Current limiting the output of a power supply.
Current limiting the output of a power supply.
(OP)
--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?
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?





RE: Current limiting the output of a power supply.
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
RE: Current limiting the output of a power supply.
RE: Current limiting the output of a power supply.
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.
RE: Current limiting the output of a power supply.
...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
RE: Current limiting the output of a power supply.
----------------------------------
If we learn from our mistakes,
I'm getting a great education!
RE: Current limiting the output of a power supply.
TTFN
RE: Current limiting the output of a power supply.
--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.
RE: Current limiting the output of a power supply.
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
RE: Current limiting the output of a power supply.
...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.
RE: Current limiting the output of a power supply.
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.
h
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.
RE: Current limiting the output of a power supply.
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!
RE: Current limiting the output of a power supply.
TTFN
RE: Current limiting the output of a power supply.
RE: Current limiting the output of a power supply.
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