Doug Kerr
Well-known member
This relates to a potential problem with flash units that have not been operated for some while.
This seems to be the closest to an appropriate forum for this topic.
**********
In almost all flash units, the storage capacitor (which stores the electrical energy that energizes the flash tube) is of the electrolytic type. In that type of capacitor, the dielectric (the insulating layer between the two electrodes of the capacitor) is a thin film of aluminum oxide on one of the aluminum electrodes, created by an electrochemical process.
If the capacitor isn't charged for a long time, that dielectric layer can "deform" (degrade), causing the capacitor to electrically "leak", with the result that (depending on the nature of the charging circuit) it will not fully charge and/or may heat up internally.
I had been aware of this phenomenon for over 60 years. It used to be regularly discussed in amateur radio and "radio experimenter" magazines in the 1940's and 1950's. As I recalled, curing an occurrence typically required a careful "reforming" process, generally involving the cautious, gradual application of voltage to the capacitor with a variable voltage power supply.
The matter came back into my view a week or so ago when Will Thompson old me of a problem he was having in rehabilitating a studio flash power pack, in which the storage capacitor got quite warm when the unit was energized.
I reminded him of the matter of "dielectric deforming" in electrolytic capacitors, and we felt that this might in fact be involved.
Shortly after, Will called me back and said he had found in the manual for a Novatron studio flash unit the advice that, if a unit had not been used for a long while, when putting it back into service, to energize it just for a brief period (I think just a second or two), then let it sit for a while, and then power it up again. This is evidently a workable strategy for reforming the dielectric in at least some modern types of electrolytic capacitor.
Yesterday, I decided to set up my studio lighting rig, which had not been used in full form in over a year. It comprises two Vivitar 285HV shoe-mounted flash units, mounted on stands in reflecting umbrellas and powered by 6 V motorcycle batteries, plus a third 285HV (running from the normal AA cells) on the camera, triggering the two main units which are equipped with optical slave receivers.
The first exposure tests suggested visually that there was a dramatic disparity in the outputs of the left and right units (when set to the same "manual" output). I swapped the two units just to make sure, and the disparity traveled with them.
I decided that I had better make a more direct test of the output of the three units to be certain what I had, and I shut everything off until I could get to that.
About a half hour later, I put the three units one at a time on the camera, all set to 1/2 output, and took test frames with them aimed at my white studio background.
Then I read the exposure result from the frames, and found that the illumination was consistent to within less than 1/10 stop.
Then, just to complete the circle, I put the two units back in place on the stands for a regular test shot (this time with "Stella", my stand-in manikin - Carla said she was out of the "test subject" business for the day) and found that I had the consistency of illumination I originally expected.
Apparently having the one unit charged (for about 20 minutes, I am afraid) and then letting it rest allowed the capacitor to reform. (It is very possible that this was the only one of the three units that actually was out of service for that full time).
I suggest that anyone putting a flash unit back into service after several months of non-use apply the "Novatron" reforming doctrine described above.
This seems to be the closest to an appropriate forum for this topic.
**********
In almost all flash units, the storage capacitor (which stores the electrical energy that energizes the flash tube) is of the electrolytic type. In that type of capacitor, the dielectric (the insulating layer between the two electrodes of the capacitor) is a thin film of aluminum oxide on one of the aluminum electrodes, created by an electrochemical process.
If the capacitor isn't charged for a long time, that dielectric layer can "deform" (degrade), causing the capacitor to electrically "leak", with the result that (depending on the nature of the charging circuit) it will not fully charge and/or may heat up internally.
I had been aware of this phenomenon for over 60 years. It used to be regularly discussed in amateur radio and "radio experimenter" magazines in the 1940's and 1950's. As I recalled, curing an occurrence typically required a careful "reforming" process, generally involving the cautious, gradual application of voltage to the capacitor with a variable voltage power supply.
The matter came back into my view a week or so ago when Will Thompson old me of a problem he was having in rehabilitating a studio flash power pack, in which the storage capacitor got quite warm when the unit was energized.
I reminded him of the matter of "dielectric deforming" in electrolytic capacitors, and we felt that this might in fact be involved.
Shortly after, Will called me back and said he had found in the manual for a Novatron studio flash unit the advice that, if a unit had not been used for a long while, when putting it back into service, to energize it just for a brief period (I think just a second or two), then let it sit for a while, and then power it up again. This is evidently a workable strategy for reforming the dielectric in at least some modern types of electrolytic capacitor.
Yesterday, I decided to set up my studio lighting rig, which had not been used in full form in over a year. It comprises two Vivitar 285HV shoe-mounted flash units, mounted on stands in reflecting umbrellas and powered by 6 V motorcycle batteries, plus a third 285HV (running from the normal AA cells) on the camera, triggering the two main units which are equipped with optical slave receivers.
The first exposure tests suggested visually that there was a dramatic disparity in the outputs of the left and right units (when set to the same "manual" output). I swapped the two units just to make sure, and the disparity traveled with them.
I decided that I had better make a more direct test of the output of the three units to be certain what I had, and I shut everything off until I could get to that.
About a half hour later, I put the three units one at a time on the camera, all set to 1/2 output, and took test frames with them aimed at my white studio background.
Then I read the exposure result from the frames, and found that the illumination was consistent to within less than 1/10 stop.
Then, just to complete the circle, I put the two units back in place on the stands for a regular test shot (this time with "Stella", my stand-in manikin - Carla said she was out of the "test subject" business for the day) and found that I had the consistency of illumination I originally expected.
Apparently having the one unit charged (for about 20 minutes, I am afraid) and then letting it rest allowed the capacitor to reform. (It is very possible that this was the only one of the three units that actually was out of service for that full time).
I suggest that anyone putting a flash unit back into service after several months of non-use apply the "Novatron" reforming doctrine described above.