Power Rails voltage drop

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neil.johnson
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Re: Power Rails voltage drop

Post by neil.johnson » Wed Aug 26, 2020 9:03 am

cackland wrote:
Wed Aug 26, 2020 8:34 am
neil.johnson wrote:
Wed Aug 26, 2020 5:50 am
An alternative to diodes are MOSFETs, which will have a lower voltage drop if you really need to squeeze the last few 100mV or so out of the rails.

Neil
Yes, you must of missed this mentioned above:

I've seen some circuits with these mosfet diodes in place of the 1N5819.

https://au.mouser.com/ProductDetail/621-DMP3099L-7
https://au.mouser.com/ProductDetail/621-DMN3404L-7

Assuming these are more efficient?
Sorry, yes, more efficient in that the forward voltage drop will be significantly less due to very low Rds(on) (the devices you linked to are in the region of 10s of milli-ohms). The other difference is price: MOSFETs are about 50-100% more expensive than the diodes. Again, for a hobby project this is not an issue, but for production it can matter.

Neil
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Synthiq
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Re: Power Rails voltage drop

Post by Synthiq » Wed Aug 26, 2020 2:11 pm

cackland wrote:
Wed Aug 26, 2020 8:34 am
neil.johnson wrote:
Wed Aug 26, 2020 5:50 am
An alternative to diodes are MOSFETs, which will have a lower voltage drop if you really need to squeeze the last few 100mV or so out of the rails.

Neil
Yes, you must of missed this mentioned above:

I've seen some circuits with these mosfet diodes in place of the 1N5819.

https://au.mouser.com/ProductDetail/621-DMP3099L-7
https://au.mouser.com/ProductDetail/621-DMN3404L-7

Assuming these are more efficient?
This is a better technical solution than the Schottky diode. Just make sure they are connected "backwards" so the source is connected to your module and the drain to the power connector. This ensures the parallel diode is reverse biased and not conducting if you reverse the power connector.

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cygmu
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Re: Power Rails voltage drop

Post by cygmu » Thu Aug 27, 2020 5:05 am

cackland wrote:
Wed Aug 26, 2020 8:37 am

Currently using two LM1117MPX-3.3 voltage regulators for the stm32, one the digital VDD and the other for the VDD_A. This chip wouldn’t work without having power to the ADC and have read that it is good practice to have split power for both normal 3.3 and analog 3.3. Maybe I should go for just one??

I could definitely reduced the frequency and see how low I can go and how that affects the current draw
200mA draw on the positive rail does seem like a fair bit so I think it is worth a bit of exploration to see if that is a natural consequence of sensible design, a consequence of a slightly flawed design, or a result of a problem with the build itself.

70mA for the processor is ok; it's the additional 130mA for the other components that seems surprising. Changing the voltage regulator setup won't make much difference there: the LM1117 won't eat up much current by itself. It would be ok to use just one of those, though, and maybe put a little LC filter for the VDD_A -- take a look at the schematics for ST's evaluation boards such as the Nucleo boards for example. But that will not address the current draw issue.

You mentioned you had some LM4040s around. Depending on the resistor you have at the cathode, they could use quite a bit of current. What else is there that would draw current? Some op amps I guess, maybe some VCA ICs or OTAs? LEDs, pots, etc will all draw a bit of current but 130mA is a lot.

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cackland
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Re: Power Rails voltage drop

Post by cackland » Thu Aug 27, 2020 5:44 pm

cygmu wrote:
Thu Aug 27, 2020 5:05 am

200mA draw on the positive rail does seem like a fair bit so I think it is worth a bit of exploration to see if that is a natural consequence of sensible design, a consequence of a slightly flawed design, or a result of a problem with the build itself.
Thanks cygmu.

It could be a result of all.
cygmu wrote:
Thu Aug 27, 2020 5:05 am

70mA for the processor is ok; it's the additional 130mA for the other components that seems surprising. Changing the voltage regulator setup won't make much difference there: the LM1117 won't eat up much current by itself. It would be ok to use just one of those, though, and maybe put a little LC filter for the VDD_A -- take a look at the schematics for ST's evaluation boards such as the Nucleo boards for example. But that will not address the current draw issue.
I assume this is what you are referring to here -> BEAD to AVDD.
Screen Shot 2020-08-28 at 8.37.39 am.png
cygmu wrote:
Thu Aug 27, 2020 5:05 am

You mentioned you had some LM4040s around. Depending on the resistor you have at the cathode, they could use quite a bit of current. What else is there that would draw current? Some op amps I guess, maybe some VCA ICs or OTAs? LEDs, pots, etc will all draw a bit of current but 130mA is a lot.
Yes, I have many components: 17 Quad Op Amps and 4 VCA IC's.

And a few LM4040 IC's.
Screen Shot 2020-08-28 at 8.41.48 am.png
Screen Shot 2020-08-28 at 8.41.58 am.png
Screen Shot 2020-08-28 at 8.42.07 am.png
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Elahrairah
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Re: Power Rails voltage drop

Post by Elahrairah » Thu Aug 27, 2020 10:37 pm

cackland wrote:
Thu Aug 27, 2020 5:44 pm
Yes, I have many components: 17 Quad Op Amps and 4 VCA IC's.

And a few LM4040 IC's.
Maybe 200mA draw is fine and expected in that case. Lose the 10R on input and you'll be fixed. If its purpose was to fuse-blow it could be downsized to a 2R, since the 10R rule of thumb doesn't expect you to use 200mA in one module. You're pushing the definitions of modular here, it's sounding like several full-on synths behind that 10R.


I have seen 200mA only on modules with vacuum tubes being heated by the power rail. My highest draw (after the tubes) was a poorly designed clock divider that uses CMOS logic to dump a signal to ground occasionally. My other highest draw was a damaged arduino. I had burned out some pins with negative voltage, and saved the device for another module that did not need those pins. I didn't realize the damaged pins were actually dumping current and making the little guy get super hot to the tune of 100mA.

In any event when something is wrongly dumping power, you'll find it as heat. any part you don't want to keep your finger on are worth fixing. Regulators get heatsinks and other parts are just setup wrong.

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cygmu
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Re: Power Rails voltage drop

Post by cygmu » Fri Aug 28, 2020 4:18 am

cackland wrote:
Thu Aug 27, 2020 5:44 pm

Yes, I have many components: 17 Quad Op Amps and 4 VCA IC's.
Aha, then 200mA starts to seem reasonable. Op amp supply current might be 1.5 - 2mA per amplifier and you have 68 of them, so 130mA draw for the analog parts of the circuit might even be on the low side of things :) The datasheet for the op amp you are using will tell you of course. But I think this solves the mystery of the current consumption, and as others have said, a 10 Ohm series resistor is probably not the best plan in such a circuit.

The LM4040 cathode resistors all look fine by the way: no excess power consumption there.

In the AVDD supply you posted, yes, the bead is part of the filter I was talking about. Somewhere else on the same schematic I expect there is also a capacitor from AVDD to ground, so the bead and the cap together form a filter for high frequency noise.

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cackland
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Re: Power Rails voltage drop

Post by cackland » Fri Aug 28, 2020 11:11 pm

Thanks guys. I'm definitely looking at ways to reduce the power consumption. Appreciate the feedback.

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