RE: [CAD_CAM_EDM_DRO] Dropping a few volts ?

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@...]
> 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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