What Is This Component - Page 1
Forum section: electronics / projects

author: bostonman
author_rank: Super Contributor
timestamp: July 11, 2022, 01:09:31 pm
content: I have a really old 10A car battery charger and assumed it had a rectifier and capacitor inside to generate 12V (or maybe 14.4V). Knowing it's old, I wanted to test the output voltage under a load, so I connected it to a variable DC load box and discovered the output voltage was all over the place; along with high pitched whistling.When I opened it, I found it only had a transformer, what appears to be a miniature light bulb, either a terminal block or some sort of fuse, and the obvious front panel amp meter.Does anyone know what the miniature light bulb is and why it's in the circuit? While looking at the picture, is that some sort of temporary fuse or just a terminal block?On a side note, it appears this charger (and maybe all car battery chargers) rely on the impedance and capacitance of the car battery to filter and regulate the voltage. I expected to see a blown capacitor along with a simple bridge rectifier, but all I saw was basically a transformer.With all the electronics in cars, is this type charger still practical to use? I'm uncertain how to calculate the capacitance and impedance of a car battery. I'm sure if I measure the drop across a car battery I'll get the resistance, but not sure if it's safe to rely on the battery to act as a capacitor.

author: mariush
author_rank: Super Contributor
timestamp: July 11, 2022, 01:14:54 pm
content: It's not just a transformer.There's probably two big diodes right there in the center of your first picture doing the rectification of your center tap transformer. Or maybe it's a bridge rectifier package. Can't tell from picture.you have one wire directly from the center tap of transformer to the battery output, and the other goes through the on/off switch and through that neon bulb / fuse somethingThe whole case is probably used as secondary ground

author: bostonman
author_rank: Super Contributor
timestamp: July 11, 2022, 01:20:01 pm
content: Oh, you mean the red thing that I assumed was a fuse block could be two diodes?That would make sense. I actually should have considered that since the output wouldn't be allowed to swing negative.As for an on/off switch, there isn't one.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 11, 2022, 02:28:00 pm
content: Looks like something bimetallic strip inside that bulb. Oh, maybe a thermal circuit breaker, hence also being positioned on the heatsink? And yeah definitely some manner of diode, unconventionally packaged there, not a terminal block.I don't have a clue what would be "squealing", it should just emit a hum, and the output is unfiltered DC -- shitty, but par for the course for lead acid charging. Ripple doesn't actually matter all that much in the grand scheme of things. Just remember to use a timer, it'll likely overcharge if left alone.And as such, no regulation to speak of, output proportional to mains voltage. Which fluctuates up and down over time scales of minutes to hours.Tim

author: tunk
author_rank: Super Contributor
timestamp: July 11, 2022, 03:52:51 pm
content: I'd guess the regulation is by the transformer, i.e. the transformeroutput voltage drops when you draw more current. How did you set theDC load? If set to constant power, then it tries to draw more powerthan the transformer can deliver, which could explain the whistling.

author: bostonman
author_rank: Super Contributor
timestamp: July 11, 2022, 11:15:42 pm
content: I must have got somewhat confused in the process of looking at the transformer and that it has to be somehow rectified. For many reasons, I should have considered that red thing to be a rectifier, but I think the output getting rectified got mentally thrown aside when I didn't see a capacitor; and that made me forget I was looking at a 0 to +X volts charger.I forgot the sound it was making, but it wasn't a typical slight hum, this was quite loud. The variable load box is something I bought off Amazon a few years ago. It's a 180W maximum I believe and that car battery charger is 10A, so I was able to load the charger.When I heard the humming, I was interested to know what the spikes were, so I measured it with a scope, but the spikes were huge, so, again, I thought the capacitor(s) were shot and didn't put much mental thought into what I was looking at. Eventually I grabbed a very large capacitor that had a 200 ohm resistor across the leads to avoid it self charging. Once I connected the capacitor (and resistor), the noise stopped.I'm curious whether this (and other old chargers) are safe for modern cars. The spikes I saw on the scope were huge, but I didn't take time to actually measure them because I figured I'd see giant capacitors blown up. Car batteries come in various sizes, so I assume this changes the impedance and capacitance causing the battery charger to vary based on the battery its connected to. Large spikes can obviously damage the electronics in modern cars, so I wonder if modern chargers are regulated to 14.4V (or something close) to avoid such spikes).Obviously old cars didn't have electronics, so, other than blowing up the battery, you couldn't overload anything in the car unless you turned on power such as the radio. My assumption is this battery charger may be better used as a paper weight because it's unregulated and dependent on the battery loading it.Measuring/calculating the internal resistance of the battery is fairly easy, but what about calculating the capacitance?

author: BrokenYugo
author_rank: Super Contributor
timestamp: July 11, 2022, 11:42:56 pm
content: This is pretty typical construction, yours is fancy enough for the panel meter, but not fancy enough for a slow/fast or 6/12V tap and switch. Power transformers must be measured under load, I wouldn't doubt 20+V peak unloaded. Auto electronics are ruggedized against far worse (look up "load dump", easily double that), not that they'd see it because the battery will load it down to more like 14. It is still a perfectly serviceable tool, though the automatic electronic type is more convenient.Regarding noise, not much in there to make noise, I'd guess either some disagreement and oscillation with the presumably electronic load, or loose core lamination (those rivets look cheap and weak). Stick it on a discharged battery and see how it behaves.

author: james_s
author_rank: Super Contributor
timestamp: July 12, 2022, 12:43:06 am
content: That little glass thing is an automatic resetting thermal circuit breaker. If you overload the charger it will heat up and open the circuit.