How to drop 170V DC to 50V?

I have old flash that generates about 170V on it's trigger when it's charged and ready to fire. When flash fires, voltage drops to zero, and rises to 170V as flash charges...

I need that flash connected to 4N35 optocoupler. As i can tell from datasheets, it can withstand only 70V, so i need a way to drop voltage.

So, whats the best way to do it? Use resistors? Power adapter? Or simply use other optocoupler?

Schematic should like similar to this one:

I am interested in what's going on with 4N35 which has F ( flash ) marking pin 5. Except, this one is obviously made for newer flashes, and they do not generate high voltages on their terminals.

• "I need that flash connected to 4N35 optocoupler." Explain how you plan to connect it, show us a schematic. With this limited information it is anyone's guess. Commented Mar 3, 2016 at 12:15
• Usualy trigger needs very high voltage approx 20kV. A transformer is used. Commented Mar 3, 2016 at 12:31
• You really need to be clear what it is you want. Do you mean the 170V (across the capacitor) is for firing the flash tube and are you intending use this voltage to signal the arduinothat the flash is ready to trigger. As Marko points out the falsh is normally triggered through a small transformer that generated a trigger pulse of about 20kV. Commented Mar 3, 2016 at 12:34
• So what you're actually looking for is something which can switch that 170V and is controlled by the 4N35 opto? Commented Mar 3, 2016 at 12:55
• @zbrka Is F for focus (as per the connector pin-out), or is F for Flash, as per your text "I am interested in what's going on with 4N35 which has F ( flash ) marking pin 5"? I'm guessing what you're really wanting to control the 170V flash voltage on is the SH/shutter terminal, correct? Commented Mar 3, 2016 at 13:29

Possible solution for the problem is using cascade transistor connection. Use the 70 volt optocoupler to control a high voltage transistor which in turn will switch the 170v.

In this way your optocoupler will see only about 0.8V. You may need to modify the codes so that the microcontroller will provide active low output.

I see 4 options for accomplishing what (I think) you're going for here:

1. Replace one of your optoisolators with a 5V drive, 170V+ load relay, such as this one. The major disadvantages here being the need for a freewheel diode to protect your 'duino from inductive voltage spikes from the coil, and contact wear/welding on the relay itself (and relay noise, if that's a significant concern).

2. Replace one of your optoisolators directly with a 170V+ logic-level drive power MOSFET, like IRL640STRLPBF. The down-side to this option is that you lose your isolation (arduino gnd must be tied to signal gnd & failure of the MOSFET has a small chance of exposing your arduino/circuit to 170V).

3. Add a 2nd power supply, such as an A23 battery to drive a "standard" 170V+ power MOSFET (like FDD7N20TM), using the optoisolator to drive this "intermediate voltage." This method maintains your full isolation for the arduino & its power supply without the i ductive problems & contact wear of an electromechanical relay, at the cost of needing to add a small battery.

4. Just replace the 4N35 with a higher voltage optoisolator & call it a day.

(All digi-key links are provided for convenience only and are for example parts only. Feel free to source any similar part anywhere you like; my only affiliation with digikey is that I tend to buy more of my own components there out lf habit.)

Background

Most of the older film SLR (single-lens reflex) cameras had a mechanical contact for the flash trigger. These generally gave a brief contact closure to trigger the flash (and the more advanced cameras had the option of triggering the flash when the shutter first curtain opened or just prior to closing the second curtain). This contact was suitable for any kind of flash including units with a high-voltage trigger such as yours. Modern DSLRs (digital SLRs) tend to have low-voltage switches suitable for modern electronic flash units only and users need to be careful not to connect HV flash units despite the standard flash shoe mount.

Opto-triac

simulate this circuit – Schematic created using CircuitLab

One potential solution to your problem is to use a triac opto-isolator. Triacs have the characteristic that, once triggered, they remain on until the current through them falls below the holding threshold. They seem a good fit for your application as they have a high-voltage rating. Everything else in your schematic can remain the same.

A brief web search showed up an Instructables article with the same idea.