"David Harmon" wrote in message
...
On Tue, 27 Dec 2005 05:58:18 GMT in sci.electronics.design,
(John Crighton) wrote,
This link tells you how to use the TL494 IC
http://focus.ti.com/lit/an/slva001d/slva001d.pdf

So why is the guy from TI showing NTE transistors for the power
switching? Nothing in the TIP line good enough?



Probably good enough for his measly 32 volts. I need to modify that
circuit to work on outputs of 90 volts and 6 volts.

Buy a DC-DC converter that includes other protection
features (ie overvoltage and overcurrent protection) and
outputs known parameters (ie ripple voltage). I doubt you
will find a single DC-DC converter for both voltages. But a
small converter for each voltage should work. Many sources
including this one that just arrived on my desk -
www.astrodyne.com .

BTW, power from automotive systems should also worry about
load dump. Technically load dump can be a transient as much
as 270 volts on a 12 volt system. In reality, such events may
only be 50 volts or less. Transients that would not damage
other automotive electronics already designed for this rare
and so destructive event.

That is a problem with some regulators such as 78xx series.
Maximum voltage is typically not sufficient for automotive
purposes - would require additional protection.

"James F. Mayer" wrote:
Probably good enough for his measly 32 volts. I need to
modify that circuit to work on outputs of 90 volts and 6 volts.

On Wed, 28 Dec 2005 02:20:02 -0500, w_tom wrote:

On Wed, 28 Dec 2005 02:20:02 -0500, w_tom top-posted:
....
BTW, power from automotive systems should also worry about
load dump. Technically load dump can be a transient as much
as 270 volts on a 12 volt system. In reality, such events may
only be 50 volts or less. Transients that would not damage
other automotive electronics already designed for this rare
and so destructive event.

That is a problem with some regulators such as 78xx series.
Maximum voltage is typically not sufficient for automotive
purposes - would require additional protection.

This is what transzorbs
http://www.vishay.com/docs/88301/15ke.pdf
and hash chokes are for.

I'm not affiliated with Vishay, formerly General Semiconductor,
just a satisfied customer. I've used them to protect circuits
against the transients caused by a 5 KV arc in an ion gun. :-)

Cheers!
Rich



"James F. Mayer" wrote:
Probably good enough for his measly 32 volts. I need to
modify that circuit to work on outputs of 90 volts and 6 volts.

Actually SGS Thompson that specifically makes protectors for
automobiles and load dump (do what 'too small' transzorb would
also do) acknowledge in their app notes that:
" ... there are several existing products products able to
clamp this overvoltage at the board level, for example the
LDP24 or RBO series. The protection at the alternator level
is a quite new concept and all the technical problems do not
seem to be completely solved."

This would explain why an 'up to 270 volt' transient is not
routinely observed. Still, automotive electronics typically
should withstand about 50 volts ballpark without damage.
Transzorb could be but one part of that protection system. Or
a DC to DC converter that specifically has such protection
could be obtained. A weakness of Transzorbs are their low
power abilities. The advantage - those avalanche diodes can
handle so much more power than conventional zeners. My first
design that used Transzorb was maybe 20 years ago. GS was
selling Transzorbs even long before then.

Load dump does not occur frequently. But that one time can
be so destructive. Best to consider load dump when attaching
any electronics to automotive power because even trivial
protection does so much.

Rich Grise wrote:
This is what transzorbs
http://www.vishay.com/docs/88301/15ke.pdf
and hash chokes are for.

I'm not affiliated with Vishay, formerly General Semiconductor,
just a satisfied customer. I've used them to protect circuits
against the transients caused by a 5 KV arc in an ion gun. :-)