Strange Mosfet Results

[Robert C Monsen]

"Active8" <reply2group@ndbbm.net> wrote in message
news:fu35lkxr0c09.dlg@news.individual.net...

On Thu, 05 Aug 2004 07:53:13 GMT, Robert C Monsen wrote:

"Peter" <pmolsen@one.net.au> wrote in message
news:6ba442b.0408041245.2ddf3995@posting.google.com...
35VAC > Bridge rectifier > Lights > Mosfet > Gnd.

How are you arranging the 5V logic supply? Battery?

Good question. He mentioned that turning on one string causes them
all to turn on under whatever incarnation of the circuit he was
talking about. IIRC it was the one with the gd cap.

Regards,
Bob Monsen
^^^^^^^^^^^^^^^^ add a hyphen or two and readers will clip your
title block for replies.

--
Best Regards,
Mike

The issue I see is that when the mosfet is conducting, the voltage
across it is small. Thus, its not going to be a very good source for
generating 5V. Thus, he needs another source, which could be a
battery, or another bridge. However, if its a bridge, and he connects
the grounds, he is in trouble for obvious reasons. The second bridge
will conduct, causing lots of wierd behavior.

I like PWM for this application, actually, using a triac. The basic
stamp would have to use two pins rather than one, though. One for an
input to determine zero crossing, and one to control the triac.

--
Regards,
Bob Monsen

 

[Active8]

On Thu, 05 Aug 2004 20:44:09 GMT, Robert C Monsen wrote:

"Active8" <reply2group@ndbbm.net> wrote in message
news:fu35lkxr0c09.dlg@news.individual.net...
On Thu, 05 Aug 2004 07:53:13 GMT, Robert C Monsen wrote:

"Peter" <pmolsen@one.net.au> wrote in message
news:6ba442b.0408041245.2ddf3995@posting.google.com...
35VAC > Bridge rectifier > Lights > Mosfet > Gnd.

How are you arranging the 5V logic supply? Battery?

Good question. He mentioned that turning on one string causes them
all to turn on under whatever incarnation of the circuit he was
talking about. IIRC it was the one with the gd cap.

Regards,
Bob Monsen
^^^^^^^^^^^^^^^^ add a hyphen or two and readers will clip your
title block for replies.

--
Best Regards,
Mike

The issue I see is that when the mosfet is conducting, the voltage
across it is small. Thus, its not going to be a very good source for
generating 5V. Thus, he needs another source, which could be a
battery, or another bridge. However, if its a bridge, and he connects
the grounds, he is in trouble for obvious reasons. The second bridge
will conduct, causing lots of wierd behavior.

I like PWM for this application, actually, using a triac.

That would work with an AC source for the lamps, but then it's not
really the pulse width that's being controlled, but the conduction
angle.

I still think a single pwm output feeding a control gate for each
light string is the ticket. OP's already got a 38 VDC supply.

The basic
stamp would have to use two pins rather than one, though. One for an
input to determine zero crossing, and one to control the triac.

You can get zero crossing opto couplers or build it into the triac
circuit. Any significant delay from zero cross to turn on cill
generate EMI.
--
Best Regards,
Mike

 

[Peter]

How are you arranging the 5V logic supply? Battery?

The 5v supply is derived from the 35v AC input using a typical
transistor/zener arrangement. Transistor is MJE3055, zener is 5.1v.
Output sits on 4.8v with almost no ripple on scope.

Rectifier > 1000uF 63V cap to gnd > MJE collector > MJE emitter >
100uF cap to gnd > PIC and 595's.
Zener between MJE base and gnd. 10k resistor between collector and
base.

 

[Active8]

On 5 Aug 2004 15:09:00 -0700, Peter wrote:

How are you arranging the 5V logic supply? Battery?

The 5v supply is derived from the 35v AC input using a typical
transistor/zener arrangement. Transistor is MJE3055, zener is 5.1v.
Output sits on 4.8v with almost no ripple on scope.

Rectifier > 1000uF 63V cap to gnd > MJE collector > MJE emitter
100uF cap to gnd > PIC and 595's.
Zener between MJE base and gnd. 10k resistor between collector and
base.

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created by Andy´s ASCII-Circuit v1.22.310103 Beta www.tech-chat.de

a picture's worth 1000 words. That's hint #2.
--
Best Regards,
Mike

 

[Robert C Monsen]

"Active8" <reply2group@ndbbm.net> wrote in message
news:jp5rs6symuwm$.dlg@news.individual.net...

On Thu, 05 Aug 2004 20:44:09 GMT, Robert C Monsen wrote:

"Active8" <reply2group@ndbbm.net> wrote in message
news:fu35lkxr0c09.dlg@news.individual.net...
On Thu, 05 Aug 2004 07:53:13 GMT, Robert C Monsen wrote:

"Peter" <pmolsen@one.net.au> wrote in message
news:6ba442b.0408041245.2ddf3995@posting.google.com...
35VAC > Bridge rectifier > Lights > Mosfet > Gnd.

How are you arranging the 5V logic supply? Battery?

Good question. He mentioned that turning on one string causes
them
all to turn on under whatever incarnation of the circuit he was
talking about. IIRC it was the one with the gd cap.

Regards,
Bob Monsen
^^^^^^^^^^^^^^^^ add a hyphen or two and readers will clip your
title block for replies.

--
Best Regards,
Mike

The issue I see is that when the mosfet is conducting, the voltage
across it is small. Thus, its not going to be a very good source
for
generating 5V. Thus, he needs another source, which could be a
battery, or another bridge. However, if its a bridge, and he
connects
the grounds, he is in trouble for obvious reasons. The second
bridge
will conduct, causing lots of wierd behavior.

I like PWM for this application, actually, using a triac.

That would work with an AC source for the lamps, but then it's not
really the pulse width that's being controlled, but the conduction
angle.

I'd say he just waits for some time after each zero crossing, and
triggers the triac. By increasing the time after each zero crossing,
he dims the bulbs in a way that doesn't depend on the current or RC
constants. The pulse width of on time for the lamp is being
'modulated' by the PIC

I still think a single pwm output feeding a control gate for each
light string is the ticket. OP's already got a 38 VDC supply.

The basic
stamp would have to use two pins rather than one, though. One for
an
input to determine zero crossing, and one to control the triac.

You can get zero crossing opto couplers or build it into the triac
circuit. Any significant delay from zero cross to turn on cill
generate EMI.

That is true. The MOC3063 does a good job of this. You can then
control the output by keeping it off for more cycles each time.

OTOH, the other scheme is actually pretty easy to do. I once built a
hands off lamp dimmer that controlled the brightness of a lamp that
way using a PIC, a triac, and an IR proximity detector. Move your hand
closer to dim the lamp. Two quick swipes turns it on or off at the
same value it had before.

It used a 4.7MEG resistor from Line to measure the zero crossings.

--
Best Regards,
Mike

 

[colin]

"Peter" <pmolsen@one.net.au> wrote in message
news:6ba442b.0408032148.61766b93@posting.google.com...

Well that had a less than interesting effect!
First I tried the idea of just connecting a capacitor and resistor in
parallel between the gate and ground. Tried a few combinations but the
result was exactly the same as I was experiencing with just a cap on
the gate, namely that increasing the RC did not increase the "fade"
period. It just stretched the fully-on time then it faded off rapidly.
That seems to be indicating that the mosfet is very efficient. ie. it
is fully on over a fairly wide range then rapidly goes to fully off.

I therefore tried the idea of using the so-called Miller effect. ie. I
connected a cap from gate to drain plus the resistor from gate to
ground. Very interesting. The mosfet promptly died (internal short so
always hard on). It had gotten fairly hot so I then installed a new
mosfet with heat sink attached. Same result. Died immediately. It did
not even wait for a signal to the gate and did not bother getting hot.
Just died the moment power was applied to the drain (indicated by
coming hard on even though no gate signal). The cap I used was a 0.1uF
(104) with a 10k resistor from gate to ground.

As I said before the source is at 40v (actually rectified 38vAC). The
max rating specs for the mosfet (2SK2175 - equivalent to STP16NE06)are
as follows:
"Vdgr Drain-Gate voltage (Rgs=20k) 60V
Vds Drain-Source voltage (Vgs=0) 60V
Vgs Gate-Source voltage +/- 15V."

sorry for the delay, im not sure why the capacitor made your mosfet die, but
isnt the mosfet going to be running a bit close to its specs?
if the mosfet is dieng it seems that the mosfet is not capable of diming the
lamps as it gets too hot or exceeds its safe operating area.
what power is it going to disipate when its half on ?

just thought is your rectified 38vac smotthed? if not it might explain the
destruction, you wld need to put in a gate protection diode with anode to
gnd and cathode to gate, u cld use a 12v zener to give even beter
protection, this wld stop the negative transients on the supply cuasing the
gate to be driven far too negative by the capacitance, this might be a good
idea anyway.

the mosfet has an inherent parasitic capacitor from its gate to drain
anyway, but this is usualy in the order of tens or hundreds of picofarads.
usualy its responsible for slow turn off times but this is what u actualy
want anyway.

Colin =^.^=

 

[colin]

"colin" <no.spam.for.me@ntlworld.com> wrote in message
news:XI4Sc.1079$K84.372@newsfe3-gui.ntli.net...

"Peter" <pmolsen@one.net.au> wrote in message
news:6ba442b.0408032148.61766b93@posting.google.com...
Well that had a less than interesting effect!
First I tried the idea of just connecting a capacitor and resistor in
parallel between the gate and ground. Tried a few combinations but the
result was exactly the same as I was experiencing with just a cap on
the gate, namely that increasing the RC did not increase the "fade"
period. It just stretched the fully-on time then it faded off rapidly.
That seems to be indicating that the mosfet is very efficient. ie. it
is fully on over a fairly wide range then rapidly goes to fully off.

I therefore tried the idea of using the so-called Miller effect. ie. I
connected a cap from gate to drain plus the resistor from gate to
ground. Very interesting. The mosfet promptly died (internal short so
always hard on). It had gotten fairly hot so I then installed a new
mosfet with heat sink attached. Same result. Died immediately. It did
not even wait for a signal to the gate and did not bother getting hot.
Just died the moment power was applied to the drain (indicated by
coming hard on even though no gate signal). The cap I used was a 0.1uF
(104) with a 10k resistor from gate to ground.

As I said before the source is at 40v (actually rectified 38vAC). The
max rating specs for the mosfet (2SK2175 - equivalent to STP16NE06)are
as follows:
"Vdgr Drain-Gate voltage (Rgs=20k) 60V
Vds Drain-Source voltage (Vgs=0) 60V
Vgs Gate-Source voltage +/- 15V."

sorry for the delay, im not sure why the capacitor made your mosfet die,
but
isnt the mosfet going to be running a bit close to its specs?
if the mosfet is dieng it seems that the mosfet is not capable of diming
the
lamps as it gets too hot or exceeds its safe operating area.
what power is it going to disipate when its half on ?

just thought is your rectified 38vac smotthed? if not it might explain the
destruction, you wld need to put in a gate protection diode with anode to
gnd and cathode to gate, u cld use a 12v zener to give even beter
protection, this wld stop the negative transients on the supply cuasing
the
gate to be driven far too negative by the capacitance, this might be a
good
idea anyway.

the mosfet has an inherent parasitic capacitor from its gate to drain
anyway, but this is usualy in the order of tens or hundreds of picofarads.
usualy its responsible for slow turn off times but this is what u actualy
want anyway.

Colin =^.^=

actualy if its unsmothed rectified ac then the miller cap wont work anyway.
instead you cld try puting a source resistor so that the curent varies more
slowly with respect to the changing gate voltage. i not read all the other
replies yet maybe youve fixed it already tho ..

Colin =^.^=

 

[colin]

"Peter" <pmolsen@one.net.au> wrote in message
news:6ba442b.0408031248.61a7094b@posting.google.com...

My first attempt was simply connecting a big resistor from gate to
ground with no cap. But that gave almost instant turn-off, even when
using a 10M resistor! That was why I removed it and used a cap
instead. But I think I can now see that if I use a resistor and a
bigger cap then it will ramp the voltage down more slowly.

Colin can you please explain the Miller effect a bit more. And are you
saying a cap from gate to drain (which is sitting at 40V) plus also
the resistor to ground?

the idea of using the miller efect is that the gate voltage varies only very
slightly from full on to full off, and the point at wich it does this can
vary a lot, so any capacitor from gate to ground is going to have much more
efect on turn on time than fade time.

a much smaller capacitor from gate to drain will not have much efect while
the device is fully on or fully off so wil not impose much delay, but as
soon as the the drain voltage rises the capacitor wil charge up and this
charge curent wil rob the gate of its drive stoping it from rising as fast,
thus the rise time of the drain voltage is limited to the suplied gate
current. dv/dt = I/c.

sorry it seems u have problems with it, see my other posts too, this thread
seems to be totaly screwed up on my news server for some reason, 100s of
duplicates that all arive many days late, and all disconected.

Colin =^.^=