Nikon f2 TTL flash

[BeBu Lamar]

"So how many ohms should I need to measure the trigger voltage?"

 

- Loads!

Seriously; to measure directly you need an instrument called an electrometer, which has an input resistance of 1000 Megohms or greater. Alternatively a storage oscilloscope allows you to capture the open-circuit voltage before it discharges and decays.

 

However, the addition of a simple resistor of 10 Megohms, plus a bit of maths, allows you to calculate the true voltage with two readings, using a cheap Digital Multi Meter (DMM).

 

The method is to measure the trigger voltage using a 10 Megohm input DMM. Call this reading V1. Then add the 10 Megohm resistor in series with the meter, and take a second reading V2.

 

You then need to solve two simultaneous equations to find the true, open circuit voltage.

 

Luckily, the unknown voltage calculation simplifies to (V1*V2)/(V1-V2) provided the added series resistor is equal to the input resistance of the meter.

 

With old designs of flash circuit, you'll often find the O/C trigger voltage to be somewhere in the region of 330 to 350 volts!

 

Although this can give you a nasty 'tickle' the current is limited to a safe value by a series resistance of several Megohms. It's this safety resistor that prevents getting a true reading with a DVM or moving-coil meter.

 

But all the scopes I know only have 1 Megaohm input impedance.

[rodeo_joe1]

"But all the scopes I know only have 1 Megaohm input impedance."

 

- Not if you switch the probe to x10.

 

Anyway, it's not the input resistance that's important, but the fact that a storage 'scope can catch the voltage in the first few milliseconds or microseconds before it decays.

 

Old trigger circuits have a small capacitor of 10 nanoFarads or so across the trigger terminals. If you can read the voltage on that capacitor before it discharges through the instrument resistance, then you get a good indication of the open-circuit voltage.

 

IME, those old trigger circuits almost always take the triggering voltage straight from the main storage capacitor at around 350 volts via a many-megohm resistor.

 

Better to be safe and measure the trigger voltage carefully and accurately, than to kill the flash circuit on an expensive digital camera. (Botzilla take note!)

 

It's voltage that damages semiconductor devices, by puncturing the insulation or junction layer(s). So it doesn't matter if the current is limited to a few micro- or milli-amps by a large series resistor.