I have a 15v x 2 transformer, 30VA which i want to use to make a variable power supply. i only need a single rail supply that delivers max 1A so i was thinking of joining the 2 secondaries of the transformer and using a LM317 regulator. i am just a bit worried about the voltage from the secondaries being too high once rectified. (30v * 1.414 = 43v)

i have tried to look for regulators that can handle a higher voltage but have not found anything. i also looked at making a dual supply with lm 317 and lm337, but then i need a dual pot - which i can get only 100k.

the supply only needs to give out max 24v, so i would like to use lm317 if i can, has any1 got ideas to help me??

 

I suggest using switching power supply regulator instead of the linear reguator cuz the power dissipation of the linear regulator IC could be too high ((43-24)V x1A=19W). Goolge the circuit, you can find one that suits your needs, I believe.

 

Why don't you wire the transformer secondaries in parallel instead of series? That way you'll have more current available, but at a lower voltage.

You'll need a pot with much lower resistance than 100k. Look in National Semiconductor's datasheet for the LM117/LM317 for applications circuits.

Be certain to use a good-sized heatsink. Note that at a given current, the lower the output voltage (greater voltage differential across the regulator), the more power will be dissipated in the regulator.

 

if i connect the secondaries in parallel i only get 21v out, and i would like to have at least 24v out. i don't need much current, max 1A

 

if i connect the secondaries in parallel i only get 21v out, and i would like to have at least 24v out. i don't need much current, max 1A

You said you don't need much current. But saying so is not meaningful.

The 24V @max. 1A is probably nearly the max. your 30VA transformer can provide to you anyway.

One additional point, if you connect the windings in series and use bridge rectifier configuration, you will probably won't be able to get the negative supply required for the LM337, which is only available if you use the 15-0-15 full wave configuration.

 

What you want, with what you want to use, is probably not a good idea. At 1 Amp, best case (24V out) your linear supply will be dissipating about 20W, worst case will be greater. The dissipation of the TO220 and TO3 package devices are internally limited at 20W.

Alternatives: Use a different transformer; Use a switch mode supply; change your requirements.

 

At 1 Amp, best case (24V out) your linear supply will be dissipating about 20W

At 30V 1A, the transformer is giving out 30VA output, , DC load taking 24Vx1A=24W, dissipation=20W ???

The figures does not add up.

 

A 30VAC/1A (30VA) transformer makes a peak of 42.4V. If it is rectified and filtered then the transformer is overloaded when the DC current is more than only 680mA.
The bridge rectifier heats with about 1W.
The regulator heats with about (40.4V - 24V) x 680mA= 13.2W.
The 24V load heats with 16.3W.
The total heat is 30.5W.

 

Your concerns about the input voltage implies that you want the output to be able to go below about 4 volts.

If this is the not the case then your concerns are unfounded but you do need to bear in mind the current limitation of the simple capacitive input filter, as mentione above.

A possible solution that you may like to consider is the use of a choke input filter. This would give a d.c. across the filter capacitor of 0.9Vac and allow you to draw 94% of the transformer's current capacity.

 

A 30VAC/1A (30VA) transformer makes a peak of 42.4V. If it is rectified and filtered then the transformer is overloaded when the DC current is more than only 680mA.
The bridge rectifier heats with about 1W.
The regulator heats with about (40.4V - 24V) x 680mA= 13.2W.
The 24V load heats with 16.3W.
The total heat is 30.5W.

Thanks for the clear breakdown on the power disspation. When the transformer is overloaded, it still works with more copper and iron loss as long as the magnetic core is not saturated. So the regulator could heat with a power less than 20W but larger than 13W.

 

If the voltage is set low and the output current is 0.68A then the LM317 will try to dissipate 26.6W and it will overheat and shut-down.