Thyristor Gate in a camera flash - Page 1
Forum section: electronics / repair

author: PhilC
author_rank: Newbie
timestamp: February 05, 2020, 04:05:06 pm
content: Hello,First time poster here, I recently replaced the Xenon flash tube in a friends Yongnuo YN600EX-RTII camera flash, it has been working great.However it developed a louder pop than usual recently and I found the the magic smoke has been released from a small SMC capacitor that is across the Thyristor gate and Cathode and in parallel with an SMD LED. The flash is still working.I have removed the damaged capacitor from the board to stop the sparking/electrical flash (the loud pop).My question is what is the purpose of this capacitor is it just for the LED, e.g can I just remove it and not have to replace it. The LED is not functioning either.As from my limited knowledge it is not essential for the Thyristor gateI have requested the circuit diagram from the manufacture, however I don't expect a response from them.Included is a photo of the component and the link to the spec sheet for the thyristor.https://www.renesas.com/us/en/doc/products/transistor/005/r07ds0332ej0301-cr5as12a.pdfAny guidance would be greatly appreciated.Many thanksPhil

author: schmitt trigger
author_rank: Super Contributor
timestamp: February 05, 2020, 04:22:53 pm
content: I can't read any numbers from the "thyristor" package.Are you sure it is a SCR and not an IGBT?IGBTs allow tho control of the flash duration, thus controlling its total integrated light output.

author: PhilC
author_rank: Newbie
timestamp: February 05, 2020, 05:01:55 pm
content: According to the linked spec sheet it is a Thyristor it is a CR5AS12A

author: schmitt trigger
author_rank: Super Contributor
timestamp: February 05, 2020, 05:40:47 pm
content: I have read the datasheet...it is a SCR.Let me re-phrase my question: Since there are no visible numbers or letters in the DPak device, how did you surmise it was that particular Renesas part number?

author: PhilC
author_rank: Newbie
timestamp: February 05, 2020, 07:21:19 pm
content: The photo doesn't show the part number unfortunately, but the part number is clearly visible and hence my reference to the data sheet.Thanks for being so through and checking.

author: flolic
author_rank: Frequent Contributor
timestamp: February 05, 2020, 07:40:34 pm
content: Take a look here:https://impulsite.ru/viewtopic.php?f=10&t=1222&sid=26c9b95fb40a8e994bbeb722c57a7d46You can use google translate

author: PhilC
author_rank: Newbie
timestamp: February 05, 2020, 08:36:46 pm
content: Thanks @flolic for the link, that has help with the circuit diagram now attached to this post.I my instance it is the capacitor 302 that has failed. What I am trying to identify is what is the purpose of this capacitor is, is it only there for the LED D303.Thanks all so far for you helpPhil

author: TerminalJack505
author_rank: Super Contributor
timestamp: February 05, 2020, 09:47:52 pm
content: The cap is there to dump a little extra charge into the gate so that the thyristor turns on fully. I'm guessing it is around 100nF. The LED must be there as a poor-man's voltage clamp so that in the case the thyristor doesn't trigger the voltage on the gate (and across the cap) is limited. Most thyristors will trigger at ~800mV so the cap doesn't need to have a particularly high voltage rating and the Vf of the LED will need to be greater than this. The circuit may work fine without these components but I would replace them just to be on the safe side.