How does this idea sound for a simple b+ delay with a soft start.

IXYS makes a mosfet rated for 1200vdc at 3a (IXTP3N120). Use a large
value resistor to charge a cap of maybe 47uf at the gate. The slow
voltage ramp should control how the fet conducts. Put this in the
negative line of the b+. If a delay is needed then use a monostable 555
timer to control the fet. Any good reasons not to try this?


Adam

On Tue, 06 Sep 2005 01:03:48 GMT, Adam Stouffer
wrote:

How does this idea sound for a simple b+ delay with a soft start.

IXYS makes a mosfet rated for 1200vdc at 3a (IXTP3N120). Use a large
value resistor to charge a cap of maybe 47uf at the gate. The slow
voltage ramp should control how the fet conducts. Put this in the
negative line of the b+. If a delay is needed then use a monostable 555
timer to control the fet. Any good reasons not to try this?

Sounds good. You might want to include a mechanism
to make the ramp restart at any power interruption.
Short (time) interruptions can be deadly to semi's.

'Course, your power supply caps will still take the
inrush beating. Maybe not a whole lot more circuitry
to ramp them up instead?

Good fortune,

Chris Hornbeck

Chris Hornbeck wrote:

Sounds good. You might want to include a mechanism
to make the ramp restart at any power interruption.
Short (time) interruptions can be deadly to semi's.


Yeah at $4 each those fets can add up. This looks like a promising idea
for a delay. The author claims instant off.

http://www.diyaudio.com/forums/showt...914#post689914




'Course, your power supply caps will still take the
inrush beating. Maybe not a whole lot more circuitry
to ramp them up instead?

I was thinking of putting this before the filter caps for that very
reason. Although the ripple will be pretty high at this point. Not sure
what that will do to a FET.


Adam

On Tue, 06 Sep 2005 03:53:54 GMT, Adam Stouffer
wrote:

'Course, your power supply caps will still take the
inrush beating. Maybe not a whole lot more circuitry
to ramp them up instead?

I was thinking of putting this before the filter caps for that very
reason. Although the ripple will be pretty high at this point. Not sure
what that will do to a FET.

If the tubes are cold, the FET will only have to
deal with current to charge the capacitors. This
is the sum as the *peaks* of both polarities of
rectifiers, so not trivial. And, of course, the
FET must never be reverse-polarized, an issue
on shut-down.

But if the tubes are warm, the FET will also have
to dissipate significant power during ramp-up.
Do-able, but you may break some eggs making this omelet.

Big fun,

Chris Hornbeck

"Adam Stouffer"
How does this idea sound for a simple b+ delay with a soft start.


** STUPID.

IXYS makes a mosfet rated for 1200vdc at 3a (IXTP3N120). Use a large value
resistor to charge a cap of maybe 47uf at the gate. The slow voltage ramp
should control how the fet conducts.


** Nonsesne.

The MOSFET will conduct quite hard as soon as the gate threshold voltage is
reached - at around 4 to 6 volts.

Go find the full data and see what he turn on curve is like.

Also, dissipation during turn on could exceed heatsinking capacity and blow
the fert.




........... Phil

I've been doing this for years. Examples may be found on my site
www.triodeguy.com
(There is a guide to where various PSU topologies are used at the end of the
power supply article.)
I use the mosfet after the reservior cap. I suspect that if no reservior
cap is present before the fet, it (the fet) may be susceptible to transient
failure due to inrush but I have not confirmed this. (If you try to use the
fet before the cap, remember you will need a higher voltage source to bias
the zener. This can be done using a single diode such as a UF4007 or better
from the rectifier into a smallish cap to form a peak capture circuit. The
cap only need be small since the zener bias current will only be around 1mA
or so.) Also remember that the output voltage will be lower than you will
get from a capacitor inpit arrangement since the fet will truncate the peak
voltage.
The fet will offer up to 60dB of ripple reduction. The ripple filtering is
non reactive so the ripple current will cause heating, this should be
accounted for in sizing the heatsink as well as the DC level drop across the
fet. Try to ensure that the negative troughs in the ripple are above the
output voltage so that troughs do not appear in the output current. I bias
a zener from the input side using a resistor value for about 1mA, the zener
is coupled to the gate (don't forget a gate stopper, 100ohms or larger)
using 4.7meg charging resistor into a film cap. (The cap must be film, the
leakage current of an electrolytic will drop the voltage at the charging
resistor.) 4.7meg and 4.7u will give a charge time constant of 22 seconds,
a nice steady turn-on. The big resistor and low inductance of a smallish
film cap also does a nice job of removing HF hash at the gate which improves
the HF/RF rejection of the mosfet.
Richard.


"Adam Stouffer" wrote in message
news:UV5Te.7681$ia7.1271@trndny08...
How does this idea sound for a simple b+ delay with a soft start.

IXYS makes a mosfet rated for 1200vdc at 3a (IXTP3N120). Use a large value
resistor to charge a cap of maybe 47uf at the gate. The slow voltage ramp
should control how the fet conducts. Put this in the negative line of the
b+. If a delay is needed then use a monostable 555 timer to control the
fet. Any good reasons not to try this?


Adam

"francis"
I've been doing this for years.


** Bull**** - read the OPs post.

He intends the mosfet to be in the ground circuit.



............ Phil

On Thu, 8 Sep 2005 13:00:49 +1000, "Phil Allison"
wrote:

** Bull**** - read the OPs post.

He intends the mosfet to be in the ground circuit.

It's a series circuit. What's the beef?

Chris Hornbeck

Hmm. Actually it is not BS but never mind. If you do go to my site, you
will be able to find some examples of the proposed soft-start method that I
HAVE had in use for years. To find these go to the "Tube Audio" section: I
have an article on PSU issues, at the end are directions to where various
psu topologies are to be found on the site. I don't understand what Phil
means by "he inteneds the mosfet to be in the ground circuit." If you do
take a look, you will see that I do not 'intend' any such thing and that the
mosfets actually ARE used as series elements in the + side of the supply.
Peace.
Richard.
www.triodeguy.com


"Phil Allison" wrote in message
...

"francis"
I've been doing this for years.


** Bull**** - read the OPs post.

He intends the mosfet to be in the ground circuit.



........... Phil

"francis"


** Top posting is the unmistable sign of ****ing, nut case, asshole.

" IXYS makes a mosfet rated for 1200vdc at 3a (IXTP3N120). Use a large
value resistor to charge a cap of maybe 47uf at the gate. The slow
voltage ramp should control how the fet conducts. Put this in the
negative line of the b+. If a delay is needed then use a monostable 555
timer to control the fet. Any good reasons not to try this? "


** Read it 100 times if you like.



** Bull**** - read the OPs post.

He intends the mosfet to be in the ground circuit.



........... Phil

Phil Allison wrote:
"francis"


** Top posting is the unmistable sign of ****ing, nut case, asshole.

" IXYS makes a mosfet rated for 1200vdc at 3a (IXTP3N120). Use a large
value resistor to charge a cap of maybe 47uf at the gate. The slow
voltage ramp should control how the fet conducts. Put this in the
negative line of the b+. If a delay is needed then use a monostable 555
timer to control the fet. Any good reasons not to try this? "


** Read it 100 times if you like.


http://www.bittermen.net:808/~tesla/douchebag2.jpg


Adam

Adam Stouffer wrote:
How does this idea sound for a simple b+ delay with a soft start.

IXYS makes a mosfet rated for 1200vdc at 3a (IXTP3N120). Use a large
value resistor to charge a cap of maybe 47uf at the gate. The slow
voltage ramp should control how the fet conducts. Put this in the
negative line of the b+. If a delay is needed then use a monostable 555
timer to control the fet. Any good reasons not to try this?


Adam

Hi Adam,

I've used a 6D22S damper diode in the negative side of the B+. It takes
about 30 seconds to warm up and start conducting, and it has the added
bonus of another glow bottle. I run them from a separate 6.3V winding.

Raymond