LTspice is the most widely used free electronic simulation software.
This website offers free downloads of electronic circuits for simulation mainly in LTspice, but also in Qspice and Micro-cap.
this allows you to see the voltage and current waveforms throughout the circuit without having to build the circuit or buy any test equipment.
In fact you don't need to spend any money at all.
start off by downloading and installing LTspice from www.analog.com
then click on the text or the graphics, to open the below sections,
then download the circuits to download and run with LTspice
any comments to keith@analogsimulation.co.uk