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HLMP-1700 low current 3 mm red LED [message #87745] Sat, 07 April 2018 16:31
FL152 is currently offline  FL152
Messages: 29
Registered: April 2011
Many tubes work with relative low current (0,5 ... 2 mA) and have relatively low Ugk voltage („bias"), say -1 ... -2 V. Examples are 12AX7 and 6SL7 „families" of tubes.
HLMP-1700 Φ 3 mm red LED is redily available solution for low current applications (data: 1,7 V / 2 mA). I put one „specimen" on test. The aim was finding (un)suitability for cathode bias purpose. Graphical „tangent line" way is used to determine dynamic resistance rD .

Figure 1: Forward voltage Uf against applied current If for HLMP-1700 3 mm low current red LED

Figure 2: Dynamic resistance rD vs. applied current If for HLMP-1700 3 mm low current red LED

As can be seen from the graph, dynamic resistance rD has values in tens of Ohms, especially for the currents less then 1 mA. For 1 mA it is (disappointing) about 47 Ω. But to put things in perspective, 12AX7 with OP Ia = 1 mA / Ugk = - 1,67 V would have Rk = 1,67 kΩ, and (Rk fully bypassed with Ck) anode resistance of over 60 kΩ. Additional (μ + 1) x rD < 5 kΩ can be acceptable.
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