RE: [CAD_CAM_EDM_DRO] Re: Dropping a few volts ?

I misread the intent, sorry.

I thought that the desire was to drop the supply permanently to keep
it under the voltage limit.

Mosfets are nice (with their near zero static gate current) but there
is a wide variation in Vgs_threshold which means that the "regulated"
output level will be device (as well as load) dependent (for example,
the MTP10N10E has 2.0V <= Vgs_threshold <= 4.5V). I've therefore only
used them in closed-loop regulator applications.

I'd prefer a power darlington NPN for an open-loop application (or I'd
design a proper closed loop regulator).

JohnH

> -----Original Message-----
> From: turbulatordude [mailto:davemucha@...]
> Sent: Sunday, 28 September 2003 11:02 PM
> To: CAD_CAM_EDM_DRO@yahoogroups.com
> Subject: [CAD_CAM_EDM_DRO] Re: Dropping a few volts ?
>
>
> Hi John,
>
> Using a few diodes will offer a voltage drop, but it will be
> constantly in the loop so the voltage will always drop a few volts
> regardless of the actual voltage.
>
>
> the Mosfet idea was more as an over protection limit. if the voltage
> were under the zener/mosfet limit, the mosfet would be on constantly
> and not drop anything more than it's resistance value.
>
> only when a rise in voltage occured, the mosfet would start limiting
> the voltage.
>
> Dave
>
>
>
> --- In CAD_CAM_EDM_DRO@yahoogroups.com, "John Haddy" <jhaddy@c...>
> wrote:
> > Hi all,
> >
> > It seems to me that if you only wanted to drop a couple of volts
> then
> > it'd be far simpler to just put 3 diodes in series, rather than hack
> > around with mosfets and their required protection. Just make sure
> > that you choose the diodes to cope with both the maximum current
> > requirement as well as the maximum power. Don't forget to derate
> > if the environment will have an elevated ambient temperature.
> >
> > Whichever method is chosen, just remember that the same power is
> > always being dissipated, just that you're spreading it out across
> > multiple devices, so the system needs to be able to dissipate the
> > load no matter what you choose (e.g. a voltage dropping element like
> > a diode or mosfet, if attached to the same heatsink as the voltage
> > regulator, will result in the heatsink rising to the same
> temperature
> > above ambient as it would have if the regulator was doing all the
> > dissipation by itself).
> >
> > Cheers,
> >
> > John Haddy,
> > Sydney, Australia
> >
> >
> >
> > > -----Original Message-----
> > > From: turbulatordude [mailto:davemucha@j...]
> > > Sent: Saturday, 27 September 2003 2:41 PM
> > > To: CAD_CAM_EDM_DRO@yahoogroups.com
> > > Subject: [CAD_CAM_EDM_DRO] Dropping a few volts ?
> > >
> > >
> > > In the circuits folder, there is a way to drop a high voltage to
> a
> > > voltage regulator.
> > >
> > > /CIRCUITS/HIGH INPUT VOLTAGE REGULATOR.pdf
> > >
> > > it uses a zener diode and a mosfet to drop the voltage to the
> voltage
> > > regulator.
> > >
> > > what if one used a 70 volt zener to a 30 amp mosfet without the
> > > voltage regulator ? would that limit the maximum voltage so we
> could
> > > use a little higher power supply and stay in the safe range of
> the
> > > Gecko ?
> > >
> > > Assuming a 48V transformer, less 1.2 volt for the rectifier and
> then
> > > 1.414 for the DC voltage, one would expect about 68 volts.
> > >
> > > The 70 volt zener would drop to 68V with the 2 volt drop in the
> > > mosfet and therefore the mosfet would add little resistance, but
> the
> > > drop would also not generate significant heat.
> > >
> > > On the surface, it seems that each volt higher would yield one
> watt
> > > per amp of the power supply. I didn't look at the resistance
> thru
> > > the mosfet for the additional voltage drop and the heat
> associated
> > > with no additional drop of the supply voltage.
> > >
> > > so, dropping 2 volts at 20 amps would create 40 watts of heat.
> Not
> > > much considering the benefit.
> > >
> > > Dave
> > >
> > >
> > >
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http://groups.yahoo.com/group/jobshophomeshop I consider this to be a
sister site to the CCED group, as many of the same members are there,
for OT subjects, that are not allowed on the CCED list.

NOTICE: ALL POSTINGS TO THIS GROUP BECOME PUBLIC DOMAIN BY POSTING THEM.
DON'T POST IF YOU CAN NOT ACCEPT THIS.....NO EXCEPTIONS........
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