5 volt chip from 24 volt transformer

[Gaston]

i am trying to run a 5 volt chip that draws 30mA from a 24 volt transformer.

this is how i calculate it.

24 volt ac rectified and filtered 24 * 1.44=34 volts dc

34-5=29 volts to be dropped by the regulator

so 29v * .030A =.87W that the regulator will have to dissapate.

will a lm7805 be able to handle this? I don't think they can drop that much voltage but don't understand why.

 

[justDIY]

the datasheet i have for the Fairchild LM7805 says Vin <= 35vdc for under 7818 and below, and 40v for over 7818 and above.

so 29v is technically within the specs, and at 30mA, a big beefy (and inexpensive) to220 regulator shouldn't have any problems

might want to look at ldo regulators, something in a sot-223 or super sot3 package if 30ma is all you really need.

 

[mneary]

If you do use surface mount regulators, pay careful attention to the design guides. 0.87W can be a lot for a tiny package. For the Fairchild LM7805, the quiescent current with 25V, (no value given for 34V) is typically 6mA, adding 0.204W (or more) to the total.

A bare TO-220 can handle almost 2W in free air, but it'll feel awfully warm even with 1.07W. The case will be between 90 and 100c.

 

[on1aag]

Hi Gaston,

A standard 7805 or LM340-5 would do the job but if you feel a bit
uneasy about it you can always use a 7824 to preregulate the input
voltage of the 7805 to 24 volt and also bring down the dissipation
of the 7805 to a more moderate level. (19 * 35E-3)
Alternatively you could take a look at the datasheet of the TL783.

https://www.alldatasheet.com/datasheet-pdf/pdf/28835/TI/TL783.html

on1aag.

 

[Hero999]

What voltage does the transformer give when no load is connected?

About 28V?

That will give a bit more about 40V which is too much for the LM7805.

You could use an LM317 and add two extra resistors and can handle a voltage drop of 40V.

 

[Gaston]

do they make a switching regulator that may be able to handle it a little better?

 

[mneary]

Sure. A lot of them are great for this job. LM2575 has been around a while and is cheap ($1.21 @ 1K).

 

[Hero999]

do they make a switching regulator that may be able to handle it a little better?

You could build your own but for 30mA it hardly seems worth it.
https://www.romanblack.com/smps/a03.htm

 

[blueroomelectronics]

24*1.414 = 34V
Subtract the votage drops in the diodes = 32.4V which is below the 35V maximun rating. It's going to get really hot though.

 

[Gaston]

sorry .. i got the 1.44 wrong and forgot to subtract the diode drops. thats why i like to post my math in case i make a mistake someone can point it out. thanks. so is .87 watts a lot of heat for a to220? i didnt see in the datasheet where it specified wattage. how do you know how hot that is? just from experiance i guess?

 

[justDIY]

the data sheet puts thermal resistance to air at 65C/w ... so lets round your .87 up to 1w ... that means the body of the regulator will increase 65C over ambient with 1 watt of power dissipation.

If you're in a cool room, 25C, that's well withing the 125C operating temperature. Even if it is hot out, like 35C, you're still within spec.

however, 90-100C is damn hot. even a small heat sink would make the regulator safer to work around, in case you touch it, you don't get a nasty surprise.

 

[Gaston]

so just add 65c per watt over ambiant temp and that gives you the temp?

 

[Hero999]

24*1.414 = 34V
Subtract the votage drops in the diodes = 32.4V which is below the 35V maximun rating. It's going to get really hot though.

But that's ignoring the fact that the transformer's off load voltage could be as much as 20% higher than the rated voltage.

 

[Gaston]

But that's ignoring the fact that the transformer's off load voltage could be as much as 20% higher than the rated voltage.

so mabey i would do better going with the lm2575?

 

[Hero999]

You could do but is it really worth it for just 30mA?

I'd go for the LM317 if I were you.

 

[blueroomelectronics]

Why the 24VAC in the first place?

 

[justDIY]

there are newer chips than the 2575 - they run at higher frequencies so they work with smaller inductors and smaller caps. I couldn't find a SOT-23 or TSOT-3 5 pin switcher that accepted 40v input, but there are lots of TO-220 5 pin and DIP 8 pin switchers out there.

The "solution finders" at TI and National both recommend linear regulation for such small current, Linear's solution finder recommends tiny leadless smt chips which aren't at all friendly.

 

[mneary]

If you can, a lower voltage source would make this simpler.

 

[Gaston]

i have amplifiers with two 24 volt transformers in them. i dont want to have to buy twenty more transformers to run the voice chips. thats why i am trying to supply the chips with what i have already

 

[blueroomelectronics]

Are the transformers center tapped? You can use two diodes and the center tap for gnd.