Bob: Barry is trying to say that a "dummy" power load is only resistive at the power frequency, and may not truly represent the EUT at all of the other frequencies within the test range. He's right.
Bob suggests a powerline filter be added to my El Cheapo lightbulb load bank to more closely approximate an EUT. He's right, too. My bank of lightbulbs is versatile, but it IS spread out over a few square feet. There are stray lead inductances and capacitances, and the lamp filaments are also inductors. At a low frequency starting point of 10KHz (or 30Hz for me), this is negligible. But yes, as we go up to 10MHz or 30MHz, these undefined and uncontrolled variables will start causing some weird effects. A better load bank could be constructed of non-inductive bulk silicon carbide resistors, but these are expensive ($150 each) and not as versatile. (OTOH, I have NEVER had one burn out during a test!) Practically, I assume that the load is mostly resistive, having had the experience of using ordinary light bulbs on the end of a coax cable for loading amateur transmitters in the HF and VHF regions. Lamp filaments do a good job of converting RF to heat and light, so I trust that they retain a considerable resistive component of the impedance at any of these frequencies! But, I don't think we need to know what the impedance curve of my loadbank is, because there is no standard to compare it against. What we are groping toward here is that there seems to be a need to define a standard "dummy" power load. This load would have an impedance similar to the typical EUT. Placing this dummy load into the test setup would then allow test conductors to verify the quality of the supplied power and the sensitivity of the measurement equipment. So, if there is a need to have a defined dummy load impedance, then we'll have to start with a survey of typical EUT's to find the value and variation of the impedance. Maybe we'll need several categories of dummy loads (appliance, computer, entertainment)? But, as far as I'm concerned, I still have a few cases of light bulbs to use up first! ;-) Ed ------------------------ From: Robert Macy <[email protected]> Subject: Re: Shield Room Grounding Date: Fri, 9 Oct 1998 16:27:11 -0700 (PDT) To: [email protected], "Bailin Ma"@unspecified-domain Cc: [email protected] > > Lightbulbs make great AC loads, but use a line filter in series with them > and you'll probably duplicate most EUT's since they'll be using filters. > > - Robert - > > > On Fri, 9 Oct 1998 [email protected] wrote: > > > Hi Ed, > > > > I appreciate your kindness to share your experience with us about > > constructing equivalent load: "I constructed a load bank consisting of 16 > > surface mount light bulb sockets, all wired in parallel. I just screw in an > > array of 25/60/75/100 Watt rated lamps until I get the necessary current. > > Sure, there's some unknown slight lead inductance and capacitance. But all > > I want to do is draw a few amps DC" > > > > I have two questions: (1)What is the impedance of your bulb array at 30 > > MHz? i.e., Zb=? @30 MHz. (2)What is the impedance of EUT at 30 MHz? > > i.e., Ze=? @30 MHz (DETAILS SNIPPED) > > Suggestion: We might need to check the equivalence of Zb and Ze @30 MHz by > > using an Impedance Analyzer, e. g., HP4191A(?). > > > > > > Thank you. > > Please correct me. > > Best Regards, > > Barry Ma > -------------------------- Ed Price [email protected] Electromagnetic Compatibility Lab Cubic Defense Systems San Diego, CA. USA 619-505-2780 List-Post: [email protected] Date: 10/12/1998 Time: 10:47:22 -------------------------- --------- This message is coming from the emc-pstc discussion list. To cancel your subscription, send mail to [email protected] with the single line: "unsubscribe emc-pstc" (without the quotes). For help, send mail to [email protected], [email protected], [email protected], or [email protected] (the list administrators).
