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]
