Precision Chopper Circuit

  • @Alan Smith
    I have some 16 Ga already, thanks.
    15 Ga was mentioned in one of the IH-Rossi patent applications, so I was looking for some of that size for a while.
    Seems to be special order stuff. And a pain to use as heater in short lengths.
    I was just attempting to see if a Lugano Coil as described in the application was feasible. I managed with 14 Ga, so it is.
    Except for the 2.650 Ω/ft specifcation, which does not seem possible in wire of anything like 15 Ga.

  • New to this thread, but I saw several inaccuracies from the previous posts.
    Power measurements cannot use the average (mean) for current or voltage unless it is DC or a perfect square wave. Only RMS values can be used because power is I-squared * R or Vsquared/R.
    For instance, the average of a triangle wave is .5, but the RMS value is 1/sqrt3 = .577. Using .5 instead of .577 to compute power input would give you 15% apparent excess power. The apparent excess depends on the actual waveform.
    The best way to measure is to simultaneously measure V and I, and then average the V*I samples. That is what the power meters do. But the sample rate must be much higher than the highest frequency you are measuring.

    It is also not correct to multiply RMS voltage times RMS current unless the load is purely resistive. If the load is inductive, the current and voltage are out of phase and Vrms*Irms does not equal the actual power delivered.

    The Spice model shows some series inductance, but is not accurate because it assumes that resistance remains constant. The resistance of a heater element changes by a large amount as it heats or cools and you cannot use the cold resistance alone to determine power.

    Also, for a heater, the resistance of the leads to the heater element should not be neglected. The voltage should be measured with a separate set of test leads connect at the ends of the heater element. This effect also makes measurement of wall outlet power inaccurate, although at least this error is in the right (conservative) direction - it will overestimate input power by the amount of power delivered to the heater leads and the electronics.

  • @Robert Horst

    While I basically agree with all you say, the error from ignoring the coil inductance is very likely OK. That is because:

    (1) It is conservative: P = Vrms*Irms*cos(phi)
    (2) For a small reactive phase shift the correction is second order (cos(phi))

    You do have to be careful because the power waveform has significant components well above 100Hz due to the Triac drive. And I agree that for the same reason any digital sampling meter (which includes all true RMS meters) needs to have a high enough sampling rate to capture the spikes.

  • Using a programmable power supply and measuring the current and voltage (at the load) would give the most accurate measure of applied power but would not achieve your requirement for high harmonic content. Measuring the (IV) power coming out of the power supply would give a reasonable measure of the power supplied to the pulse circuit. However, if you are pulsing the load current you will probably have some unexpected loss of power due to the ripple current flowing in the output capacitors on the power supply due to their ESR.

  • I use a home brew 4 channel power supply- up to 2.5kW at 50V max. 3 channels have variable voltage current-limiting PWMs, which read-outs that give output (conveniently) in watts as well as VA. The 4th channel is a fixed 9V output dedicated to power my data-logging stuff.

    The pictures show 2 channels feeding into H-Bridges which create square wave AC over a huge range of frequencies. The square waves go in turn to supply the heater coils in two closely matched reactors.

    All good fun, even better than watching football.