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In Reply to: Wow, you copped a hiding on this one ... posted by Naz on May 7, 2007 at 07:26:30:
. . . I don't really see it like that. It seems more to me like a bunch of concerned and experienced people sharing their knowledge, in the hope of preventing someone from being misled into making a regrettable mistake. Actually, I wasn't really supporting the idea, just interested in it, since it came from a 'usually reliable source'. It sure led to a lot of responses, though, so that's good!I take it that your suggestion to ramp up the voltage slowly would not be satisfied by an RC time constant on the gate, to allow it to reach the reference voltage from the zeners slowly? That may explain why MOFET serial regulators usually have a series resistor of ~100 ohms, and why Satoru-san uses a delay relay to bypas a resistor in series between the bridge and the smoothing cap.
Follow Ups:
> > I take it that your suggestion to ramp up the voltage slowly would not be satisfied by an RC time constant on the gate, to allow it to reach the reference voltage from the zeners slowly? < <This can be done but it means using a large C in order to achieve a high enough time constant with a low enough value of R. The time constant needs only to be large enough to limit inrush current caused by having to charge the OP Caps.
> > That may explain why MOFET serial regulators usually have a series resistor of ~100 ohms> >
The series (100R) that people hang off the gate is to prevent HF oscillation ... similar to a grid stopper. It's also important to place a zener from S to D to limit potential over-voltage to a safe level.
BTW, there was a comment made by JS about the voltage rating of the Mosfet. If the INPUT voltage is ramped up slower than the regulator's ability to charge the output C then barring a short or sudden instant VERY heavy load on the PS the voltage rating needn't be all that high because the differential across any of its terminals will never be greater than the input V minus the OP V at any given moment. Staging the input voltage is also a reasonable technique here.
"The series (100R) that people hang off the gate is to prevent HF oscillation"
I know, but I didn't mean in series with the gate, I meant in series with the source. I've sometimes seen it described as a 'safety resistor'. I suppose it's to limit the current if the output is short-circuited (as it would be, for a short time after switching on, when the following reservoir cap has to be charged up).The time constant I had in mind was a 2.2Meg resistor, from the top of the zener (or VR tube, if you prefer) stack to the gate stopper, with a 2uF cap from the gate stopper to ground. Since the MOSFET has such a high gate impedance, a very high value of resistance can be used if required. This would cause the gate voltage to rise slowly, say about 10 seconds from 0v to the reference voltage.
Sorry about the misinterpretation but I've never seen a 100R used in series with the source. As to its purpose I'm sure it's as you suggest but unless it's being used in a low current application I think it would be detrimental to sound.
> > The time constant I had in mind was a 2.2Meg resistor, from the top of the zener (or VR tube, if you prefer) stack to the gate stopper, with a 2uF cap from the gate stopper to ground. Since the MOSFET has such a high gate impedance, a very high value of resistance can be used if required. This would cause the gate voltage to rise slowly, say about 10 seconds from 0v to the reference voltage. < <The time constant is OK and even lower will do but IME the high value R can affect the HF response if a good quality cap is not used.
A resistor after the source would negate the regulation. Put it before the drain, though, and it has no effect on reg'n provided that it's not too big.
I first tried this when I added a series reg on my PP ref 600s. I kept blowing up huge Mosfets and even Bipolars, so I thought I'd try a resistor in series with the drain. It absolutely annihilated Dynamics and the amp sounded dead ... unlistenable (I know that’s not a word)! Removed the R and the whole thing came back to life! It obviously just slows things down too much.
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