AD/DC adapter for Battery Powered Device

JonReeves

Hi guys,
I stumbled across this forum via the following post:
Making a battery powered device work with an AC/DC adapter?

which is very similar to the question I have.

The Project
I'm basically trying to power my Photography Flash Heads (which require 4 AA batteries) from the mains. I'm doing this because I use them 90% of the time indoors, and am tired of recharging batteries. I have found similar projects:
http://blog.davidburren.com/2003/03/...ower-pack.html
http://www.wonderhowto.com/how-to-ma...-285hv-298680/
which have given me a good starting point, but they mainly refer to building an adapter that will use an external battery pack, rather than an AD/DC plug.

My Key Questions

• Is it possible/safe?
• I know that I need a 6V AC to DC converter, but how do I establish the current (amps) required (the one I've found is 1800mA)?
  (The flash itself can be powered by 4x AA Alkalines (1600mAh) or 4xAA NiMh (800-2700mAh). The problem is I don't know how that converts to required current, or if indeed the current is important at all, or if it will just affect the charge time of the flash. Is there a way to gauge the Max and Min Currents the device will accept.)
• Is there a way to determine the best to Wire Guage I need for a particular Voltage / Ampage?
• How can I establish what terminal on my Flash is Positive and which is Negative, will a voltmeter help?

Thanks for any advise / help you guys can offer. I guess my main concern is really about understanding how Amps are at work here in this battery device. As I said, it requires 4xAA batteries so the Volts are in the region of 4.5 and 6, but the Ampage must vary from battery to battery, so I just don't get what the device expects / can handle?

Here are the two types of devices I'm building this for:
http://www.amazon.com/Vivitar-285HV-.../dp/B00004TVSP
http://www.amazon.com/Speedlite-YN56...5519395&sr=1-2

cheers,
Jon

dmxtothemax

Photographic flash heads do require a healthy power source,
But exactily how much varies from unit to unit,
You would be better off measuring the current required
using an amp meter.

If the power supply is too small,
it will just take longer to charge up.
So you can experiment,
Try a 1 amp supply,
see how long it takes to charge up,
If it takes too long,
try a larger supply, say 2 amps,
You will soon find which is about right.

a7ecorsair

I'd think the 1800mA converter would work fine. I doesn't hurt to have a power supply with a higher current rating. This is just saying that the power supply can deliver 1800 mA without dropping below 6 VDC. It would probably work with a 1000 mA supply.

JonReeves

Thanks for the info. So it will draw what it needs (current wise) rather than overloading it? In the same way a 500Watt powersupply for a PC can produce upto 500W when requested, but will also power anything upto that?

I grabbed my Multimeter to see what 4x AAs give me in general current wise, but I'm a little confused to what its telling me:

when set to 20mA range. the readout says: between 4.2 and 6.7
when set to 200mA range: the readout says: between 45.3 and 67.8
when set to 10A range: the readout says: between 3.7 and 4.2

I'm not sure how to interpret the units from my multimeter. Anythoughts?

JonReeves

I kinda figured out how to measure what the AA batteries can push (but not sure I fully understand the numbers. But how would I go about testing the device itself using something like a MultiMeter. I've been trying to find a way to test the terminals while the batteries are attached, but it seems to prevent the flash from firing. I must be shorting something.

JonReeves

So I think I figured out the answer to this, good video by the way:
http://www.youtube.com/watch?v=bF3Oy...eature=related

I'll try and fashion a few wires to allow for me to place the multimeter in-circuit.

a7ecorsair

How are you connecting the meter?
If you want to see how much current is being delivered to the flash you will need to pick up a battery clip that holds four AAs in series. The clip will have two wires that can be connected with alligator clips to the battery contacts in the flash and your meter.

http://www.radioshack.com/product/in...filterValue=AA

Your meter is connected "in series" with the batteries and your flash.

JonReeves

Yeah I used the Battery bracket that comes with the flash to connect all four together and then just sampled the power from the two terminals left exposed.

I just managed to rig it so the batteries are connected to the Flash via the Multimeter.. I'm showing 2.3A (when in 10A range), 23.1A (when in 200mA range)... still not sure what the display is trying to say, but sounds like its drawing 2.3Amps (or at least for a split second, cause it drops rapidly down to 1.8, and then to almost 0.1 once fully charged).

I'm gonna swap out these used Alkaline Batteries for 4 Freshly changed 2000mAh NiMh and see if there is any difference in draw.

mpoulton

You need to look at what the units are on the meter's display. I'm quite sure it says somewhere on the display.

The peak current is what matters, and that will be drawn immediately after the flash is triggered, just as it starts to recharge the capacitor. The peak current may only be drawn for a fraction of a second, but this is the current that matters when choosing a power supply. Your meter may not update fast enough to read the peak current. Does the meter have a "peak hold" function? That would be the easiest way to capture the maximum current draw.

Another way to figure this out is to calculate it. Does the flash have a rating in joules or watt-seconds (W-S)? If so, then measure how long it takes (in seconds) for the flash to recharge between flashes. Divide the watt-seconds rating by the number of seconds taken to recharge. Now multiply this by about 5 to get the approximate peak input power draw in watts. Then divide by the voltage (6V) to get the maximum current required. Example: a 10 W-S flash that recharges in 2 seconds would require about 25W peak. At 6V, that's 4.1A.

JonReeves

Interestlingly with fresh Rechargables I'm getting 4.2A (when looking at the 10A range).

Seems like alot of current from 4x AA batteries, but then again I know nothing about batteries. This might explain the earlier numbers I was getting for testing the 4x batteries directly.

Seems like the flash-head is able to pull alot of current, probably as much as its given. No idea how I figure what the max is, but I can live the the recharge rate I'm getting, so I guess 1800mA might be ok.

I'll have to test my other brand's unit and see how that copes.

Any thoughts?

JonReeves

Unfortunately the display is a bit crap, and it doesn't have a peak hold option. But I like your thinking with this calculation.

Now the flash head doesn't seem to have a watt-second listed, but I've found a few references to it being 60 online... and it takes a little over 12seconds to charge, so thats 4.166A. Which beautifully enough is very close to the 4.2A that I catch a glimpse of via my multimeter.

Awesome.

mpoulton is there anything dangerous/damaging about providing less Ampage in my situation, or am I just going to experience longer charge times?

The DC adapter I have can do 1800mA, but I think the most I can find for 6v is 3000mA.

JonReeves

The YN560 Flash head I'm hoping to test tomorrow is allegedly arround the same Watt-Seconds. However every indication is that its able to charge in 2seconds, which would mean it pulls 25A... which seems a little mad.

I'm concerned to test it via the multimeter as mine maxes out at 10A. Is 25A likely?

Thadius856

A 2 second charge time is quite likely. I used to use a pair of Alien Bees that would charge in much less time.

I can't speak to the amperage, though.

mpoulton

Using a power supply with a lower current rating could potentially damage the power supply. It's unlikely to do anything bad to the flash unit, and will probably just lengthen the charging time. An 1800mA adapter can probably source a few amps for a few seconds at a time with no problem. So that's probably fine. If the charge time is substantially longer than on batteries, consider upgrading the power supply.

mpoulton

25A from AA batteries is not likely. In fact, it's mostly impossible unless they are nicd or nimh. If that unit does use high-quality nicd or nimh cells, then 25A is definitely possible. What's more likely is that the power supply is carefully designed to draw current more evenly during the charge cycle, which results in a much faster charge for a given peak current draw. Thus, a 2-second 60 joule charge might only require 8A or so, but draw it consistently during the whole charging period. This is better for batteries with limited current capability.