Testing trigger voltage

AVO, they'er Test meters (Amps Voltage Ohms) but we knew them as the
'bees knees' of testmeters - built like a tank and normally used a
moving coil meter as opposed to a digital read out.

Digital meters usually have an input around 10 M ohms per volt,
meaning that if you are measureing 2 volts the circuit will see a 20
M ohm resistor across it's terminals.

A moving coil meter could be anywhere from 1k ohm per volt to arounf
100 k ohms per volt, meaning when we measure say two volts an 100K
ohm testmeter will represent a 200K resistor across the circuit your
testing.

The lower the current drawn by a testmeter the more accurate the
readiing and modern circuits use very low currents.

It's not easy to measure the flash voltage because of the rise time -
the moment of applying voltage to trigger the flash and the time it
takes for the meter to register the voltage, moving coil or analogue
meters as they are known are totally useless, digital meters fare no
different although better, so slow not to give any readings that is
anywhere near accurate, by the flash has gone the poor ol' meter will
still be responding.

The correct way is to use an oscilloscope, not only could you measure
the voltage you could also measure the 'rise time'.

I am curious to know the reason why you need to measure a voltage,
it's something that normally isn't done even in a microwave oven it's
never necessary to know the magnetron's supply voltage...!

Flash tubes normally use a 'trigger transformer', charging a capacitor
to around 2,000 V (2kV), the firing circuit which could be transistors
or thyristors.

A little off topic ....

Davy

lid (Davy) wrote:
AVO, they'er Test meters (Amps Voltage Ohms) but we knew them as the
'bees knees' of testmeters - built like a tank and normally used a
moving coil meter as opposed to a digital read out.

Digital meters usually have an input around 10 M ohms per volt,
meaning that if you are measureing 2 volts the circuit will see a 20
M ohm resistor across it's terminals.

The voltage you are measuring has *nothing* to do with it.

If you use a 2 volt scale, the meter will have 20M Ohms across
the terminals (whether you are measuring 2 volts or 20 volts).

A moving coil meter could be anywhere from 1k ohm per volt to arounf
100 k ohms per volt, meaning when we measure say two volts an 100K
ohm testmeter will represent a 200K resistor across the circuit your
testing.

And it is also true that a DVM might be just about any value too.

The lower the current drawn by a testmeter the more accurate the
readiing and modern circuits use very low currents.

It's not easy to measure the flash voltage because of the rise time -

The OP wanted to measure the trigger voltage, not the flash voltage.

....
I am curious to know the reason why you need to measure a voltage,
it's something that normally isn't done even in a microwave oven it's
never necessary to know the magnetron's supply voltage...!

Untrue.

Flash tubes normally use a 'trigger transformer', charging a capacitor
to around 2,000 V (2kV), the firing circuit which could be transistors
or thyristors.

And that is exactly why the OP wants to know what the voltage
is! The electronics in the camera may, or may not, be able to
handle any given voltage on the circuit it uses to trigger the
flash. Modern flash units will have less than 15 volts on the
trigger terminals, while 20-30 year old units might have
hundreds of volts, and may damage the electronics in a modern
camera.

A little off topic ....

No.

--
Floyd L. Davidson http://www.apaflo.com/floyd_davidson
Ukpeagvik (Barrow, Alaska)

"Allan" writes:
Yes, DVM is Digital Volt Meter. AVO is the trade name of a British company
(AVO Megger) that makes high quality multimeters and other lab grade test
equipment - considered by many as the multimeter of choice.

Never heard of them in North America, so AVO probably means nothing to a
lot of readers.

It doesn't matter what specific brand of DVM you use, but it should be
something with at least 10 megohm input impedance to measure the
trigger voltage accurately. Any *good* lab or portable DVM should meet
this, though I wouldn't assume it was true of a cheap meter bought from
an auto store.

Dave

lid (Davy) writes:
AVO, they'er Test meters (Amps Voltage Ohms) but we knew them as the
'bees knees' of testmeters - built like a tank and normally used a
moving coil meter as opposed to a digital read out.

Digital meters usually have an input around 10 M ohms per volt,
meaning that if you are measureing 2 volts the circuit will see a 20
M ohm resistor across it's terminals.

Actually, no. Most digital meters are either 10 Mohm or 11 Mohm on
*all* DC ranges except perhaps for the lowest (200 mV) range. The
measuring circuit itself has extremely high input impedance, thousands
of megohms, and the impedance across the probes is the impedance of a
voltage divider. For an 11 M input meter, the divider is the series of
resistors 10 M, 1M, 100k, 10k, 1k, ... connected in series, with a
selection of taps that allow input voltage, 1/10 input, 1/100 input, ...
to be connected to the measuring circuit. The 10 M input meter is
similar, but the voltage divider chain is 9 M, 900 k, 90 k, 9 k, 900, ...

A moving coil meter could be anywhere from 1k ohm per volt to arounf
100 k ohms per volt, meaning when we measure say two volts an 100K
ohm testmeter will represent a 200K resistor across the circuit your
testing.

A moving coil meter is driven by current; one rated at 100 kohm/volt
really means that the meter is 10 uA full scale. Resistors are
connected in series with the meter to convert voltage to current.
So if you select the 10 V *range*, a 1 Mohm resistor is used to provide
10 uA at 10 V. But the resistance depends on the setting of the range
switch, not the current measurment. Measuring 2 V on a 10 V range, the
resistor is still 1 M, not 200 K.

It's not easy to measure the flash voltage because of the rise time -
the moment of applying voltage to trigger the flash and the time it
takes for the meter to register the voltage, moving coil or analogue
meters as they are known are totally useless, digital meters fare no
different although better, so slow not to give any readings that is
anywhere near accurate, by the flash has gone the poor ol' meter will
still be responding.

True, but he's trying to measure the trigger voltage, not the flashtube
or flash capacitor voltage. The trigger voltage is DC.

I am curious to know the reason why you need to measure a voltage,
it's something that normally isn't done even in a microwave oven it's
never necessary to know the magnetron's supply voltage...!

Flash tubes normally use a 'trigger transformer', charging a capacitor
to around 2,000 V (2kV), the firing circuit which could be transistors
or thyristors.

Triggering the flash is normally done by charging a small capacitor,
then dumping that capacitor through a switch into the trigger
transformer primary. The transformer generates a few kV to ionize the
gas in the flashtube, which then conducts the main flash current.
The voltage in this trigger capacitor can be a couple hundred volts,
and sometimes this is present right on the flash trigger contacts.

The mechanical switches in old film cameras weren't bothered by
switching this, but some newer digital cameras fire the flash using a
transistor, not a mechanical switch. The safe trigger voltage for these
cameras can be as low as 6-12 V. So the flash needs to provide a low
voltage at the sync terminals which is used to trigger an SCR which
switches the trigger capacitor internally.

If you connect an old high-voltage trigger flash to a new camera with a
low-voltage flash interface, you'll damage the camera. Permanently.
That's why he cares.

Dave

The voltage drop across the trigger is NOT measured during the brief
interval that the flash is being discharged. AMPERAGE would be measured
then. The voltage is continuously present -- for MORE than enough time
to be measured by even the SLOWEST response meter.

High-voltage damage will occur to a camera's circuit as soon as it is
applied, whether the flash is triggered or not.

Voltmeters are all high impedance. Although a HIGHER high impedance will
theoretically draw less power, this application does NOT require it. It
makes no difference whether the meter shows 185 or 300 volts -- if it is
over 6 volts, it is too much.

On Sun, 26 Mar 2006 21:08:01 +0000 (UTC), (Dave
Martindale) wrote:

good stuff snipped


The mechanical switches in old film cameras weren't bothered by
switching this, but some newer digital cameras fire the flash using a
transistor, not a mechanical switch. The safe trigger voltage for these
cameras can be as low as 6-12 V. So the flash needs to provide a low
voltage at the sync terminals which is used to trigger an SCR which
switches the trigger capacitor internally.

If you connect an old high-voltage trigger flash to a new camera with a
low-voltage flash interface, you'll damage the camera. Permanently.
That's why he cares.

Dave

Well said, Dave. Best response in the thread.

Tom

(Floyd L. Davidson) writes:

The voltage you are measuring has *nothing* to do with it.

Correct, for both types of meters.

If you use a 2 volt scale, the meter will have 20M Ohms across
the terminals (whether you are measuring 2 volts or 20 volts).

Not for the DVM/DMM. Its input impedance is generally 10 or 11 Mohm on
*all* scales, because the same voltage divider is always bridged across
the input leads. The resistance is a shunt resistance, not a series
resistance, and it doesn't change with the scale selected. So it's just
"10 Mohm", not "10 Mohm/volt".

(The exception is some meters that remove the divider from the circuit
on the lowest range, often 200 mV, giving an impedance of hundreds or
thousands of Mohm).

Dave

In article , Roy Smith
writes
In article ,
"Allan" wrote:

I was just doing this before I looked at these posts.

One of the other posters and the site mentioned both suggest a DVM to
measure the voltage and that was my first choice.

However, I have discovered a problem. The problem is the readings on the
DVM's (two of them) do not agree with my AVO. For those who know, AVO's do
not lie. Both DVM's measure 94 volts on my Vivitar 283, while the AVO says
212 volts.

I'm assuming DVM is Digital Volt Meter, but what's an AVO?

Amp volt Ohm.
--
Ian G8ILZ

In article , Fred McKenzie
writes
It must be Analog Volt Ohmmeter, which explaines the reference to 20K
Ohms per volt.

Nah, it predates digital meters by so much that analogue was the only
sort and there was no distinction to make.
--
Ian G8ILZ

In article , Davy
writes
A moving coil meter could be anywhere from 1k ohm per volt to
arounf
100 k ohms per volt, meaning when we measure say two volts an
100K
ohm testmeter will represent a 200K resistor across the circuit
your
testing.

The Avo 8 & 9 are 20k ohm / volt, which means that on the 1000 V range
it would be higher than the 10M ohm of a typical DVM.

The lower the current drawn by a testmeter the more accurate
the
readiing and modern circuits use very low currents.

Usually, I once had inaccurate readings due to contamination acting as
an electrolyte and causing an offset voltage which collapsed when a
lower impedance meter was used.

--
Ian G8ILZ