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Author Topic: [SOLVED] 6V regulator putting out 3.3V  (Read 1885 times)
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Do you still have the logic supply of the H-bridge connected to the 5V line?
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Yes.
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I am running out of ideas but I don't think you have tries a capacitor yet. It says about the output capacitor:-

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CO is not needed for stability; however, it does improve transient response. Values of less than 0.1 uF could cause instability.

Which is odd because if the capacitance of your load is less than this, then it suggests you are going to get instability. They might be assuming that any load is going to have 0.1uF decoupling capacitors on it anyway so are saying you don't need any extra. But, if you have none at all that could lead to instability.
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I'll scrounge around and find a capacitor to place across the output. Either way, it's late here, so I'll sleep on it and try again in the morning, hopefully finding a solution. Thanks for sticking with me on this so far. smiley
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I've retested - Still sticks at 6V when disconnected, drops to 3.3/3.2 and draws 40mA when connected, and bizarrely, when disconnected, seems to jump between 6V and 9.5V.

Any ideas as to why it jumps to 9.5V when disconnected? None of my other regulators do this. Maybe it is the regulator...
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Maybe it's miswired?
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Maybe it's miswired?

I get the feeling this might be the case...  smiley-sad-blue Either way, here's a dodgy circuit diagram:

  +------------+----------+                +--------------------------+
 |    +----+  |  +----+  |  +----+        |    +-------------+       |
 |    |3.3V|  |  |5V  |  |  |6V  |        |    |  H-bridge   |       |
 |    |    |  |  |    |  |  |    |        |    |             |       |
 +---+|Vin |  +--+Vin |  +--+Vin |        +----+1,2EN   Vcc1 +-------+--+---+
 |    |Vout|     |Vout+-+   |Vout+-----+       |1A        4A |          |   |
 |   +|Gnd |    -+Gnd | |  -+Gnd |     |       |1Y        4Y |          |   |
 |   |+----+    |+----+ |  |+----+     |       |Gnd      Gnd |          |   |
 |   |          |       |  |       +---|-------+Gnd      Gnd |          |   |
 |   |          |       |  |       |   |       |2Y        3Y |          |   |
 |   |          |       |  |       |   |       |2A        3A |          |   |
 |   |          |       |  |       |   +-------+Vcc2   3,4EN +----------+   |
 |   +-------+--+-------|--+-------+           +-------------+              |
 |           |          |                                                   |
 | +----+    |          |                                                   |
 +-+10V+|    |          |                                                   |
   |10V-+----+          |                                                   |
   +----+               |                                                   |
                        |                                                   |
                        |                                                   |
                        |                                                   |
                        |                                                   |
                        |                                                   |
                        |                                                   |
                        +---------------------------------------------------++

     3.3V goes to some other components. A second H-bridge with identical
     connections to the first is present. xY are not connected, xA=LOW
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If that's how it is actually wired, I don't see what the problem is.
VCC2 can be 5V also, why not simplify as a test and ditch the 6V regulator?
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If that's how it is actually wired, I don't see what the problem is.
VCC2 can be 5V also, why not simplify as a test and ditch the 6V regulator?

Ok, the 6V is still attached to the circuit, just no longer supplying Vcc2. The H-bridges still draw 40mA, still a little high, but the 5V regulator stays at a solid 5V when connected as Vcc2. Seems like something is up with the 6V regulator, or at least how I've wired it.
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Ok, that current draw doesn't seem out of line with the spec sheet for the outputs enabled and sitting low.
Sounds like something with your 6V regulator.
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Well, luckily RS Components gives free overnight shipping anywhere in Australia, so I should be able to try a new one by tomorrow. I'll keep you posted...
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Also, how often do componenets like this fail? It seems a little off.
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My guess is still lack of capacitors. Do you have access to a scope kr are all the voltage measurements done with a meter. A scope will show if the output is rearlly low or if it is oscillating.
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Nope, just a multimeter. In AC/RMS mode, it shows 12.6 for the normal and 4.5 when connected to the bridge.

I found a 0.1uF capacitor, put it across the voltage output and... It worked?! Now, without the capacitor, the regulator is putting out 6.6V, and with, it's putting out a solid 6V when connected to the bridge. There was a point where I accidentally short circuited it (got 0.3V) while testing the capacitor and almost burnt my hand on the regulator heatsink, but now it seems to be working...


I understand how a filter capacitor works, but I'm completely confused as to how it fixed the issue. The other 2 regulators are working completely find without external caps, even the 5V worked fine when connected to Vcc2. And isn't 2.7V a fairly large swing for a regulator? I mean, if it is AC, which it shouldn't be...

I'm basically confused as to why that works.
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So if it reads 4.5V on AC that is the RMS value. To get the peak value you multiply by root two. This gives a reading of 6.3V, so it shows it was oscillation.
Quote
The other 2 regulators are working completely find without external caps
That is more of a surprise than this regulator needing a cap.

Quote
but I'm completely confused as to how it fixed the issue
A regulator is in essence a feedback amplifier. It monitors the output and feedback adjusts the output high or low to make it a stable voltage. Any feedback amplifier is potentially an oscillator so you need capacitors to reduce the frequency response of the feedback circuit to a point below the natural resonant frequency. What was happening was not so much AC as going from zero to six volts repeatedly.

Put capacitors on the other regulators, just because you can't see trouble at the moment doesn't mean they are not needed.

Lesson - decoupling capacitors are good - I did suggest capacitors a long way back in this thread.   smiley-wink
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