Jump to content
Electronics-Lab.com Community

Dual Tracking Supply Design Not Working as Expected


seanacais
 Share

Recommended Posts

Hello Everyone:

I've designed and built a Dual Tracking Supply circuit (Schematic attached as PDF).  I feel like I'm too close and am either not understanding, or missing something that is causing my confusion.  I'm hoping somebody can take a look and set me straight.

I'm feeding the circuit 36VDC (floating) at the input and am seeing some measurements that I do not understand. 

Measuring everything with reference to pin 2 on the output terminal (the nominal ground of the circuit) I get +Vout and -Vout within about 0.2 V but it is only about 2/3 the value I expected.  I expect to see around 17.5V but only see 12.5

What bothers me more is that value I see at the top of Z1 and the bottom of Z2.  I expect those to be about +12V and -12V respectively.  What I see is about +6V and -12V instead.  Similarly, on the actual input terminals of J1 I see +12V and -24V.  I really don't understand why the voltage is not distributed evenly?  Although this explains why I'm not seeing full voltage at the output.

So  I figure that something in the circuit is pulling the voltage down but  I don't know what it is. 

I'd read in some forums that you should put a resistor in place instead of tying an op-amp input to ground so this is why I put R31 in place.  I thought I misunderstood it's purpose so tied some clips in place to remove it.  While that had no effect on the circuit I had initially made a mistake and had tied pin 9 of U4 (the inverting input of the same op-amp) to ground instead.

This mistake did make the circuit appear to run correctly although the regulation between +Vout and -Vout was significantly worse.  If the point between the Vout sense resistors (r32 and r33) is supposed to be tied to ground, I'm not sure what the purpose of opamp 4c is at all.

Anyway, any and all help, suggestions are VERY welcome.

Kevin

PowerSupply.pdf

Link to comment
Share on other sites


Assuming you have replaced all of the opamps, the uneven voltage distribution seems very even. Since the circuit is designed by yourself, it may easily be the wrong offset voltages of any of the opamps, and the voltage distribution only looks to be half. A saturated opamp is easy to identify.


Sorry.  I didn't follow that at all. 

I didn't replace the opamps but I have no real reason to think any of them are bad.  I also  don't think any of the them are saturated.  If I change the value of R35 I can vary Vout (and it tracks on -Vout). 

I'm willing to look to see if any of the opamps are saturated but I'm not sure how to go about that. 

I'm also not sure how to interpret "the unevn voltage distribution seems very even".  Could you elaborate?

Kevin
Link to comment
Share on other sites


Okay, if J2 is the output, then Q1 needs to be an NPN and Q3 a PNP. The emitters are then connected to
Vout.


Q1 is a PNP and Q3 is NPN with the collectors on the output side.  I do have current sense resistors between the collectors and Vout, but that scheme should still work.

Vset and Vsense are fed into opamp U3C.  If Vset > Vsense the output goes a little higher, forcing more  current through CE of Q2 and BE of Q1 which in turn causes the Voltage across Q1 CE to drop increasing Vsense until Vsense = Vset.  If I'm looking at something the wrong way please let me know.

The circuit does respond to changes in the Vset and Iset.  My main question is still why doesn't the input voltage center around the ground (J2-2)?

Thanks again for all the help!

Kevin

Link to comment
Share on other sites

I've implemented the changes that you suggested.  Q1 is now a TIP41 (NPN) with the collector to Vin and the emitter facing the sense resistors.  Q3 is a TIP42 (PNP) with the collector and emitter swapped as well.

The circuit is still showing the same readings.  The voltages on the input are pulled off center (+12/-24 VDC).  The output still tracks although it is much lower than expected.

Could you explain why the input voltages aren't centered around the ground as I would have expected?  Could you elaborate on the earlier comment "the uneven voltage distribution seems very even"?

I've checked the outputs of the opamps.  None of them are driven to the rails.

Thanks,

Kevin

Link to comment
Share on other sites

  • 2 weeks later...

It sounds like a grounding problem to me.

You said that you fed this circuit from 36VDC floating. What you need to do is feed the circuit from +/-18VDC referenced to 0V.


Hi Hero

That's really the central question.  I don't understand why the 36V doesn't balance itself in the circuit?  Even if I gave the input +/-18V with a 0V reference the whole input would still be floating.  What would prevent the input from being pulled down the way the 36V is?
Link to comment
Share on other sites

That's your problem, with a dual power supply, the 0V terminal is fixed at 0V regardless of the current flow, with a single supply the 0V terminal can float ay any voltage between either supply rail.

If you connect the circuit to 36VDC and leave the ground connections unconnected they will float at a voltage determined by their resistances between the +V and 0V rail.

If you connect the ground connections to 0V they will be a 0.

To illustrate this, build a circuit consisting of two resistors, say 10k and 22k arranged in a potential divider configuration and call the point where the two resistors join 0V. Now try connecting the resistors to a 36V supply and measuring the voltage between the 0V terminal and either supply rail and repeat the experiment with the 0V terminal connected to the 0V point of a +/-18V supply.

Link to comment
Share on other sites


That's your problem, with a dual power supply, the 0V terminal is fixed at 0V regardless of the current flow, with a single supply the 0V terminal can float ay any voltage between either supply rail.


AHHH!!!  Got  it.  By using only 0-36V the current flow has to go from +Vin to -Vin.  The only way the circuit would work as it is now is if the current flow from +Vout to 0V was equal to -Vout to 0V (meaning the resistance of the positive load was equal to the resistance of the negative load).  The bigger the current mismatch between + and - the more my "0V" would move toward one pole or the other.

Thanks for taking the time!!!!! 

Kevin
Link to comment
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

Loading...
 Share

×
  • Create New...