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I often have to use my cobra flash indoor, as a studio flash, but I always have batteries problems : not enough charged batteries, no new batteries for backup, battery charge too low after some flashes...

My flashes use 4x 1,5V batteries, so they need 6V.
And each of the battery I use is 2600 mAh.
I've been told that my flashes can support up to 4000 mAh.

I see that I have many portable power supplies that give 12V or 6V power, for 1A or 2A (like portable computer chargers).
But nothing that gives at least 2600 mAh. I guess it's possible to build a custom power source by plugging some dummy batteries into my flash connected to the wall electrical outlet, for some instant use, without having to think about having or not having charged batteries at home.

Do you know how I could do that or find that ?

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1 Answer 1

up vote 3 down vote accepted

Powering a Nikon SB-28 or other flash with an external not portable power supply

Update: Oliver now advise that he will be using a Nikon SB-28 flash. This is an excellent older Nikon flash with a high guide number and reasonably well suited to his application. See added notes at end specific to this flash.

This response applies to ANY flash that uses 4 x AA cells
and which can use Alkaline or NimH batteries.

** Thermal issues are usually of significant concern when any flash is used in this way with a willing power supply and heavy use. The comments at the end and Ken Rockwell's cited review are worth noting when using a flash in this manner.

Summary:

  • A 6 Volt regulated supply capable of 2 amps will work well.

  • A 6V regulated 3A or 4A supply MAY allow faster recycling or may not, but will do no harm.

  • A 6V regulated supply at 1A will probably work OK.

  • A 5V regulated supply at 1A to 4A will probably work OK but a 6V supply would be better.

  • The 2600 mAh figure is an indication of battery capacity, not of discharge rate.

  • I have recommended a regulated power supply. This is required so that as high a Voltage can be provided as is safe but so that the Voltage does not rise to dangerous levels on light loads.
    Most but not all modern power packs are regulated.


First some basic definitions are needed:
Ah or mAh are a measure of battery energy capacity.
A or mA are a measure of battery current.
W or mW are a measure of power
V is battery voltage

Ah = amps x hours is the product of current (A) and the time which the battery can provide this current for. Actual Ah ratings of a battery vary with load and temperature and age, but I'll assume for now that the value is constant.
A 2600 mAh battery (= 2.6 Ah) could supply: 2600 mA current for one hour (as 2600 x 1 = 2600) or
1000 mA current for 2.6 hours (as 1,000 x 2.6 = 2600) or 5,000 mA for 0.52 hours (as 5,000 x 0.52 = 2600)
SO the Ah rating of a battery does not give an exact indication of its CURRENT capability.
There will be SOME correlation, but it's not precise.
A 2600 mAh battery may be able to provide say 5000 mA usefully when newly charged.
This may drop as the battery gets near fully discharged.

Your flash will probably work with NimH (Nickel metal hydride) batteries and with Alkaline batteries.
4 x NimH batteries will provide about 4.4V of Voltage across much of their discharge time.
4 x Alkaline batteries will provide about 6.5V when new and about 4V when almost fully discharged.
SO the flash needs to be able to operate OK on battery voltages from say 4.4V to 6.5V (and ideally 4V to 7V to be safe).
Any power supply that provides say 5.5V to 6.5V will be safe. At the higher end will usually give a faster charge rate. A REGULATED 6V supply will do well. A REGULATED 5V supply will be OK but 6V is better).

The flash will have an upper current drain when provided with a capable supply but will operate on less current with lower cycle times if the supply is less capable than the best that the flash requires.
A 2A supply at 6V regulated will probably be adequate.
A 3A or 4A supply at 6V regulated MAY produce a fatsre recycle time depending on the flash design.

You do not say what model Cobra flash you are using, and they sell many.
They may well "badge engineer" models available for general sale.
Here is a typical Cobra flash on amazon.

Photos: Left: Cobra 480AF . Right: Nikon SB28.

enter image description here enter image description here

If you provide a specific model and a website link I may be able to refine this recommendation BUT the above applies to any flash that takes 4 x AA cells and can use Alkaline or NimH batteries.


Nikon SB-28 biased comments - still of general applicability:

Oliver now advise that he will be using a Nikon SB-28 flash.

Here is a Nikon SB-28 speedlight review written in 2004 by Ken Rockwell. Unlike many reviews it usefully lists current drain under various conditions and makes comment on thermal issues. It will be very worthwhile reading this review when using this flash in this manner.

Ken advises the current consumptions listed below from 4 x AA NimH cells.
It is not clear what causes the 7A peak figure - it may be a measurement anomaly or a power supply input capacitor charging initially. The actual "inverter" that boosts the voltage for the flash circuitry does not seem to be responsible directly for this peak. I 'm commenting on this in detail as I think it means it does not matter if your supply cannot fully meet this brief requirement.

Most important is the "500 mA when charging" which occurs at any time that the flash is charging whether from fully discharged or during topup cycles when on standby. This is lower than I'd have expected and means that a 6V regulated (in all cases) x 1A supply should be more than adequate, a 2A supply may be slightly better and a 3A supply is not needed but should do no harm. All supplies will have an output capacitor which can provide a brief current peak above rated level but only for a very short period. It MAY help to add a say 1000 uF capacitor across the power supply output, but probably not needed.

  • 7A peak (approximate)
    500mA charging
    111mA with the LCD illuminator on

  • 28mA on but not charging, plus
    500mA for 0.8s each 32s as capacitor is topped off (40mA average)

  • 0.75mA in STBY mode, plus
    500mA for 0.8s each 32s as capacitor is topped off (13mA average)

  • 0.5 uA when off


Thermal issues:

Ken's article suggests that the flash will recycle in 5 seconds from fully discharged with good batteries (but much faster rates can be achieved in normal use). This seems to equate to an input power of P = V x I = 6V x 500 mA = 3 Watts when used continuously. This is not a vast amount of power thermally but this and other flashes have overheating problems if very heavy use is attempted.

Ken notes

  • ... I believe the manual warns that you can melt the plastic of the SB-28 if you pop off more than ten full-power flashes in a row without waiting ten minutes for it to cool down.

That seems an excessively low thermal capability and is probably well below actual, but do read what the manual says and do some thermal tests of your own. (Touching the warm flash should be enough to get a thermal sanity check in most cases.)

If you are keen to push the cycle rate with any flash when used in a studio setting you may be able to get useful gains by adding a fan. This can be quiet and have quite low airflow and still achieve major gains compared to "still air" use. When an external battery is used you can direct air into the open battery door and across the flash body. Cooling the actual flash tube area will probably have no ill effects but be wary. At extreme limits thermal shock to the tube may push it over the edge.

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Thank you for those details. The flash I plan to use for the told purpose is a Nikon SB-28. My problem is still to find a good power supply as you described in your answer. I saw those 2 : cgi.ebay.fr/ws/… and cgi.ebay.fr/ws/… What do you think about them ? –  Oliver Jul 30 '12 at 14:50
    
@Oliver - Both those look tres bon. As they are the same price you might as well buy the 3A one. They SHOULD be regulated BUT I'd test what I bought with a voltmeter before use. Vout should never be more than 6.5 Volts, even when unloaded. Chances are that the flash will accept slightly higher Voltage, but maybe not. –  Russell McMahon Jul 30 '12 at 15:36
    
I'm not an expert, but this one cgi.ebay.fr/… seems to be regulated, isn't it (noise <= 120mV (1%) of the industry-standard output voltage) ? –  Oliver Jul 31 '12 at 8:43
1  
@Oliver - Yes - statements indicate it's regulated - a supply of that rating and size will be a switched mode supply so regulated as of right (almost :-) ). Worth checking when you get it or whatever you buy. Connect voltmeter across unloaded output. Vmax not over 6.5V. PROBABLY safe to 7V and maybe quite a bit more but this is uncertain. If above 6.5V a small permanent load will probably load it down enough. An eg 560 ohm resistor will draw ~= 10 mA and dissipate under 1/8th Watt so any 560 R would do. Smaller resistors will load it more and heat more. Report measurement when available. –  Russell McMahon Jul 31 '12 at 12:00
1  
@Oliver - I hadn't been shown the 2nd one until now. Both are potentially OK. The 2nd one does not mention ripple voltage BUT when buying cheap Chinese products the claims and reality are not certain to match. So, whatever you buy, check the voltage as noted above. –  Russell McMahon Jul 31 '12 at 14:50

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