Reduce recycle time on camera flash (charge a capaitor faster or control its output)

Thread Starter

yt_1300

Joined Feb 18, 2023
22
Hi, I am trying to modify some Andoer speed flashes to decrease the recycle time.

The circuit (see photo) is pretty simple. The flash is not adjustable (no brightness settings) and uses a 330V 110 micro farad capacitor (runs on 2 AA batteries and uses a transformer to boost to 330V). My problem is that it takes about 6 seconds for the capacitor to recharge between flashes, which is much too slow. So what can I do to get the capacitor to charge more quickly?

I tried using a slightly smaller capacitor (330V 85 micro farad) hoping that would at least shave a little time off, but it made no noticeable difference.

Somebody suggested that maybe a different transformer would speed things up. Could that be the bottleneck?

Related to this, it is unclear to me how adjustable flashes control the output of the capacitor (or do they actually control the input?) Is there a way that I could, for example, get a 330V 1100 micro farad capacitor and get it to only dump 1/10th of its charge each time the flash fires, that way I could fire it multiple times before it needed to recharge. In an ideal universe, could this be adjustable? Ultimately, cost, size, and level of skill/knowleedge required are my big limiters.

Thanks for the help
 

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MrChips

Joined Oct 2, 2009
30,714
Welcome to AAC!

The charging time and the flash output are both directly dependent on the capacitance value.

Changing the charge storage capacitor from 110μF to 85μF is about a 22% reduction. It is not surprising that you did not experience any reduction, or at least it should have changed the charging time from 6 seconds to 5 seconds.

Try a 10μF 330V capacitor and see if both charge time and flash output change. The charge time should be less than 1 second.
 

MrSalts

Joined Apr 2, 2020
2,767
If you're ok with less illumination power (less energy per flash), then you can play with the storage capacitor's capacitance.
otherwise, you need to look at the charge circuitry. Typically, an AA battery is limited to 250mA by the circuit designer to prevent the battery from overheating and popping open or leaking. In your camera's charging circuit, there will be some type of "astable multi-vibrator" circuit to pulse the DC. This pulse train is likely fed to the base pin of a transistor that feeds a transformer.

The DC resistance of the transformer primary coil is likely the current limiting feature of the circuit. You'll need to either,
A)find a transformer with similar turn ratio but with lower DC resistance on the primary and power with something more than AA batteries (D-cells or wall adapter with 1 or more amps output capacity), or,
B) use a higher voltage to power the whole circuit. It may "blow up" but, depending in the price of a flash unit these days, it may be a fun experiment. I'd step up by 20-50% each time.
 

Pyrex

Joined Feb 16, 2022
247
Two AA batteries will provide too small power for quick charging. And , in addition, it seems the circuit is designed for slow charging, according to the power source capability
 

BobTPH

Joined Jun 5, 2013
8,813
110 uF charged to 330V is 6 Joules. Charging it in 6 seconds requires 1W of power, or 333mA at 3V. Given less than 100% efficiency it is probably drawing 1/2 Amp. You won’t get much more from AAs.
 

Hymie

Joined Mar 30, 2018
1,277
With great caution, you could try replacing both 1.5V alkaline batteries with two 3.7V lithium batteries. Although this will increase the voltage from 3V to 7.4V, such an increase is unlikely to exceed any circuit component rating but might result in an excessive current causing damage.

It may surprise many that AA size batteries are available as a 3.7V lithium; if replacing four 1.5V cells with lithium types, this would increase the battery voltage from 6V to 14.8 V. I’ve not heard of any fires resulting from someone doing this, but it is only a matter of time.
 

nsaspook

Joined Aug 27, 2009
13,086
With great caution, you could try replacing both 1.5V alkaline batteries with two 3.7V lithium batteries. Although this will increase the voltage from 3V to 7.4V, such an increase is unlikely to exceed any circuit component rating but might result in an excessive current causing damage.

It may surprise many that AA size batteries are available as a 3.7V lithium; if replacing four 1.5V cells with lithium types, this would increase the battery voltage from 6V to 14.8 V. I’ve not heard of any fires resulting from someone doing this, but it is only a matter of time.
With great caution is an understatement.
 

MrSalts

Joined Apr 2, 2020
2,767
With great caution, you could try replacing both 1.5V alkaline batteries with two 3.7V lithium batteries. Although this will increase the voltage from 3V to 7.4V, such an increase is unlikely to exceed any circuit component rating but might result in an excessive current causing damage.

It may surprise many that AA size batteries are available as a 3.7V lithium; if replacing four 1.5V cells with lithium types, this would increase the battery voltage from 6V to 14.8 V. I’ve not heard of any fires resulting from someone doing this, but it is only a matter of time.
Are you the guy who's been recommending nitroglycerin as a replacement for gasoline?
 

Thread Starter

yt_1300

Joined Feb 18, 2023
22
Thanks everyone for the suggestions, I will try some of these and see how I go. A couple of follow ups.

First, I should have clarified at the start that the end product needs to fit in a small underwater housing, so size is a big limiting factor (thus no D cells and I can't hook up to an external power source).

Second, the information about the limitations of system are useful, but does anyone know how higher-quality flashes achieve faster recycle times? I have another flash that also runs on 2AA, but on its lower settings (it is adjustable) the recycle time is well under 1 second, and even on its highest setting (much brighter than the ones I am modifying) it recycles in under 3 seconds. That one is more expensive, so I don't want to dismantle it for this project, but I opened it up once to have a look, and it also uses a single 330V capacitor, but it is much larger (several hundred microfarads [I forget the exact value]), which brings me back to wondering how the adjustable flash circuit works, and what is allowing it to recycle so quickly while using the exact same batteries.

thanks again
 

Thread Starter

yt_1300

Joined Feb 18, 2023
22
The DC resistance of the transformer primary coil is likely the current limiting feature of the circuit. You'll need to either,
A)find a transformer with similar turn ratio but with lower DC resistance on the primary and power with something more than AA batteries (D-cells or wall adapter with 1 or more amps output capacity), or,
Upgrading the transformer seems like a really good idea. My knowledge of them is pretty limited though. What should I look for when comparing them?
Thanks
 

MrSalts

Joined Apr 2, 2020
2,767
Upgrading the transformer seems like a really good idea. My knowledge of them is pretty limited though. What should I look for when comparing them?
Thanks
You'll need to measure the DC resistance through the primary and secondary coils. Then, you'll need some low voltage AC signal to estimate the turns ratio. Put a 1V signal into one side and see what voltage comes out the secondary. Once you have that, you can look for a transformer with a similar ratio but lower DC resistance - as a first iteration.
 

MrSalts

Joined Apr 2, 2020
2,767
The transformers of these older style flash units have a characteristic whine as they charge. I'm guessing that your modern, fast charging flash is silent because it runs at much higher frequency. Is that true?
 

Thread Starter

yt_1300

Joined Feb 18, 2023
22
The transformers of these older style flash units have a characteristic whine as they charge. I'm guessing that your modern, fast charging flash is silent because it runs at much higher frequency. Is that true?
The higher quality flash gives a very quiet whine if I am shooting on the highest brightness (otherwise it is quiet), whereas the cheap one I am modifying always whines really loudly, so I think you are on the right track.

Regarding the turns ratio, I don't have a means of supplying 1V of AC (my equipment is pretty limited), but I know that these flash bulbs use ~300V and the capacitor is rated at 330V, so given the 3V input from the AA, is it sufficient to estimate the ratio simply as 3/300 (1/100)? or have I missed something.

Thanks
 

Pyrex

Joined Feb 16, 2022
247
Transformer upgrading is not so easy, as it seems at first glance... To estimate transfer ratio, you need a high frequency voltage source, as it works at HF, not at 50 or 60 Hz. In addition, it may be installed an air gap inside, it's thickness is important, too.
Anyway, you can experiment with other transformer. Ratio is about 1/100, as you mentioned. Worst thing what can happen is output transistor exploding, so make a photo of it ,to know it's type to replace
 

Thread Starter

yt_1300

Joined Feb 18, 2023
22
Update: I tried running it on an 18650 battery (3.7V). Ran it for several minutes firing the flash as frequently as possible and nothing released its blue smoke, so that's promising. Even better, the 110uF capacitor recycled at ~3 seconds, while the 85uF recycled at ~2 seconds. So that's a big improvement. I've ordered a 60uF capacitor, but I'm concerned about going lower than that as the flash output is noticeably lower at 85 compared to 110.

I'll keep looking into transformers. If I could get it down to a 1 sec recycle time that would be optimal.

Thanks again
 

BobTPH

Joined Jun 5, 2013
8,813
Update: I tried running it on an 18650 battery (3.7V). Ran it for several minutes firing the flash as frequently as possible and nothing released its blue smoke, so that's promising. Even better, the 110uF capacitor recycled at ~3 seconds, while the 85uF recycled at ~2 seconds. So that's a big improvement. I've ordered a 60uF capacitor, but I'm concerned about going lower than that as the flash output is noticeably lower at 85 compared to 110.

I'll keep looking into transformers. If I could get it down to a 1 sec recycle time that would be optimal.

Thanks again
I was going to suggest that. The lithium battery not only has a slightly higher voltage, but also can supply more current.
 

Thread Starter

yt_1300

Joined Feb 18, 2023
22
Related question: now that I have 3 different capacitors with different capacities (and therefore brightnesses), I'm considering making the flash "adjustable" by wiring all three capacitors into the circuit, with the positive ends running through 3 points on a rotary switch (sorry if I'm not using the correct terms). So basically, on any of the 3 positions, only 1 capacitor would be completing the circuit, while the other two would be open, so I can turn the dial to select among the 3 capacitors. Does anyone have any thoughts on why that would be a bad idea?
 

MrSalts

Joined Apr 2, 2020
2,767
I think it's a great idea. The problem is, the active capacitor will charge as soon as the flash happens, then you'll switch to another capacitor and that one will change (leaving the previous capacitor charged). This means you'll need to be very careful when you open it up - more than one capacitor will be charged and, at a level that can hurt or be bad for your heart if you happen to touch a capacitor lead with each hand (the discharge round of the 300v will be (likely be?) through your heart.

Also, you can use a small slider switch - just be sure the switch can handle 300 vDC.
 
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