# won't work, right?

Discussion in 'General Electronics Discussion' started by rebeltaz, Nov 23, 2012.

1. ### rebeltaz

33
0
Nov 22, 2012
OK, so I have a center tapped transformer, about 15v open circuit side to side. I need 3.3vdc with full wave rectification , ~12vdc and about 7.5vdc. I did this somehow, but I can't remember how.

In the circuit, the LEDs from the half wave rectifier do not share a common ground with the rest of the circuit.

Will this circuit work or am I going to blow something up?

Thanks.

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2. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,418
2,788
Jan 21, 2010
I'm pretty sure you don't know what you're doing.

The LEDs are getting AC, not DC
R14 in superfluous
R16 is almost certainly not required (or doesn't do what you might think it does)
C1 is very small, unless your current requirements are also very small (under 50mA total?)
There is no 7.5VDC anywhere.

3. ### rebeltaz

33
0
Nov 22, 2012
Well, since I have been a professional electronics technician working on televisions and stereos for the past 25 years, I am pretty sure that I have some semblance of an idea of what I'm doing.

How are the LEDs getting AC through a rectifier? They are getting pulsed DC.

The transformer puts out ~15.5v open circuit, with ~7.5v from each side to the center tap. The output at C1 is ~7.5 x 1.414 = ~10.6, hence both R14 and R16

R14 is to lessen the load on the voltage regulator so that it doesn't have to work as hard and to reduce the amount of heat dissipated.

R16 is to lower the voltage as well. I know that the schematic says 12v, but I am driving a 9v relay with that tap.

The voltage do not need to be exact except for the 3.3 going to a MSP430 microprocessor, so yes.. the current drain is minuscule.

4. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,418
2,788
Jan 21, 2010
Sorry I missed the diode there (oops).

R15 will drop just over 2V at 50mA, so it's probably not going to help a great deal. (Although it will help a little)

R16 is obviously chosen with the load in mind, so I'll accept it's the right value.

It's really hard to answer questions when there is a heap of other hidden requirements.

Sorry about missing the diode though.

Seriously, we have too many people coming here assuming that there is a resistor that will regulate voltage.

5. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,418
2,788
Jan 21, 2010
I'd also check that the 78L33 is stable with only 0.1uF output capacitance. I would presume so though since it doesn't appear to be a low dropout regulator.

6. ### rebeltaz

33
0
Nov 22, 2012
No problem I understand.

The datasheet for the L78Lxx does call for a .1uf cap, so that's what I'm going on...

The part I'm really curious about though is the full-wave bridge and the half-wave rectifier on the same transformer. Something about it just doesn't look right to me, but I can't put my finger on it. Maybe it's just too late/early or... What do you think?

7. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,418
2,788
Jan 21, 2010
No, everything looks fine. I think you have already observed that the half wave regulated voltage for the LEDs has no common ground with the other voltage rails.

8. ### rebeltaz

33
0
Nov 22, 2012
I just wanted to say thank you. I finished my project last night and everything is working fine. No earth-shattering-kaboom

Now off to the next project... Thanks again.