Hello, I am currently using two full wave rectifiers for my senior project. One is being used on my current dimming circuit 120v, and the other for a low voltage (12v) 555 timer circuit.
Is there anything available that I can use instead of using two full wave rectifiers? Maybe a 120VAC (ripple) to 12VDC output?

If I could find an alternative I could reduce my circuit by a minimum of 4 diodes and a transformer.

Thank you!

 

If the 120V is filtered then you can use a switched mode power supply to step it down to 12V.

 

The best way to get the 12 volts is to use a transformer. A little wall transformer is cheap and, most importantly, safe.

 

You can even get them in module form, in the form of a wall wart. Filtered and regulated in a cheap package.

 

You can even get them in module form, in the form of a wall wart. Filtered and regulated in a cheap package.

"Wall warts" are typically rectified, but not regulated nor filtered.

 

Hello, I am currently using two full wave rectifiers for my senior project. One is being used on my current dimming circuit 120v, and the other for a low voltage (12v) 555 timer circuit.
Is there anything available that I can use instead of using two full wave rectifiers? Maybe a 120VAC (ripple) to 12VDC output?

If I could find an alternative I could reduce my circuit by a minimum of 4 diodes and a transformer.

Thank you!

There is an old old old trick for doing this....called the "economy" power supply. Very tricky..wish I'd invented it. You need a transformer with a low voltage tap on the secondary. What you do is you put a full wave bridge across the "main" secondary winding, and put a filter on that. This is your high voltage supply Then you take the tapped winding, which will now have DC on it relative to ground, but it's still got a lot of ripple. You put a filter cap across that and, voila...low voltage dc! Two voltages with one rectifier.

Cool!

eric

Eric

 

Thank you for your suggestions everyone. My current design consists of a low voltage circuit (timer) and a HV current mode amplifier that will drive LEDS.
The low voltage circuit currently has a 120/12v transformer that is then rectiifed to a 12v ripple and finally input to a 12VDC voltage regulator. The regulator output obviously feeds the timer circuit.
Maybe the "Buck Mode" powersupply is the best idea? Are there any designs online that I can look at?

Thanks
Darrin

 

Have a look here for the design of switched mode power supplies:

http://www.nxp.com/acrobat_download/applicationnotes/APPCHP2.pdf

 

"Wall warts" are typically rectified, but not regulated nor filtered.

Hmmm, we must be using different sources then, cause I have quite a few. I also have wall warts that are AC out, so you have to read the specs on them. For those with DC out filtered is the minimum spec.

 

There is no need for a full wave bridge except to reduce ripple. Loose the diodes, increase the caps. If the cap. gets too big go to full wave.

russ_hensel

 

There is no need for a full wave bridge except to reduce ripple. Loose the diodes, increase the caps. If the cap. gets too big go to full wave.

russ_hensel

How can I rectify 120VAC to 12VDC without using a full wave rectifier? If I use a half wave it's going to affect the operation of the timer circuit isn't it?

 

The easy way is to use a transformer that has a center-tapped secondary, like 12v-0v-12v. You ground the center tap, and use two diodes for the ends of the transformer, anodes connected to the windings, cathodes connected together.

 

I appreciate the suggestion but I am already using a 120VAC/12VAC transformer. From there I am using a fullwave rectifier and the output is going to a 12VDC regulator. I was trying to see if there was a possibility of eliminating the transformer and second fullwave rectifier, but it doesn't seem to be possible.

Thanks

 

Do you rectify the 120V AC directly and then smooth it with a capacitor filter or just rectify it?

I though something 'bad' which may work!!

 

If you are happy with the 12v then the leds can be driven directly from AC how use a triac photoisolator and a diode in series like 1n4006 to cut the saturation current during [-] swing.

 

Do you rectify the 120V AC directly and then smooth it with a capacitor filter or just rectify it?

I though something 'bad' which may work!!

I have two circuits. One circuit (120VAC) essentially uses a switch mode puwer supply as the driver for the LED's, the other circuit is just a 12V circuit that is used for a 555 timer circuit. I am using a full wave rectifier without a capacitor. The timer circuit works I've tested it. I'm just trying to find a way to eliminate the transformer and second rectifier if possible.
I was thinking of another SMPS circuit?

 

If you are happy with the 12v then the leds can be driven directly from AC how use a triac photoisolator and a diode in series like 1n4006 to cut the saturation current during [-] swing.

It's my Senior Project and the circuit is already designed. I am just trying to reduce components where possible.... If I can reduce 120VAC to 12VDC without using a transformer and second full wave rectifier I can reduce some of the cost of my circuit.

 

How many amps does the timer draws? What does it drives?

 

How many amps does the timer draws? What does it drives?

The timing circuit solely consists of a regulator (12v) and the 555 timer setup to output a 1kHz signal. That will be used as an input to an "opto-isolator" for the current mode controller.

 

I won't tell you the idea because it is not safe at all. Better to use a small 6VA transformer and a 1 Amps bridge rectifier to do the job rather get killed.