Shutter trigger - suggestions for batteries

[Harold_4074]

Knowing nothing about your circuit, but having played for a few years with electrically operated Packard shutter controllers, I expect that you need the 1 ampere current draw for only a fraction of a second at a time. The solution may be a very large (electrically) capacitor, which will probably be considerably smaller (physically) than a pack of batteries capable of delivering the peak current that you need. To give an idea of the concept: I have a 12-volt solenoid which draws about 6 amperes. A 5 ampere-hour gel cell will operate it, but only for a fraction of a second before the voltage has dropped too far. Since a cylindrical solenoid exerts minimum force at the beginning of the stroke and maximum at the end, it needs a big surge to get going; two nine-volt batteries in series plus a 10,000 microfarad capacitor (gross overkill, but that's what I had lying around) slam the solenoid through its stroke with great vigor, to the extent that I should probably put some cushioning at the end. Given that the interval between operations is typically a minute or two, I could probably operate this thing with a stack of six lithium coin cells.

This approach may be recognizable to veterans of the flashbulb era, who used the so-called BC flash guns--- the name derived from "battery-capacitor"---to deliver from a small 22.5V dry cell battery the multi-ampere pulses needed to fire the bulbs. The big press-camera flash guns used three D-size dry cells to do what the BC components could do in a volume of about a cubic inch.

If you can go this route, you will probably find that your battery life is greatly extended; the large currents are flowing between the capacitor (which has a relatively low internal resistance) and the load, instead of through the higher internal battery resistance; your working lifetime may well be limited not by the remaining charge in the battery but by its ability to deliver enough peak current. Keeping the recharge current just high enough to accommodate the interval between operations would give the best battery life.

[Jac@stafford.net]

[...] This approach may be recognizable to veterans of the flashbulb era, who used the so-called BC flash guns--- the name derived from "battery-capacitor"---to deliver from a small 22.5V dry cell battery the multi-ampere pulses needed to fire the bulbs. The big press-camera flash guns used three D-size dry cells to do what the BC components could do in a volume of about a cubic inch.

If you can go this route, you will probably find that your battery life is greatly extended [...]

Excellent idea. Indeed, the #412 BC flash battery unit lasted way longer than the D-cell battery flashes, and in addition they were much smaller; easy to keep a spare in a pocket for another month of daily newspaper shots.

The unit should be designed so the circuit is not closed until activation, otherwise it will drain the battery in a day or so. (Back in the day with the BC units, the photographer did not load a bulb long before he needed it. Just the mild current in the circuit, through the bulb, would continue to bleed the battery.)

Note that an alleged replacement for the Eveready/NEDA 412, the Excell 412A is just a bit too fat to fit in some original flashes, so for a device today model the unit around the Excell 412A, not the original. Then the unit will work with either battery.

[Tin Can]

3 BC by moe.randy, on Flickr


3 similar working BC flash. Biz card for scale. I use these all with stacks of cell batteries. Best to buy the batteries in bulk.

[barnacle]

Hi Neil,

Have you used these batteries much?

No. Our product, for which I'm doing the research, uses AA cells and we recommend normal alkaline cells; I'm trying to find out what happens when they use other cells - the ones we *don't* recommend!

Small rechargeable lithiums are the kind of thing many aero modellers use, so they're easily available, along with external chargers. That might be the way to go, but as you said people may not have the charger.

Neil

[Kevin Harding]

Kumar, have you thought about using the small portable cell phone charger batteries? I have some that can provide up to 2A draw and have upwards of 10,000mAh capacity. I use one to power an Arduino.

If someone has one of these, they could plug in via USB to the charger (you could use a USB plug only for power) and then the charger itself is useful for multiple uses (cell phone, shutter trigger, etc.)

[B.S.Kumar]

@Bill Burk: Yes, as I already stated, I'd prefer to use batteries that photographers already use. We don't need to worry about carrying another special battery/charger.

@Harold_4074: Yes, the circuit draws 1A for a short time. We're already looking at using a capacitor. Thank you.

@Neil: Thanks for the further information. The idea is that the user should already have the batteries/charger, so he/she would not need to use special stuff.

@Kevin: Thanks. The 3.7V battery I linked to in my first post is indeed a cellphone battery. I don't want in-device charging due to regulatory issues. External charging is fine.

Please keep the suggestions coming!

Kumar

[Kevin Harding]

Kumar, I mean something like this. A self-contained battery cell with recharging circuitry onboard. You plug it into a AC-DC converter with a USB port (or a nearby USB powered port) to recharge and plug a USB cable into it to draw power.

The linked product is exorbitantly priced; you can find them for considerably cheaper (and with higher capacity) elsewhere.

[B.S.Kumar]

Kevin, okay, you meant a power bank / phone charger. I thought you just meant the batteries inside. I looked at those earlier and found only larger ones, which would not fit into my case. This one looks promising. I'll check it out. Thank you.

Kumar

[BetterSense]

Just use CR123. 1 Amp is not the slightest problem, they can do more than that continuously. They have a flat discharge curve and a long shelf life, and can easily be had for less than 2 dollars each, and they are also "photo" batteries.

CR2 would be fine too.

[B.S.Kumar]

That's a good idea. Thanks.