I am creating an adapter to provide external power to a small digital camera. I have designed and printed a dummy pair of AA batteries, with space carved out at the positive and negative contact points for two springs, and channels for wires attached to the springs to exit out the slot provided next to the battery compartment door. It all fits quite neatly.

I intend to power it with a mains adapter, but for testing I am using a pair of AA batteries in a carrier, as being nearest to the original setup. But when I connect it up I get a current draw of about 800mA, without the camera turned on, so there is obviously a fault somewhere.

I have confirmed there is no fault within the adapter. The resistance across the adapter when inserted is either 1.7kOhms or 48kOhms, depending on the polarity of the multimeter, neither of which would suggest a current drain of 800mA at 3v. The power switch is electronic, so a measurable resistance is expected for a device like this.

There is some fancy molding and associated metalwork at the battery interconnect on the compartment door to provide physical isolation if a battery is inserted backwards, so I wondered if there was something similar at the internal end (the interconnect is completely isolated from either end of the supply). But there is nothing visible, and the measured resistance across the camera contacts is the same for any way I measure it - adapter just touching the contacts, fully pushed home, or multimeter leads direct to the contacts.

The attached photo shows the adapter and the battery compartment. If you look closely you can see from the label that the adapter is placed upside down - I have checked many times that it is being powered with the correct polarity!

If anyone has any experience with this arrangement or ideas as to what might be happening I would be very grateful for the comments.

 

What do the contacts at the other end of the battery holder look like?

 

If on the adapter RED is positive and BLACK is negative then you have the polarity backwards.

 

What do the contacts at the other end of the battery holder look like?

They are identical strips mounted at the side to provide spring. They both contact at the centre on a pad about 3mm across, as would be required for the positive button. The are above a partial metal plate, which made me think that perhaps there was a detection if they were overflexed, but AFAICT a standard battery pushes them the same distance as my adapter, and in any case the measured resistance does not change as I move the contacts from a light touch to fully pressed.

 

If on the adapter RED is positive and BLACK is negative then you have the polarity backwards.

Red is positive and black is negative, but as mentioned in the original post the adapter is laid out backwards for the photo. The red connector goes to the side the has the negative battery symbol at the top of the compartment; the black connector goes to the side that has the positive battery symbol at the top of the chamber. I think that's the right way around.

 

I’ve done the same. Some older Canon models have a notch in the door for that reason. Sure the springs aren’t overriding and shorting the source. Does the camera still work with regular batteries. It likely wouldn’t if it took 800ma in a reverse polarity.

 

Does the camera still work with regular batteries. It likely wouldn’t if it took 800ma in a reverse polarity.

The camera still works just fine, which suggests something physical. But then why just 800mA and not an obvious burnt contact point? Nothing has ever been connected reverse polarity.

I might re-do it as one dummy with two contacts wired, and one other with two contacts direct connected internally. That's how the commercial units are made, but I suspect that's only because it makes it easier to manufacture for any number of batteries. But if I can't see what's wrong with this setup it's likely I'll run into exactly the same problem.

 

Depending on what type of AA batteries you are using, 800mA might just be the short-circuit current! That would definitely be the area I would be checking out. I would advise against connecting your 100A mains power supply to it just yet!

 

Does the 3V collapse when the 800mA is drawn? If not, then where's the 2.4W being dissipated (something should be getting hot)? If it collapses then it looks like it's simply a short circuit.

 

A photograph of the battery contacts inside the camera would be helpful. Also I don't think the coil spring contacts are a good idea as they may add enough resistance in series with the battery to cause a problem. I think it would be a good idea to make the connections physically the same as the contacts on a battery. If you look at the design of the cover that connects the batteries in series you will see the clever design that prevents making a connection to a cell when inserted the wrong way.

Les.

 

I agree with Anniel. The spring could be shorting the adapter +ve to surrounding -ve copper indside the camera.. If you look at the battery compartment door you will see the contact for the battery +ve teminal is smaller than the terminal itself.

 

In fact, real batteries do not have springs, and so the adapter is not what it should be. So you need to simulate the ends of actual batteries.

 

Does the 3V collapse when the 800mA is drawn? If not, then where's the 2.4W being dissipated (something should be getting hot)? If it collapses then it looks like it's simply a short circuit.

A battery pack that reads 2.6v open circuit drops to 1.75 when the adapter is inserted, with a current draw of ~750mA. According to my calculations that's not a dead short, as the internal resistance of the batteries is quoted as less than 0.1Ohm for a single battery, so no more than 0.2Ohm for the pack.

 

A photograph of the battery contacts inside the camera would be helpful. Also I don't think the coil spring contacts are a good idea as they may add enough resistance in series with the battery to cause a problem.

If that was the problem I would be happy! The problem is that there is too much current flow without the camera turned on.

I think it would be a good idea to make the connections physically the same as the contacts on a battery. If you look at the design of the cover that connects the batteries in series you will see the clever design that prevents making a connection to a cell when inserted the wrong way.

There are many ways I could redesign it, but I would like to get an idea of what might be going wrong before doing that, so I have some confidence the changes would work. The design of the cover was mentioned in the post - that design is one reason I bypassed the cover entirely.

 

If the camera functions correctly with normal batteries, then a simple test would be to put in the normal batteries but substitute a current meter across the back ends as a temporary replacement for the jumper in the cap. Then read the draw when it runs on the regular batteries. That 750 mA might be a short time current, or there might be a problem. But it is a simple and cheap check.

 

Your positive spring diameter is too large.

If that was the case I would be able to detect a change in resistance as the contacts were pressed onto the backing plate. That doesn't happen.

 

If that was the case I would be able to detect a change in resistance as the contacts were pressed onto the backing plate. That doesn't happen.

It would not be a resistance issue, but rather a "contacting the wrong thing" issue. OR it may be pressing too hard and causing some sort of short circuit. Not a "dead short", just something behind the contact being touched that should not be getting touched. Things like that do happen.

 

If the camera functions correctly with normal batteries, then a simple test would be to put in the normal batteries but substitute a current meter across the back ends as a temporary replacement for the jumper in the cap.

An interesting thought, but it gives what is expected. Without the camera switched on there is no current draw. When switched on it is 450mA. That's with the digital multimeter: with the analogue meter, when the camera is not switched on there is a small bump in the needle every second. I think that might be the power button electronics testing whether it's been pressed, and the pulse is too short to register on the digital meter.

 

A photograph of the battery contacts inside the camera would be helpful.

This is the best I can do for a photograph. Note that I have confirmed that the backing plates are directly connected to the leaves above them, and are not connected to each other. Also the measured resistance across the adapter terminals does not change whether I press hard or just lightly contact them, and the current draw with the adapter is the same whether it just lightly touches or is pressed fully home. So the current seems to be flowing through those spring leaves, but I can't figure out why.

 

In fact, real batteries do not have springs, and so the adapter is not what it should be. So you need to simulate the ends of actual batteries.

OK. But why? I don't want to go to that trouble without some understanding of what's happening and what I need to change to fix it. There's no point in just making random changes and hoping they will work.