Dave Plowman (News) wrote:
In article ,
AJH wrote:
On 10/04/2020 06:33, Brian Gaff (Sofa 2) wrote:
Are we sure the coil is not just knackered?
Brian

It may be, it shows a few ohms resistance and draws over 250mA with a
1.5 cell across it and doesn't spark the plug when the circuit is
broken.. I'll need to pull one off another to check but I don't have one
here atm.

Ignition systems generally run at a few amps average. Try flicking a 12v
car battery etc across the coil while looking for a spark.

Coils ain't designed to have volts applied constantly for any length of
time.


Transformers are designed to work with AC signals.

Applying DC to coils isn't always a safe thing to do.
Depends on numbers, resistance of coil, and so on.

If you measured the resistance of the primary on the
coil, and it was 2 ohms, you'd expect 12V/2ohms = 6 amps
to flow, and 6 amps * 12V = 72 watts. Which is likely
to be too much for the physical design of the coil,
if left there for any period of time.

It's hard to control the "intermittent" connection of
a battery well enough, to ensure safety or a lack of damage.
You might use or need an "interruptor" ("joy buzzer"),
as a means of exciting the coil.

Drivers for coils might involved the usage of capacitors,
if you knew what voltage to use. Like what voltage was normal.

Another option, would be to place a 1V amplitude AC signal
on the primary (small enough that the SCR might not interact),
then measure the voltage on the secondary (which is unloaded).
And from that, determine the functioning turns ratio. Since
ignition coils can develop wind-to-wind shorts, the
number of turns still functional in a coil, might not
be the original value.

Much of these tests rely on your knowledge of what would
be "normal" for the circuit, in response. I bet, given
my drothers, I could make that coil throw a spark, but
that's not going to make the device run properly
exactly. Such a test would prove there wasn't a dead short
or an open in the coil. Like if the secondary was shorted
out completely, you wouldn't expect to see a spark or any signal
come out.

And based on my experience of being thrown across the basement
floor by a car ignition coil, remember your elfen/safety.
In my case, I'd thrown a cloth over something (because the
workbench was dirty), and I happened to lean on the cloth, and
the damn red high tension wire just happened to be
underneath there. I swore an oath that day, that I would
never again subject myself to something like that, and
so far, it worked. The oath worked. No more HV accidents.
No more throwing shop rags where they don't belong. And I've worked
on HV stuff since then, with zero incidents to report. It's
funny how coming to on the basement floor and wondering how
you got there, clears the mind.

Paul