Few weeks ago I bought online an old flash gun Philips 25CB. I was very excited about my first flash gun capable of bouncing up to 90 degree and having three power level setting. When I received the flash, I hurriedly inserted three freshly charged cells and turn it on. I waited for few seconds to let the flash fully charged. There was no response. I tried couple of times pressing test button but all in vain. Unfortunately flash was dead!
I was very disappointed. I called to the first owner of the flash to file a complaint but his cell phone was powered off. After thinking a while about the current situation, I decided to give it a try to repair. In short repair was very straight and simple as I was excepting. I reassemble the flash inserted the cells and turn it on. This time it started charging with a whistling sound and after 5 seconds ready lamp was lit. I pressed the test button and there was a very powerful flash illuminating the whole room.
As I was already searching on the internet about using old flash gun on new DSLR, I was very conscious about the HIGH TRIGGER voltages on hot shoe that could FRY by gear. I set my DMM at auto range and measured the voltages of the only pin of the flash gun hot shoe. I was shocked looking at the screen of the DMM. It reads 150VDC. The excitement of the successful repair vanished by this measurement. According to available information on the internet, it was not safe to use this flash on my Pentax K-r. Being an electronic engineer, I was not going to let the flash go unserved so easily. So, I decided to modify its circuit and made it work.
I was completely in dark about how flash actually works. Some further searching reveals that camera triggers the flash mounted on the hot shoe by shorting the central contact pin of the flash to ground. I shorted the flash contacts to verify my freshly gained knowledge and it worked. My next thought was “what is inside the camera which does this job????”
I further searched on the internet to get service manual of my Pentax K-r for an exact answer of my question but could not succeed. At that moment a thought flashed into my mind. How much damage a 1.2V cell can cause to my camera??? Hold on I know we are talking about HIGH VOLTAGES but don’t forget our main power source is three or four 1.2V cells assemble in parallel to increase current capacity. To discharge the Xeon lamp in the flash we need 1000V to 4000V. To get such high voltage from a DC power source, an oscillator circuit oscillates the DC power which drives a step up transformer. The stepped-up voltages are then stored in a bulky capacitor which ultimately became our main power source to discharge the flash. All flash guns use the same principle. A friction of this voltage appears on the hot shoe contact.
Does only high voltage enough to destroy or damage an electronic circuitry? The answer is NO. We need a power source capable of delivering high current to damage a component as well. According to electrical law “stepping up voltage reduces the current in the safe friction it is stepped up”. Means when we step up 1.2 volt to say 1000 volt, we actually do almost 1000 times step up ultimately reducing the current capacity 1000 times. In my case I used 2100mA three cells in parallel thus total current capacity of 6300mA which reduces to 6.3mA after step up procedure. Most of the current flows in trigger plate to discharge the Xeon lamp. When I measured how much current flow between the hot shoe contacts, I was astonished to know that it was merely 130uA. So little current even at 150V is not enough to damage an electronic component. Now, I was not worried at all putting my flash gun on my Pentax K-r hot shoe. I made my first flash mounted on the camera and waited for any smoke or any other abnormality. Nothing happened. I made couple of further shots to test the camera and flash. Everything was working fine.
The last words, for electronic switching inside camera we have three options to use. A
1. BJT
2. FET
3. MOSFET
In my opinion a MOSFET is the most probable solution to be used for HSS. Even a small signal MOSFET can handle 350V plus 500mA current safely. In my opinion, high trigger voltages are not a problem as long as it belongs to a flash gun. So, keep enjoying old flash guns you got without any worry next time.
“THIS IS ALIJAFARY SAYING YOU KEEP FLASHING”