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Yesterday, 02:49 PM
Hi Steve
With PS set for maximum output voltage there should only be about 4V drop across the mosfet and the waste heat will be pretty low. For example, 50mA load corresponds to only 200mW of heat and a very low temperature rise.
As the threads about potentiometer tolerances, PS sweep and the compromises that result regarding possible dead spots of sweep, the issue comes down to the pot itself and the circuit voltage. The 1M pot can be 1M2 down to 800k at its 20% tolerance extremes. In spice, a 900k pot gives the nearly expected sweep in the SV2-M36 where R23=36k, R5=332k, R3=100k and D5=4v7. R3 and D5 do not effect the overall sweep, there role is to set a minimum output voltage. "Nearly" means that the high-end voltage drop is closer to 11V rather than 4V.
R5 is a power resistor in the physical circuit with a value of 330k; at 5% it can be 346k5 to 313k5. R23=36k is 1%.
With the 900k pot and R5=330k, changing R23 from 36k to 38k gets us to a 4V drop but a short dead spot of sweep covering about 20k (22% of the total pot resistance). In the simulation there is a "sweet spot" value between 36k and 37k5 where the dead sweep is reduced to zero.
If the pot is 800k, R23=37k5 works well. Most of the 1Ms I have measure around 900k, with very few lower than that.
For a 1M2 pot and stock value of 36k, maximum Vout has a 6V drop.
Back to R23=36k and a 900k pot: If R5=313k Vout drops 4V and there is a small dead sweep; R5=346k Vout loses 23V, which can be reduced to 4V with R23=39k.
To summarise the tests above, we see that the two loose-tolerance components in the divider dominate the initial behaviour. Fortunately their values are stable over time and changing the tight-tolerance R23 can produce pleasing results.
With PS set for maximum output voltage there should only be about 4V drop across the mosfet and the waste heat will be pretty low. For example, 50mA load corresponds to only 200mW of heat and a very low temperature rise.
As the threads about potentiometer tolerances, PS sweep and the compromises that result regarding possible dead spots of sweep, the issue comes down to the pot itself and the circuit voltage. The 1M pot can be 1M2 down to 800k at its 20% tolerance extremes. In spice, a 900k pot gives the nearly expected sweep in the SV2-M36 where R23=36k, R5=332k, R3=100k and D5=4v7. R3 and D5 do not effect the overall sweep, there role is to set a minimum output voltage. "Nearly" means that the high-end voltage drop is closer to 11V rather than 4V.
R5 is a power resistor in the physical circuit with a value of 330k; at 5% it can be 346k5 to 313k5. R23=36k is 1%.
With the 900k pot and R5=330k, changing R23 from 36k to 38k gets us to a 4V drop but a short dead spot of sweep covering about 20k (22% of the total pot resistance). In the simulation there is a "sweet spot" value between 36k and 37k5 where the dead sweep is reduced to zero.
If the pot is 800k, R23=37k5 works well. Most of the 1Ms I have measure around 900k, with very few lower than that.
For a 1M2 pot and stock value of 36k, maximum Vout has a 6V drop.
Back to R23=36k and a 900k pot: If R5=313k Vout drops 4V and there is a small dead sweep; R5=346k Vout loses 23V, which can be reduced to 4V with R23=39k.
To summarise the tests above, we see that the two loose-tolerance components in the divider dominate the initial behaviour. Fortunately their values are stable over time and changing the tight-tolerance R23 can produce pleasing results.
