Capacitor to reduce DC motor start up current draw.

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Hi folks,

I'm a noob when it comes to electronics but am trying to learn.

A while back I bought a cheap battery powered rotary tool / large engraver.  
It didn't run long on a chanrge and didn't have much power so I opened it  
up. It had three NiCad cells end-to-end as it's energy store, a simple  
switch and charged through a jack to which you'd connect the 5v DC power  
supply.

I decided to replace the NiCads with an 18650 Li-Ion cell and use the  
following module to charge the cell and prevent discharging;  
http://www.dx.com/p/219454

I've used those modules before with cells taken from old laptop battery  
packs and LEDs and they work well. However after I modded the case, soldered  
everything together and tried it (yeah I should have tested it outside the  
case...) I found it wouldn't switch on. :(  However if I momentarilly  
shorted the DC out to the battery connection the motor would run just fine  
except if put under very heavy loads, in which case it'd stop. (I killed a  
couple of the modules messing around trying to fit a momentary contact  
switch to short across to start the motor.)

Some guy in a forum on that site suggested it was because of the high  
current draw on start-up of the motor and that I put a capacitor in parallel  
(?) with the motor it might fix it. However I was unable to engage him any  
further to get him to tell me more, what size and type of cap etc.

I'm not rich and don't have the funds to buy electronic components and try  
whenever possible to use salvaged parts. I keep old PCBs and tend to use  
them as my supply of parts whenever possible...

Can anyone tell me if doing this is likely to work and if so what type and  
value of capacitor to use please? I seem to recall seeing something similar  
somewhere, what appeared to be a tantalum capacitor wired across a DC motor.  
I can see quite a few of those on the various PCBs that I have. If someone  
can confirm that it might work them I'll get to unsoldering and try to work  
out what values the caps are that I can find.

Cheers, and TIA.
--  
Shaun.

"Humans will have advanced a long, long way when religious belief has a cozy  
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Re: Capacitor to reduce DC motor start up current draw.
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problem is your module isn't tough enough to start the motor.  
adding capacitors will likely make matters worse.  

Adding a resistor in series with the motor might help, say 0.47 ohm 5W  
then put a switch parallel with the resistor to bypass it once the
motor starts.

--  
  \_(?)_

Re: Capacitor to reduce DC motor start up current draw.
Once upon a time on usenet Jasen Betts wrote:
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Ok, thanks. Like I said I didn't understand what the guy meant and I'm quite  
new to electronics (and an 'old dog' - finding new tricks not as easy to  
learn as they used to be).

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Thanks. There's very little room in the case. I've already crwoded it with  
the longer cell and charging module. The momentary swith I fitted was a thin  
thing, a recessed reset switch scavenged from a dead ADSL modem and  
hot-glued into the rotary tool case.

Cheers.
--  
Shaun.

"Humans will have advanced a long, long way when religious belief has a cozy  
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Re: Capacitor to reduce DC motor start up current draw.
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I can't help but point out that even the sales picture shows a board
with an LED misaligned and almost shorting to a nearby resistor, which
looking at the board traces, would probably damage the rearby IC.
I'm not saying that the board is the problem, I just think it says
something about the sellers of this stuff that they use a board like
that as the demo.

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That wouldn't work unless the capacitor's connection to the battery voltage
bypassed the power switch, so that it was charging all the time. Even then
it may trigger the current limiting circuitry, which may or may not cause
a problem depending on design. If it uses foldback current limiting, then
it would drop the voltage until the point that less that the maximum
current is flowing. As the capacitor charged, the current flow at that
voltage would be reduced, and as such the current limiting circuit would
progressively raise the voltage until both charge circuit and capacitor
were effectively at their full voltage. with a normal electrolytic
capacitor, this would always take less than a second to occour.

If however, the current limiting actually cuts off the power completely
when the maximum current is exceeded (for protection in the case of a
short perhaps), you would need to limit the power going to the capacitor
when it charges. This could be achieved by placing a current limiting
resistor (47 ohms should allow a quick enough charge time with most
large value electrolytics) in series with the capacitors connection
to the battery voltage (which is still before the power switch), and
a power diode (eg. 1N4004) in parallel with its Anode towards the
capacitor. The diode would supply current from the capacitor when the
power switch is turned on.

The capacitor value required would depend on the peak load exerted by
the motor (Storage Oscilloscope measurement), but I'd recommend just
trying the largest capacitance value electrolytic you have which is a
suitable size for the application. You might need to try some in
parallel if that isn't enough. You'd want the total capacitance well
above 1,000uF in any case. Quite likely above 10,000uF.

Note that in both cases, the connection of the capacitor bypassing the
power switch will cause a constant (though not relatively large) drain
on the battery. The exact current would depend on the leakage of the
capacitor, and therefore its capacitance, quality and age. The other
suggestion of a switched resistor in series with the motor might be
preferred if this is undesired. From the battery/module's point of
view, that is simply a manual equivalent to the capacitor solution.

Indeed a circuit could be constructed to automate the switching of
the motor series resistor if so desired. :)

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Perhaps they were actually for reducing electrical noise from the motor,
ceramic capacitors are sometimes used for that purpose.


Sorry for the long post. And I just realised that I can't really speak
because I've got a cheap rotary tool I was given with a dead NiCad
battery, and it's still sitting disassembled somewhere. :)

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Re: Capacitor to reduce DC motor start up current draw.
Once upon a time on usenet Computer Nerd Kev wrote:
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Actually I've noticed that with a lot of their products and so far nearly  
all of the things that I've bought have been better made than the pictures.  
I considered that maybe they get a prototype for the pic?

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Yeah. That's a bit too complicated and fiddly for my liking. ;)

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So about as big as the 18650 cell than? ;)

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Ok. However the case is quite tight...

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Heh, yep if I was up to the task and there was room.

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Thanks.


LOL, sounds like me. It was well worth it re-powering with the Li-Ion cell.  
It's more powerful (and doesn't seem to overheat or anything) and of course,  
even with a salvaged cell last longer between charges and can sit in the  
drawer for a month between uses and still have charge.

I originally modded this with a simple Li-Ion charge module and it worked  
great. But then I got worried about over-discharging so changed to this  
thing. I'm starting to think that the easiest answer might be to run the  
motor from the battery terminals and connect an LED to the 'load' terminals.  
When the LED goes out then it's time to stop using the tool.

There is continuity between the load out + and battery + so I could switch  
positive to both the LED and the motor. Then have the LED - going to load  
and motor - going to the battery teminals of the module. When the battery  
drops below 2.5v the LED should go out. That should work don't you think?

I dunno, I'm winging it here. ;)
--  
Shaun.

"Humans will have advanced a long, long way when religious belief has a cozy  
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Re: Capacitor to reduce DC motor start up current draw.
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Huh, you never can tell with this Chinese stuff.

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You know most of that was describing the technicalities of how it
worked? The actual circuit is just the capacitor with the existing
power switch and control module:

Argh, I couldn't be bothered to do this last time...   _._
                      CAP                             |   |
                 _____| |_________     + _________    |   |
 --------  +    |     | |      *__|_____|         |   | C |
|        |______|__________*\  ON       |         |---| E |
| MOTOR  |     POWER SWITCH- \          | CONTROL |   | L |
|        |                    *OFF      | MODULE  |---| L |
|        |------------------------------|_________|   |   |
 --------  -                           -              |___|

The only added component is the capacitor.

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                  CAP   RESISTOR(47R)
                  | |    ______
                 -| |---|______|--                     _._
                | | | |           |                   |   |
                |     | |\ |      |                   |   |
                |     |-| >|------|    + _________    |   |
 --------  +    |DIODE> |/ |   *__|_____|         |   | C |
|        |______|__________*\  ON       |         |---| E |
| MOTOR  |     POWER SWITCH> \          | CONTROL |   | L |
|        |                    *OFF      | MODULE  |---| L |
|        |------------------------------|_________|   |   |
 --------  -                           -              |___|


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Quite likely (having just looked up the size of one of these mystical
Lithium beasts). Possibly smaller if you can find 6V or 10V rated caps
in the right range. Much smaller if you use a couple of Supercaps.
  
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Oh boy, here we go again...
                     LED    ~120R
                     | /|   ______
                 ----|< |--|______|-                   _._
                |    | \|   ________|   + _________   |   |
 --------  +    |          |    *_______|         |   | C |
|        |______|          *\  ON       |         |---| E |
| MOTOR  |     POWER SWITCH- \          | CONTROL |   | L |
|        |                    *OFF      | MODULE  |---| L |
|        |------------------------------|_________|   |   |
 --------  -                           -              |___|

Hmm, if that's what you meant then we are in trouble...

I think this is really what you want:
                     LED    ~120R       _This is a bridge,
                     |\ |   ______     / not a join
                 ----| >|--|______|-  /                _._
                |    |/ |          | / + _________    |   |
 --------  +    |               *__|____|         |   | C |
|        |______|__________*\  ON  |    |         |---| E |
| MOTOR  |     POWER SWITCH- \     |    | CONTROL |   | L |
|        |                    *OFF |    | MODULE  |---| L |
|        |------------------------------|_________|   |   |
 --------  -                     /     -              |___|
                                /
                        That's the join (Resistor to -)

It's drawn a bit confusingly because I'm getting lazy(er), but
the LED should be on until the voltage gets below ~2V, or the
power switch is turned off.

Now looking at the notes from the a Lithium battery controller I
recently designed (but also haven't got around to doing yet...),
I had the cut-off voltage at 3V. Perhaps against best advise, I
trust myself in my past reasoning and would suggest that 2V is
too low a cut-off point. However I remember I got all my
information from the website "Battery University" (I'm out of
time now, so can't look it up myself to check).

In any case, a regular diode in series with the LED would make
the cut-off point ~2.7V, and that might be acceptable. In
practice it will grow dimmer towards the cut-off point instead
of suddenly going out.


Sorry for the whining, don't feel you put me under any
obligation to do all this.

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Re: Capacitor to reduce DC motor start up current draw.
And before you redefine your understanding of capacitor theory, the
capacitor in those diagrams should be connected with its negative to
the battery negative, not to the motor positive. eg.
                                                       _._
                      CAP                             |   |
                 _____| |_________     + _________    |   |
 --------  +    |     | |+     *__|_____|         |   | C |
|        |______|__________*\  ON       |         |---| E |
| MOTOR  |      |    POWER-  \          | CONTROL |   | L |
|        |      |    SWITCH   *OFF      | MODULE  |---| L |
|        |------------------------------|_________|   |   |
 --------  -                           -              |___|

Sorry about that.

--  
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#_ < |\| |< _#

Re: Capacitor to reduce DC motor start up current draw.
Once upon a time on usenet Computer Nerd Kev wrote:
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Thanks for taking the time to help and sorry it's been so long since I  
visited here.

I was thinking of something along the lines of:


                     LED    ~120R       _This is a bridge,
                     |\ |   ______     / not a join
                 ----| >|--|______|-  /                _._
                |    |/ |          | / + _________    |   |
 --------  +    |               *__|____|         |   | C |
|        |______|__________*\  ON  |    |         |---| E |
| MOTOR  |     POWER SWITCH- \     |    | CONTROL |   | L |
|        |                    *OFF |    | MODULE  |---| L |
|        |                          ----|_________| | |   |
 -------- ------------------------------------------  |___|

Where the motor positive and LED positive are switched but motor negative  
goes straight to the battery bypassing the protection circuit (which seems  
to operate on the negative - the battery positive and load out positive are  
common).

That way only the LED is on the protection circuit (so motor shouldn't trip  
overcurrent protection) and would turn off when the voltage drops below the  
predefined low for the 18650 (18mm diameter, 650 long, the most ubiquitous  
Li-Ion cell there is - it's in flashlights, laptop batteries, powertool  
batteries and even powers Prius and Tesla cars).

Cheers,
--  
Shaun.

"Humans will have advanced a long, long way when religious belief has a cozy  
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Re: Capacitor to reduce DC motor start up current draw.
Once upon a time on usenet ~misfit~ wrote:
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D'oh! Of course 18650s aren't 650mm long they are 65mm long. Sorry about  
that. :-/
--  
Shaun.

"Humans will have advanced a long, long way when religious belief has a cozy  
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Re: Capacitor to reduce DC motor start up current draw.
On 30/11/2015 9:08 AM, ~misfit~ wrote:
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A lot of 18650s on the internet have a capacity only a fraction of that  
claimed. It is probably better to pay the extra for known brands (and  
hope that they are actually from that manufacturer).

Re: Capacitor to reduce DC motor start up current draw.
Once upon a time on usenet keithr wrote:
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Thanks, I know. The only ones I've bought (other than ones supplied with  
flashlights etc.) have been Panasonic NCR18650s from a reputable NZ source.  
Otherwise I've obtained quite a few by opening laptop battery packs and  
indivudually testing each cell. I've got a couple of dozen that way (Sanyo,  
Sony and Panasonic) that hold in excess of 1.5 A/hr out of bad battery packs  
where it's turned out only one cell (or row*) was dead, the rest of the  
cells being still quite usable.

[*] It seems that most laptop batteries have electronics in them that use  
electricity, even on the shelf. Usually a PU for controlling charge /  
discharge parameters and a ROM which records data such as date of  
manufacture, design capacity, cell manufacturer and power cycles. It's been  
my experience that most packs run their cells in a mix of parallel and  
series (i.e. a six cell pack would be 2P3S). All the of the 'dead' packs  
that I've opened have run their internal circuitry from a single parallel  
row of cells. Often if a pack is left flat for a time then that row will  
drop below the voltage threshold that the PU will recharge so the pack is  
essentially 'dead'. However the other parallel rows are often salvageable if  
trickle charged and regain most of their capacity.

(Needless to say I have a charging / discharging set up that records how  
much energy a cell takes to reach 4.2v and how long it takes to discharge to  
2.6v using various drains - usually LEDs.)

Now I just need to find a use for a few dozen ~1.5 A/hr 18650s. I have a  
couple of flashlights but they're not using all of the cells which, once  
tested and capacity recorded, are currently just sitting on a shelf.
--  
Shaun

"Humans will have advanced a long, long way when religious belief has a cozy  
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Re: Capacitor to reduce DC motor start up current draw.
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OK, that's alright if you actually don't mind pulling over 3A from the Lithium
cell. For the breif moment at start-up it probably wouldn't be too much of a
problem (but you might want to check the technical specs, because I'm no
battery expert). The issue might be if you put too much load on the tool and
cause it to stall, then the full stall current will flow and may damage the
cell.

You've also left out the second diode in series with the LED, that I
recommended to get the low voltage warning to trigger closer to 3V.

I know there's a page about the correct discharge cut-off voltages for
Lithium-Ion bateries at this website:
http://batteryuniversity.com/
But the bookmark is on another computer and I can't find it in the index.

OK, just tried looking on other computer, written notes, forehead. Can't
find that bookmark anywhere, perhaps it doesn't exist, I'm sure it does
though. Argh, try another website.

--  
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#_ < |\| |< _#

Re: Capacitor to reduce DC motor start up current draw.
Once upon a time on usenet Computer Nerd Kev wrote:
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The cell can handle it. Also unlikely to have full stall current for more  
than a second if that as it's hand-held. Even 'low draw' 18650s can handle  
5A continuous draw (although it will shorten their lives somewhat) and the  
higher ones as much as 20A.

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I wouldn't mind a slightly higher trigger voltage to extend cell life but  
it's a trade-off with storage capacity. As it is I've got so many usable  
salvaged cells that they're likely to die of old age (even stored at ~40%  
charged in the fridge) before I find uses for them all. ;)

Cheers, and thanks again for the input.
--  
Shaun.

"Humans will have advanced a long, long way when religious belief has a cozy  
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Re: Capacitor to reduce DC motor start up current draw.
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One of the things it said on the page was that 95% of the energy is exhausted
from the cell when it reaches 3V. I actually did finally find the page
yesterday (they'd changed it on me so I didn't realise when I found it), but
of course I bookmarked it on another computer and now I don't have time to
find it again... It's in the index under some fairly obvious name.

--  
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#_ < |\| |< _#

Re: Capacitor to reduce DC motor start up current draw.
Once upon a time on usenet Computer Nerd Kev wrote:
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Yes, you're right - it was the other end that I was thinking of (~4.2v). I'd  
be quite happy to stop draining my cells at 3v. Trouble is almost all  
control modules I can find pull them down to around 2.6v.

Cheers,
--  
Shaun.

"Humans will have advanced a long, long way when religious belief has a cozy  
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