On Fri, 6 Feb 1998 13:47:24 +0200 Pasi T Mustalahti writes: >On Thu, 5 Feb 1998, Robert Nansel wrote: > >> On 2/2/98 Pasi T Mustalahti wrote: >> >> -------8<-------- >> > >> > ------------------------------------- +U (0.6..1.55 V) >> > | | >> > R20K || < >> > | || < >> > ----------------> || < >> > | > || < >> > = 10nF | /------>|--| >> > | ----| BC337 | >> > | \ | >> > | | | >> > -------------------------------------- >> > >I connected it to a AA-cell with 1.452 V in it at about 18:00 last >friday >and left it there. This morning there was 0.73 V and the LED was just >as >bright as in the begining. So it has lasted nov 7 days (minus 4 hours) >and >still going strong. > >The LED is a 3500 mcd device and you can read A4 in its light. >It costs about nothing, so you can make some tries. I'm interested if >you >find some optimisation to this. Or a SIMPLE method to make a current >limiter without spending current. The circuit is best described as a self-oscillating flyback converter. Each time the transistor turns on, it charges the transformer with current until it saturates. When it saturates, the voltage induced in the base winding decreases, causing the transistor to turn off. The energy stored in the transformer is then dumped to the load (LED in this case). For a given frequency of operation and transformer core, the circuit will deliver approximately constant *power* to the load, regardless of the load voltage. The power is distributed in pulses having the energy that the transformer core can hold before it saturates. Since it always charges until it saturates, the output power doesn't depend on the input voltage, if (and this is a big "if"), the frequency doesn't vary. If the DC supplied to the transistor base circuit is adequate, the circuit will oscillate continuously at the highes practical frequency (depends on the input voltage and the inductance of the winding). More likely, Pasi's circuit is running in "relaxation mode". The base current required by each cycle discharges the capacitor somewhat, to less than Vbe so the transistor doesn't turn on right away after the transformer voltage reaches zero (all energy having been delivered to the LED). There is a delay during which the resistor has to charge up the capacitor to start the transistor conducting again. So the frequency probably decreases significantly with input voltage as the current available thru the resistor decreases. Varying the resistor should vary the frequency, and thus the brightness of the LED. A diode in parallel with the transistor base (to keep it from going too far negative) would supply current to the capacitor while the transistor is off, and probably get the circuit to oscillate continuously rather than in relaxation mode. However, the resulting output power would likely be too high, and difficult to control. However, the experiment shows that it had satisfactory performance over the life of the battery. The self-regulating properties of the circuit are apparently working well enough. _____________________________________________________________________ You don't need to buy Internet access to use free Internet e-mail. Get completely free e-mail from Juno at http://www.juno.com Or call Juno at (800) 654-JUNO [654-5866]