Posted on 05/10/2023 8:07:46 AM PDT by Paul R.
There are many surplus and refurb chargers / adapters for powering laptop computers on the market these days. Ebay, for example, has loads of them at good prices. And adapters from good brand names like Dell and HP seem to be quite reliable.
By their ratings, these would appear to be useful for other projects. However, I was curious as to how consistent the low or no load voltage output was vs. the voltage at rated output. In the case of my Dell DA130PE1-00, a 130 watt adapter, there is just the "simple" rating if 19.5 DC volts @ 6.7 amps. So, I first tried using my Fluke multimeter to measure the output voltage of the adapter at its output plug, with its input plugged into a live wall socket, and... nothing. Voltage = "0".
Check the meter by measuring a battery -- good. Plug the adapter back into the laptop - the laptop is charging.
I can only guess the adapter either detects "no load" and suts down its output in that event, or, the adapter "talks to" the laptop via data riding on the would DC supply lead, and only provides current if the laptop tells it to (and, then, probably how much to provide -- a computer controlled charge.)
That's as far as I got.
Maybe that's why they are cheap, on eBay?
Anyone have any knowledgeable insight here?
Try one and see what happens.
Chinese wall-warts and power supplies, especially ‘bargain’ ones are notoriously noisy in the RF department. Get an SDR, plug in a chinese power adapter and observe bands of noise from DC to cosmic rays.
Bkmk
Very true. An AM radio tuned to a weak station makes a good detector.
My understanding is the same as what you were speculating on - that it is a controlled charge. In the old days, the adapter just pumped power into the batteries, whether they needed charging or not. “Modern” adapters when paired with the matching battery bank will attempt to optimize the charge to what the battery needs to maintain a charge, provide a full-charge, or just maximize battery life.
It’s logical then to assume that you need to match the power-adapter you purchase to the device you’re charging, following the manufacturer’s specifications.
Most if not all “brick” power supplies and any power supply inside a standard desktop computer is a “switching” power supplies. They develop their output by rectifying and filtering a high-frequency (maybe 100 KHz) oscillator inside. One of the several advantages of these types of power supplies is that they do not need a heavy-with-iron power transformer. If you have some device that uses a “wall wart” type of external supply, maybe 2” x 2” x 2”, you may note that such a supply is fairly heavy. That’s because it has an iron transformer inside. Those are called “linear” supplies. Should you wish to develop 6.7 amps out of a linear supply, you’d probably be looking at a fairly heavy item. Switchers are more efficient; but they tend to fail a bit more and they almost inevitably place residual noise (from the internal oscillator) on the DC power they are designed to create. In most cases, that noise does not matter, or, stated differently, the thing the power supply is designed to power up is immune from such noise by design.
Without going into further minutiae, you have the right idea. Switchers almost universally require a load to work. Linears do not. They do their voodoo without any load placed across the output. If you go to youtube and search for “PC desktop bench supply” you find multiple videos where folks converted a PC power supply into an experimenter benchtop supply. Which is pretty cool, because presumably you can get an old PC power supply for free from a discarded computer....unless the supply itself is the cause of death. You might have to pay $100 for a benchtop experimenter power supply....but of course, you don’t have the meters, the pretty case, the carry handle, the convenient binding posts. But that’s “experimenter” tradition. My point is, that EVERY one of those projects requires placing a load resistor across the output wires of the adapted PC power supply. The load resistor fools the switching power supply into thinking that the device it is supposed to power is actually there when it isn’t.
You might have to introduce a small load (resistor) across the output to get the PS to fire up. Then you can measure actual voltage.
Yeah, the charger detects if the battery is low so it can stop charging when it is no longer needed. That’s why some chargers have a light on them, so you can see (if it is lit) that it is still charging, and if the light is off, the battery is fully charged.
I assume you are aware that on Dell chargers the center pin is NOT part of the power supply output but some sort of sense line so the laptop knows how many watts the power supply is?
Look closely at the plug and you’ll see that the outer barrel is double-sided, with metal contacts on both sides—that’s where the output of the power supply is connected.
Some pictures here show exactly what I just described:
“Switchers almost universally require a load to work”
Wow! I connected a 12 volt muffin fan to a little car battery charger. Fan didn’t work but it the charger works on car batteries
Do you suppose that muffin fan looked like “nothing” to the charger?
That should work, assuming the fan wants to see 12 VDC and not AC. (AC would be rare) I’ve powered muffin fans to blow away solder fumes on my bench with wall warts plenty of times. I usually power a 12 volt fan with a 9 volt supply so the thing isn’t so screaming loud.
Without knowing anything at all about your situation, yeah it’s possible that the fan isn’t enough of a load to turn on a switcher. A car battery is practically a dead short, especially when mostly discharged. Pick up your little charger. If it’s really lightweight, it’s likely a switcher and it may need to see a much larger load than a little fan. Those fans use about 1 amp at 12 volts. A fully charged car battery draws 2-3-4 amps. Should be enough to start the switcher but if it isn’t, it isn’t. You might have to place a 10-12-20 ohm resistor across the fan wires but said resistor would have to be a biggy, like a 20-25 watt one. Does the fan work? Maybe try placing it across your car battery.
Most of the newer car battery chargers won’t work unless they detect some voltage from a battery.
This means that they won’t work to charge a completely dead car battery.
A good test if the adapter is charging is to have it plugged into the wall outlet then using your tongue, touch the tip of the other end....
Actually, a voltage of zero indicates a depleted battery and signals the beginning of a charge cycle. In that case, a good charger will go through a recovery procedure prior to charging.
The float voltage is at its highest following a charge, still below the charge voltage. A charger stops when the current drawn by the the reverse-biased battery drops below a threshold.
Not the case. A charger will pulse at zero volts in order to sense the current. A totally depleted battery looks like a dead short. For an open circuit, that pulse will draw no current. That makes the charger assume there is no battery connected.
Hi, All!
Sorry to post and run. It was “one of those days” starting about a minute after post #2.
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