Stun gun charging mysteriously

Discussion in 'General Electronics Discussion' started by StealthyBunny, Apr 14, 2013.

1. StealthyBunny

2
0
Mar 22, 2013
Hey guys, i'm totally new here, kind of newbie in electronics and this question is very probably dumb.

I opened my stun gun to see what was inside, and was confused by the battery charging process.

The unit is to be plugged directly into a 110V AC outlet, and the battery inside is made of 4 little batteries packed together, each one being 1.2V (according to the multimeter), for a total of about 5 volts.
Then it seems to me that in order to charge those batteries, the current has to be turned from 110V AC to 5V DC.

The thing is, I followed the path from the charging prongs to the batteries, and it only passes through a capacitor and a bridge rectifier.
So 110V AC enters, and my batteries receive 110V DC ?
Does the combo capacitor/bridge rectifier lowers the voltage (I don't believe so, I read that it just turned AC into DC then smoothes the output) or is it okay for the batteries to receive more than 20 times their voltage ? Help me understand how it works

The capacitor is a 400V HXDCBB22 and the bridge rectifier is a DB107.

I tried to draw the circuit:

(The resistor is for the LED, I don't think it interfers with the rest)

And by the way, do you think I can plug it into a 220V power outlet ?
Thank you !

2. galaxy

28
2
Nov 3, 2012
I beleive the impedance of the Capacitor at 60hz will drop the 105 Volts.

And no, I wouldn't stick it in 220v, unless fireworks were the desired result.

Last edited: Apr 14, 2013
3. KrisBlueNZSadly passed away in 2015

8,393
1,271
Nov 28, 2011
Yes, the series capacitor acts as a current limiter.

With alternating current, a capacitor acts a bit like a resistor. It exhibits a behaviour called "capacitive reactance", which is like resistance in some respects, and is measured in ohms. The formula for capacitive reactance is:

Xc = 1 / (2 pi f C)

where
Xc is capacitive reactance, in ohms;
f is the frequency, in hertz;
C is the capacitance, in farads.

For example, at 60 Hz a 1 uF (one microfarad) capacitor has a capacitive reactance of about 2650 ohms. At 110V this corresponds to a charging current of about 40 mA. The bridge rectifier converts the AC into DC so the batteries are always charged in the same direction.

As galaxy says, you can't plug that circuit into 220V. The charging current will be doubled and the battery could be damaged.

BTW the capacitors used for this function are a special type, designed for continuous operation from the AC mains. So if you want to halve the capacitor value, so you can use 220VAC, make sure you use a suitable part.

If you can post the actual value of the existing capacitor ("HXDC8822" doesn't mean anything to me) I can suggest a suitable replacement if you want to convert to 220V. (Make sure the connectors and cable are rated for 220V first.)

4. StealthyBunny

2
0
Mar 22, 2013
Thank you for your answers, and the detailed explanation.

So from what I read, no need for big transformers and stuff if I want to transform some home AC current to low DC ?, just a suitable capacitor and a rectifier would do the trick ? That's cool. (Well I guess this can only be used on few circuits, or else the capacitor would overheat/burn)

Anyways thanks again, I think I will use some external thing to turn my 220VAC to 110, so I don't temper with the unit and can still charge it on 110VAC later on. (And for the capacitor, part of the label was hidden by the bridge rectifier kinda glued to it, and I put everything back together)

(By the way, if you know a simple/non expensive circuit to halve 220VAC current to use it with american appliances (at least those for which the frequency doesn't matter), i'm interested

5. KrisBlueNZSadly passed away in 2015

8,393
1,271
Nov 28, 2011
That trick can only be used when (a) the required current is fairly low, and (b) everything connected to the circuit is fully enclosed and insulated and there's no way for a person to make contact with it. IT IS NOT SAFE as a general way to power or charge devices. That technique is used in some LED-based light bulbs because it's cheap and compact.

Good idea about keeping the gun in its original condition.

If the gun was designed for 120V 60 Hz mains and you're using 120V 50 Hz with it, it will take 20% longer to charge, because at 50 Hz that capacitor's reactance is 20% higher than it is at 60 Hz so the current is 20% lower.