Design problem for scientific lamp current source.

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 Miller c apacitance 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 a n eon lamp, scientific or not, it is hack simple. A lot of your circuit valu es are screwy.

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

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

Jon

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.

ith this circuit is that at above currents

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.

In my disco days I dimmed neon by feeding the transformer whole cycles of AC... dimming by skipping full cycles.

(Duty-cycle schemes with TRIAC's proved to be transformer killers :-) ...Jim Thompson

and why not increase the 68R

-Lasse

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.

be

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.

n isolation resistor between the OA output and transistor base such that th e rolloff pole formed by that resistor and the worst case transistor Miller capacitance 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 a neon 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.

agreed...

Mark

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

is

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.

?-)

n isolation resistor between the OA output and transistor base such that th e rolloff pole formed by that resistor and the worst case transistor Miller capacitance 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 a neon 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 wrote in news:XnsA10ABEB9996F0Geoff@88.198.244.100:

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

low currents and at higher (>

EDIT: 194 to 210 Volts.