Sub-C, or not to Sub-C. That is the question!

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PrIsMaTiC
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Sub-C, or not to Sub-C. That is the question!

Post by PrIsMaTiC »

Yea, I know, really bad Shakespeare. However, it
might take the genius of Shakespere to figure this
one out!

I keep two or three of these gadgets charging
on a small table in my bedroom. It uses a 4/5-SC
1.2 vdc-1200 mAh NiCad. The battery powers a
motor which looks like it belongs in a child's
toy. The motor has always run sluggishly, so I
decided to do a current check. I was guessing
around 200 milliamps or less. When my DVM
displayed "overload" I was puzzled. After switch-
ing to a higher setting I was amazed to see my
"toy" motor pulling an average of 1.2 amps.
That Sub-C is being taxed to the max!

I hooked it up to a regulated suppy and discovered
that at 3 vdc the motor ran faster, smoother, and
generated less heat.

I've been searching for some kind of substitute
battery (or batteries) that will boost the voltage
while providing at least the same mAh rating.
Obviously, it has got to fit inside my gadget.
I realize I'll have to build or buy a different
charger, but that's no problem.

A simple task, to be sure! After you wizards tell
me the perfect battery to use in my gadget, you
might as well cure cancer, build a time machine,
etc.

I'm calling this device "my gadget" because the
history of why I'm stuck with this junk goes
back 25 years. Let's just say my life would be
very uncomfortable without it. I wish it were
something exciting like some strange sex toy.
Watching grass grow or paint dry would be closer
to the truth. Sorry to disappoint all you guys!
Mike
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Location: Illinois
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Re: Sub-C, or not to Sub-C. That is the question!

Post by Mike »

The same exact thing happened to me a while ago. I had a 48VCT 1A Transformer. I wanted to measure amperage with my DVM, so my first thought was hook the DVM up between + and -. I did that, and it showed a reading of over 10A of power. I was curious and posted here about that.

Well it turns out that unlike voltmeters, you do not connect ampmeters in parallel with the power supply. Instead, you connect them in series with the circuit. Think of the DVM in ampmeter mode as a light switch. Connect it between the positive on the battery and the positive on the motor. That will give you the correct reading.

What im assuming, is if that motor is from a toy, it either (a) runs that slow, (b) is designed to run off of 3V (two AA batteries) or (c) designed to run off of 9V.

If its A, then theres nothing you can do besides swap out the motor for a faster one at the 1.2V of the battery you have. If it's B, you will need another one of those batteries in series to power it at full speed. Finally, if it's C, you will need a 9V Ni-Cd.

What voltage does your power supply provide? As long as it's more than the battery you are charging (over 2.4V if you simply need two of those batteries, or over 9V if you need a 9V power source).

You most likely will simply have to add another one of those batteries in series, especially if you are satisfied with the speed of the motor at 3VDC. in that case, get another battery, hook it in series, and charge them using a 3VDC charger with a max of about 500ma to prevent charging them too rapidly. it will take a few hours to charge, but it will prolong battery life and prevent explosions, leaks, etc.
dyarker
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Re: Sub-C, or not to Sub-C. That is the question!

Post by dyarker »

2 "D" size NiCads in series. But check the specs before buying. Some Ds are just a C in a bigger can (1200mAHr). You want the kind of D that is 4500 - 5000mAHr.
Dale Y
L. Daniel Rosa
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Re: Sub-C, or not to Sub-C. That is the question!

Post by L. Daniel Rosa »

If you have space for it, I suggest switching over to NiMH AA batteries. You'll have the same voltage as before, but space permitting four (2*2 series/parallel) of them you'll have 3000-4000mAH. I've found that the best engineering went into AA batteries. Then it's just a matter of getting a charger that can take care of 12 to 16 cells at once.
PrIsMaTiC
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Re: Sub-C, or not to Sub-C. That is the question!

Post by PrIsMaTiC »

Thanks for taking the time to reply, Mike, Dale Y,
and L Daniel Rosa.

Hey Mike, you almost make me cry. You're a
charter member at this forum and you didn't
know how to take a current measurement with
a DVM? Please tell me it ain't so, Mike!

I learned basic electronics when everything was
analog. I've been using analog and digital test
equipment for many years. I think I know how
to measure current with an analog or digital
meter. I've done it several hundred thousand
times!

Here's a good way to take a current reading
without removing the battery from your device.
Cut a small piece of double sided copper clad
circuit board. Trim it down so it will break
contact between the positive battery terminal
and the plate inside the battery compartment.
Set your meter to DC amps. Touch the positive
probe to the battery side of the copper clad
and the negative probe to plate side. This will
complete your series connection without having
to remove the battery.

I looked at very long lists of exotic batteries
hoping to find something that would fit inside
the compartment that holds the Sub-C. There
just isn't anything available that would boost
the voltage while maintaining the same mAh
rating.

This gadget of mine was engineered with very close
tolerances. The battery gives the motor what it
needs without leaving any room for error! When
you're building something for yourself you always
make sure your power source is overrated by at
least 20%. That's not what they do when companies
are trying to save every penny so they can sell
their product in Wal-Mart and still make a profit.
Mike
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Re: Sub-C, or not to Sub-C. That is the question!

Post by Mike »

Well it does only take I believe around 17-18 posts to become one.

And I asked that question a long time ago.

As long as you did make a series connection, there is no way that a little toy motor draws 1.2A. That's why I suggested you were improperly measuring the amperage. If it was connected properly to the circuit, then something must be wrong with the motor
rshayes
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Re: Sub-C, or not to Sub-C. That is the question!

Post by rshayes »

A stalled or slowly running motor can draw a suprising amount of current. When the motor isn't turning, the only limit to the current is the brush resistance and armature resistance. The total can be in the one ohm range even fot small motors, so a current in the one amp range isn't too startling.
Dean Huster
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Re: Sub-C, or not to Sub-C. That is the question!

Post by Dean Huster »

Actually, Mike, if one knows what they're doing, connecting an ammeter in parallel with a supply can provide some valid measurements and can be considered proper and normal practice. A good example is if you want to set the current-limit point of a power supply to a specific value. Start with the current limit at its lowest value, adjust the supply voltage to the desired value, connect the ammeter across the supply terminals and adjust the current limit for the desired reading on the ammeter. So don't feel too bad about an improper measurement technique.

Dean
Dean Huster, Electronics Curmudgeon
Contributing Editor emeritus, "Q & A", of the former "Poptronics" magazine (formerly "Popular Electronics" and "Electronics Now" magazines).

R.I.P.
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