# Half wave rectification question

#### Hk606

Joined Oct 19, 2014
19
My project requires 12 volts DC 29mA. The input voltage to the project is 24 volts Ac 2.0 amps this is a must.
As my load is so small, For simplicity purposes I'd like to use a simple half wave but I don't know what size filter cap to
use across on the output. to control any ripple. I would be grateful for any help. Thanks

#### Dodgydave

Joined Jun 22, 2012
10,483
Any size from 470 to 1000uF , the voltage after rectifying will be approx 16V open load.

Last edited:

#### alfacliff

Joined Dec 13, 2013
2,458
he input current at 2 amps is decieving, just because a power supply or wall wart is rated 12 volts at 2 amps does not mean that 2 amps is necessary, the current depends on the load;. the current rating just tells you tghe maxumum it will put out. your 24 volt ac transformer will be putting out 29 ma RMS, the load current.

#12

#### #12

Joined Nov 30, 2010
18,223
(Square root of 2) C Er(peak to peak) F = I

#### wayneh

Joined Sep 9, 2010
17,172
... to control any ripple.
You need to define what you mean by "control". For 100% ripple, do nothing. For 0% ripple, give up now. For something in between, there are options.

#### Hk606

Joined Oct 19, 2014
19
he input current at 2 amps is decieving, just because a power supply or wall wart is rated 12 volts at 2 amps does not mean that 2 amps is necessary, the current depends on the load;. the current rating just tells you tghe maxumum it will put out. your 24 volt ac transformer will be putting out 29 ma RMS, the load current.
Yea I just wanted to give all the info on the project. Thanks for getting back

#### Hk606

Joined Oct 19, 2014
19
Any size from 470 to 1000uF , the voltage after rectifying will be approx 16V open load.
Thanks for the help. If the 24 volts weren't needed for other relay control it would have been easier, not that what I'm doing is complicated for sure!

Joined Dec 4, 2009
328
What is the tolerance of your project requirement voltage of 12V, 29mA? The half wave rectified voltage will have a peak of around 33V.

#### MikeML

Joined Oct 2, 2009
5,444
Here is how I would do it.

Note the voltages at the three named nodes. Note the little bit of ripple left in the load current...

#### wayneh

Joined Sep 9, 2010
17,172
R1 limits the current into the cap?

#### MikeML

Joined Oct 2, 2009
5,444
I screwed up the load current. Here is revised circuit that will make 29mA in the load.

#### #12

Joined Nov 30, 2010
18,223
I calculate 285 uf to keep the ripple voltage down to 10% of 12 volts, but there is no good reason to mess with Mike's design unless you have some difficulty with the amount of ripple he has presented.

#### alfacliff

Joined Dec 13, 2013
2,458
what is r5 for?

#### MikeML

Joined Oct 2, 2009
5,444

#### MikeML

Joined Oct 2, 2009
5,444
I calculate 285 uf to keep the ripple voltage down to 10% of 12 volts, but there is no good reason to mess with Mike's design unless you have some difficulty with the amount of ripple he has presented.
Actually, the ripple helps keep the power dissipation in R1, R2 and D2 a bit lower.

#### #12

Joined Nov 30, 2010
18,223
Actually, the ripple helps keep the power dissipation in R1, R2 and D2 a bit lower.
I know.

#### Hk606

Joined Oct 19, 2014
19
I screwed up the load current. Here is revised circuit that will make 29mA in the load.

View attachment 81877
Just for clarity purposes the circuit is to operate a miniature air solenoid SMC model: vqz 100 series. Question: Is the R3 420 ohm resistor shown the resistance
of the load or is this resistor added across the solenoid terminals. By the way. Thanks for the efforts on this!

#### #12

Joined Nov 30, 2010
18,223
Mikes R3 is pretending to be your load. It's only there for the simulation program.