A Look Inside #1 - Emergency LED Light

[MJLorton]

Some snippets / pictures from the first "A Look Inside" video:

Video description: Teardown of an emergency LED light. An explanation of the circuit, a look at the diode bridge rectifier wave forms on an oscilloscope.

Link to video: http://www.youtube.com/watch?v=htODAdLb2Rk

[SeanB]

That 6V SLA is a fake. Too light, and you will find it is a lot lighter than the replacement. Inside you will find a single plate and a polystyrene spacer taking up the rest of the space inside.

[SeanB]

Tips for rework.

Voltage regulation - Take the 22R resistor and put in series with the diode to battery, to provide some current limiting. I have added simple voltage regulation using 1W zener diodes, a 3V3 and a 3V6 zener in series ( buy around 10 of each and use a power supply and a 100R from 6v to select pairs of diodes that have 6V9 drop, and place them from the junction of the 22R resistor to ground via a 1N400x diode. this will clamp the max voltage to the battery to 6V9, enough to keep it charged and not too high to keep the battery life long. You do not need extra filtering, a little ripple on a SLA charge is not a worry. You probably would replace the OHL 100uF cap with a bigger size same capacitance unit to have a longer life, use a 25V unit to have better voltage margin.

To the light side, you probably want to add a 1R2-1R5 1W resistor in series with the big panel array, and a 10R 1W in series with the small array. Current limiting to make the LED's last longer, though they are already degraded.

To flash the array you take a standard 555 astable, running at 0.5Hz or so, with a diode to have a short high time and a long low time, and use a Mosfet to do low side switching to the common of the array ( this then allows the selection of what flashes via the switch) or make the 4 LED array flash always when power is lost, the switch then enabling use of the big array or not, small always ready to flash on power loss. When using a 555 Ic you will need to have a 1000uF 16V capacitor across the battery terminals for extra smoothing. This will make the 555 happy.

If you go for the ultracaps you can use the zener limiter, and just do a direct replacement of the battery with the 3 ultracaps in series. You do not need the extra capacitance with the flasher in this case.

Addons would be a mains fuse in the marked place on the PCB, and a 240v varistor across the transformer primary after the fuse.

Edit, looking at power dissipation replace the 22R 3W resistor with a 47R 5W wirewound unit, as this will run cooler, and will limit the zener current to 100mA when the battery is full, or disconnected. Will also limit the current to what the transformer can really supply, not the 200 plus ma you were drawing from a 100mA unit. Leave the leads on the zener diodes long to allow them to dissipate heat, or solder on some copper heatsink tabs near the diodes.

[Antraciet]

I think many people like those kinds of vids. Maybe boring for the professional, i don't know, but for me, is more then just a vid. Very usefull.
This vid : http://www.youtube.com/watch?v=cyhzpFqXwdA informative too, with a great sense of humour explained, like all his vids, but yeach...where and what is he measuring ?
That is what the hobbyist, like me, wants to see. There is a problem : open the stuff, and measure it. And above all : let's see where to measure, where to put the probes : the meter, the scope next to it.

[MJLorton]

Tips for rework...

Loads of handy feedback and ideas SeanB...I will look into these options.

Thanks for the post.

[SeanB]

What makes you think I have had some bad experience with these.......................

[runem]

When I saw this circuit yesterday I got a little bit shocked, so I sat down and played around with a circuit that at least has some kind of safety built into it.
Man, charging (and discharging) lead-acid batteries without any fuses or voltage/current limiting is a VERY bad idea in my opinion. Sheesh, wholy smokes!!
I kinda works like this:
A simple voltage regulator, as described by the books, is formed around Tr1 (BD139-10 NPN) with a base resistor (IsetC) and a zener diode to ground. When the mains supply voltage from the transformer is cut off, the current flow through will also stop. The current flow through Tr2 (BD140-10 PNP) will start, with it's base tied to ground via a 2.2k current limiting resitor (IsetL), and deliver power to the load (lamp or LEDs).
Note that both charge and discharge is current limited with the 1 Ohm resistor (IsetLC) and the base resistor for the respective transistor that is active. The battery is also protected with a fuse, if something should go horribly wrong with the circuit or the load.
The circuit is hardly any cheaper than the one in Martins lamp, but it does at least provide some protection, and I'll bet all of the components used costs only a few cents on the Chinese marked, so maybe it's worth it after all(?)


If someone wants to play with the circuit, here is the link to the Falstad simulator (must have Java installed and enabled):
http://tinyurl.com/8msatz2

[SeanB]

Poor transistors...... Run with negative base bias on the off one, and with a 6V SLA battery they will have reverse leakage that is high, enough to degrade them badly with time. You will have to put a series diode in with the base leads, so as to reduce the reverse leakage if you want the transistors to have a life close to the battery.

[runem]

Yes.. I have probably done many wrong things here, Sean. I actually thought about the reverse bias on the PNP, but I'm not experienced enough to know if that was important or not. I'm just a simple hobbyist. I'm not sure I understand why you want a diode on the NPN also.
Maybe the whole thing is just silly and should be ignored..

NB. The circuit is up and running in real life, so it's not just simulated (in a perhaps a little bit doubtful simulator). It does work, but how well can of course be discussed. I have also added a diode on the base of the PNP.

[SeanB]

It will work, until you get a batch that actually breaks down within spec at 5V reverse bias, where they will be on permanently. Reverse bias on a BE junction is not good parameter wise, it causes the gain to drop  and leakage to increase with time. Using it in reverse is also very noisy, it is often used as a broadband noise source, though the transistor will never be good after a year or three in this mode for any other use.

Running in reverse breakdown will also make the transistor act as a poor transistor. To prove this take any small transistor and use a multimeter transistor gain checker to see that the gain is high in the forward direction ( C to C, B to B E to E)  but in reverse ( C to E B to B E to C) that you will still have some gain shown. Not much, but it is there.

[MJLorton]

Yes.. I have probably done many wrong things here, Sean. I actually thought about the reverse bias on the PNP, but I'm not experienced enough to know if that was important or not. I'm just a simple hobbyist. I'm not sure I understand why you want a diode on the NPN also.
Maybe the whole thing is just silly and should be ignored..

NB. The circuit is up and running in real life, so it's not just simulated (in a perhaps a little bit doubtful simulator). It does work, but how well can of course be discussed. I have also added a diode on the base of the PNP.

Hi Runem,

Please don't pass off what you are doing as silly as I think it is a great post and we can all learn from the process. I certainly am and thanks to Sean for his input too.

Cheers,
Martin.

[runem]

Don't worry Martin. As a Transistor Chauvinist and a Relay Hater, I haven't given up yet. In fact, I'm at it again with another attempt (Oh no!). let's see what the Yoda's of Electronics (just kidding) are saying this time.



The circuit in the Falstad simulator:
http://tinyurl.com/8hcb6jc