[sdiy] DCO ramp core / capacitor discharge transistors

[harrybissell at wowway.com]

The transistor will eventually be limited by the voltage drop
across it, and the FET will limit by on resistance. Eventually
the FET will cause a complete discharge. Either one will work...
analog switches are another possible choice.

I'd use the NPN if it will work, its cheap and probably less fussy than the FET.

H^) harry




On Tue, 16 Dec 2008 22:00:47 +0000, Tom Wiltshire wrote
> Hi All,
> 
> I'm having one of my periodic "Should I build a DCO?" moments...
> 
> Consequently, I've been looking at the Electronotes/ASM ramp core,  
> and various schematics for DCOs, including the JX8P.
> 
> I'd have thought that since the ramp core of both of these circuits  
> is identical, you'd see similar components, but you don't. The ASM  
> VCO uses a FET chosen for low on resistance (2N4856 seems favourite) 
>  whereas the JX8P uses a normal NPN, the 2SC945. The JX8P also uses  
> half the capacitor value (1n versus 2n2) so probably doesn't need  
> such a low on resistance.
> 
> Can anyone tell me why we need to use a FET for the VCO, if the DCO  
> can get by with a NPN?
> 
> Thanks,
> Tom
> 
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Harry Bissell & Nora Abdullah 4eva