Current limiting the output of a power supply.

[drpappy]

--What approach should I take to limiting current?--
I have a constant voltage power supply into a DC converter and a load. There is also a backup battery being trickle charged. My trouble is that traces and components are burning and the 15A fuses are too slow.

--RF load
|
28VDC--f--D---------DCto12VDC
|
24V Batt

The power supply is a Lambda SE-500, 500W.
The fuse is AGC15, 15Amp.
D is a Schottky diode, 30A.
The DC buck converter is a Power Products WS series, 100W.
RF load is roughly 50W.


I don't know where the damaging energy is coming from (probably when AC is lost), but am thinking of resistor current limiting. Should I be trying to crowbar the power supply instead?

 

[IRstuff]

STOP..

You need to answer whether you are overloading the PS in the first place. Is there something wrong with your load now that didn't exist before?

Why not get a bigger supply if the load is working correctly? Your patches are going to cause a shutdown in an overload condition.

TTFN

 

[OutToLunch]

I can't see how the fuse would blow when the AC is lost. I can see some damaging effects occuring when AC is restored, though. You have a 24V battery that is tied to the 28V source through a schottky diode. When the AC kicks on and the 28Vdc comes up there will be a large inrush current from the 28V supply to the battery through the path of the fuse and diode. The simplest solution is to place a resistor in series with the battery to limit charge current - size the resistor value for the lowest voltage that the battery will go to. As the battery charges up, the charge current will decrease. Don't forget power rating on the resistor, either. Do you have a cut-off circuit that will disconnect the battery if it gets too low when the 28V is gone?

 

[itsmoked]

drpappy; OutToLunch is correct that battery will look like a very large load once power is restored. Easily enough to fold back your supply or blow the fuse. You can also have the condition that if the battery drops low enough to stop your RF load when the AC is returned even if the fuse doesn't blow you might not get your RF-load supplied for a while.

Unless your system actually disconnects the battery once the battery is dead (about 24V if lead acid) you will need to actually install a resistor to limit the current into the battery to the amount of power available from the supply minus the RF load and the buck converter.

AC PS - RF load - buck load = power remaining for batt charging.

 

[drpappy]

...The load hasn't been changed, however if it's sagging then that might explain a noise/modulation problem I just heard about on another unit. I just changed the 100AH battery manufacturer because it wasn't lasting more than 18 months -- meaning too high a trickle (float) voltage, or just bad batteries. I'll check into the RF load some more.

...I do have a cutoff circuit set at 80%. A current limit resistor only on the battery leg sounds reasonable. But, I wasn't able to measure a significant amount of current when AC kicks back on, using a clamp meter.

Perhaps the problem with this design is that the RF load doesn't have it's own DC-DC regulator? I've got users probably setting the float voltage too low to save battery longevity, and maybee messing up the RF load.

I'm determined not just to add another fuse, there's already three scattered around this tiny circuit.

(more detail of 24V batt circuit)
..............
|
f
|
12V batt
|
f
|
12V batt
|
xstr to GND

--Thanks for the good ideas; back to the bench I gor

 

[ScottyUK]

A 100AH battery is an expensive bit of kit - is the PS suitable for battery charger duty? If not you should consider a dedicated charger. Battery chargers tend to be a breed apart because they are designed to work around the non-ideal battery characteristics.


----------------------------------

If we learn from our mistakes,
I'm getting a great education!

 

[IRstuff]

Your 100 W DC-DC supply is capable of drawing nearly 14A at full load and 60% efficiency. Any additional load would quite easily blow your fuse.

TTFN

 

[itsmoked]

You might want to think about this mod;


--RF load
|
28VDC--f--D---------DCto12VDC
|
|
-----
| |
^ R
D |
| |
24V Batt

Where D is a diode
R is a charge only current limiting resistor.

No current limit when using the battery only when charging.

 

[elfgriper]

drpappy,
Some people use a soft start circuit where they place a current limiting resistor in series with the output of the PS. When the voltage comes up to the proper value they energizes a relay that shorts out the current limting resistor and connects the load at the same time. this allows the power caps (pass stage or switcher ps) to charge without having to drive the output, a possible poor load under low voltage startups conditions. This is fairly simple circuit to implement.
-elf

 

[drpappy]

...Scotty, Have you been getting decent life out of your gel cels lately? The old lead calciums lasted 15yrs I hear.

...IRstuff, the fuses are'nt protecting my 0.2"/10A traces. Someone suggested adding more wire in parallel!

...itsmoked, niice
I had been thinking of something similar but using a NIS5101 hot swap current limiter in a kind of H-bridge. But, they only handle about 6 amps. I'll try your way. I need to find a low drop diode, to not mess up the backup battery runtime. Yesterday, I dialed in 0.4 Ohms in series before the cutoff circuit kicked in (24.24V) while on battery discharge.

...elfgriper, That sounds neat. I need to look that over.

Thanks for the design advice.

 

[itsmoked]

Hi drpappy;

The two most common killers of gelcell PB batteries are:

1) They very rapidly, permanently sulfate, losing huge chunks of capacity which means the next cycle is going to be even worse. I have found that many gelcells will be utterly ruined after being discharged down to 11.5V and left for 24hrs... That's it toast! So all you need is one AC power fail event that drags them down below about 12V (12V battery) and you may have a ruined/damaged battery.
Any reliable UPS system like yours absolutely positively needs a circuit to disconnect the batteries the moment they are *just* fully discharged, never allowing further discharge.

2) They die over time, weeks or a few months, because the batteries have their charge profile violated. If you are going to float them you MUST not exceed the recommended float voltage or they will just heat up and evaporate away their electrolyte.

I run many UPS. They all run well and do their jobs for years usually over 6 before the battery capacity is down to about 50%. This is because they have well designed charge circuits. They have staged chargers that limit initial charging rate then go to bulk charge then fall back to a much lower maintenance float.

Your set up is like the bare raw functional hook up. I would expect lousy battery life with this system. Your system would be much better served by having a true battery charger minding the battery with something like a relay that drops out when AC is lost, only then hooking up the battery bank. This would probably work glitch free because your original AC->DC supply will hold up the load long enough for the relay to drop all the way out (or in as it were).

You still need something to disconnect the battery at the fully discharged point.

Here is the absolutely best cheap charger I have found.

http://batterytender.com/default.php?cPath=11_3&osCsid=11ec48b514c4c983cba0047e4876f877

http://batterytender.com/default.php?cPath=11

It even has an idiot light on it that tells you important useful stuff. Like charge and floating.

 

[ScottyUK]

drpappy,

ItSmoked has posted some good comments regarding cell life. I consider five years service for gel cells to be reasonable, although the better brands are often good for a lot longer if an intelligent charger is used and they are not cycled too deeply. Most of ours are in critical applications such as switchgear tripping batteries, and running the battery until it fails is not an option: 15 years to failure might be possible; it's just not something we can afford to find out. Gel cells aren't the first choice cell type for this application, but the high capital cost of plante cells worked against us and we lost the battle with the accountants. We use the best gel cells we can find. The Hawker SBS range is pretty good.

Personally I'd be budgetting for cell replacement after five years on critical batteries. We have a planned replacement at five year intervals because experience shows that the capacity decreases and failure rate increases beyond this point.



----------------------------------

If we learn from our mistakes,
I'm getting a great education!

 

[IRstuff]

if the current running through your traces are normal, then your board is underdesigned and should be re-done.

TTFN

 

[itsmoked]

IR is correct, but you can solder external wires on for a stop-gap measure.

 

[drpappy]

Yes, our big accountants switched us to lighter gel cels that you can't add water back in.

The typical current is 9Amps [Fluke DC current probe]. I'm shying away from adding more wire. Our safety guys would hari-kari; can't have that. I checked the repair records on the RF load and the DC converter. RF load almost nil, except for a little cheap mica keeps going out. DC-DC module may be the culprit with it's failure record.

I still want to put in a variable resistor to the battery, so users won't be tempted to turn down the power supply to meet battery float voltage maximums. I like the relay instead of a diode to put in parallel. -thanks