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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