NiCad charger getting hot

[Randall]

"You could test it with a hair dryer if you like- PTC, so resistance should increase with temperature. Does it locate near the heat sink as I conjectured?"

It is about 1/2" from the heat sink, but I still don't see how this would make the transformer heat up like this.

"But where are you measuring the 3.6mA ? I don't see how that kind of draw would make the transformer get hot. That's like one of the LED's turned on."

Measured across the open fuse at the secondary of the PT. No LEDs are on.

[Helmholtz]

Measured across the open fuse at the secondary of the PT. No LEDs are on.

What matters regarding transformer heating is primary current.

Please measure with and without secondary load.

Do you have a "Kill a Watt" meter that reads real input power?

[Randall]

Current measured by placing ammeter in AC line is 0.093A loaded and unloaded, that is with secondary fuse in/out, but not charging a battery. 11.6 watts.

[Helmholtz]

Current measured by placing ammeter in AC line is 0.093A loaded and unloaded, that is with secondary fuse in/out, but not charging a battery.

At 120V that means an apparent input power of 11.2W, which is quite a lot for a small transformer (what are the core dimensions?). Unfortunately without a "Kill a Watt" we don't know the percentage of real power (heating up the transformer) and reactive power.

[nevetslab]

The BR and the X0405 thyristor are the components mounted to the heat one sink. They are not real close together. I don't see a thermistor.

"Those bats are OBSOLETE!"

Tell that to the 40 -50 year old satellites that are still running on NiCads. They can be rejuvenated, I just did it to a "dead" Ryobi 18v battery. Totally dead, wouldn't even turn my drill after 1 hour on charger. Put the battery in the charger with red light on, unplug/replug the charger several times, surging the cells until the grren light comes on. Now a dead battery charges up to 20v, and holds a charge for days. How's that for obsolete?

Now if I can just get this charger to cool down, I'm good for another few years.

I have a pair of NiCd batteries in a vintage General Radio 1564A Sound & Vibration Analyzer (1/3 Oct, 1/10 Oct, All Pass) that I bought used in Aug 1992 for $40 at a Radio Amateur's Technical Swap Meet TRW in Redondo Beach, CA, and those NiCd batteries are STILL going strong. I don't know who GenRad found as a battery supplier, but they picked a great one! (Gould 9.6V x 2)

[Randall]

I have 125v here, puts it at 11.6W. Laminate dims are 2.5" x 2.25" x 1.15".

I just ordered a Kill A Watt meter.

[Leo_Gnardo]

I just ordered a Kill A Watt meter.

You're bound to have some fun with that! Excellent piece of kit to have around the workshop & house.

[g1]

Measured across the open fuse at the secondary of the PT. No LEDs are on.

My thinking was that during normal operation, when a LED was on, drawing that kind of current, the supply should not overheat. 3.6mA at the secondary should not pull approx. 100mA from the primary, in any case.
I think we may be seeing a limitation of the neon bulb test for transformers, as you said it passed.

[Randall]

"I think we may be seeing a limitation of the neon bulb test for transformers, as you said it passed. "

I was thinking the same thing.

[Helmholtz]

Shorted turns should show in the neon bulb test and would most probably cause way higher input current.

I think it's just a poor transformer design with relatively high magnetizing current. This is caused by low primary inductance. I also assume relatively high primary DCR.
DCR times input/magnetizing current squared heats up the winding. (Any chance that the core laminations loosened allowing Es and Is to separate?)

I just measured a smaller transformer having a core size of 2.15"x1.79"x0.8". Apparent input power at idle is 8.4W, real (heating) power 2.1W. Gets barely warm.

[Randall]

So what is the bottom line here? I don't remember it always getting hot like this. Is the transformer malfunctioning? I have included a couple of shots of it, but it doesn't look to me like there is anything amiss with the laminates or anything else. I looked it up, it is used in microwave ovens as well as Ryobi chargers, and they are available used for a price.



edit: after being plugged in for over an hour out of the case and with secondary fuse out, the laminates measure 128*F at the top of the transformer, somewhat less at the bottom.

[Helmholtz]

Bottom line is that a PT that gets too hot even without a load should be replaced because it is a potential safety risk. Even more alarming if it ran cooler before, as that means that something inside the PT must have changed and might continue changing.

The neon bulb test shows that there is no problem with the windings. I rather assume that for some reason (temperature, vibration, mechanical shock, corrosion) the effective airgap of the core has increased. This would explain increased input current and transformer dissipation/heating.

[Randall]

"I rather assume that for some reason (temperature, vibration, mechanical shock, corrosion) the effective airgap of the core has increased. This would explain increased input current and transformer dissipation/heating. "

Well it does live in my garage always plugged in, in SW Florida where it is often hot and humid. Time for a new one I guess.

[nosaj]

"I rather assume that for some reason (temperature, vibration, mechanical shock, corrosion) the effective airgap of the core has increased. This would explain increased input current and transformer dissipation/heating. "

Well it does live in my garage always plugged in, in SW Florida where it is often hot and humid. Time for a new one I guess.

Man if it's been heat indices of 105 to 113 down there like it's been here they need a new word for hot n humid maybe F'ing hot!

nosaj

[Randall]

I scored a good used charger for $15 shipped. It doesn't get nearly as warm, even while charging. Buh-Bye potential fire hazard!