some quick electrical sanity checking

adolf hamster · 14 Jun 2018 at 23:46

ok, so i have a DC motor that's ordinarily powered by 4x aa batteries, however the device it's in has its own seperate power supply from a 11.1 lipo battery.

basically i want to replace the aa battery pack with a voltage divider and a supply from the main battery.

So i'm looking to approximately halve the input to the motor, supposedly "its ok" to take 11.1 directly, but given its a component that will be a massive pain to replace i'd rather not kill it prematurely.

So the plan is to switch main power to the motor then drop it with a potential divider. However i'm not sure how i should size it.

I have a bunch of 10k resistors, but i'm not sure if that'll drop the current capacity too much if i use 2 to make a potential divider. I dont know what current this motor takes, but i'll investigate tomorrow and see.

And hints on how i should be wiring/sizing this?

Chris Wilson · 15 Jun 2018 at 11:03

To give a meaningful answer on the practicality of a resistive voltage divider I need to know (as you surmise) the current draw of the DC motor. From that I can calculate the wattage of the required resistors. If the current draw is significant some heat will be generated, and the resistors may be physically large. A cheap DC to DC converter from Ebay may be a more efficient way to do it. I am also thinking it may be possible to "tap off" a lower voltage from the Lipo cells?

Chris Wilson · 15 Jun 2018 at 11:32

10K are far too high.

150 Ohms and 175 Ohms gets you close. 175 Ohms is not a standard value though

The divider itself would consume about 65mA idling

Tefal · 15 Jun 2018 at 11:56

What's the device I'm curious why it uses 2 differnt batteries ?

adolf hamster · 15 Jun 2018 at 12:30

Its an airsoft gun with a seperate electrically wound mag.

The magazine motor is tiny in comparison to the main motor (that needs the lipo to spin it) but i'm currently trying to improve the longevity of the electrical system.

I've replaced the main switching element with a mosfet and the plan is to tap the power from the main motor directly to the magazine, it'll only wind when i'm firing which is exactly what i want.

I forgot i have an adjustable voltage supply so i'll run that at 6v and try to figure out the mag draw and report back. I'm guessing whatever wattage the motor takes is also what the potential divider needs to take?

Chris Wilson · 15 Jun 2018 at 12:34

At least double up the resistor Wattage compared to how many Watts the motor draws. a resistor at its full spec Wattage will be mad hot. The divider resistivity total needs calculating in association with the current draw of the motor. Lower Ohms resistors draw more idling current of course, but higher Ohms ones may well be incapable of supplying the current without significant voltage drop. Need to balance this! Hence a DC to DC converter may be far better. Need more info!

Rroff · 15 Jun 2018 at 12:41

I wouldn't use a resistor divider for that kind of use personally - too easy to become unbalanced with changing current demand and spikes. You definitely want a switching regulator or buck converter IMO.

A lot of these setups won't be particularly battery efficient mind - probably why they have the split power supplies with the AAs in the first place.

AtaRo · 15 Jun 2018 at 13:03

You can use an XL6009 Step Up Down DC-DC Converter cost about 1- 3 quid, there is also the LM2596 based buck converters but prefer the XL6009 due to better efficiency.
If you need a really tiny converter look for mp1584 buck converter though I have not used this for driving motors.
You can get this from the auction site.

adolf hamster · 15 Jun 2018 at 13:04

ok, so the motor is drawing 0.22A with no load on it. if we want i can try loading the mag up and testing it while properly operating (the bb's come out a feed tube so i can just pop that back into the mag and create a loop)

basically this system only needs to function when the trigger is pressed and the main motor is firing, so whatever system it uses will either need to run direct from the main motor power in paralell or be switchable by an 11.1v signal.

mechanically this system doesn't have much load, all it does is life bb's from the mag into a feeding tube. although when the tube is full it'll just stall the motor. which seems to work ok given the default setup works fine.

i don't know exactly what the main motor draws, but i've heard estimates in the 16A range which seems beleivable.

AtaRo · 15 Jun 2018 at 13:25

What is the operating voltage of the motor? you say it is powered by 4 AA batteries, assuming the baterries are in series then it is about 6Volts.

Alkaline AA baterry is rated around 2800 mAh, if the motor actually can pull 16A then it is like shorting this batteries out.

It would be best to actually measure how much current the motor draws from the AA battery.

adolf hamster · 15 Jun 2018 at 13:30

AtaRo said:
What is the operating voltage of the motor? you say it is powered by 4 AA batteries, assuming the baterries are in series then it is about 6Volts.

Alkaline AA baterry is rated around 2800 mAh, if the motor actually can pull 16A then it is like shorting this batteries out.

It would be best to actually measure how much current the motor draws from the AA battery.

the batteries are in series and it is 6v.

this small motor only draws 0.22A, it's the main motor that runs off the 11.1 lipo that draws 16A when in use.

what i want to do is be able to run the small 6v motor off the same 11.1v supply as the main motor, so the pair will drive at the same time. thus eliminating the need for a seperate battery pack for the small motor.

in terms of space if it needs a regulator there's plenty, as long as it's no bigger than a quad AA battery pack.

AtaRo · 15 Jun 2018 at 14:02

You really need to measure how much current the motor draws at 6V, otherwise well only be able to guess here.

If you want to try your luck you can try this converter .

adolf hamster · 15 Jun 2018 at 14:17

AtaRo said:
You really need to measure how much current the motor draws at 6V, otherwise well only be able to guess here.

If you want to try your luck you can try this converter .

0.22A is it's draw at 6v, albeit with no load.

Tefal · 15 Jun 2018 at 15:08

adolf hamster said:
Its an airsoft gun with a seperate electrically wound mag.

The magazine motor is tiny in comparison to the main motor (that needs the lipo to spin it) but i'm currently trying to improve the longevity of the electrical system.

I've replaced the main switching element with a mosfet and the plan is to tap the power from the main motor directly to the magazine, it'll only wind when i'm firing which is exactly what i want.

I forgot i have an adjustable voltage supply so i'll run that at 6v and try to figure out the mag draw and report back. I'm guessing whatever wattage the motor takes is also what the potential divider needs to take?

How are you going to electrically connect it to the gun?

Contacts might become frustrating if they become lose or worn

adolf hamster · 15 Jun 2018 at 15:10

Tefal said:
How are you going to electrically connect it to the gun?

Contacts might become frustrating if they become lose or worn

i'm thinking the main power comes in paralell from the mosfet, it came with connectors by default which are a bit of a pita to connect, it's easier to safe the gun by removing the battery than the magazine.

Tefal · 15 Jun 2018 at 15:17

adolf hamster said:
i'm thinking the main power comes in paralell from the mosfet, it came with connectors by default which are a bit of a pita to connect, it's easier to safe the gun by removing the battery than the magazine.

I mean is the battery for the magazine in the magazine and seperat e electrically from the gun. And are you going to make a set of contacts to transfer power from gun to mag.

I'm guessing this is something like a imagine gun set up with a huge capacity may so you won't be changing it that often?

But if the connection isnt great the mag cutting out would get frustrating. Does the mag plug in as it were or jist have to exposed metal contacts that slide against each other

If it's for lightness the simplest solution may be replacing the 4xaa with a small Lipo paxk of thier own?

adolf hamster · 15 Jun 2018 at 15:23

Tefal said:
I mean is the battery for the magazine in the magazine and seperat e electrically from the gun. And are you going to make a set of contacts to transfer power from gun to mag.

I'm guessing this is something like a imagine gun set up with a huge capacity may so you won't be changing it that often?

But if the connection isnt great the mag cutting out would get frustrating. Does the mag plug in as it were or jist have to exposed metal contacts that slide against each other

If it's for lightness the simplest solution may be replacing the 4xaa with a small Lipo paxk of thier own?

it's an mg42, the mag is mahoosive and you can re-fill it without removing it from the gun. it only gets removed at the end of the game day.

in it's stock config there's a secondary microswitch on the trigger that switches the mag motor, and it has an external connection to join the 2 together.

thing is the secondary microswitch is very flimsy and the weak point in the system. seeing as i have the whole thing apart atm i figured it'd be the right time to eliminate it. plus by powering it off the main battery i won't need to worry about keeping 2 sets of batteries charged.

lightness isn't a problem- its already a heavy beast

AtaRo · 15 Jun 2018 at 16:49

You can try using this mp1584 step down converter, it is capable of 2A at 6 volts output.

adolf hamster · 15 Jun 2018 at 16:52

AtaRo said:
You can try using this mp1584 step down converter, it is capable of 2A at 6 volts output.

how quickly will that output properly from being turned on? this system will involve being turned on/off constantly when firing short bursts.

AtaRo · 16 Jun 2018 at 06:46

To removed the delay the converter can be wired in a way that it is always powered on when the airsoft is inoperation, then insert the on/off switch for the motor at the ouput of the converter in series with the motor.
A main power switch for the converter then can be installed in series at its input to put it, in its always on state.