How bad is this phone charger?

[lti]

This is a Just Wireless Blackberry/Treo charger that was purchased a few years ago. It produces 5VDC with an unknown maximum current (they put the wrong label on it).

There is no fuse. The bridge rectifier diodes are 1N4007s. The primary filter cap (C1) is a 4.7uF 400V Fullcon (the logo reminds me of Fujicon), and the small cap next to it (C4) is a Jakec 33uF 50V. The output filter cap is a CA 470uF 16V and is probably not low ESR. The largest cap is 8mm in diameter. It has a very small transformer.

There is also a current sensing circuit on the output. It is a voltage divider in parallel with a diode. The voltage divider is connected to the base of an S9015 transistor that switches on a yellow LED if the current draw reaches a certain level. The LED is a bicolor yellow/red part, and the red section is powered constantly.

I tested this charger by connecting it to my cheap computer speakers. The amplifier produced a buzzing noise that sounded like a nest of angry bees. I turned the volume up and turned the charging indicator into a level meter. The charger didn't like that, judging by the smell it produced.

Is this charger dangerous? Is there anything that needs to be improved? I am not currently using it, but I am keeping it as a spare.

[b700029]

Re: How bad is this phone charger?

Looks similar to this one, but no optocoupler?
https://www.badcaps.net/forum/showthread.php?t=11786

[lti]

Re: How bad is this phone charger?

Should I add a zener diode on the output for protection?

How much power will it be able to produce? I don't think it could handle a current draw of 500mA. That transformer is too small.

Should I install a low ESR output filter cap? I have a 1000uF 10V cap that is low ESR and can fit in the case.

[lti]

Re: How bad is this phone charger?

I just opened this charger again and need to make some corrections:
The primary filter cap is a Fullcap RD series. The brand name is printed in the same font that Fujicon uses. I don't think the two brands are related.
The Jakec cap is actually 0.33uF.

This charger produces 5.98V with no load. With my headphone amp connected to the charger, the output voltage is 5.20V. When the charging LED turns on, the voltage is 5.14V. This appears to be an unregulated power supply with no feedback circuit.

[japlytic]

Re: How bad is this phone charger?

Some serious corners have been taken in this charger...

[PlainBill]

Re: How bad is this phone charger?

And your point is???

It is apparent the Blackberry / Treo has it's own power regulator that both charges the battery and provides the proper voltages to the electronics. The 1N4007 is a reasonable choice as an input rectifier for a low current power supply. The power supply was not designed to provide power to a headphone amp. Complaining about the high ripple is pointless.

Is it dangerous? Only in the hands of someone foolish enough to make modifications without understanding it's design.

PlainBill

[Toasty]

Re: How bad is this phone charger?

Ouch!

[lti]

Re: How bad is this phone charger?

I know that it was designed to charge a phone and that the phone has its own regulator inside. I connected it to an amp because it would allow me to measure the output voltage of the charger under load. I realize that the components they used will work fine for this application.

I was wondering if its output voltage could rise to a level that would damage a phone and how much power it might be capable of producing. It should work fine as a phone charger until something fails. The charging circuit in a phone might not be able to handle nuch more than 5V.

I actually did connect it to a phone. It didn't get very warm, so it doesn't need upgraded. The charger works well for its intended purpose.

[Th3_uN1Qu3]

Re: How bad is this phone charger?

I was wondering if its output voltage could rise to a level that would damage a phone and how much power it might be capable of producing.

Yes, but phones have protection diodes. Unless corners have been cut in the phone as well...

The charger isn't able to supply enough current to melt something if that's what you're asking. Okay, maybe it'll melt itself. But not anything more than that. Instead of a fuse you likely have a fusible resistor somewhere in the primary. It is usually in series with the AC input, before the rectifier, but that isn't always the case.

[lti]

Re: How bad is this phone charger?

Instead of a fuse you likely have a fusible resistor somewhere in the primary. It is usually in series with the AC input, before the rectifier, but that isn't always the case.

There is a resistor before the bridge rectifier.

And your point is???

It is apparent the Blackberry / Treo has it's own power regulator that both charges the battery and provides the proper voltages to the electronics. The 1N4007 is a reasonable choice as an input rectifier for a low current power supply. The power supply was not designed to provide power to a headphone amp. Complaining about the high ripple is pointless.

Is it dangerous? Only in the hands of someone foolish enough to make modifications without understanding it's design.

I never said that the diodes were too small. The DVD recorder I tried to repair used the same diodes in its bridge rectifier. I know that phones have regulators and a charging circuit inside them. As I said in my previous post, the amp simply provided a load that would allow me to measure the output voltage. I don't rememmber saying that I planned on using it for anything other than charging phones.

Ouch!

[Th3_uN1Qu3]

Re: How bad is this phone charger?

The fusible resistor is used to limit the current that the charger can draw from the mains. If you try to draw more, it doesn't allow you to. If you keep trying, it eventually blows. It's the reason why it's not feasible to squeeze more current out of this type of supply (also why it is inherently safe for its intended use - even if the feedback loop goes bonkers, the current draw of the phone will always keep voltage in check).

It also allows the use of a really lame switching transistor... If you change the fusible resistor for a regular fuse, the transistor will likely blow to bits before the fuse does.

[lti]

Re: How bad is this phone charger?

I am not going to upgrade the switching components to try to get more power from it.

This will be my emergency replacement charger. I will use it when another 5V charger fails. I might be able to adapt it to use the tips that came with a Philips USB charging cable along with the included tips.