Appropriate current for modification of a bubble machine - Page 1
Forum section: electronics / beginners

author: Enlargee79
author_rank: Contributor
timestamp: May 10, 2019, 05:04:07 am
content: Hello EEV blog,I have a kids' bubble machine that uses 10x AA batteries. It was always my intention to modify this and feed in a 15V supply from an AC/DC wall wart or similar. The only one I have available provide 400mA of current, and this has lead me to the question of determining how much current the toy is likely to draw.I say 15V because 10xAA at peak charge is ~15V (all are in series). This would probably drop to ~11V fairly rapidly, but I want to maintain close to peak AA power levels for ultimate bubble spewing power. A 12V adaptor would probably be at least adequate, I suppose (and I have those with up to 2A capacity, so there's no likelihood of overload there).Aside from throwing in 10x AA and measuring the actual draw via multimeter, is there a less empirical way of doing this?Also:I've also contemplated using an R/C lipo but then scrapped that thought because bubbles = water + children.Naturally there will be appropriate IP65/66 connections to the power brick.

author: Brumby
author_rank: Unknown
timestamp: May 10, 2019, 05:37:08 am
content: My short answer is: No.With a bubble machine you have electric motors and their current draw is closely coupled with the mechanical load they must drive. I would not try any "empirical" processes, just a piece of double sided PCB slipped between two batteries, with a meter to directly measure current.The one way you might be able to try is to put in a set of fresh batteries, turn the machine on and monitor the terminal voltage of the cells. You then map the voltage drop against discharge charts for the batteries used and deduce the current draw. It is important that you not use any Batterisers.Quote from: Enlargee79 on May 10, 2019, 05:04:07 amWhy scrap this idea?The only danger with a LiPO is when charging or if it is shorted or damaged mechanically. I don't see a great risk to kids with a supply around the 15V mark, especially if it is properly contained.

author: Enlargee79
author_rank: Contributor
timestamp: May 10, 2019, 10:31:31 am
content: Hey there. Thanks for the fast reply.By empirical I actually meant testing the active circuit for real-world data. But your idea of measuring before and after voltage drop is exactly the piece of information I needed. Would it not be similar to measure loaded vs unloaded terminal voltage, or is it better to go by the battery data? (I'm thinking Vdiff= Voff- Von-> then Ohm's law).As for lipo, my concern particularly IS to do with shorting and or physical damage - they are kids. If you're not looking for 23 nanoseconds they will drop the entire thing into the kiddy pool because "I don't know". Even though they're not to use the bubble machine without daddy supervising, they will still have grand ideas.

author: Brumby
author_rank: Unknown
timestamp: May 10, 2019, 10:52:02 am
content: Quote from: Enlargee79 on May 10, 2019, 10:31:31 amMy idea of using the battery manufacturer's discharge charts to compare to actual discharge and identify the current drain that matches is not a really practical one. If you want to know the current required for an alternate power supply, then you should just measure it directly.

author: DDunfield
author_rank: Regular Contributor
timestamp: May 10, 2019, 01:26:55 pm
content: Quote from: Enlargee79 on May 10, 2019, 10:31:31 amIf you know the internal resistance of the batteries (+ the contact resistance between the 10 cells and holder), then by measuring the voltage drop under load vs unloaded you could work out how much current it's drawing.But... this won't tell you exactly how much current it will draw from your 15v supply, because the internal resistance of the supply (and contacts) will be different. 10 AA's at room temp. will likely be somewhere in the vicinity of an Ohm .. probably not significant in this application (Bubble machine presumably runs for a while, so not huge current draw?) .. but it's always a good idea to compare the current draw on batteries to the current draw on a substituted power power supply - if for no other reason than a sanity check. Assuming you go to the bother of doing this, why not just measure current draw in the first place?Dave

author: rstofer
author_rank: Super Contributor
timestamp: May 10, 2019, 03:05:20 pm
content: Measuring the actual current should be trivial. The suggestion above to use a piece of double sided PCB might not work because it will probably be too thick to fit between batteries in a holder but a thin piece of plastic with some thin metal may work. It doesn't even have to be plastic, a piece of card stock will probably work.There's no real requirement for metal foil on both sides of the plastic strip, you can complete the circuit with your multimeter probes. Not ideal but workable.It's just too easy to get the right answer to bother trying to get the wrong answer.

author: mvas
author_rank: Contributor
timestamp: May 10, 2019, 06:01:16 pm
content: How long does it take to discharge your AA Batteries?Some AA discharge curves ...https://www.powerstream.com/AA-tests.htm2 amps ... < 1 hour1 amp ... ~ 1 hour500 ma ... > 1 hour100 ma ... ~ 2 hoursThis does not account for any motor start surge.The AA Battery Voltage drops fast!

author: Enlargee79
author_rank: Contributor
timestamp: May 13, 2019, 10:42:59 am
content: So instead of just theorising... I went and tore this thing down. It's a simple, yet clever design that produces multiple different voltages by using batteries at different points of the bank.If I had just rigged up a 15V power supply, nothing would have worked.I discovered there was more going on while putting some batteries in - the fan and LEDS started up before I was done putting in batteries, since I'd bumped the on switch at some point, unintentionally.Within the battery compartment, there are actually two separate circuits. One pair of batteries is purely for the motor that rotates the multiple bubble rings. These rotate and fall into the bubble mixture as they pass a holding vat, and then continue on around the merry go round over the main fan.There's a 6V circuit that draws partially from the lower 4 batteries and drives a strip of 4 multicoloured LEDs.The main fan uses a 12V supply, using all 8 of the other batteries. It indeed draws >500mA, although I can't measure beyond that at the moment while I wait for my replacement multimeter fuses to arrive. (The 10A circuit on my MM is presently fuse-less, owing to an act of stupidity several years ago).I've sketched up the actual circuit now:So now my plan is to use a couple of DC-DC converters and a 12V 2A supply, which realistically should have enough oomph.