1 Topic by superbaka (edited by superbaka 2024-12-27 06:24:13)

  • Registered: 2020-02-15
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Topic: Power Fail tech question

RME,

I'm building a regulated linear PSU (adjustable 7v-20v, 2A) and as the filter capacitance and choke in it increase in value, I eventually trip the Power Fail.

I have a large capacitance (12,000uF) and choke (1.5mH). Given that many PSUs have Farads of capacitance these days, I am suspecting its over current protection from the unusually large choke.

Ultimately I need to examine the situation on a scope, but I don't believe there is any interruption in voltage or current - in fact the opposite. I theorize without knowing how the protection circuit works that the uninterrupted supply of current stored in the choke is giving the ADI an unexpected current supply and trips it. However, I am able to leave the choke but reduce the filtering capacitors, so it seems related to power reserves in general. (The design is very simple, LM regulator with feedback for stability.)

I'm wondering if you can offer any clues.??
thanks

2 Reply by MC

  • MC
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Re: Power Fail tech question

That starts with writing clear descriptions: Should this mean 'the power fail warning shows up on the display of the ADI'?

If so check the start-up time (ramping up on power on) of the voltage. If the 12 V come up too slowly the ADI will briefly think voltage is too low.

Regards
Matthias Carstens
RME

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3 Reply by superbaka (edited by superbaka 2024-12-28 03:33:21)

  • Registered: 2020-02-15
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Re: Power Fail tech question

Yes, it means the power fail warning shows on the display. (Is there another "Power Fail" possibility?)

Whether I add additional filter capacitance before the ADI starts up, or WHILE it's running (in realtime via a switch in the PSU), it will trigger the Power Fail. And during this time, it emits a quiet high pitched whine.

Note: If I switch a capacitor out of circuit, the ADI will close the warning and go back to operating, in realtime. But I measure 12v output the whole time.

Another brand of film caps I used with the same capacitance caused it to power cycle off and on with a black screen. Does this behavior offer any clues? My supply has a cascade regulated 9-15V and can provide 2A. It works and measures as expected on other gear.

With a smaller mH, I can add as many filter capacitors as I want with no Power Fails. It seems somehow the choke is slow to respond and possibly reducing the voltage momentarily. But it's strange how I can "switch it back" in realtime and the ADI will work again, while outputting 12v continuously and there is no in-rush going on.


thanks

4 Reply by MC

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Re: Power Fail tech question

Please use the Oscilloscope instead of speculating. And please understand that we can not support such a DIY project.

Regards
Matthias Carstens
RME

MC's Website

5 Reply by KaiS (edited by KaiS 2024-12-28 16:28:55)

  • KaiS
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Re: Power Fail tech question

• A coil is not only an inductance, but it’s a resistor too.
1 Ohm resistor @ 1 Ampere current = 1 Volt voltage drop (Ohms Law).
Measure the resistance of the coils to get an idea.


• Switching in an unloaded capacitor while running means, this cap needs to be charged first before normal operation comes back.
In the first moment the PSU’s output voltage drops to almost zero!
The inductance/resistor of course slows down the recharge, this is the nature of it.


• A relatively small coil with ferrite or metal core will likely saturate and loose most of it’s inductance if high DC current runs through.
This means it doesn’t have as much of it’s expected positive effect of smoothing the DC.
If you use a coil, make sure it’s spec’d for 2 A DC, or use a core-less air-coil.


• The coil/resistor is responsible for most of your observations.
E.g. it prevents the voltage from keeping stable the moment you plug in ADI-2, as it limits the current during the load phase of ADI-2’s internal caps.


• Your simple voltage measurements can’t show all these dynamic reactions of what’s going on, DMMs have a too slow response.

Get an oscilloscope to see what’s up.
If you need one, I have some spare Fluke ScopeMeters (located in Germany, PM me).


• With all the stress you put on ADI-2, it’s a wonder (or thanks to RME’s built in protections) that it’s not already toast.
Use a 6 Ohm / 40 W resistor (or 2 pc. 12 Ohm 20 W in parallel) as dummy load to simulate 2 A @ 12 V for further experiments instead, until you have a solution.

Under-voltage will not break ADI-2, but high voltage spikes when switching possibly loaded caps, or removing coils, might.

KaiS's Website