[sdiy] CA3046 Heater / Substrate Question

Roman Sowa modular at go2.pl
Fri May 6 10:44:49 CEST 2016

You say, Q3 base is at -1.8, but with no heating current (at start) Q3 
emmiter is at about -0.7, determined by 2 small resistors and BE 
junction of Q2. So the substrate is higher than Q3 base and all 
transistors are connected now, no substrate isolation.
Loose the LED and it should work, but substrate will be at very low 
negative voltage now, I'm not sure if it will not degrade expo pair 
BTW, "better" is an enemy of "good". What is wrong with just 1 
transistor heater?


W dniu 2016-05-05 o 19:46, Nils Pipenbrinck pisze:
> Hi folks!
> I'm working on my expo converter again. Did some traces of the Moog
> Prodigy Heater circuit. It works as it should, but long story short I
> want to heat with two transistors instead of just one.
> Here is what I came up with, and I can't find out why it doesn't work.
>    http://torus.untergrund.net/synth/heater.png
> My part is in yellow.
> So the circuit around Q1 and Q2 is directly from the Prodigy. R3 has
> been raised because I'm using dual 15V.
> In my part I generate a small negative voltage using the LED and R5.
> This puts the base of Q3 at -1.8V and the emitter at roughly -2.4V.
> That's the most negative point in my circuit.
> R6 allows some bias current to flow at any time and R4 limits the total
> power to about 280mW.
> That should be fine, right?
> The problem:
> ============
> If I apply power the measurement I get from my temperature BE junction
> (not shown) directly jumps to about 500mV and the heat cycle never
> starts. I have doubts that this measurement is correct. Since I've
> messed around with the substrate connection I think this could be the
> problem, but I have no idea how to debug this.
> If I just remove R4 everything starts to works as expected, albeit with
> just one transistor heating.
> So, do I have to maintain some minimum impedance at the substrate pin?
> Do you have another idea what could be the problem?
> Best,
>    Nils
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