Nuts and Volts - October 2010

QUESTIONS & ANSWERS

when I tried to use it for a passive circuit, I got an error message: No IC in the circuit. I did a Google search and found 5spice which looks to be easy to use but costs $199. There is a free demo but it is not useful. I also found Opus spice which is free but I have not evaluated it or figured out how to install it.

LTspice has made some modifications that minimize “time step too small” errors in switching circuits so it is good for those types of circuits. In any simulator, the results are only as good as the model, but the more complex the model, the more likely that the program will fail. Always use the simplest model that will give the results you need and keep in mind that if you use an IC in an atypical way, it may not work because of assumptions that the model designer made.

Purchased heat sources are mega bucks and I’d like to do this cheaper. Would you please help me?

— Mark Hoffman

be much less than that if it is well insulated. I will use the step down circuit of the datasheet and calculate the parameters using the Design Formulas from the datasheet.

ASince the 78S40 has been doing the job, we may as well continue with that. I don’t know how much power is required but with 12 volts and 9. 5 ohms, it can’t be much more than 10 watts. I expect the power to Vo=10V (that’s the most we can get with 12V supply) Vd = 1.5V (diode drop at 1 amp) Vi=12V (battery) Vsat=1.3V (transistor drop at 1 amp)

Ton/Toff = 12 (from datasheet formula)

Set Fmin to 20Khz (so you can’t hear it)

BLACK BODY HEAT SOURCE

QI need to calibrate some infrared thermometers. I hacked an old black body heat source, modifying the well to accommodate the IR thermometers and an NIST traceable RTD thermometer. My issue is that the source only produces a single (100 deg C) temperature. My goal is to be able to construct a stable multiple point heat source (e.g., 50, 100, 150 deg C) or a variable source of the same range.

Here are some particulars about the old black body heat source, some of which I want to reuse: The heat is provided by a coil of resistive wire which measures 9. 5 ohms at 75 deg F, which is wound around a copper bobbin with a well painted flat black. It is powered by a 12V 7 AH SLA battery (nice because it makes it quite portable) and it uses a temperature sensor which measures ~800 ohms at 75 deg F, and uses a uA78S40PC IC to do the control.