paulb,LM317,338 etc,are ''floating'' and they only see the input to output differential voltage,so,several hundreds volts can be regulated as long as the maximum input to output differential is not exceeded.At maximum differential the heat is also at it's maximum.The 18volts differential is not I believe too big to worry.It will reach temperatures within their limits and with the help of a good heatsink they will run even cooler.I use a 24vdc output supply fed by a 18vac transformer which has 31vdc after rectifier and the regulator runs really cool.It would run hotter only if the transformer had higher ac output,something that would increase the input to output differential.For our friend sunny it would perhaps be better if his transformer had some volts less,but since that's the one he has it's ok,and although it may run hotter yhan say mine,still I think it will be well within the regulator's limits.
It's not the differential voltage, it's the power dissipation I'm talking about. The differential voltage multiplied by the current being supplied. See Andrew's calculations. Your supply that is running cool is not supplying as much current.
A really big heatsink will increase temperature by typically 1 deg C per watt of dissipation. 45 watts means 45 degrees, added to a room temperature of 25C makes the heatsink 70 degrees C, too hot to touch and the regulator will be hotter still. This will reduce its reliability.
Here's a better idea:
http://www.national.com/pf/LM/LM2678.html
A really big heatsink will increase temperature by typically 1 deg C per watt of dissipation. 45 watts means 45 degrees, added to a room temperature of 25C makes the heatsink 70 degrees C, too hot to touch and the regulator will be hotter still. This will reduce its reliability.
Here's a better idea:
http://www.national.com/pf/LM/LM2678.html
Hi,
that's the boy, some sideways thinking.
How about the very efficient switcher as a pre-regulator to get down to near 15Vdc and feed this to the 12Vdc linear regulator.
You get the advantage of nice, fairly clean linear reg as your final output and cooler running (7W still needs a big sink) from the fixed input voltage.
The dirty high power stuff is done by the switcher and at 90% efficiency it also stays reasonably cool. It removes all the mains variations and glitches but has a bit of ripple on the nearly fixed DC output.
that's the boy, some sideways thinking.
How about the very efficient switcher as a pre-regulator to get down to near 15Vdc and feed this to the 12Vdc linear regulator.
You get the advantage of nice, fairly clean linear reg as your final output and cooler running (7W still needs a big sink) from the fixed input voltage.
The dirty high power stuff is done by the switcher and at 90% efficiency it also stays reasonably cool. It removes all the mains variations and glitches but has a bit of ripple on the nearly fixed DC output.
16v3A solution reply
Hi Sunny
I would like to point out if U use a 12vx2 Xfmr to build a 16V3A
reg. d.c. I think 12Vac can't do the job. 12Vx1.414=16V and regulator like LM338 needs at least 3V input output diffierential, therefore no extra voltage to make resorvior, no regulation can be achieved. If U use 24V bridge rectifier, the output dc will be at 36V, 36V-16V=20V at 3A current, the power dissipation is 60W.
the best way is to use a 16vx2 Xfmr. 16x1.414=22vdc, therefore 6v for resorvior would be fine for Ur application. the power lost dissipation is 6vx3A=18W which is resonable for ps design.
new member ( Mitwrong )
Hi Sunny
I would like to point out if U use a 12vx2 Xfmr to build a 16V3A
reg. d.c. I think 12Vac can't do the job. 12Vx1.414=16V and regulator like LM338 needs at least 3V input output diffierential, therefore no extra voltage to make resorvior, no regulation can be achieved. If U use 24V bridge rectifier, the output dc will be at 36V, 36V-16V=20V at 3A current, the power dissipation is 60W.
the best way is to use a 16vx2 Xfmr. 16x1.414=22vdc, therefore 6v for resorvior would be fine for Ur application. the power lost dissipation is 6vx3A=18W which is resonable for ps design.
new member ( Mitwrong )
paulb,you might be right about heat,however when I was experimenting with my 24vdc type,I made two identical boards,with two toroidal transformers,one 24vac /300mA and the other 18vac/400mA.The 24vac/300mA version gets a lot hotter(that is LM317T).The only major difference between the two versions,due to the transformers is the resulting voltage differential.Maybe there is also something else I don't know.
Have you measured the actual voltage of the transformer? Does it measure close to 12vac? I know from messing around with small transformers that they sag quite a bit under load. The voltage their rated at is usually for a specific current draw.
So say that the rectified voltage measures 34vdc, with a current draw of around 3 amps it could actually sag down well under 30v.
You mentioned they are 12-0-12 5 amp transformers, so unloaded it could be closer to 30vac or even higher. With a load of only 2.5 amps the rectified voltage might even be closer to 40v.
So say that the rectified voltage measures 34vdc, with a current draw of around 3 amps it could actually sag down well under 30v.
You mentioned they are 12-0-12 5 amp transformers, so unloaded it could be closer to 30vac or even higher. With a load of only 2.5 amps the rectified voltage might even be closer to 40v.
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