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

Dodgydave

Joined Jun 22, 2012
11,285
A small rotary switch or slider will do, the capacitor that takes longest to charge will give the brightess flash output and vice-versa,.
 

Pyrex

Joined Feb 16, 2022
247
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?
yes, you can do like this. I suggest , connect a resistor of 470 kohm or so across the pins of the capacitor, each of them. This is for safety .
As about the switch, it's not easy to find a switch for 300VDC. You may use a convenient 250VAC switch. It will commute the circuit with no current, so a general purpose switch can do that.
 

MisterBill2

Joined Jan 23, 2018
18,176
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
Some flash units switch in an additional capacitor to increase the power. Many increase the voltage to increase the power. And in all instances, the recharge speed is limited by the capability of the charging power supply. The reality is that the ony place the flash energy comes from is the power source that charges the capacitor. And the only way to charge a capacitor faster is to supply more current.
 

MrSalts

Joined Apr 2, 2020
2,767
Some flash units switch in an additional capacitor to increase the power. Many increase the voltage to increase the power. And in all instances, the recharge speed is limited by the capability of the charging power supply. The reality is that the ony place the flash energy comes from is the power source that charges the capacitor. And the only way to charge a capacitor faster is to supply more current.
More current by using higher input voltage which, may increase the maximum charge and, net-net, the same amount of charging time if there is no on-board voltage reference as many of these very cheap flashes don't have. .
More current by lowering the internal resistance of the battery will yield somewhat faster charging time (lithium vs alkaline batteries),
More current by using lithium battery with a transformer with much lower DC resistance will get you very far.
 

MisterBill2

Joined Jan 23, 2018
18,176
The current capability of a power supply is rather independent of the output voltage. Mostly, any powr supply is designed with a specific output voltage and a specific current capability. Some may be adjustable, many are not adjustable. And in all cases the current at a voltage is limited by the supply effective internal resistance.
 

Thread Starter

yt_1300

Joined Feb 18, 2023
22
Thanks everyone, this thread has been super useful.

Another follow up, how concerned should I be if the capacitors get a bit hot when I leave the flash on for a while with the 3.7V battery? I'm assuming that's not a good sign
 

Pyrex

Joined Feb 16, 2022
247
Thanks everyone, this thread has been super useful.

Another follow up, how concerned should I be if the capacitors get a bit hot when I leave the flash on for a while with the 3.7V battery? I'm assuming that's not a good sign
A hot capacitor is not a good sign, indeed. Get a multimeter and check the voltage
 

MrSalts

Joined Apr 2, 2020
2,767
It is the charge/discharge that makes the heat, not the staying charged. So leaving it on, not flashing, should not overheat the capacitors.
Well, if you described the "should-be" case, and he is observing something else, then he might have a problem, right?
 

MrSalts

Joined Apr 2, 2020
2,767
I'm assuming that's not a good sign
If the higher voltage battery is causing the cap to charge to a proportionally higher voltage, then you may be exceeding the rated voltage of the capacitor. Or your faster charge/discharge cycles may be causing resistive heating in the charge and discharge events. Replacing with a lower ESR rated capacitor could reduce the heating.
note that most caps are rated at some temp. Check the cap and make sure it feels less than the rated temp.
 

MisterBill2

Joined Jan 23, 2018
18,176
Leakage current IS certainly going to cause capacitor heating, usually until it causes capacitor insulation failure and breakdown. And certainly, operating a capacitor above it's intended operating voltage WILL lead to failure. Not always immediate failure, but almost always eventual failure.
 

Thread Starter

yt_1300

Joined Feb 18, 2023
22
Unfortunately, my cheap multimeter doesn't go past 300V, so I haven't been able to actually measure the voltage, but if the transformer is indeed a 1:100, then using a 3.7V battery is probably exceeding the caps' rating (330V).

In a failed attempt to be clever, I tried running the battery through a diode in the hopes that I'd still get the advantage of the increased amps from the 18650 while the drop down from the diode would get the volts back to 3V. This, of course did not work. The flash now takes 8 seconds to recycle (worse than with the AAs). My understanding of diodes is still pretty limited. Can anyone explain why that made things so much worse (I thought it would recycle slower then the 18650 but faster than the AAs)?

I'm still hunting for a better transformer, but I'm having trouble finding what I need, and on many electronics sites, the number of options are bewildering. Any advice on how to narrow in my search?

Thanks again. I'm quite new to all of this and you all have been very helpful.
 

Thread Starter

yt_1300

Joined Feb 18, 2023
22
More specifically, I'm struggling to understand the number of pins on transformers. The one on the flash has 5 (3 on the primary side and 2 on the secondary). Most of the ones I'm seeing online are symmetrical (same number on both sides) and many have more than 3 pins per side. Can anyone explain?
 

MrSalts

Joined Apr 2, 2020
2,767
More specifically, I'm struggling to understand the number of pins on transformers. The one on the flash has 5 (3 on the primary side and 2 on the secondary). Most of the ones I'm seeing online are symmetrical (same number on both sides) and many have more than 3 pins per side. Can anyone explain?
Some designs have two primary windings to allow the designer to connect them in parallel dorm more current or series for higher voltage (e.g. allow a 115vAC input using two connected in series) or 230vAC (two input windings connected in parallel - or using only one of the two primarily coils).
on output, something similar can be done whether you want 6 or 12v out (as an example). Those with more outputs can, for example, provide a center tap so you can reference the center to ground and have -15, 0 and +15v after rectification and filtering, or reference to the negative pin after rectification and filtering to have 0, 15 and 30 volts.
Some have multiple secondary coils to give 0-5v and 0-15v after rectification, filtering and regulation.

I have an audio receiver with three secondary coils for 0-5v for the electronics controls, 0 to +/-15v for the op amps in the pre-amp stage. And 0 x +/-51v for the unregulated output stage.
 

Thread Starter

yt_1300

Joined Feb 18, 2023
22
I got a slightly better multimeter today, so I can provide a few more specific stats.

With the 3.7V battery, the capacitor is charging to ~365V, so this is a 1:100 transformer, and I am exceeding the cap's rated V (330), which I imagine means blue smoke in my future.

The resistance on the transformer's secondary coil is only 1.2 ohm, which actually doesn't seem too bad. I'm struggling to find a 1:100 transformer much better than that unless I get a substantially larger transformer (which will no longer fit in my housing).

Likewise, I looked into getting 400V caps, but by that voltage, they are much too large for my purposes.

So my latest idea is to replace the transformer with one with a slightly lower ratio. 1:80 should give me ~296V with the 3.7V battery which, unless I've missed something, should let me use the fast-recycle battery with my current capacitors without blowing anything up.
 

Thread Starter

yt_1300

Joined Feb 18, 2023
22
Hi everyone, sorry to be a bother, but I'm still struggling a bit with transformers. I'm looking at the three in the links below, but shipping to where I live is pretty pricey, so I'd like to make sure I get the right thing. If anyone has time to take a look and weigh in on whether you think it is what I need, I'd appreciate it.

I also confess that I'm still a bit confused on how (or whether) I can wire a 5-pin (per side) transformer into my circuit (which has 3 pins on the primary and 2 on the secondary). Any advice would be great.
Thanks again
https://au.mouser.com/ProductDetail/Wurth-Elektronik/7492540070?qs=QNEnbhJQKvabfQHT0m3yxA==
https://au.mouser.com/ProductDetail/Triad-Magnetics/CSE5-100701?qs=LASUwJSZkDn%2BD2QnUSsvqg==
https://au.mouser.com/ProductDetail/Coilcraft/CST2010-080LD?qs=chTDxNqvsyl/rmpiWm5T8w==
 
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