Re: Transformer Current
- From: Ross Herbert <rherber1@xxxxxxxxxxxxxx>
- Date: Fri, 06 Feb 2009 07:00:51 GMT
On Thu, 5 Feb 2009 08:50:59 -0800 (PST), usenet2@xxxxxxxxxx wrote:
:Nutshell:
:
:I have a lead acid 12V battery charging device that used a wall
:transformer rated as 120 VAC 26W input, 12 VAC 1.6A out. The xformer
:failed (primary opened) and I made a replacement (12.6 VAC 3A out) but
:it is drawing 4.5A and blowing the secondary fuses I put in place.
:
:Synopsis:
:
:I have two "Basement Watchdog" battery operated sump pumps. The
:batteries are 6 cell 12V lead acid batteries. The control panel on
:each unit charges the batteries as well as giving status and alarming.
:The controller accepts a dry contact (float) to trigger a pump cycle.
:It has some led's, a piezo and a button to silence alarms/run pump.
:The main load runs from the battery all the time.
:
:One of the devices gave me an AC failed indication. I checked the
:transformer and was getting nothing from it. I removed it and went on
:to other things. Several days passed and w/ rain in the forecast I
:decided to top the charge of on the battery by moving the wall
:transformer from the (still) good unit to the one that had failed. Not
:too bright on my part because poof went the other transformer. Turns
:out there is a problem in the one unit that caused the transformer to
:fail.
:
:I pulled the failed control unit out and peeked inside and found a
:shorted 6A bridge rectifier diode. I went to RS to get a replacement
:and noticed they had a 12V 3A xformer so I decided to build a
:replacement, one that would run both units. I made a simple box, wired
:up the xformer (full voltage, not using the center tap), paralleled
:one secondary leg to two fuses (one for each control unit). I
:connected the wires to the control unit to one of the fuses and the
:other secondary leg. Secondary was giving 14.6 VAC, no load.
:
:Power up and connect to the pump control unit and all seems ok but
:then after about 20 seconds the fuse goes. Investigation shows that
:the control unit is drawing about 4.5 A. Remember, this unit is the
:one that had zero problems at all, the repaired one is still on the
:bench.
:
:Can someone offer any ideas as to why the charger is drawing far more
:than the original transformer was rated? I cut one open and could not
:find any identifying marks on the transformer. The primary had failed
:and the secondary was fused at 5A. The fuse was intact. Input was
:labeled as 120 VAC 26W, looks like 0.2A roughly. Stepped down to 12V
:that would be 2A.
:
:I am not certain why the high draw - maybe the initial charging cycle
:draws much more than 2A, for some period of time, and then bleeds
:down? That would mean the original transformer was rated for much
:higher than what the wall transformer was labeled. It was fused at 5A
:on the secondary.
:
:Thanks.
Those "wall transformers" (we call 'em plug-packs) will invariably have a
thermal fuse built into the primary winding. If a prolonged overload occurs,
which may not be sufficient to cause an external fuse to rupture, the thermal
fuse blows in order to prevent a fire or other hazardhous situation.
Unfortunately, this also means the transformer is now defunct and unrepairable.
It appears to me that whatever else is connected to the output of the
transformer then it must have a low impedance or short circuit in it. When you
replaced the 6A diode did you investigate to see what caused that to blow? If
you simply replaced the rectifier without checking for other failed components
then you may now have another dead rectifier as well.
.
- References:
- Transformer Current
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