Re: Voltage Regulators or Diodes to drop voltage?

Hi Simon,
I most definitly agree with Hans and Tim. Exceeding the MAXIMUM
voltage on many semiconductors can immediately destroy the part (over
current is sometimes allowed as long as is not too much or for too
long). I read (somewhere) that the highest safe operating voltage
for parts such as the A3977SED (used on my board), is 90% of maximum
(MAX=35V) which is just about 31 Volts. This gives a little extra
room for BEMF created by the motor during deceleration.
A simple (but perhaps expensive) solution would be to put a
variable transformer between the mains and your present transformer
so you can limit the voltage to a safe number.
If you do find a way to bring your present supply down a couple
of volts (at or just below 35V) I highly recomend a regen circuit to
dump any extra voltage that may be created by BEMF of the motor
during deceleration. Mariss of www.geckodrive.com has posted an
example of such a circuit at:

http://groups.yahoo.com/group/CAD_CAM_EDM_DRO/files/CIRCUITS/RETURNED%
20ENERGY%20DUMP.pdf

(make both lines all one long line)

Being on the safe side now will help to keep the circuitry safer.
The cost of repair can exceed the precautionary safety
circuits/variable transformer, when you add in downtime while waiting
for replacement parts.

Jeff
http://www.xylotex.com


--- In CAD_CAM_EDM_DRO@yahoogroups.com, "abbylynx <abbylynx@y...>"
<abbylynx@y...> wrote:

> Hi again,
>
> Well, my linear power supply is working well. Too well in fact..
> after the bridge recifier and capacitor, it's pumping out 38 volts.
> This would be fine, only that the maximum voltage limit on my

driver

> board is 35V. Aaah!
>
> Anyway, I need to drop off about 4V or so, just to be safe. Any
> advice? I know I could just open the transformer and start taking
> windings of the secondary, but (A) it's tedious as hell, (B) it's
> probably defeats the isolation (my duct-taped result would

certainly

> not be as isolated as the original) and (C) ... it's just plain not

a

> good idea.
>
> So I was wondering about other avenues? I'd heard someone talking
> about using Diodes to drop the voltage on the line? What diodes do
> you use for this and how many? (I can no longer find the message)..

I

> have some diodes around, but they're all meek li'l 1AMP ones,

they'd

> certainly become a small short-lived heating element on the 7AMP
> power supply.
>
> Also, what about voltage regulators? I know they don't make

anything

> above the 7805 and 7812 (no 7824).. but I know there are variable
> ones available where you can set the output voltage with a

resistor.

> Again, looking on the web I've only been able to find some meek

li'l

> 1.5ampers - which'll no doubt be carbon-toasties(tm) by the time my
> steppers have vamped it. A voltage regulator would seem good -

since

> it might just save the driver board in the event of the AC power
> supply jumping up a few volts in a surge etc.
>
> Any advice welcome!
> Thanks. :)