View Full Version : repairing / modifing Yongnuo Triggers

31-12-2009, 10:10am
Hi All,
I received a set of Yongnuo Triggers and Receivers for Christmas and managed to blow two of the receivers within 30 minutes. It turned out that 2 of the old flash guns I connected them to had trigger voltages around 170V. Needless to say the receivers stopped working very quickly after that after first trigger.

The high voltage blows the trigger transistor which is situated on the edge of the PCB just under the negative battery terminal. It can be replaced with a BC817 (surface mount NPN transistor for about 20 cents).

The problem is you still can't use those old flash guns......

I have modified the circuit and added a optoisolator with a triac driver output (part number MOC3020). These devices are good for around 400V and seem to be able to handle the trigger current. One track has to be cut and one resistor is required for the optoisolator input. (see the pic below)

I've modified both the 'faulty' receivers and so far I've had no problems. I've triggered the high voltage flash guns well over a hundred times at full power and all is working well.

I hope that maybe this can help someone else because other than the high voltage, the transmitter / receivers have been working very well.




31-12-2009, 10:21am
Thanks for that. How would you determine the trigger voltage of a non canon flash? I have an old cobra unit

31-12-2009, 10:40am
Hi Coolie21,
A multimeter is the safest, but make sure you get a good connection. (That's how I got caught, I didn't really get to the edge corrector correctly so the first time I measured it the volts looked ok.)

Be careful when you measure the pins, if it is 170V you will get a small zap if you touch the two trigger points with your fingers.

31-12-2009, 2:51pm
well done greg and thnks for the tip !

I @ M
31-12-2009, 4:52pm
A very useful post Gregory :th3:
I will remember this post for when someone runs into troubles like you did.

31-12-2009, 6:11pm
A very useful post Gregory :th3:
I will remember this post for when someone runs into troubles like you did.

Why not make it sticky at the top of the Strobist forum area ??

26-01-2010, 3:51pm
Somehow I've messed up the link to the photo. The resistor used is a 180R. 1/4watt.


28-04-2010, 10:57am
That's really helpful thanks a lot for the Info.

19-11-2010, 2:46pm
Hello Gregory.

I found your post on this forum about modifying the RF602 flash trigger, while searching to find if they are safe for high voltage flashes. Which apparently they arent! I really like your idea of adding the MOC3020 opto-isolator!!

I cannot tell exactly from your description which pins are going to what on the trigger board though. Hopefully you can clarify?

I found this link describing the isolator: http://uk.rs-online.com/mobile/search/searchBrowseAction.html?method=getProduct&R=1954116

From you picture, I cant tell which pin is the anode.. And not completely understanding the circuit and how it is functioning, I'm at a bit of a loss how to wire it up.

Also, where is the wire on the far left going to (from the resistor to..?). Will I have to remove the board and connect to something on the other side?

I hope you can provide me with a little guidance for this amazing modification.
Might want to think about creating a little tutorial and posting it on the strobist.com forums? I think a lot of people could benefit from this mod!

Thanks for your time,


PS because I am a new forum user, I cannot send a private message. I hope Gregory reads this. If anyone would be able to draw this post to his attention, I'd be quite grateful! Thanks!