Design problem for scientific lamp current source.

[Bill Sloman]

200 V at 60 mA is 12 W!  Must be a big neon lamp!> The trouble I have with this circuit is that at above currents

My guess is that the lamp exhibits negative resistance at this
current.  You will find it pretty difficult to control the current
through a negative resistor.  I think the only hope is to add
series resistance greater than the value of the negative resistance.

Negative resistance wouldn't be a problem. The circuit is a constant
current source, sensitive - in the first instance - to the current
through the sensing resistor. The voltage drop across the lamp does
control the base-collector voltage, but that has very little effect
(less than 0.1%) on the base-emitter voltage which the op amp sees.

If the discharge turned right off, then there would be a problem. If
the control loop is oscillating, and drove the controlled current down
to zero this could happen, but stopping the control loop from
oscillating would be the area to concnetrate on, rather than the non-
linearity of the lamp when operating way off the desired stable state.

 

[[email protected]]

That's about the rating of a few elemental reference low pressure
emission lamps. A low pressure sodium lamp is pretty bright even at 12W.


Seems reasonable to me. Replacing the 330R with ~3k9 30W ought to do it
by dropping about half the HT voltage across the external resistor after
the lamp strikes and the extra series resistance should then discourage
it from becoming a negative resistance relaxation oscillator.

and why not increase the 68R

-Lasse

 

[Bill Sloman]

OK, I think that's you answer, then.  it is negative resistance,
and any simple control loop will be confounded by this.

This seems unlikely to be correct. The circuit involved is a simple
constant current source, and could be expected to cope with a negative
resistance across the lamp without difficulty. If the current source
is not properly frequency compensated, and thus oscillates, it could
turn the lamp right off, and restarting the discharge could be messy.

 

[Bill Sloman]

SOA says you can only run 600V up to a hair over 10mA, maybe 12mA.  BJTs
are a bad idea for dropping high voltages due to second breakdown.

Why not choose a tube like 6L6GC?  Current production, doesn't take much
heater power, and takes the voltage easily.  Okay, a MOSFET would be
smaller and cheaper (though probably about even if you include the
heatsink, come to think of it).

Note you're trying to carry a maximum of (800V - 200V) * 0.06A = 36W,
which is a moderate amount for a small transistor.  Just in case, you'dbe
better off with a TO-247 package, or two TO-220s in parallel.

Switching methods are much more efficient, even if you can't run the thing
directly on AC (I assume that's part of the design requirement).

Switching supplies always leave some ripple on the lamp output, which
may not be acceptable in the application.
Enough filtering can reduce this to what ought to be acceptable levels
- though you may end up still seeing the effects if convection
currents inside the lamp, or something equally odd - but a big
transistor on a big heatsink is usually a cheaper and easier solution
than a switching supply.

 

[George Herold]

You seem to be missing the obvious for the circuit and that is you need an isolation resistor between the OA output and transistor base such that the rolloff pole formed by that resistor and the worst case transistor Millercapacitance is at least a decade higher in frequency than the RC feedback of the 10n and 1K (really). There's nothing exotic about a crummy VCS for aneon lamp, scientific or  not, it is hack simple. A lot of your circuit values are screwy.

Ouch I missed that.. no base resistor.

George H.

 

[Mark]

My guess is that the lamp exhibits negative resistance at this
current.  You will find it pretty difficult to control the current
through a negative resistor.  I think the only hope is to add
series resistance greater than the value of the negative resistance.

Jon

agreed...

Mark

 

[[email protected]]

This seems unlikely to be correct. The circuit involved is a simple

constant current source, and could be expected to cope with a negative

resistance across the lamp without difficulty. If the current source

is not properly frequency compensated, and thus oscillates, it could

turn the lamp right off, and restarting the discharge could be messy.

Yep- the collector isolates the nonlinearity from the control loop- the basic VCS circuit is bad no matter what he's driving.

 

[josephkk]

Well, it is by definition a stability problem because you're seeing
oscillation where you want to see steady operation. The only question is
where the oscillation is coming from, which is why I was suggesting
looking at other points in the circuit than just the op-amp output.

The circuit topology is correct assuming that there's not a lot of
coupling between the collector and base of the transistors, that the
Darlington stage isn't too slow. That 10nF cap is in the right place,
but Bill is correct that it may be too small.

If an integrator is really the correct topology Bill may be right, however
i do not think it is the correct topology; i would replace it with 1n.
Moreover a 100R resistor in place of the diode should do far better at
turning off the transistor / Darlington.

It is very much to the point to look at the transistor base when it gets
wonky.

?-)

 

[[email protected]]

You seem to be missing the obvious for the circuit and that is you need an isolation resistor between the OA output and transistor base such that the rolloff pole formed by that resistor and the worst case transistor Millercapacitance is at least a decade higher in frequency than the RC feedback of the 10n and 1K (really). There's nothing exotic about a crummy VCS for aneon lamp, scientific or  not, it is hack simple. A lot of your circuit values are screwy.

No need for the darlington either. That just makes Miller worse.

If he needed the drive, then an emitter-follower to Vc, not a
darlington.

 

[Geoff]

http://i251.photobucket.com/albums/gg302/triode101/sink.jpg

This circuit is to drive a scientific lamp which is low
pressure neon of strike volts 600V and typ run volts of
about 200V. The design intent is DC current from 0 to 60mA.
This circuit traditionally runs to about 30mA with no
problem and is well proven at these currents. Normally it
is not darlington pair but it is something I have tried..

The trouble I have with this circuit is that at above
currents of about 35-40mA the lamp starts to flicker
indicating a higher current than is dialed in by the pot.
An oscilloscope at the opamp output confirms that at the
point where it starts to flicker, the loop starts to try to
lower the current since the opamp output volts drops. The
onset can be made to be a periodic dip (tens of hz dip
about a volt) before the lamp begins to flicker. When the
lamp flickers on and off more severely, the opamp tries to
go negative to compensate. This to me shows that the output
transistor is conducting, not the failure of the loop
control.. I have played around with the drive impedance, ie
with/without the 1k base resistor. The point of flicker can
be less severe with the resistor, or latches hard without.
Maybe the VCEO spec is being stretched here. I have tried a
variac and reduced the volts to about 580V, still does it
but onset is higher current (50mA). I have tried a BUL 416
transistor also. I might just be applying too much voltage,
but I can't see why, might have to look at the op curves a
bit harder. Maybe I could use a MOSFET or IGBT.

I grabbed a IRFPE50 which had a marginal SOA and it worked
perfectly well. Confirming that the problem was indeed
transistor breakdown, as I suspected. I swapped it for a
2SK1317 when I had stock and it went well too. For curiositys
sake I checked the lamp voltage at a few currents. Basically
it had a positive resistance at low currents and at higher (>
40mA) currents tended towards constant voltage ie from 194 to
210 mA range. These lamps do eventually run out of gas
pressure, and will begin to turn off/on under these
conditions. But this is not what I saw, which was a huge
increase in current. The astronomy market now has a new
product anyway.

 

[Geoff]

low currents and at higher (>

40mA) currents tended towards constant voltage ie from 194 to
210 mA range.

EDIT: 194 to 210 Volts.