Hi Yes this is happening at max current limit 4A. The bulbs draw more than this close to 5A.Incandescent lamps present a much lower resistance cold,
than hot. Are you flickering due to current limit? Can you
measure the load current in the time domain ('scope and
sense resistor)?
Hi. As I mentioned, there is a 10K resistor missing. It is connected between D4 anode and D17 Anode.The schematic does not make sense to me. I can't see how Q7 ever switches off since some current always seems to be flowing through the R6-D4 branch into Q7 base regardless what Q5 or Q4 does to Q6. R12 is too large to affect the current through R6. Maybe there are some errors in the schematic.
Hi,
.. A warm welcome to you at edaforum
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try this:
connect a 100R within the case from +_Output to +V_sensing
and a 100R from 0V_Output to -0V_sensing.
This mainly prevents from high outut voltage in case one of the sense lines is disconnected externally.
With this you can use the power supply in two wire mode. (without external sense wires)
As soon as you connect the external sense wires the resistance of the connected sense wire is much lower than the 100R and the true "sense mode" is active.
Try if the problem goes away wiht the two resistors. There is a little change because of decreased (and faster) feedback.
If the problem persists, then connect a 100nF ceramics capacitor across the just installed 100R resistors (two capacitors).
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If the problem still persists, then one needs further analysation on your complete regulation loop.
The problem with incandescent light bulbs is, then when cold (at the time when powered ON) they draw much more current than when alight. Mybe too much current.
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Let´s try .. and see what happens.
Good luck
Klaus
The primaries aren't in series. Its just the way my friend Paul drew it. The 18V sec is just a separate winding, but there is only one Prim winding. This is one transformer. its a big ass 240VA traffo and is toroidal. I did a circuit to switch the taps automatically. Was a pain in the balls to do, but its working great.It's certainly an unusual design and there appears to be at least one error in the schematic in that the two input transformers have primaries in series instead of parallel. If it has been wired that way, there is a good chance there is insufficient internal supply voltage/current anyway.
The floating 20V supply is referenced to 10V above output voltage and 10V below it so looking at worst case load - a short circuit - the top of R6 has 10V on it, if you ignore the bias feed to Q7 from Q6 (as it should be cut off) and subtract the Zener voltage, the maximum possible base current to Q7 would be (10 - 5.6)/10K = 440uA. That also ignores the Vbe drop in the output transistor so you are right, it never switches off but it is starved of enough base current to fully conduct.
Brian.
The capacitors alone sorted the issue, but I will add the 100ohm resistors in parallel with the caps, as you have suggested.What wattage resistors should I use?
Also why are the caps solving this issue?
All of the previous discussion tells me that the circuit's feedback loop is marginal.
Unfortunately, and as others have also noticed, the circuit is quite unusual and overly complicated. It is not readily obvious which of the several resistor/capacitor networks is actually performing the feedback compensation.
There are circuit features that I don't quite comprehend...for instance, what is the purpose of the R14/C5 time constant?
I am going to ask the following question: Has someone else built this circuit, exactly as shown, and made it work? Or is this the very first iteration?
Let me finish with the following statement: I applaud and commend anyone that attempts to build a circuit using only discrete components. Doing so gives you an insight of electronic circuits that an IC does not provide.
But if you are planning to build a discrete transistor supply, why don't you use the "classic" circuit as your starting point? This is a proven circuit that has worked for over 60 years. You may add additional features and enhancements to the basic circuit.
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