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Topic: Suggestion for 3.3 volts (Read 1 time) previous topic - next topic

Jassper

Sep 10, 2009, 09:10 pm Last Edit: Sep 10, 2009, 09:11 pm by Jassper Reason: 1
I have 11 - 15 volts, I need 3 or 3.3 volts @ 600ma , 1 amp preferred.
I could use 2 of these http://www.cd4power.com/data/meters/dms-78xxsr.pdf (assuming I can parallel them)

But they are a bit pricey - any other suggestions?

kg4wsv

The standard TO-330 sized LM series (you'll be wanting the 7833) are generally capable of 1A.

The LM317 adjustable regulator is capable of 1.5A.  It's a different pinout than the 78xx and you'll need a pair of resistors to set the output voltage (google lm317 calculator for help with that).

You refer to a switcher; all these I mention are linear regulators, so if you're very worried about efficiency or heat dissipation keep looking.  :)

-j

Oracle

#2
Sep 11, 2009, 01:44 am Last Edit: Sep 11, 2009, 01:45 am by Oracle Reason: 1
Putting 15V at 1A through a linear regulator to get 3.3V makes mother nature cry.

And you can probably use the regulator in place of a power LED ;)

ill_switch

I asked a similar question here:

http://www.arduino.cc/cgi-bin/yabb2/YaBB.pl?num=1252535815

except I want 5v from 24v. The consensus seems to be several regs in series, such that the dissipation is spread out.

Ran Talbott

Quote
I have 11 - 15 volts, I need 3 or 3.3 volts @ 600ma , 1 amp preferred


If you use a linear regulator or regulators,  you'll need to dissipate about 5-10W of heat,  depending on on where your actually wind up within the ranges you specified.

As others have pointed out,  that's a lot.

For about $10 in onesies,  you can get a switching regulator that looks like an overgrown 78xx chip in a TO-220 package.

That's a lot simpler,  and not hugely more expensive,   than using multiple 78xx regulators with heat sinks,  resistors,  etc.

Ran

Jassper

Thanks, I'll look at all this. It might be best to power each section individually. I have 3 sections of the circuit each drawing about 200ma, so for heat dissipation reasons It might be best to use 3 regulators.

Papabravo

#6
Sep 11, 2009, 04:25 am Last Edit: Sep 11, 2009, 04:32 am by Papabravo Reason: 1
You don't really need regulators in series.  A power transistor in an emitter follower configuration will easily drop half of the voltage from Vin to 3.3 or 5V and give you another package to help disipate the heat.  I think a power transistor, a resistor, and a zener diode should beat the cost of a 7815 or a 7812 in front of the the 7805
We never have time to do it right; but we always have time to do it over.

Jassper

Quote

You don't really need regulators in series.  A power transistor in a common base configuration will easily drop half of the voltage from Vin to 3.3 or 5V and give you another package to help disipate the heat.  I think a power transistor, a resistor, and a zener diode should beat the cost of a 7815 or a 7812 in front of the the 7805


Do you have a quick schematic?
I assume then that I would use the Base to Emitter voltage drop across a resistor to feed to a v reg, am I looking at that right?

Papabravo

#8
Sep 11, 2009, 05:25 am Last Edit: Sep 11, 2009, 05:52 am by Papabravo Reason: 1
I do, but I have no clue as to how I can get it to you.

Edit: Try this

Replace the load resistor(R3 - 20 Ohms) with your regulator.
Use any suitable power transistor you have.
The 10 Ohm resistor (R1) limits the current through the transistor
The Zener I showed is a 15V. You can use other values.
R2 provides base current for the power transistor and zener current for the diode.
Make sure each of your components can dissipate the required power
It is easy to play with this circuit in a simulator like LTSPICE
We never have time to do it right; but we always have time to do it over.

Jassper

#9
Sep 11, 2009, 05:27 am Last Edit: Sep 11, 2009, 05:35 am by Jassper Reason: 1
I'll message you my email
[Edit]
Thanks

elwing

#10
Sep 11, 2009, 10:51 am Last Edit: Sep 11, 2009, 10:52 am by elwing Reason: 1
you're needing too much power to use linear regulation, it is of course possible to use a single 7833 or a good To3 transistor with an huge heatsink, but to me it seem much better to use switching regulator like LM2825

Jassper


Jassper

I was going over my circuit and changed a few resistors from 1K to 10K, and a POT from 10K to 100K.

After recalculating, My entire circuit now only draws 210ma (70ma for each section, actual meter reading = 72.3ma +/-0.1ma) So I can go with a little bit lighter regulator, 0.5a should do just fine.  :D

Thanks for all the suggestions! Will come in handy on my next project.


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