Oscilloscope Output Issues with Radio Shack 276-1152 Rectifier & 273-1385 Transformer

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Why does the rectifier output look wrong on the oscilloscope when I connect a 12 V AC transformer and bridge rectifier without a filter capacitor?

The strange spikes are caused by measuring the rectifier with no real load; add a load resistor such as 1 kΩ to the output, and the waveform should look like the expected rectified output [#21661724] Real diodes have reverse junction capacitance, so when they are reverse-biased this charge can build up and then discharge on the next forward cycle [#21661724] With only the oscilloscope connected, its 1 MΩ input makes that effect noticeable on the scope [#21661724] With a normal load, the capacitance effect becomes negligible and the waveform looks normal [#21661724]

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#1 21661707 08 Apr 2012 18:26

Binary Nation Binary Nation

Anonymous
Post #1
21661707 08 Apr 2012 18:26

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

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forums-bridge-rectified-12v-1333841193.jpg (8.6 KB) You must be logged in to download this attachment.
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#2 21661708 08 Apr 2012 19:05

Otto Sponring Otto Sponring

Anonymous

Post #2
21661708 08 Apr 2012 19:05

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.
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#3 21661709 09 Apr 2012 04:16

Ashesh Sharma Ashesh Sharma

Anonymous

Post #3
21661709 09 Apr 2012 04:16

u need A capacitor bro as it help to filter the waves
#4 21661710 09 Apr 2012 05:25

Mark Harrington Mark Harrington

Anonymous

Post #4
21661710 09 Apr 2012 05:25

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
#5 21661711 09 Apr 2012 09:59

David Figueroa David Figueroa

Anonymous

Post #5
21661711 09 Apr 2012 09:59

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.....
#6 21661712 09 Apr 2012 11:29

Mark Harrington Mark Harrington

Anonymous

Post #6
21661712 09 Apr 2012 11:29

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
#7 21661713 09 Apr 2012 11:48

David Figueroa David Figueroa

Anonymous

Post #7
21661713 09 Apr 2012 11:48

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.
#8 21661714 09 Apr 2012 13:56

Binary Nation Binary Nation

Anonymous

Post #8
21661714 09 Apr 2012 13:56

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.
#9 21661715 09 Apr 2012 14:37

Binary Nation Binary Nation

Anonymous

Post #9
21661715 09 Apr 2012 14:37

Something I just noticed, the wave generated from the negative current looks out of shift to me.
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#10 21661716 09 Apr 2012 23:04

Ashesh Sharma Ashesh Sharma

Anonymous

Post #10
21661716 09 Apr 2012 23:04

check the connection specially at the side of oscilloscope or from where u have given the supply
#11 21661717 10 Apr 2012 04:12

Mark Harrington Mark Harrington

Anonymous

Post #11
21661717 10 Apr 2012 04:12

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
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#12 21661718 10 Apr 2012 13:28

Rinkesh Kurkure Rinkesh Kurkure

Anonymous

Post #12
21661718 10 Apr 2012 13:28

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
#13 21661719 11 Apr 2012 09:03

Samir Guessmi Samir Guessmi

Anonymous

Post #13
21661719 11 Apr 2012 09:03

hi, i seen onetime this problem, please can you post the wiring diagram, or schematic of your DC power supply, i need to see.
#14 21661720 11 Apr 2012 09:10

Samir Guessmi Samir Guessmi

Anonymous

Post #14
21661720 11 Apr 2012 09:10

see this picture.
#15 21661721 12 Apr 2012 10:36

Mark Harrington Mark Harrington

Anonymous

Post #15
21661721 12 Apr 2012 10:36

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
#16 21661722 13 Apr 2012 10:20

Rinkesh Kurkure Rinkesh Kurkure

Anonymous

Post #16
21661722 13 Apr 2012 10:20

i did nt understood what u mean to say may it be possible for u to eloborate
#17 21661723 10 Jan 2015 22:06

Bob Alexander Bob Alexander

Anonymous

Post #17
21661723 10 Jan 2015 22:06

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
#18 21661724 12 Jan 2015 09:24

Bob Alexander Bob Alexander

Anonymous

Post #18
21661724 12 Jan 2015 09:24

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.
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Topic summary

✨ The discussion addresses issues with the output waveform observed on an oscilloscope when using a Radio Shack 276-1152 bridge rectifier and a 273-1385 12V 300mA transformer to build a 120V to 12V AC to DC converter. The primary problem is an abnormal waveform lacking the expected full-wave rectified shape, with distortion and spikes. Key causes identified include the absence of a bulk smoothing capacitor (recommended at least 100µF to 1000µF, 16V or higher) and the lack of a load resistor (e.g., 1kΩ) on the rectifier output. Without a load, the reverse capacitance of real diodes causes charge accumulation and release, producing spikes and waveform distortion visible on the scope. Additional troubleshooting points include verifying correct bridge rectifier orientation, ensuring oscilloscope grounding and probe connections are proper, and confirming measurements are taken on the DC output side of the rectifier. Suggestions also include adding RC filters and voltage regulators (such as LM317 or LM337) for cleaner DC output. The waveform should show a positive full-wave rectified signal with rounded peaks after filtering. The discussion emphasizes the importance of proper circuit configuration, load application, and filtering to achieve a stable DC output from the transformer and rectifier setup.
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FAQ

TL;DR: Weird oscilloscope spikes from a 12 V/300 mA supply vanish when you add a real load—“connect a load, such as a 1K resistor.” The effect comes from diode reverse capacitance interacting with a 1 MΩ scope input. [Elektroda, Bob Alexander, post #21661724]

Quick Facts

OP parts: Radio Shack 273-1385 transformer (12 V, 300 mA) + 276-1152 bridge rectifier. [Elektroda, Binary Nation, post #21661707]
Typical smoothing: ≥1000 µF, ≥16 V electrolytic across DC output for lower ripple. [Elektroda, Mark Harrington, post #21661712]
Add a load (e.g., 1 kΩ) to stabilize waveform; 1 MΩ scope alone can show spikes. [Elektroda, Bob Alexander, post #21661724]
Expect about 0.6 V drop per conducting diode in the bridge path. [Elektroda, Mark Harrington, post #21661710]
Mains frequency reference discussed: 60 Hz in the USA, 50 Hz in the UK. [Elektroda, Mark Harrington, post #21661717]

Who is this FAQ for, and what problem does it solve?

Hobbyists debugging odd waveforms after a 12 V AC transformer and 276-1152 bridge. It explains why spikes appear on scopes without a load and how to get clean DC with proper loading and filtering. [Elektroda, Bob Alexander, post #21661724]

Why does my rectifier output show tall spikes on the scope?

With only a 1 MΩ scope load, the bridge diodes’ reverse capacitance stores charge and releases it as visible spikes when the polarity flips. Adding a modest load, like 1 kΩ, damps this effect and the waveform looks as expected. [Elektroda, Bob Alexander, post #21661724]

How do I fix the weird waveform quickly?

Connect a real load across the rectifier’s DC output. Start with 1 kΩ, then observe the change. Add a bulk electrolytic (≈1000 µF, ≥16 V) if you also want smoothing. “Connect a load, such as a 1K resistor.” [Elektroda, Bob Alexander, post #21661724]

Do I need a smoothing capacitor, or is a load enough?

A load fixes the spike artifact. A capacitor reduces ripple. Many builders use around 1000 µF at ≥16 V for a 12 V secondary to get a steadier DC level under light loads. Size it for your current draw. [Elektroda, Mark Harrington, post #21661712]

What forward-voltage drop should I expect from a bridge rectifier?

About 0.6 V per conducting diode segment in silicon bridges. A full-wave bridge places two diodes in series during conduction, so expect around 1.2 V total drop in the DC path. [Elektroda, Mark Harrington, post #21661710]

Could my oscilloscope grounding cause strange readings or shocks?

Yes. A mains‑earthed scope can create ground potential issues if you clip to the wrong reference point. Mis-referencing can show distorted waveforms or cause sparks. Verify isolation and clip ground only to circuit return. [Elektroda, Mark Harrington, post #21661710]

How should the full-wave rectified waveform look before smoothing?

You should see both half-cycles flipped positive—clean, evenly spaced humps centered on a DC level. If it looks asymmetrical or clipped, check rectifier orientation and where you’re probing. [Elektroda, Mark Harrington, post #21661710]

Could I have wired the bridge rectifier incorrectly?

Yes. If you measure at the AC input pins or swap DC and AC connections, the scope will not show the correct full-wave output. Verify the two ‘~’ pins go to the transformer, and ‘+’/‘–’ feed the load. [Elektroda, Mark Harrington, post #21661710]

What capacitor value should I start with for a 12 V/300 mA supply?

Start around 1000 µF at ≥16 V, then increase if your load draws more current or ripple is still high. Larger values reduce ripple but raise inrush current. [Elektroda, Mark Harrington, post #21661712]

Why did measuring the wall outlet show spikes around certain voltages?

If the scope or probe reference is incorrect or the setup lacks isolation, you can see artifacts and even get small shocks. Always confirm safe probing practices before connecting to mains. [Elektroda, Mark Harrington, post #21661710]

What is “reverse capacitance” in diodes?

When reverse-biased, a diode’s P‑N junction behaves like a small capacitor. It stores charge and then releases it when the polarity changes, causing visible spikes on high‑impedance measurements. [Elektroda, Bob Alexander, post #21661724]

Is a series resistor required in addition to the load?

A separate series resistor is not required for a basic bridge-and-cap supply. The key is a proper load across the output and a suitable smoothing capacitor if DC ripple must be reduced. [Elektroda, Bob Alexander, post #21661724]

How do I wire this Radio Shack transformer and 276-1152 bridge to get DC?

Connect the transformer’s 12 V secondary to the two ‘~’ bridge pins.
Take DC from ‘+’ and ‘–’; add a 1 kΩ load across them.
Add an electrolytic across ‘+’/‘–’ for smoothing, observing polarity. [Elektroda, Mark Harrington, post #21661710]

What’s an edge case that can still make it look wrong after adding a load?

If you clip the scope ground to a live point or probe the AC side by mistake, you may still see distorted shapes or get sparks. Recheck probe reference and measurement node. [Elektroda, Mark Harrington, post #21661710]

What did the original thread conclude about the strange waveform?

The solution was to connect a load on the rectifier’s output. That eliminated the spikes caused by diode reverse capacitance interacting with the scope’s high input impedance. [Elektroda, Bob Alexander, post #21661724]