Nuts and Volts - January 2009

■ FIGURE 6

current, say 10 amps into the device being measured and then measuring the voltage drop across the device in question. Is this possible and if so, can you come up with any circuit suggestions to accomplish this?

— Jeff

AIn the circuit (Figure 5), I chose a supply voltage of five volts to keep the power dissipation in Q1 low and still allow measurement of up to 0.1 ohms. To measure up to one ohm, reduce the current to one amp by increasing R1, to one ohm. There is a small error due to the base current of Q1 flowing through R1, but this is less than 1% in the worst case and less than 0.1% typically. I would glue the diodes to the transistor case so they are at the same temperature; wait for thermal stability before taking a measurement.

The diodes will not exactly match the Vbe of the transistor, so you will need to adjust R4 for exactly one volt across R1. The transistor is in a TO- 3 case and will need a good heatsink. Wakefield part number 423-A would be a good choice but I don't know who stocks it. Mouser has 567-423-K which doesn’t have holes, so you will have to drill your own.

FM SHUTDOWN PROBLEM

QFM translators consisting of a receiver and a transmitter on another frequency still in the FM band require a transmitter shutdown if the signal from the receiver fails.

28 January 2009

The problem is that when the signal fades or drops out below the receiver capability, the discriminator output turns to random noise. It seems that a variation of the night light control (Oct 08; page 23) would do the job if I can get a signal from the AGC or some other signal from the FM receiver. The transmitter load is less than 2.0 amps at 120 VAC.

— Bob Ziller

AI don't see how the night light control circuit applies, and since an FM receiver does not have AGC, we have to look elsewhere. If you have a decent signal-to-noise ratio ( 6 dB or

better), you can tap off some signal from an intermediate IF stage, probably just before the limiter. The circuit of (Figure 6) shows how this could work: The capacitor, C1, "remembers" the signal level, so when the signal drops out the switch is flipped. Since FM can still work when the signal is down at the noise level, the only alternative is to insert a pilot tone that can be detected and used to turn the transmitter off if the tone is not present.

The LM567 phase lock loop was made for this application. (Figure 7) is similar to a circuit I built some years ago to replace a tuning fork filter that was used in a business band radio for squelch control. The signal was 7 Hz and low level so it was not audible, but it was enough to be detected. The oscillator would be placed in the originating transmitter and added to the voice at a low level.