Power Supply being loaded down. Why? 1

[MarcSylex]

Hello everyone,

Totally new.

I have a test rig that operates on 28V and powers (10) electronic components. Typically when we run the system we used the power supply that wil be used on the final system and powers everything up with no problem.

My problem is when I power the test rig externally using a power supply set to 28V (voltage limit at 28V) and turn the PS output ON I botice the voltage on that PS go down to 5V and current reaches a limit set to 1A.

Do I need to raise the current limit on the ext.PS to ensure I have enough power going to the system to power all (10) units? IS what I described called "loading down the PS?"

Thanks,

 

[waross]

What is the normal current demand of the devices you are powering and do they have an energization surge?
respectfully

 

[dpc]

Agree with waross - you need some idea of the anticipated current demand on your test rig. Who knows if 1A is enough or not?

Are you sure this external PS is functional?

 

[jghrist]

You might have a short-circuit that is not cleared by any protection because the power supply limits the current to 1A.

 

[sreid]

If there is a large inrush current to the load (load capacitors), the power supply can go into fold back current limiting.

 

[MarcSylex]

The system works fine for the regular power supply.


Actually for this test I'm running there should only be three devices powered. I was thinking about the whole system when I should not have.

There is one electronic component that draws around 0.120-0.150ma and two others that draw ~0.300ma each. By the way these three are all on 1 channel and each channel should not require more than 1A as I just learned.

It just doesn't make sense that this PS would get loaded down by so much leaving only 5V, it has a power rating of 290watts.

If I take one of the electronic units out and power it removed from the rig I can power the unit fine and keep my 28V @ 0.120 mA

 

[MarcSylex]

In response to waross and sreid, yes these devices do have an energiztion surge.

My power supply may not be able to handle the initial surge. In that case is there some way to set it so it can handle the surge current?

thanks for ur help by the way

 

[Skogsgurra]

It usually helps to connect a large electrolytic capacitor parallel to the output. Make it large enough to deliver that switch-on surge without voltage dipping too far.

Another way is to apply voltage slowly. That may not always work if the load contains switchers, which usually draw more current when voltage is low.

I would test with a large electrolytic. But remember to disconnect it if you use the PSU for other purposes. Sensitive loads, that need the current limit, may be damaged otherwise.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[sreid]

With all due respect to soggsgura, I think that adding an electrolytic capacitor will only make the inrush surge worse.

Switching power supplys have a soft start who's speed varies from manufacturer to manufacturer. At start up there is also a current trip higher than the power supply current rating. This also varies from manufacturer to manufacturer. And often it is not spec'd or it is burried in the spec sheet somewhere.

Linear power supplies are usually much more forgiving on inrush current. A 28 V, 2 A supply would not cost much.

 

[Skogsgurra]

Thanks for the respect.

The role of the capacitor is to deliver the As:s needed to cover the inrush. And, yes, it increases the current surge, but that current surge will not affect the current limit.

It is a tested and proven technique.

Gunnar Englund

http://www.gke.org

--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[itsmoked]

The five volts as mentioned is HOW the supply limits the current. It automatically reduces the voltage to whatever value is required to not exceed the current limit.

I have read your post several times and cannot figure out if the supply you are using is the same type you ship it with or another type altogether. Is it possibly a LAB supply that lets you turn a knob to set the voltage and another knob to limit the current?

If this is the type you have and you suspect nothing is wrong with your other stuff, since it all powers up individually, then YES turn up the current. Turn off the power supply, turn up the current limit dramatically, and turn the supply back on. You can also remove the load, turn up the current limit and reapply the load. Take your pick.

I would not just turn up the current limit while in your stalled mode as this is often an undefined state for your loads and you may get strange happenings that let smoke out.

Skog's capacitor works great if the load is applied to the running PS. If it is just added, then the supply is turned off and on, I would expect little if any improvement in this described scenario.

The device I am working on at this moment needed me to add 4,700uF at the input terminals or my 65 pound 1.2kW lab supply goes in to a terrifying oscillation mode where I start fearing for my health, as I believe it may jump off the shelf and land on me.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[Skogsgurra]

I see a switch between PSU/CAP and load. Capacitor charged all the time.

Gunnar Englund

http://www.gke.org

--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[MarcSylex]

In resposne to itsmoked:

Yes the PS does have a dial to adjust voltage, current and power limits.

In my first post I did want to increase the current limit but I just wasn't sure how high was safe enough to cover the inrush without casuing damage to the rig.

At my job someone more familiar with the system would agree with you since he says to raise the current limit to about 3A.

I still don't understand what difference there is when powering teh unit on the rig versuses powering the unit individually. The only thing I can think of is our rig does have alot of wires, I means alot, several feet worth.

thanks again all. I appreciate the expert advice.

 

[itsmoked]

And you still haven't clarified to me whether this 'rig' supply is the same as the 'shipped with' supply.

But..

If you've tried the individual loads and they seem fine, AND, the supply just limits to 5V rather than 0V. I'd turn up your rig supply to MAX. That's what I'd do. I'd leave it set there until you've finished loading all the individual loads.

Yeah Skogs that would probably do the trick - I agree.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[waross]

The current limit does not control the current, it limits the current. The load determines the current until the limit is reached. Then the voltage drops as required to limit the current to the set limit.
The current limit is more to protect the power supply than to protect the load, and possibly to limit collateral damage if a load starts to let the smoke out.
The current limit will normally come into operation only when a device is failing.
When skog's capacitor is initially energized the 1 amp current limit will probably hold the voltage at a low level but the voltage will rise as the capacitor charges. When the capacitor is charged the voltage will be normal and you are good to connect the load. Be aware that in the event of a load problem there will be no current limit until the capacitor discharges into the fault.
As a first solution, just crank up your current limit.
If you can not energize the load at the maximum current limit setting then add skog's capacitor and switch.
respectfully