Departments

March 3, 1997

Signals & Noise

Corrections and updates

In our cover story, "For want of circuit protection devices..." (EDN, Feb 3, 1997, pg 46), we discussed a few references but neglected to list them. Here they are:

"Circuit Breaker Handbook," Bulletin 9001, Eaton Corp.
Lundy, Len, "The fuse-selection checklist: a quick update," EDN, Sept 26, 1996, pg 121.
Daynov, Irina and Barry Breen, "How to choose the correct Accu-Guard fuse for circuit protection," Technical Information Bulletin, AVX Corp.
Valencia, Mauricio, "Self-resetting PTC-polymer devices guard against overcurrent damage," EDN, Jan 18, 1996, pg 131.

EDN regrets omitting these references from the article.

EDN incorrectly listed the telephone number for Aries Electronics in the Nov 21, 1996, issue (pg 49). The correct telephone number is +44 1908-260007. You can also call Aries' Frenchtown, NJ, office at (908) 996-6841. We apologize for the error.

Phone becomes fax

The phone number we printed for Inresco in "For want of circuit-protection devices..." (EDN, Feb 3, 1997, pg 49) is, in fact, the company's fax number. To contact the company, call Inresco at (908) 974-2889; fax (908) 974-2774.

Power-supply-rejection correction

I read with interest "Inherent power-supply-rejection scheme improves sensor-system performance" (EDN, Nov 7, 1996, pg 161) by Eric Jacobsen. In my opinion, the author missed one important aspect of the circuit's operation. If you look at Figure 1 in the article, you can see that the resistance of the supply-voltage divider (R₁/R through 128R) is comparable to R_PU (28 kV). Switching on Q₁ significantly reduces the V_DIV voltage that the MC33072 comparator senses.

The algorithm that Jacobsen proposes searches for the correct division ratio with Q₁ switched on and stops after detecting logic 0 from the comparator. Unfortunately, the same logic 0 switches off Q₁, causing V_DIV to increase above its limit, and the circuit oscillates. If I am right, you can solve the problem by searching from the lowest possible voltage (that is, with Q₁ off) or by buffering V_DIV going to the R_PU resistor.

Piotr Mazur
via the Internet

Author's note: This observation is 100% accurate, as are the corrective-action options discussed. Although the current implementation includes a buffering circuit for R_PU, I noticed immediately after publication that I omitted the buffer circuit (an older design of the circuit was published in the article) for R_PU and V_DIV, and I was unsure how to proceed. I apologize for any inconvenience this may have caused.

As I have done in the current circuit implementation, I recommend buffering R_PU and performing the calibration as described in the original article. Because this method uses a more disciplined engineering approach to the fundamentals of circuit design, I believe this corrective action is better than starting the search for V_DIV from the lowest possible voltage and using the circuit shown in the article.

Again, I am sorry for the inaccuracy. I should have noticed the out-of-date schematic before it was published.

Eric Jacobsen
Motorola Inc
Phoenix, AZ

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