discharging capacitor to ground

[Peter]

Hi folks,

I want to discharge a 1uF monolytic capacitor charged at 5V through a 2N3904
(collector to cap, emitter to ground). The transistor driven into saturation
to do that.
It is not an oscillator, the cap is permanently charged through a 1K
resistor to a 5V source.

But should I add a limiting resistor (like 33 ohms) to limit peak current
under 200mA? Or the limiting resistor is not needed because of capacitor
internal resistance or transistor resistance?

ThankS!
-- Peter --

 

[John Larkin]

On Sat, 22 May 2004 12:52:25 -0400, "Peter" <peterm@wanad00.com>
wrote:

But should I add a limiting resistor (like 33 ohms) to limit peak current
under 200mA? Or the limiting resistor is not needed because of capacitor
internal resistance or transistor resistance?

ThankS!
-- Peter --

No need. The energy stored here is far too small to damage the
transistor. Let it discharge as fast as the transistor can yank it
down.

John

 

[Jamie]

yes, limit the current with a resistor.
a cap can really generate some heavy
current in very short periods.
and some transistors can not handle that.
they will actually blow/crack the case sometimes
with hardly no heating due to the sudden current
impact that can cause pressure expansion.



Peter wrote:

Hi folks,

I want to discharge a 1uF monolytic capacitor charged at 5V through a 2N3904
(collector to cap, emitter to ground). The transistor driven into saturation
to do that.
It is not an oscillator, the cap is permanently charged through a 1K
resistor to a 5V source.

But should I add a limiting resistor (like 33 ohms) to limit peak current
under 200mA? Or the limiting resistor is not needed because of capacitor
internal resistance or transistor resistance?

ThankS!
-- Peter --

 

[Peter]

Good! I don't like adding unecessary components, even a single resistor

Thanks!

On Sat, 22 May 2004 12:52:25 -0400, "Peter" <peterm@wanad00.com
wrote:


But should I add a limiting resistor (like 33 ohms) to limit peak current
under 200mA? Or the limiting resistor is not needed because of capacitor
internal resistance or transistor resistance?

ThankS!
-- Peter --



No need. The energy stored here is far too small to damage the
transistor. Let it discharge as fast as the transistor can yank it
down.

John

 

[John Popelish]

Jamie wrote:

yes, limit the current with a resistor.
a cap can really generate some heavy
current in very short periods.
and some transistors can not handle that.
they will actually blow/crack the case sometimes
with hardly no heating due to the sudden current
impact that can cause pressure expansion.

As an exercise you might calculate the die temperature rise above
ambient if the transistor dumps the cap and innediately turns off and
repeats this process as fast as the 1k pull up resistor can recharge
the capacitor. By the way, the instantaneous peak current is limited
by the beta of the transistor times the base current. There are a few
simplifications in doing this math.

--
John Popelish

 

[John Larkin]

On Sat, 22 May 2004 19:22:18 -0400, John Popelish <jpopelish@rica.net>
wrote:

Jamie wrote:

yes, limit the current with a resistor.
a cap can really generate some heavy
current in very short periods.
and some transistors can not handle that.
they will actually blow/crack the case sometimes
with hardly no heating due to the sudden current
impact that can cause pressure expansion.

As an exercise you might calculate the die temperature rise above
ambient if the transistor dumps the cap and innediately turns off and
repeats this process as fast as the 1k pull up resistor can recharge
the capacitor. By the way, the instantaneous peak current is limited
by the beta of the transistor times the base current. There are a few
simplifications in doing this math.

Ballpark 5 milliwatts; not enough to worry about.

John

 

[Kevin Aylward]

John Larkin wrote:

On Sat, 22 May 2004 12:52:25 -0400, "Peter" <peterm@wanad00.com
wrote:


But should I add a limiting resistor (like 33 ohms) to limit peak
current under 200mA? Or the limiting resistor is not needed because
of capacitor internal resistance or transistor resistance?

ThankS!
-- Peter --


No need. The energy stored here is far too small to damage the
transistor. Let it discharge as fast as the transistor can yank it
down.

Well, if it were me, I would use a resistor to limit the current to the
max specified of the transistor, which I think is around 1A peak. One
always wants to be able to actually *do* the sums to ensure that the
data sheet specs are met. Without doing this, all bets are off. V/0 is
infinite. For example, a 2N3904 is not speced to be able to take, say,
10A, so make *sure* that it is never in a situation that will cause this
to occur. Its simply good design practise.

Unless one has an absolutely overriding reason *not* to ensure that a
transistor is within what is specked for the transistor, not doing the
basic V/I calculations is pretty daft. It certainly gets one into the
wrong mindset for doing reliable design in general, even if one can get
away with cheating in a one off situation. If I was doing a design
review/sign off on this, I would be demanding a worst case analysis that
guaranteed that the transistor was indeed within its most *basic* of
specs (V, I, Po), even if I were to let some of the others slip through
for practical reasons (e.g. a 1u gate leakage spec, when it is really
1pa).

Kevin Aylward
salesEXTRACT@anasoft.co.uk
http://www.anasoft.co.uk
SuperSpice, a very affordable Mixed-Mode
Windows Simulator with Schematic Capture,
Waveform Display, FFT's and Filter Design.

 

[John Larkin]

On Sun, 23 May 2004 09:00:21 +0100, "Kevin Aylward"
<kevin.aylwardEXTRACT@anasoft.co.uk> wrote:

John Larkin wrote:
On Sat, 22 May 2004 12:52:25 -0400, "Peter" <peterm@wanad00.com
wrote:


But should I add a limiting resistor (like 33 ohms) to limit peak
current under 200mA? Or the limiting resistor is not needed because
of capacitor internal resistance or transistor resistance?

ThankS!
-- Peter --


No need. The energy stored here is far too small to damage the
transistor. Let it discharge as fast as the transistor can yank it
down.


Well, if it were me, I would use a resistor to limit the current to the
max specified of the transistor, which I think is around 1A peak. One
always wants to be able to actually *do* the sums to ensure that the
data sheet specs are met. Without doing this, all bets are off. V/0 is
infinite. For example, a 2N3904 is not speced to be able to take, say,
10A, so make *sure* that it is never in a situation that will cause this
to occur. Its simply good design practise.

Unless one has an absolutely overriding reason *not* to ensure that a
transistor is within what is specked for the transistor, not doing the
basic V/I calculations is pretty daft. It certainly gets one into the
wrong mindset for doing reliable design in general, even if one can get
away with cheating in a one off situation. If I was doing a design
review/sign off on this, I would be demanding a worst case analysis that
guaranteed that the transistor was indeed within its most *basic* of
specs (V, I, Po), even if I were to let some of the others slip through
for practical reasons (e.g. a 1u gate leakage spec, when it is really
1pa).

I figure that, if you can do a 10-second quick mental estimate of the
situation, and it turns out to be orders of magnitude away from any
sort of danger, then do it and move on to something more important.
Save the serious math for when it really matters.

12 microjoules dumped from 5 volts isn't going to hurt any transistor.
And there's only 6.25 milliwatts available, max. You can do both those
calcs in your head. That's what I would say at a design review.

John

 

[Geir Klemetsen]

"Peter" <peterm@wanad00.com> skrev i melding
news:bzLrc.160529$Q6.1227568@weber.videotron.net...

Hi folks,

I want to discharge a 1uF monolytic capacitor charged at 5V through a
2N3904
(collector to cap, emitter to ground). The transistor driven into
saturation
to do that.
It is not an oscillator, the cap is permanently charged through a 1K
resistor to a 5V source.

But should I add a limiting resistor (like 33 ohms) to limit peak current
under 200mA? Or the limiting resistor is not needed because of capacitor
internal resistance or transistor resistance?

ThankS!
-- Peter --

It may help to put a coil with recistance next to zero in serie with the
capacitor. I don't know how to calculate the lowest posible value (Henry)
the coil must have in order to keep the peak current below the transistors
maximum peak current limit.

 

[Kevin Aylward]

John Larkin wrote:

On Sun, 23 May 2004 09:00:21 +0100, "Kevin Aylward"
kevin.aylwardEXTRACT@anasoft.co.uk> wrote:

John Larkin wrote:
On Sat, 22 May 2004 12:52:25 -0400, "Peter" <peterm@wanad00.com
wrote:


But should I add a limiting resistor (like 33 ohms) to limit peak
current under 200mA? Or the limiting resistor is not needed because
of capacitor internal resistance or transistor resistance?

ThankS!
-- Peter --


No need. The energy stored here is far too small to damage the
transistor. Let it discharge as fast as the transistor can yank it
down.


Well, if it were me, I would use a resistor to limit the current to
the max specified of the transistor, which I think is around 1A
peak. One always wants to be able to actually *do* the sums to
ensure that the data sheet specs are met. Without doing this, all
bets are off. V/0 is infinite. For example, a 2N3904 is not speced
to be able to take, say, 10A, so make *sure* that it is never in a
situation that will cause this to occur. Its simply good design
practise.

Unless one has an absolutely overriding reason *not* to ensure that a
transistor is within what is specked for the transistor, not doing
the basic V/I calculations is pretty daft. It certainly gets one
into the wrong mindset for doing reliable design in general, even if
one can get away with cheating in a one off situation. If I was
doing a design review/sign off on this, I would be demanding a worst
case analysis that guaranteed that the transistor was indeed within
its most *basic* of specs (V, I, Po), even if I were to let some of
the others slip through for practical reasons (e.g. a 1u gate
leakage spec, when it is really 1pa).


I figure that, if you can do a 10-second quick mental estimate of the
situation, and it turns out to be orders of magnitude away from any
sort of danger, then do it and move on to something more important.
Save the serious math for when it really matters.

Well, the point of the resistor is to prevent doing any serious maths.
Its the simplest way to ensure that it is within spec. If one needs to
avoid the component, that's when a bit more work is required to see if
one can cheat reliably.

12 microjoules dumped from 5 volts isn't going to hurt any transistor.
And there's only 6.25 milliwatts available, max. You can do both those
calcs in your head. That's what I would say at a design review.

But if you can't guarantee that the transistor is within *all* rated
specs, i.e. say < 1A in this case, you still have no recourse to the
manufacture if they all start failing.

I agree, that in this case its doubtful if there will be problems, but
in general, transients into caps are an issue in transistor failures.

Kevin Aylward
salesEXTRACT@anasoft.co.uk
http://www.anasoft.co.uk
SuperSpice, a very affordable Mixed-Mode
Windows Simulator with Schematic Capture,
Waveform Display, FFT's and Filter Design.

 

[John Larkin]

On Mon, 24 May 2004 08:06:11 +0100, "Kevin Aylward"
<kevin.aylwardEXTRACT@anasoft.co.uk> wrote:

I agree, that in this case its doubtful if there will be problems, but
in general, transients into caps are an issue in transistor failures.

I see people using huge caps - as, say, 1000 uF in 12-volt
applications - as the timing caps on 555 thingies. That's scairy. 1000
mikes charged to 8 volts is 32 mJ, enough to fire a spark plug.

John

 

[Bill Bowden]

"Peter" <peterm@wanad00.com> wrote in message news:<bzLrc.160529$Q6.1227568@weber.videotron.net>...

Hi folks,

I want to discharge a 1uF monolytic capacitor charged at 5V through a 2N3904
(collector to cap, emitter to ground). The transistor driven into saturation
to do that.
It is not an oscillator, the cap is permanently charged through a 1K
resistor to a 5V source.

But should I add a limiting resistor (like 33 ohms) to limit peak current
under 200mA? Or the limiting resistor is not needed because of capacitor
internal resistance or transistor resistance?

ThankS!
-- Peter --

How do you drive the transistor into saturation, do you use
a resistor on the base?

If so, you might just recalculate the value of the base
resistor so the HFE of the transistor limits the collector
current. That way you can do it all with one resistor.

-Bill

 

[Peter]

How do you drive the transistor into saturation, do you use
a resistor on the base?

If so, you might just recalculate the value of the base
resistor so the HFE of the transistor limits the collector
current. That way you can do it all with one resistor.

-Bill

But the HFE can vary widely from one transistor to the next, even in the
same batch. Can be anywhere between 75 and 300 or so, and the collector
current further changes the HFe.
Well! I could take the maximum HFE as a start, then be sure the energy
stored can be dissipated by the junction in this short amount of time; as
John said, it is quite small with 1uF. As long as there is no other hidden,
hazardous phenomena for the transistor in the scenario of course.

-- Peter --

 

[John Larkin]

On Tue, 25 May 2004 17:36:58 -0400, "Peter" <peterm@wanad00.com>
wrote:

How do you drive the transistor into saturation, do you use
a resistor on the base?

If so, you might just recalculate the value of the base
resistor so the HFE of the transistor limits the collector
current. That way you can do it all with one resistor.

-Bill

But the HFE can vary widely from one transistor to the next, even in the
same batch. Can be anywhere between 75 and 300 or so, and the collector
current further changes the HFe.
Well! I could take the maximum HFE as a start, then be sure the energy
stored can be dissipated by the junction in this short amount of time; as
John said, it is quite small with 1uF. As long as there is no other hidden,
hazardous phenomena for the transistor in the scenario of course.

-- Peter --

The beta of a 2N3904 peaks at about 8 mA, and is dropping fast fast -
down by about 10:1 - at 200 ma. So the peak current will sort of
self-limit, given reasonable base drive. Not that it matters, with
this low energy available.

John