On Sun, 7 Oct 2007 09:25:34 -0700 Jim Cathey <[EMAIL PROTECTED]> wrote:
> The value of D9, assuming it was a glass Zener diode, is a bit of a > problem. Based on the clamping voltage of the MOV (82V), and the > maximum working voltage of C1 (25V), R2 must have at least 30 mA > through it for C1 to be protected at clamp time. That would mean that > R5 would be dropping 12 V, leaving another 12 V at D9's cathode. D9 > would have to be conducting by then, else C1 would be over-voltaged. > So it's at most a 12 V Zener, and can't be a glass Diac (which has a > breakover voltage higher than that). What is the power rating of R2? 30 mA through 2 k is 1.8 watts. Likewise, what is the rating of R1? What if the MOV is intended only to clip transients and spikes? The time constant of R2 and C1 is 0.44 s, which is a long time for transients and spikes. It also gives a pole at 0.36 Hz for the feedback loop (with the yellow winding is the feedback sense winding). > If D9 was a forward-biased signal diode R3 must have approximately 0.5 > mA through it before Q3 can start conducting, that would pin D9's > _anode_ voltage (the cathode in the schematic) at around 1 V when > regulation began. R4/R1's current would be approximately 0.7 mA, so > R5's current would be 1.2 mA. That would make the voltage on C1 > approximately 1.5 V, which seems too low. Not to mention that the temperature coefficient of a signal diode would make for lousy regulation. Note also that the R1 + R4 combination can vary 3:1 in resistance, a rather wide range for adjustment. > From this I conclude that D9 _is_ a Zener diode, of less than 12 V in > value. Zener diodes start at around 1.8 V in value, but I think I'll > first try something like a 6 V rating. The 1N6309-20 diodes are glass, > at 2.4-6.8 V in rating. 1N6319 is a 6.2 V glass Zener. 1N746A-59A are > glass, at 3.3-12 V in rating. 1N753A is also a 6.2 V glass Zener. > There are plenty of candidates to try. Indeed there are. 6.2 V is a good starting place and is near the zero temperature coefficient point of the series of diodes. I still wonder about Q6 and D11. If you're conducting through D11, Q6 has to be off. If Q6 is on, D11 cannot be conducting. In this case the stored energy in the red-green winding has to forward-bias the body diode of Q6. BTW, do Q1 and Q2 have any heat sinking? I certainly hope so, particularly if they're run in the linear region. Craig _______________________________________ http://www.okiebenz.com For new parts see official list sponsor: http://www.buymbparts.com/ For used parts email [EMAIL PROTECTED] To Unsubscribe or change delivery options go to: http://okiebenz.com/mailman/listinfo/mercedes_okiebenz.com
