Olympus BH-2 BHT Manual - Halaman 4

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Resistor R201, the current‐sensing resistor, is effectively in series with the halogen lamp. So long as the lamp current

remains within the normal range, the resulting voltage drop across resistor R201 is insufficient to initiate conduction of

transistor Q202, and the dimmer operates as described in the paragraphs above. However, if for any reason the lamp

current gets high enough that the voltage drop across resistor R201 is sufficient to turn on transistor Q202, the resulting

collector current of transistor Q202 shunts the base‐emitter junction of transistor Q201, reducing its base current and in

turn reducing the lamp current. This negative‐feedback mechanism limits the peak lamp current to approximately 7.3A.

Capacitor C202 provides low‐pass filtering for the base of transistor Q202, preventing any high frequency noise or RF

from affecting the lamp dimmer.

Figure

21,

Figure

22, and

Figure

23 show the voltage and current waveforms of the halogen lamp with the dimmer

operating at the 100%, 50%, and 20% settings of the intensity control, respectively.

Figure

24 shows the dimmer

operating under current‐limiting conditions at the 80% setting of the intensity control and with an abnormally low lamp

resistance of 0.2 Ω.

LED Bar‐Graph Display Circuitry

Figure

3 is a simplified schematic diagram of the LED bar‐graph display circuitry. This diagram contains the details

necessary for an understanding of the basic operation of the LED circuitry. Unnecessary details such as such as

switching, fusing, and electrical interconnects have been omitted for clarity. Refer to Appendix 2 and Appendix 3 of this

document for complete and detailed schematic diagrams of the BHT/BHTU electronics.

This circuit successively illuminates the four LEDs in the bar‐graph module as the intensity control is moved from its

minimum setting to its maximum setting. The variable output from the wiper of potentiometer RV101 acts as the

control signal for the LED display circuitry (as well as for the dimmer circuit described earlier), which feeds the non‐

inverting inputs of four op‐amps used as comparators. The control signal is at maximum amplitude at the minimum

intensity setting, and is at zero amplitude at the maximum intensity setting. The four comparators each have specific

switching thresholds, which are configured to allow the comparators to illuminate the LEDs at four pre‐defined lamp

voltages of 2V, 4V, 6V, and 7V, in response to the decreasing control signal from the intensity potentiometer.

Figure 3 – BHT/BHTU LED Bar‐Graph Display Circuitry

Olympus BH‐2 (BHT/BHTU) Electronics

Revision 2

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