dave wrote:
On 08/07/2013 10:07 AM, Cydrome Leader wrote:
Roger Blake wrote:
On 2013-08-07, Cydrome Leader wrote:
How large are the batteries you're charging?

12 volt lead-acid car batteries, I should have mentioned that if I didn't!

What parts are on that board you have now? Can you post a photo?

It does look like they cheaped out on a couple of rectifiers, hence
the 28V center-tapped transformer. (The transformer looks surprisingly
hefty, though.) But then again there are 2 SCRs on heatsinks and it's
surprising how many discreet transistors this thing has, enough to make
a decent transistor radio, but who knows how many are real. A bunch
of what are probably phony 1% resistors completes the components. No
ICs. No capacitors. The center tap of the transformer feeds into the
center section of the board through a circuit breaker. Each side of the
secondary then feeds into its own symmetrical section of the board.

I tried taking a picture but my camera does lousy closeups, will have
to borrow one...

Are these "SCRs" really SCRs or just large power transistors? Did the
thing sing or make horrible PWM sounds when it worked?

I have seen real SCRs in telecom rectifiers (giant battery chargers/power
supplies).


Physicists tell me that SCRs will work as transistors.

didn't mean to be a jerk with the last reply.

so I did about 3 seconds of searching and here's how that charger probably
works.

something is measuring the voltage of rectified DC and turning the SCRs on
and off when the input voltage is just right for charging the battery.

It's basically 120Hz PWM. They may even be using the negative or postive
voltage from the other leg of the center tapped transformer to shut off
the SCR on the opposing side. Cheap and simple. Sadly, I don't recall if
you just short the gate of a SCR to something to shut it off, or if it
needs a reverse voltage of some sort to really make "sure".

Failure modes as imagined by me are

1) SCR shorting and outputting full voltage into your now dead battery.
Seems not too likely- SCRs are pretty damn tough.

2) the gate circuitry getting botched up and the SCR NOT being shut off
before the voltage hits the overvoltaged and killed battery zone. SCRs
latch so if that's the case, your battery is basically being murdered 120
times as second because the gate shutoff circuity is failing 120 times a
second.

The of course, maybe the thing is a GTO, but the idea is the same either
way.