30 Volts regulator

[Ave Cox]

Hi.

I'm just doing an alimentation of 30 volts. I looked for a regulator like 78xx series but it doesn't exist in 30V. So, I search for new ideas of design of a 30V regulator. I know the very simple regulator based on a zener diode and a transistor but I don't know if it works well at 30V and what's the power in the zener (I have only zener of 5 Watts).

My input voltage is 34,8V. And I need also a -30V from a -34V input.

The power needed is over 5 to 10 W.

Thanks in advance for your answer(s).

 

[Styx]

you say the voltage is 34.8V to be that precise I am guess it is already regulated/stable

Use some diode is series with the 34.8V to drop the voltage down to 30V

 

[Ave Cox]

In fact, it's not regulated.

It's a transformator 220V/50V followed by a diode bridge and some capacitors.

34.8V is the value I measured friday but apparently now it's more thant 35V.

 

[Nigel Goodwin]

In fact, it's not regulated.

It's a transformator 220V/50V followed by a diode bridge and some capacitors.

34.8V is the value I measured friday but apparently now it's more thant 35V.

I think you need to give more details, if it's a 50V transformer with a bridge rectifer the output will be near 75V. If it's a 25-0-25V transformer with a full wave rectifier (two diodes) it will give 35-37V off load.

If it's the second option, a 25-0-25 transformer isn't really high enough to regulate down to 30V - under load the output from the rectifier will tend to drop below 30V to start with.

 

[Ave Cox]

I give you the schematic. It will be more easier.

 

[k7elp60]

You can use a couple of 12volt regulators with external resistors to set the voltage to 30V. Here is a schematic.

 

[Nigel Goodwin]

The transformer is really too low a voltage to give a regulated 30V supply, you really need something like a 32-0-32, or as an absolute minimum 30-0-30.

What are you wanting to use the PSU for?.

 

[TheOne]

Another common way with the 78XX and 79XX regulators is to stick a zener in the ground lead of the device. This will lift the regulator's fixed voltage by the zener voltage.

 

[Ave Cox]

Why did you say that the transormer is too low ? with 78xx you only need 2volts more at the input than at the output , isn't it ?

I use the transformer for some cards (5 to 10) in a eurocard bus. For the moment, I have done one of them and it needs between 4 and 5 Watts.

 

[Nigel Goodwin]

Why did you say that the transormer is too low ? with 78xx you only need 2volts more at the input than at the output , isn't it ?

That's an absolute minimum, more is preferred - but your input voltage is only 34.8V OFF LOAD. Under load this voltage will drop, depending on the load probably too low for the regulators to work, and ripple is certainly likely to drop the supply to under 30V giving hum on the outputs.

 

[JimB]

Why did you say that the transormer is too low ? with 78xx you only need 2volts more at the input than at the output , isn't it ?

I use the transformer for some cards (5 to 10) in a eurocard bus. For the moment, I have done one of them and it needs between 4 and 5 Watts.

The voltage that you have measured is the "no load" voltage, which is about 1.4 x the transformer voltage (25 x 1.4 = 35 volts).
This voltage will be a nice smooth DC voltage,
BUT,
as you start to put some load on the supply and take current from it, you will get some "ripple".
The ripple will give voltage dips every 10 mS, (assuming a 50 hz mains supply).

For a capacitor filter power supply like yours, the magnitude of the ripple can be estimated from the equation:

Ripple Volts = (Load Current x Ripple Period)/Filter Capacitance

So in this case, if you have a 1 amp load (maximum for a 78xx regulator), the ripple will be:

ripple = (1 x 10 x 10^-3)/4700 x 10^-6 = 2.1 volts

Which means that the input to your 7812 will be varying between 35 and 32.9 volts.

If you are going to take less than 1 amp from the supply, the ripple will be less.
You could reduce the ripple by increasing the value of the smoothing capacitor, say put another 4700uF in parallel with the one you have at the moment.
The mains voltage may be low, (everybody cooking their christmas turkey!), so you will have less voltage from your transformer.

etc, etc etc.

You may just get away with it if you are lucky, but this feels like a circuit which is on the edge of not working 100%.


JimB

 

[audioguru]

A 78xx regulator is spec'd with 4V to 8V across it, so its performance is good enough to be guaranteed and published.
Its guaranteed minimum dropout voltage is 2V, at which point it isn't regulating anymore.
If you get a bucket full of them, maybe you can find a few that are a little better.

 

[Nigel Goodwin]

JimB expanded nicely on my concerns, except he forgot about transformer regulation - depending on the particular transformer this could quite possibly be 10%. Basically the larger the transformer the better the regulation, it should be quoted in the specs of your particular transformer.

 

[Sebi]

If You really need to solve the PSU with these transformer, first check the output voltage with 180ohm 5W load. Maybe enough for low-dropout regulator. Anyway - with higher hum - the parallel regulator also useable.

 

[TheOne]

One problem often ignored (overlooked) with large capacity power supplies is when more capacitance is added after the rectifier, without doing proper calculations, you run the risk that as the conduction time of the rectifier becomes shorter (near the peak) in a cycle, the peak current rating of the diode may be exceeded without exceeding the average rating. This can result in unexpected failures.