I have a gadget that runs on 2 sets of 4 D batteries wired in parallel. That is, each set of 4 D batteries is in serial and the two sets of 4 batteries each are wired together at the ends.
Rather than buy tons of D batteries all the time I’d rather plug this thing in. But, what AC to DC adapter do I need to not destroy my gadget and make sure it gets enough power?
I think I need 6 Volts. How do I figure out how many amps I need in my adapter?
There seems to be a YouTube help video for almost everything; maybe someone has posted an AC conversion video for your particular gadget, if you’re lucky.
We did something similar with a “hands-free” kitchen garbage can with a sensor you wave your hand over to open the lid. Very convenient, but it used 6 AA batteries and burned through them fast. Mrs. solost found a video showing exactly how to wire up an adapter to our specific kind of can lid, saying they found that it took an adapter with (X) amount of watts (can’t remember exactly) and at least 1 amp, if I recall.
I had a big box full of old adapters for obsolete stuff. I had to go through 2 or 3 before I found one that worked, but once I did it worked great for several months before it just stopped (whether just old age, or the adapter was not quite the right voltage and wore it out I don’t know).
I found this site. I have no idea as to the quality or safety.
Can you measure the current to determine the max current the load requires?
Here is a 6 VDC regulated power supply that can source up to 1.2 A continuously. Only $10. Should work O.K. if the load requires less than that.
But things can get complicated. If the load has a max current of 1.1 A, for example, then that power supply should work fine. But if the current is 300 mA for 95% of the time and 2 A for 5% of the time, you have two choices: 1) install a capacitor across the 6 V that has enough capacitance to supply the extra current during the 5% time intervals, or 2) use a power supply that can source up to 2 A continuously.
There is a practical limit set by the device’s current usage: what’s the maximum current at 6v the existing battery of D cells can safely source? That’s the upper bound on what a DC power supply needs.
Found this lithium D cell, which specs a max pulse current of 300 mA (see second pic in product description). OP has two stacks of 4 D cells wired in parallel, so a power supply that provides 6 volts and 600 mA (300 mA for each stack) should take care of it. Here’s a 2-amp unit for $6. Says it’s high quality, so there you go. 
If OP can solder, then they can connect wires from the power supply to the appropriate tabs in the battery compartment as needed. If not, then they’ll need to device something with plastic or wood spacers to keep the wires pressed against those tabs. OTOH, if they can’t solder then its going to be difficult to to add leads to the power supply wire to feed the multiple locations in the battery compartment, so it might be better to look in to a finished solution from the link @running_coach supplied.
That’s actually an unregulated supply. Sorry, my bad. (Which still might work, depending on the load requirements.)
You could sort-of replace them with AC by buying rechargeable batteries. The current generation of rechargeables is quite good.
The current batteries are in a stand-alone enclosure connected to the electronics by pluggable wires (like a big version of this), so no soldering required.
But, I just looked at the enclosure again, and I was wrong about how it was wired. I’m not sure why I thought the two halves were in parallel, I guess my eyes weren’t working. There’s a wire across from one negative to the other positive, and the whole 8 batteries are in series which is the normal way to do things. So that’s 12 volts.
But does that mean I probably don’t need more than 1/2 amp?
Ahh, good. Because it’s easier to find a 12 V power supply than a 6 V power supply. A couple sources:
As previously mentioned by me and others, it’s best to measure the max current required by the load. You can use just about any cheap DMM to do it.
But as (also) mentioned above, things can get complicated if there are current “spikes” that the load wants, but that the cheap DMM can’t read. Furthermore, a linear power supply will always work, whereas a switching power supply may or may not work.
Once this max load is determined (at least to a conservative estimate), it doesn’t hurt to buy a power supply capable of delivery more current than needed. The excess capacity isn’t forced on the device; it would go unused, but widening the range of eligible devices can be beneficial.
Yeah, the last set of batteries I had corroded and I was hoping to avoid having to buy more. I should have done that last time it was working.
I don’t think this thing will be damaged by underpowering it, and I might have a 12V supply sitting around I can experiment with.
Oh, I agree. But it also doesn’t make sense to make it a much higher capacity than it needs to be.
Each D battery has about 4.5-8 amp-hours capacity, which means that if you aim in the middle, you’re looking at about 6.5 amps for an hour @ 1.5 volts. Or 1 amp for 6.5 hours.
If I’m remembering how to do this right, that means that your four D batteries in series are 6.5 amps for 1 hour @ 6 volts, and both sets in parallel are 6.5 amps for 2 hours.
I think that means that you need 6.5 amps @ 6 volts, but I’m not sure.
Technical sense, sure. Extra current capacity is wasted.
But it might make paradoxical economic sense. Sometimes the power supply with the higher current rating is less expensive.
How long does a set of batteries last? Assuming continuous current draw that would give you some idea of the current needed.
Agree.
One thing about the “high current” supplies is that they tend to be switchers. And you have to be careful with those due to the conducted noise they add to the DC. Some loads don’t care about the noise. Some do. By contrast, a linear supply will always work with the load. (Though of course, they tend to be bigger & heavier compared to their switcher counterparts.)
No reason to keep the gadget a secret, it’s one of these:
It’s basically a switch with a motor and a couple of circuit boards with resistors and capacitors on them. I don’t think there’s even a logic chip involved though I haven’t studied the mechanism in detail. So the load is VERY intermittent.
I don’t know about this. If I bought something cheap like this:
https://www.amazon.com/TMEZON-Power-Adapter-Supply-2-1mm/dp/B00Q2E5IXW/
Which is it?
That’s a switching PS, almost certainly. But also almost certainly a perfectly acceptable supply for an intermittently used motor.
Thanks for that. I was chomping at the bit over what the heck took 8 D cells.
I had a Mattel E-Vex video glasses. It was originally for use with RC cars but I hooked it up so I could watch TV in bed without disturbing my wife. Problem was it only ran on 3 AA batteries and went through a set every 4 hours so it was costing a small fortune to use it. I had toyed with the same idea as you but in the end just decided to use rechargeables.