Need help with electronic ballast

[stevenmahoney]

I am trying to repair an electronic ballast and do not understand some of its circuits. Can anyone help me?

1) R4 suppose to regulate the brightness of HMI light. It looks like to me as a voltage divider (I am not even sure why transistors Q3 and Q4 were used, as they do not do anything). The voltage difference from highest to lowest at pin 3 of U2A when I rotate it is 1V. However, there is hardly any voltage on pin 2, so I am confused as to how that comparator should be operating. Also, the connection to Q9, Q10 transistors is not clear... Especially when resistor R19 is so low (.1 om).

2) U4 produces a square wave. I do not understand how it turned into an oscillator. Positive bias via capacitors C1 and C2? If so, how would it work?

3) Pin 4 of U4 produces narrow square waves (pink on diagram). Is that suppose to be like this? U4 is a quad cmos NOR gate. Am I correct to assume that somehow the comparator U2A suppose to change the frequency or width of the signal on pin 4? This mechanism is also a mystery...

4) What is the purpose of transistors Q7 and Q8? How they suppose to work?

Basically I am trying to figure out how dimming works and fix it.

Any help (the more detailed the better) would be appreciated. Really want to get to the bottom of it...

Steven

Detailed schematics with oscilloscope clippings are in the attached files.

 

[KeepItSimpleStupid]

Q3 & Q4 are basically part of the start-up circuit. In other words, they only do their thing on power up. Fets are used a lot of times in audio amps to prevent turn-on thumps. With certain JFETS, they are effectively ON at the instant of power up depending on C4 and C10.

C11 is the odd one for me. It's probably charged by the leakage current of D4 and thus that comes into play on turn-off.

So, the lamp is stuck at some brightness?

I'd start somewhere in the middle. I'd look at R25, Q11, T1 and R27, Q12 and T1. You pretty much know there needs to be square waves at both places.

I would suspect that the duty cycle of the above waveforms should change with the brightness control. One way that this could be done is by generating a ramp and changing the voltage at which a comparator switches. I'll print the diagrams and look at them.

 

[stevenmahoney]

I am trying to fix a dimmer and wanted to know how the whole thing works.

Steven

 

[schmitt trigger]

I can't help, unfortunately.
However wanted to comment that, from the attached scope images, you appear to be using a B&K scope. I also have one.

 

[KeepItSimpleStupid]

PS: Heading fur surgery tomorrow. Not sure when I'll get to look at it.

 

[schmitt trigger]

Looking at the schematic a little better, I realize that U$ is essentially a two-gate CMOS ring oscillator. This is the classic oscillator used with CMOS gates. The first gate receives the dimming signal from the optocoupler, and there is a third gate which in conjunction with comparator U2 shuts down everything under certain conditions.

 

[stevenmahoney]

I use Tektronix TPS series scope.