Transformer for 5V Regulator?

Steel

I know this isn't asking specifically about a Parallax product, and I am sorry...But I am trying to power a parallax product· [noparse]:D[/noparse]

I am looking for a transformer that is most efficient for a 5V linear regulator.· The regulator is 5V, 1A for the project.

Unfortunately, there is a lot of terminology that I don't understand with Transformers.· I know that with the Voltage regulator, I will need about 6.5V-12V @1 amp to power the regulator.

Has anyone purchased any or know of a good place I can look?

Thanks
Shaun

Steve Joblin

why not just get a cheap wall wart... they are easily rated by their output voltage and amp rating... this way you get the transformer and the diodes!

Chris Savage

Steel,

It seems the most efficient transformer for a given regulator would be one that delivers not much more than a little over the minimum voltage required for the regulator and sufficient current. Now, the thing you need to be aware of is that this will not be the voltage of the transformer’s secondary itself.

Say you get a transformer with a 6V secondary. Once you’ve rectified and filtered the output it’s going to be closer to 9V. The higher the voltage the more heat the regulator generates to reduce the voltage. So an efficient design would probably involve a transformer with a secondary rated around 4V. I would recommend doing some research into linear power supply design. Take care.

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Chris Savage
Parallax Tech Support

Dennis Ferron

But if you draw the full 1 Amp from the transformer, I would assume that the voltage would sag some. This would also depend on the size of your filter caps.

mackm

Steel said...
I know this isn't asking specifically about a Parallax product, and I am sorry...But I am trying to power a parallax product· [noparse]:D[/noparse]

I am looking for a transformer that is most efficient for a 5V linear regulator.· The regulator is 5V, 1A for the project.

Unfortunately, there is a lot of terminology that I don't understand with Transformers.· I know that with the Voltage regulator, I will need about 6.5V-12V @1 amp to power the regulator.

Has anyone purchased any or know of a good place I can look?

Thanks
Shaun

Yeah, just use a wall wart of some sort.· You don't need to take the transformer out, just use as is.· You can get a multipurpose sort of power supply at radio shack for a couple bucks that can output from 3 to 12 v , unregulated I suppose.·

LoopyByteloose

Firstly, a transformer alone is an AC device. 120VAC is actual .707 of the peak voltage. You need a transformer that would have a 10 to 1 ratio and handle at least 1 amp, but having 3 amps wouldn't hurt. The only problem with a bigger transformer is more weight.

Then, after you get the transformer, you need a rectifier bridge and a ripple removal filter [noparse][[/noparse]a big capacitor [noparse][[/noparse]say 1000 uf] rated at least 1.5 times the output voltage]. Of course, you might consider fuses, voltage and amp guages, and an on/off switch.

It starts getting a bit huge and requires a box to protect you from the 120VAC.

It is nice to DIY and learn the math and the theory of both full and half-bridge rectifiers. It is also useful to learn how transformers actually isolate the AC from the DC and protect us from a severe shock. But in truth, I can just so down to the electronics shop and buy a very compact and durible wall wart rated at 1 to 1.5 amps with a lot less trouble.

On the other hand, your regulator really will tell you what you actually need. A LM7805 usually requires 7 volts or more [noparse][[/noparse]anything higher is converted to heat in the regulator]. A LM2940-5 will require only 6 volts and has features to protect it from being hooked up to the wrong polarity. I would choose the latter as I usually make some mistakes and burn up the 7805.

A 9 volt wall wart seems about the best investment and often you can find then in the trash when someone is tossing away a broken device, an old modem, or whatever.

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"If you want more fiber, eat the package.· Not enough?· Eat the manual."········

···················· Tropical regards,····· G. Herzog [noparse][[/noparse]·黃鶴 ]·in Taiwan

Steel

sorry...let me clarify. I have 120V needs on my circuitboard, as well. I need the voltage to be 120V from the wall, and once that is on the board, I have a separate circuit that is a 5V circuit.


I am using this DB102 full-bridge rectifier:
http://59.120.39.77/mccsemi/up_pdf/DB101-DB107(DB-1).pdf
Filtering caps, and the following L7805 Voltage regulator
http://www.st.com/stonline/products/literature/ds/2144/l7805ab.pdf

I am really just trying to nail down the mysteries of transformers, (there are a lot of parameters that I don't understand)

As a side note, I am surprised there are no cost-effective PCB-mount 5V regulators on the market. It seems odd that they can sell 5V wall-warts for $6.00, but a pcb-mount solution is $20+ dollars...http://www.cui.com/pdffiles/FSK-S5 series.pdf

David B

A good place to start is to nail down the current requirement of your project. The regulator may be specified to handle one amp but that doesn't mean it by itself will draw it; the regulator will pull from your transformer what your project draws from the regulator.

So estimate your current needs. Suppose it is 100 mA. Maybe double it for a good safety margin. (That doesn't mean that your circuit will draw 200 ma, just that it could, if your circuit demanded a current surge, or if you added some extra parts later on.)

Then you'll need a transformer that can supply 200 mA at about 7 or more volts to the input of the regulator. If you're cheap like me, you might even use a wall-wart on your board to transform 110 AC to maybe 9 DC for the regulator. Wall-warts usually have their current capability stamped on their housings, which makes this an easy way to design your supply. Otherwise you can look for a transformer with similar voltage and current ratings, using the AC-DC design guidelines that several others have already commented on.