LM7905 outputting positive voltage and capacitor biasing

Thread Starter

Bserar79

Joined Oct 24, 2018
16
Hello guys!

So, I'm building a project and to power my op amps I decided to use an inverting Buck-Boost converter and a linear LM7905 to generate negative voltages. The converter can be turned off, by means of the optocoupler U2, and a microcontroller, as you can see in the diagram.

Now, the problem is the following. When the Buck-Boost is off, i get about 0.2V positive voltage at its output, thus, at the LM7905 input. The latter one outputs about 0.8V of positive voltage. I don't understand how is that even possible. All voltages where measured with ground as reference.

Sorry for the lackluster explanation, but that's basically what's happening. Also, here is a diagram of the circuit, showing the voltages that I get from the two test points.

1.png

So, i was wandering why is this happening? Also, reverse biasing the caps at 0.2V is safe? I know Aluminium Electrolytics (of which I use) can whitstand about 1V of reverse bias but still... The Buck-Boost configuration is taken directly from 2595's datasheet, I don't think they wouldn't mention something like this. If it's not safe, i'll just run the converter all the time.

Thanks for the help! :)
 

Thread Starter

Bserar79

Joined Oct 24, 2018
16
Hello, thanks for answering :)Yep, everything is ok when it's on. Also, i measured the same numbers on a breadboard and on the project's PCB, so i don't think there are faulty components. The Buck-Boost outputs about -7.02V and the 7905 a steady -5V.
 

AlbertHall

Joined Jun 4, 2014
10,068
When it is off, you have the input voltage across R3, with the current flowing back to ground through D1 and L1 in parallel with D3. So I would expect the '7905 input to be about a diode drop positive. But the output of the '7905 should be less positive than the input. Are you sure you've got the 0.2V and 0.8V the right way round.
 

Thread Starter

Bserar79

Joined Oct 24, 2018
16
100% certain. 0.2V at the input and 0.8-0.9V at the output. I've just measured it again, for the 20th time today. Edit: Also 0.2V sound fine for the input, as D1 is a Schottky Diode, with about 0.18V drop across it.
 

Thread Starter

Bserar79

Joined Oct 24, 2018
16
R5 was just for simulation purposes. In the real circuit, it is connected to the negative power inputs of about 7 op amps. On the breadboard there is no load, still the same result.
 
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