Please help me understand\fix this discrete linear power supply

**Agent24** · 09-06-2015, 04:26 AM

Re: Please help me understand\fix this discrete linear power supply

Now, after playing around with it some more I don't think the IC427 Op-Amp is the problem at all - it simply triggers the problem, which seems to be something to do with the regulators themselves.

I managed to trigger the fault condition by putting a 1K resistor in place of IC427's supply pins.

I'm guessing the 7905 does come up first and neither the LF50CV (or the 78L05) like that for some reason, so the 7905 'wins' and the 5vA LDO shuts down. I wonder if D403 is there to prevent this problem happening, but when replaced there was no change. It seems odd though as I have seen the diode from output to input, but not from ground to output. Maybe that's what it needs, as stj said. But that keeps nagging me, surely such an issue would have been noticed before they shipped all these... weird.

So, perhaps there is some other fault where the 7905 comes up too soon before the 5vA LDO does.

All I can think of right now is perhaps either the 7905's output capacitors are bad (low capacitance) and it can charge them up too fast, thus the -5v rail comes up too quickly.
Or, Since the 5vA rail is dependent on the +7v rail, perhaps that +7v rail is too slow. It also uses a TIP32A transistor, just like the one that failed on the +5v rail. I wonder if perhaps that one is also going bad and is too slow to turn on?

**stj** · 09-06-2015, 05:02 AM

Re: Please help me understand\fix this discrete linear power supply

caps,
the caps have to charge before the regulators stabilise.
are they smaller on the -5v side??

**Agent24** · 09-06-2015, 05:21 AM

Re: Please help me understand\fix this discrete linear power supply

No, both -5v rail and the +7v rail that feeds 5vA have 2x 100uF output capacitors. See schematic page 19.

**fzabkar** · 09-09-2015, 02:54 PM

Re: Please help me understand\fix this discrete linear power supply

AISI, the difference between the 3 ICs is that the op-amp has no ground pin. Let's assume that the 7805 in each circuit is disabled. In the first circuit the current path for the 7905 is then exclusively via Rn and Cn whereas in the second circuit the path must go through D403 and R. The voltage of -0.7V that appears at V+ is therefore due to D403.

AISI, we need to prevent the 7805/78L05 from shutting down. Perhaps if we replaced D403 with a Schottky rectifer, then its lower forward voltage might not be enough to switch off the regulator???

Code:

     ____
     |  |
    -|7805|-  -+-------+----------------+ V+
     |____|   |    |+        |
      |    -   ---       .-.
      |    ^   ---       | |Rp
      |    |D403  | Cp      | |
      |    |    |        '-'
      |    |    |        |
      |    |    |        |    UVC3120, DS75107
     --+--------+-------+----------------+ Gnd  V+, V-, and Gnd
      |        |        |
      |        |        |
      |        |        .-.
      |        |+       | |Rn
      |        ---       | |
     _|__       ---       '-'
     |  |       | Cn       |
    -|7905|-------------+----------------+ V-
     |____|

Code:

     ____
     |  |
    -|7805|-  -+-------+----------------+ V+
     |____|   |    |+        |
      |    -   ---        |
      |    ^   ---        |
      |    |D403  | Cp       |
      |    |    |        |    TL074C
      |    |    |        .-.  V+, V-, no ground
      |    |    |        | |
     --+--------+-------+        | | R
      |        |        '-'
      |        |        |
      |        |        |
      |        |+        |
      |        ---        |
     _|__       ---        |
     |  |       | Cn       |
    -|7905|-------------+----------------+ V-
     |____|

**budm** · 09-09-2015, 06:10 PM

Re: Please help me understand\fix this discrete linear power supply

May be add the diode on the output of the 7805 (Anode to the output PIN OF 7805, Cathode is the out put) so that will isolate the output of 7805, the out put will be 0.6V lower but you can fix that by adding the forward bias diode and the small bypass cap in parallel between GND pin of the 7805 and GND.

**stj** · 09-09-2015, 07:23 PM

Re: Please help me understand\fix this discrete linear power supply

i'm sure old datasheets had a reverse diode between the input and output.

**budm** · 09-09-2015, 08:03 PM

Re: Please help me understand\fix this discrete linear power supply

I am talking about putting diode is series with output of the regulator to feed the load through this added blocking diode.
I am not talking about the typical protection diode figure 2.3.3: http://www.learnabout-electronics.org/PSU/psu23.php to prevent damage the regulator when input is shorted.

**fzabkar** · 09-09-2015, 09:26 PM

Re: Please help me understand\fix this discrete linear power supply

Like this?

Code:

       ____
       |  |  Do
    o-----|7805|--->|---+----o V+
       |____|    |
        |      |
        +---+    |
        |  |    |
      Dg V --- C  - D430
        - ---   ^
        |  |    |
    o-------+---+-------+----o Gnd

**budm** · 09-09-2015, 09:29 PM

Re: Please help me understand\fix this discrete linear power supply

Yes.

**Agent24** · 09-09-2015, 10:42 PM

Re: Please help me understand\fix this discrete linear power supply

Well, I tried changing the TIP32 on the +7v rail in case it had gone bad like the other one had.. no change.
Also tried doubling the 7905's output capacitors from 2x 100uF to 2x 220uF.. no change.

The diode idea might be worth a try, but I still can't shake the feeling that this thing must have worked fine when it came out of the factory, so why is it acting up now? I don't like the idea of adding extra parts to compensate for a problem that isn't supposed to be there when there is probably still a fault present. Still, I guess I can't think of anything else to try!

And for some reason, I still don't understand why they put D403 in the circuit at all to begin with.

**fzabkar** · 09-09-2015, 11:18 PM

Re: Please help me understand\fix this discrete linear power supply

Originally posted by Agent24

And for some reason, I still don't understand why they put D403 in the circuit at all to begin with.

Perhaps the designer witnessed the same problem you are seeing now and added the clamping diode to limit the negative excursion.

**fzabkar** · 09-10-2015, 10:15 AM

Re: Please help me understand\fix this discrete linear power supply

Originally posted by Agent24

The diode idea might be worth a try, but I still can't shake the feeling that this thing must have worked fine when it came out of the factory, so why is it acting up now? I don't like the idea of adding extra parts to compensate for a problem that isn't supposed to be there when there is probably still a fault present.

I'm not comfortable with these kludges, either. I've seen numerous dual supplies based on 7812/7912 and 7815/7915 regulator pairs and have never encountered a problem as strange as this one, nor have I ever seen a design that needed a reverse biased diode between Vout and ground.

Perhaps we're approaching the problem incorrectly. We've been assuming that it's a power sequencing issue, but maybe the 78L05/7805 regulator is shutting down due to excessive startup current. ISTM that the clue may be in the 150mA steady state current. As was already pointed out, the 78L05 can handle up to 100mA, but only with "adequate heatsinking", so 150mA should be a red flag. Moreover, the circuit's input-output voltage differential of 2V (= 7V - 5V) is at the limit of the spec, especially for the 78L05 (see the graphs in the datasheets). This means that the two-diode hack would violate this spec.

I would try to determine which component is responsible for the excessive current draw. Perhaps a resistance check would be in order (V+ to ground, V- to ground, V+ to V-).

**budm** · 09-10-2015, 11:00 AM

Re: Please help me understand\fix this discrete linear power supply

Originally posted by fzabkar

I'm not comfortable with these kludges, either. I've seen numerous dual supplies based on 7812/7912 and 7815/7915 regulator pairs and have never encountered a problem as strange as this one, nor have I ever seen a design that needed a reverse biased diode between Vout and ground.

Perhaps we're approaching the problem incorrectly. We've been assuming that it's a power sequencing issue, but maybe the 78L05/7805 regulator is shutting down due to excessive startup current. ISTM that the clue may be in the 150mA steady state current. As was already pointed out, the 78L05 can handle up to 100mA, but only with "adequate heatsinking", so 150mA should be a red flag. Moreover, the circuit's input-output voltage differential of 2V (= 7V - 5V) is at the limit of the spec, especially for the 78L05 (see the graphs in the datasheets). This means that the two-diode hack would violate this spec.

I would try to determine which component is responsible for the excessive current draw. Perhaps a resistance check would be in order (V+ to ground, V- to ground, V+ to V-).

I am thinking the same about 7805 not putting out, may be he needs to check the output current draw from the 7805, for it to shut down that fast, it must have been heavily overload. I do not believe it is the sequence up problem, that is why I suggest to add the blocking diode to see if the latch up will go away. I built so many dual supplies based on 78/79xx series without the reverse bias diode clamp for running bunch of OPAMPS, never see the latch up problems.
BTW, is the Analog Ground (AG) is tied to the Digital Ground (DG) some where on the board?

**Agent24** · 09-10-2015, 03:19 PM

Re: Please help me understand\fix this discrete linear power supply

Originally posted by fzabkar

I'm not comfortable with these kludges, either. I've seen numerous dual supplies based on 7812/7912 and 7815/7915 regulator pairs and have never encountered a problem as strange as this one, nor have I ever seen a design that needed a reverse biased diode between Vout and ground.

I have never had a problem like this before either with dual supplies.

However, this is not exactly the same in that we have a 78L05 LDO (currently my LF50CV LDO)/7905, AND the 78L05/LF50CV runs from a prior supply while the 7905 is straight from the main filter capacitor. The extra involvement of the +7v rail probably doesn't change anything, but having a "normal" regulator and an LDO in a dual-rail supply is not something I've seen before either.

Originally posted by budm

I am thinking the same about 7805 not putting out, may be he needs to check the output current draw from the 7805, for it to shut down that fast, it must have been heavily overload.

That was also my assumption, but the reverse seemed the case. Earlier on I put an ammeter on both input and output of the 78L05. When working it drew and supplied 140mA, when the output was -0.7v it drew and supplied no current at all. I did not see a current spike at switch on either (although I was using a DMM so perhaps it was not fast enough. I might try again with an analog meter)

That was when I reached your conclusion that it was being overloaded and thus swapped it out for an LF50CV which can handle 500mA or so. Alas, the problem remains.

Originally posted by fzabkar

Perhaps we're approaching the problem incorrectly. We've been assuming that it's a power sequencing issue, but maybe the 78L05/7805 regulator is shutting down due to excessive startup current. ISTM that the clue may be in the 150mA steady state current. As was already pointed out, the 78L05 can handle up to 100mA, but only with "adequate heatsinking", so 150mA should be a red flag. Moreover, the circuit's input-output voltage differential of 2V (= 7V - 5V) is at the limit of the spec, especially for the 78L05 (see the graphs in the datasheets). This means that the two-diode hack would violate this spec.

That's true, which is another reason why I figured the LF50CV would be a good replacement. 500mA current capability (easily able to handle 140mA) and the dropout voltage is only 0.45v, compared to the 78L05's 1.7v. And yet, it does exactly the same thing.

Originally posted by fzabkar

I would try to determine which component is responsible for the excessive current draw. Perhaps a resistance check would be in order (V+ to ground, V- to ground, V+ to V-).

I will check it out and see.

But so far it seems there is no specific component which causes the problem except the Opamp itself. If I remove the Opamp, the rail comes up fine.
I put the Opamp back, the problem comes back. But it does not seem to be the Opamp at fault - even if I replace the Opamp with a 1K resistor, the problem comes back.

And it's only on a cold power up too. If I switch off and then quickly switch back on, the rail comes up just fine.

Originally posted by budm

I do not believe it is the sequence up problem, that is why I suggest to add the blocking diode to see if the latch up will go away. I built so many dual supplies based on 78/79xx series without the reverse bias diode clamp for running bunch of OPAMPS, never see the latch up problems.

BTW, is the Analog Ground (AG) is tied to the Digital Ground (DG) some where on the board?

I'm not sure, I’ll have to check that. I certainly don't trust the schematic to tell me.
EDIT: I know Analog and Digital ground for the main supplies are connected: +/-7v rails which power the input front-end, and the +5v rail for the digital board share a common ground. Also the 7905 shares this ground.

That +5vA rail for the DAC\ADC might be on a completely separate ground, I don't know, I'll have to check.

**stj** · 09-10-2015, 04:15 PM

Re: Please help me understand\fix this discrete linear power supply

L05 isnt really any different, it's just a low current version that costs less because it has less heatsinking.
there are:

78s05 - 2a
7805 - 1a
78m05 - 500ma
78l05 - 100ma

**Agent24** · 09-10-2015, 04:37 PM

Re: Please help me understand\fix this discrete linear power supply

Huh... I think I thought the L in 78L05 was for LDO, maybe the 1.7v dropout vs 2v for the 7805 was part of it too.

So what you're saying is that the circuit doesn't work with either a normal regulator OR an LDO regulator? Interesting. I thought the use of an LDO may have been part of the problem.

**stj** · 09-10-2015, 04:52 PM

Re: Please help me understand\fix this discrete linear power supply

the volt-drop varies by manufacturer.

what i'm saying is the "L" does not stand for LDO, it's a rating of maximum current.

**rievax_60** · 09-10-2015, 05:42 PM

Re: Please help me understand\fix this discrete linear power supply

I tested an ST brand L7812CV. At 8ma of reverse current into the output pin, the pin stays locked at -0.75v when input voltage is applied. The output voltage becomes normal when the reverse current is reduce by a small amount.
I wonder if this is intentional by design.
Maybe a schottky diode is needed across the output.

**stj** · 09-10-2015, 05:59 PM

Re: Please help me understand\fix this discrete linear power supply

i found this in my collection.

start around page44

Attached Files

NationalSemiconductorVoltageRegulatorHandbook1980.pdf (14.45 MB, 14 views)

**rievax_60** · 09-10-2015, 06:22 PM

Re: Please help me understand\fix this discrete linear power supply

"Figure 4-2 is an example of a typical connection for obtaining a positive and negative power supply. The diodes
between output and GND are for preventing latchdown at startup and are absolutely necessary in the case of loads
shown by solid lines. Without the diodes, current flows in the separation regions between elements as described in
Figure 4-2 is an example of a typical connection for obtaining a positive and negative power supply. The diodes
between output and GND are for preventing latchdown at startup and are absolutely necessary in the case of loads
shown by solid lines. Without the diodes, current flows in the separation regions between elements as described in
chapter 2 and the output voltage does not rise (refer to Figure 4-3)."

Please help me understand\fix this discrete linear power supply

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