# Reality check- scope measurements vs SPICE simulation

#### keithostertag

Joined Oct 4, 2012
55
Could someone give me a reality check on the differences between my measurements made with a scope and my spice simulation for this simple rectifier circuit. I'm new to using SPICE simulation.

According to the SPICE simulation I should see approx 1.74 VDC with less than 2mV ripple (assuming I am reading the graph correctly). My measurements give me 1.91VDC with about 10mV ripple.

So, have I left out something? What else explains the difference?

I measured the cap and put the measured value in the SPICE simulation. The load resistor is pretty close to its stated value.

Also, looking at the scope image, what accounts for the different heights of the ripples? Slight differences in diodes? What does the shape of the ripple reveal about the capacitor performance or circuit design?

Sinewave source is a HP signal generator, using its 50-ohm output through a 50-ohm terminator to the circuit. Scope input signal (top trace) is measured directly at the circuit input. Output ripple (bottom trace) is measured by a Differential Amp plugin in my scope. DC output across resistor 1.91VDC measured with external bench digital meter.

Thanks,
Keith Ostertag

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#### #12

Joined Nov 30, 2010
18,224
You are down so low on that diode current and voltage! 191 ua at 3 volts on a set of diodes that can do an amp each at 1000 volts puts you in rare atmosphere on the datasheet. Specifically, the difference in voltage peaks is caused by mismatch in the forward breakover voltage of the diodes. The voltage and current are so low that temperature actually has something to do with the output. If you're going to expect some spice programmer to predict what your diodes are going to do, you should try using them at more that a tenth of a percent of what the programmer expected.

#### keithostertag

Joined Oct 4, 2012
55
Thanks for taking the time to respond.

OK, I hadn't thought of that. After increasing the load to a few mA (by reducing the value of the load resistor R2 ) the simulation figures come closer to the measured figures.

Thanks very much.

Keith Ostertag