The circuits that I was working with consisted of an isolation transformer, variac, and 120:12.6 step down transformer. The bridge circuits were built with four 1N4004 rectifier diodes (no filter). The load was a lamp 14V at 200mA. The rectified peak output voltage measured within 280 mV of the predicted voltage (a less than 2% deviation), but the average measured (with a Metex M-3800 DMM) DC output voltage was more than 5% less than the predicted DC output voltage. With our labs if there is more than a 5% deviation we have to explain why. Can anyone help me with the why?

 

What was the current through the lamp?

Be aware that tungsten-filament lamps have non-linear response curves.

200mA @ 14v = 70 Ohms.

But if you measure that same bulb when it's cold, using a DMM, you'll find it's resistance is much less.

 

You can recalulate the lamps real resistance hot by measuring the current, and then the voltage across the lamp. This will probably bring your measurements to within tolerances.

 

What was the voltage reading when using an oscilloscope?

I would think the rms value of the signal would have been about 11.4 volts or so under a load.