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I have the following circuit with a 10k 0.5w trim potentiometer.

schematic

simulate this circuit – Schematic created using CircuitLab

The voltage after the bridge rectifier is maintained at ~8.6 Vdc. The voltage is fine when I do not fully turn the pot to 270°. At 270° turn suddenly the voltage begins to fluctuate wildly between ~2 - ~5 V as shown by the picture below.

Oscilloscope Measurements

What is causing this effect? Is this an issue with the pot, capacitors, etc?

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  • \$\begingroup\$ Is that an actual plot of the voltages or just a drawing of what it kind of looks like? That swinging voltage is a full sine wave at 100Hz, if your power source is at 60hz, then it seems that you have some sort of 100Hz resonance going on. If you were drawing more current than the rectifer could handle, I'd expect the DC straight line to develop somewhat triangular bumps as it degrades, but without ever turning into a full sine wave. \$\endgroup\$ – Johnny Feb 17 '14 at 17:00
  • \$\begingroup\$ I couldn't pull the actual data off my oscilloscope so I guesstimated. Looking back at the data it appears closer to 60 hz then 100 hz. \$\endgroup\$ – Blackninja543 Feb 17 '14 at 17:02
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As you turn the pot, the resistance it provides is effectively lowered. This draws more current from the capacitors, lowering their voltage more quickly. When the resistance is low enough, the capacitors aren't able to maintain a steady voltage between each AC cycle.

Another way to view it is this: the capacitors and the resistors make an RC low-pass filter. When the resistor is big, this filter has a long time constant. When the resistor is small, the filter has a short time constant. As the time constant becomes shorter, this means the cutoff frequency increases, and the filter attenuates the incoming AC less.

To reduce the ripple, you can decrease the load current by increasing the resistance, or you can add more capacitance.

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  • \$\begingroup\$ The strange part is when I do a full 270° turn the resistance on 2 of the 3 pins is 10k, which would tell me I have less current going through the circuit. The pot also begins to heat up. \$\endgroup\$ – Blackninja543 Feb 17 '14 at 17:04
  • \$\begingroup\$ @Blackninja543: But at 270 the wiper connection has a resistance of 0 to one of the other connections - in other words a short circuit on the transformer output. That's probably why the pot heats up. \$\endgroup\$ – RedGrittyBrick Feb 17 '14 at 17:08
  • \$\begingroup\$ @RedGrittyBrick Sorry had the pot backwards :D. \$\endgroup\$ – Blackninja543 Feb 17 '14 at 17:11
  • \$\begingroup\$ @RedGrittyBrick Some quick math the pot at 270° has a resistance of 3.1 ohms. At the 8.6 Vdc i was pulling 2.77 amps or 23.82 watts. The only thing that really saves my pot at this point was the capacitors inability to maintain the high output of current correct? \$\endgroup\$ – Blackninja543 Feb 17 '14 at 17:18
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Maybe about 95% of the usable range of the pot will have little or no effect, the output impedance of the circuit is much lower than 10k. Somewhere the last few % of the angle the current will increase rapidly with the slightest movement. Especially if you are using a wire wound potmeter. If the power supply can deliver more than the 0.5W, the potmeter will burn and get physical damage at one end. The slightest wiggle will probably have a huge changes in impedance of the potmeter, from nearly 0 to approximately infinite.

What you probably see is the caps being drained faster by the low potmeter impedance than they get charged by the transformer and rectifier.

enter image description here

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