Problem with home made 12v dc rectifier

I'm trying to make a small 120v to 12v AC to DC converter and I have my retifier in place using the radio shack part 276-1152 along with the radio shack 12v 300mA transformer part # 273-1385. After hooking them up and looking at the output on my oscilloscope to filter out the AC ripple, with out the filter my wave does not look right. Can someone tell me what's the issue? Below is a pic. Rectifier http://www.radioshack.com/product/index.jsp?p...ge=60&filterValue=Rectifiers Transformer http://www.radioshack.com/product/index.jsp?productId=2102494

You might not have a bulk capacitor (make it 100s of uF) connected and then conenct a load resistor an measure again and adjust the capacitor for the given load.

u need A capacitor bro as it help to filter the waves

Is the load still connected ?? Is your scope running on an isolated transformer?? Is your Scope earthed on the mains supply socket?? Is your scope probe open circuit??

Because looking at the waveform it appears that you have a difference in earth potential I.e. the ground side of your oscilloscope probe and the circuit that you are studying Or you have connected the bridge rectifier round the wrong way
or you are taking your scope measurement at the ac input side to the bridge rectifier and not the output side "0V6 volt drop on each diode"


Check this first If you are seeing slight sparks or are experiencing a small electric shock whilst connecting your probe to the circuit Id say that could very well be your problem


Your scope view should be a fairly clean 100 cycle full wave waveform without the smoothing cap superimposed on a DC level All Peaks and troughs showing in the positive direction

See below

http://upload.wikimedia.org/wikipedia/commons...ifier.en.svg/600px-Gratz.rectifier.en.svg.png


Mark David Harrington
www.harrington.force9.co.uk

If you want a clean dc voltage on you output , you need a cap from circuit to ground.. Now, this is the tricky part the higher the cap the cleaner the output will be. The output waveform has to look round on the top.. I hope this helps, if anything else, post on my site.....

Yes you do need a capacitor in the order of at least 1000uf 16vwg and above , however his waveform is not right to start with which means even if he did use a smoothing bank the circuit wouldn’t work according to his scope diagram , which makes me think he’s done something wrong here

Have a look at the scope image again

Full wave rectification doesn’t render both halves of an ac waveform and its clipped at the beginning of each half cycle almost , equivalent to cross over distortion that is typically inherent in a class AB amplifier where quiescent current hasn’t been set properly, but in his case I reckon he’s either connected the bridge rectifier round the wrong way or he’s showing us a waveform present at the input of the bridge rectifier and not the dc output side

Think what fault symtom's you get when scoping a power supply with the smoothing bank open circuit I’m sure you must have come across this fault many a time

I wonder if he missing the resister , I will post a diagram for him to see if he could see what he or what he did wrong.

It's not the rectifier, the waves are not smooth humps and where the lower wave is supposed to come up by itself there is a zero voltage drop instead of a 12 volt wave "like in this pic(http://www.toddfun.com/blog/wp-content/uploads/2011/03/f0001.jpg)":http://www.toddfun.com/blog/wp-content/uploads/2011/03/f0001.jpg . I connected my oscilloscope up to my 120v AC outlet, which I measured to 108v, on the oscilloscope around 40v positive and 40v negative there was a short sudden bursts of extreme spikes, continuously on every wave at the same point. I didn't have a chance to take a pic before I left for work but I'll upload it later.

Something I just noticed, the wave generated from the negative current looks out of shift to me.

check the connection specially at the side of oscilloscope or from where u have given the supply

Thankyou one person here is getting this right What else Adding a capacitor will do what given the waveform is altenating current ??

XC = 1 / 2pi* FC springs to mind perhaps Frequency being what USA 60cycles / per second UK 50 cycles per second Cheers !! What happens to the cap

Oh dear Bang !! and what else It acts as a dead short to ac blocks DC but hang on there is no DC present Cheers !!

So uhm bye bye transformer maybe Huge puffs of smoke

Still not to worry Add add a resistor, bound to work hmmm Ok fine !!

Have another look at the scope wave form then

Wheres the Peak to peak voltage and uh what is the timer per division set at Need that infromation as well dont we ?? Or dont we ?? Might be a good idea dont you think

hey dude this wave form shows the distortion basically
u have only made an ac to dc power supply to get the good out put always remember this steps :

*_Transformer (step down) ----> Bridge Rectifier (in ur case a radio shank) -----> RC filters ------> Power Regulators_*

REGULATOR IC's
(ic 74xx series or Lm 317 (for +ve voltage ) and lm337 (for -ve voltage ) always remind here while using power regulator u decide for the output u want and add +3v to in that is minimum input u want to get to ur regulator ic

special note for ur case :
after a shank u use a 1uf tantalum capacitor in series before providing input to a shank and 10 uf capacitor at output of shank and in parallel to load , and resistance of 10k in series to input i can be in mode of 2T manner to reduce more distortion

thank u i think u may have got what i am saying

hi, i seen onetime this problem, please can you post the wiring diagram, or schematic of your DC power supply, i need to see.

see this picture.

Hmmm try using a volt meter first followed by some math’s followed by some continuity checking using ohms ranges followed by some very basic studies of what a full wave bridge circuit does and also some semiconductor , capacitance , resistance , inductance theory

This could well be an added advantage at this stage

Live in hope I guess

PS should we expect national grid power melt down when he connects his magnificent power supply to the mains and might he be better off using batteries for the general safety of the public

I hope you will treat this with the humor it deserves

i did nt understood what u mean to say may it be possible for u to eloborate

Binary Nation, did you ever find (and do you remember) the reason for this strange waveform? I am getting the same thing.

Thank you,
Bob

I've found the answer: you need to connect a load, such as a 1K resistor, to the output of the rectifier. Then you'll get the expected waveform.

The reason is that real-world diodes don't behave the way theoretical ideal diodes do. Real diodes have "reverse capacitance": when the diode is reverse-biased (as it is every half-cycle when it has an AC input), the P-N junction acts as a capacitor and accumulates a charge. That charge gets released when the diode becomes forward-biased again, and that's the funny spike Binary Nation saw in the scope.

With a normal load, like a 1K resistor, the reverse capacitance is negligible and doesn't show up on a scope. But when only the oscilloscope is hooked up, with its 1Mohm load, the capacitance is significant and its effect is sizable.