 48V is not good practice too.

[lesabb]

Yeah I missed the 40Vin rating on the 7824. But i'm using 63V caps on the other design...my bad

thanks!

[mariush]

I have other questions for you...

How much current is your preamp using at 48v and how much at 24v ?

The reason I ask is because if your current at 48v is very small (let's say 50mA) then if you're smart, you could get away with just one regulator for the 24v output.

For example, get a 36v AC transformer (2 x 18v windings). This means that after a bridge rectifier, you have a peak DC voltage of 36 x 1.414 = ~ 51v, minus about 1v in the bridge rectifier ( at 50mA, the voltage drop on each diode will be small, even smaller if you use 4 shottky diodes).
So you will have up to 50v, and by figuring out the size of the capacitor, you can make sure the voltage will never drop below a particular minimum, in your case let's say 48v.

C =  Current / ( 2 x ac mains frequency x Vripple)  where vripple is how much you're willing to let the voltage drop from the peak.

For 50mA and 50 Hz mains that would be : C = 0.05 A /  [ 2 x 50 Hz x (50v-48v)]  = 0.05 / 200 = 0.00025 Farads or 250uF .  So a minimum of a standard 270uF 63v rated capacitor should be enough to guarantee you'll always have at least 48v coming from the transformer, as long as you don't pull more than 50mA and unless your AC mains voltage goes down a lot.

If your preamp really going to care if the voltage will vary between 48v and 50-something volts? I doubt it would be that sensitive.  In a pinch, you could use a basic LM317HV that supports up to 60V at input to get 48v... as long as you have about 1v above 48v, the LM317 will output smooth 48v.

You could also tap one of the windings and get a nice 25-26v from that. You would then be able to use a regulator like 7824  to smooth it out.

Mainly my issue with your first design is that you're throwing a lot of power away as heat... you have 70v in, 24v out .. 100mA current, you basically waste  (70-24)x0.1 = 4.6 watts
The second design isn't much better.

[lesabb]

Mariush, very little current drawn from 48V, phantom power. Maybe 15mA total?. The thing is that the 48V has 6.8k current limiting so I think it has to be stable? If I use a 18-0-18 transfo, I would need 2 bridges right? 1 for the 36V and one for the 18V? How did you get the 100mA at the 7824?

thanks!

[mariush]

I used 50mA and 100mA numbers off the top of my head, just to be able to give you some examples.

You could use just one bridge rectifier, or two bridge rectifiers, it's fully up to you.

Again, to make this easier to follow, let's say you need 48v at maximum 20 mA (you say 15mA but let's be safe and size it for 20mA) and let's say you need 24v at maximum 50 mA.

The way linear regulators work is by wasting away the difference in voltage in the form of heat. So, if you want a linear regulator to produce 24v at up to 50mA, that regulator will draw up to 50mA from the input, no matter what voltage is there.

So, if we want to use the 48v to get 24v from there using a linear regulator, it means that the 48v output must be able to supply both the 20mA and the 50mA, so in total at least 70mA. Just to be extra safe, we could round this number up a bit to.. let's say 75mA.

36v ac -> bridge rectifier -> capacitor - > 48-52v DC at up to 75mA ->

A bridge rectifier converts AC wave to DC wave, and you'll have a DC output but with lots of waves, voltage going up towards a peak equal to AC voltage x 1.414  (square root of 2) and then again down towards 0, up to 100-120 times a second, depending on your mains frequency.
In a bridge rectifier, there's always two diodes conducting so there is some voltage drop on each of those two diodes, but the exact amount varies with the current and how warm the bridge rectifier is. With regular diodes, at 75mA, the voltage drop on each of those diodes would be about 0.5-0.6v but if you were to use shottky diodes the voltage drop would be much low, at around 0.25-0.4v. 

So, your 36v AC will become about 51v DC maximum, minus about 1v in losses in the bridge rectifier, less if you use shottky diodes. But let's make it easy and say we will have a peak of 50v DC.
But as I explained, 100-120 times a second the voltage will go down towards zero, and that's where that capacitor comes in... it stores energy inside it when the wave goes up to 50v and gives it back when the voltage goes down. So if this capacitor is big enough the voltage will never go down below a particular level.

Let's say we never want to have less than 49v. In this case,  I already told you the formula  C  = 0.075 A / ( 2 x 50 Hz x (51v - 49v) ) =  0.075 / 200 = 0.000375 Farads or 375 uF which is the minimum we would need to make sure we'll always have at least 49v in a country with mains frequency of 50Hz (europe etc,  with 60 Hz even less capacitance would be needed).  So the next standardized value capacitor would work.

You can use a larger capacitor, for example 470uF or 1000uF, and that will simply make the voltage stay much closer to the peak of 50v instead of staying near the 49v value.

Now from here, if you really think you need 48v, you can use a linear regulator like LM317HV (which can handle up to 60v) to have the 48v.

As for 24v, you can use another LM317HV to get the 24v up to 50mA. The wasted power which is up to (48v-24v) x 0.05 = 1.2w is not that big to be worth messing about with another bridge rectifier and capacitor.


 

[lesabb]

Yep, I see it. Thanks