[NTLUG:Discuss] OT battery substitute

[agoats at compuserve.com]

First, let's understand batteries:
AA sets the size and format and a nominal 1.5V charge based on a carbon 
type cell. All other chemistries have different voltage and current 
ratings even though they use the same size and format.
Based on the chemistry of the battery, the current capability varies, 
but is measured in mA/hr (milliamps per hour). This seems to range from 
about 1200mA/hr to 2800mA/hr (or 1.2 to 2.8 Amps/hr). Current draw is 
limited to the device and not the battery. A short causes the battery to 
discharge completely, quickly, overheating the battery with a 
possibility of catching fire or exploding. Voltage seems to vary from 
about 1.2 Volts to 1.6 Volts based on the chemistry. NiCd, NiMH, Li-ION, 
Carbon, Alkaline, Zinc and many other types of chemistries exist.

Next, power supplies:
The wall outlet is 120V AC and has to be converted to some voltage, DC. 
This can be done with switching power supplies like the ones in 
Computers and for most laptops. These are more efficient and generally 
operate above 60Hz.

The other type uses a transformer to drop the voltage down and other 
circuits to set the DC voltage, it does typically have either a 60Hz 
ripple or hum if single rectifier is used or doubled to 120 Hz if a 
bridge rectifier is used. These depend on large value capacitors on 
their output to reduce the amount of ripple or hum that exists at DC 
levels. 

Current draw is limited to what the device pulls from the power supply 
and no more. If the device is shorted, current draw is a maximum and the 
power supply should have over current protection that shuts off the 
power supply or trips a circuit breaker. If this fails, it will either 
catch fire, explode the capacitor(s) in the power supply or trip the 
house circuit breaker.

EMI/RFI Chokes:
These are typically ferrite based toroids that you wrap the cable 
through so that AC or momentary surges are suppressed by interfering 
(buck) or absorbing magnetic fields. If the choke is not designed for 
the frequency you're trying to suppress, it won't work for you (it will 
help with spikes, i.e., light switch current surges, lightening strikes, 
etc but not your hum). A lot of the switching power supplies operate at 
very high frequency and use ferrite chokes to block the high frequency 
AC from your device.

All that said.... you have a noisy AC power supply and need something 
with less noise.  Higher current power is no issue unless the device is 
bad.  Too high a voltage can be bad as it typically causes your device 
to overheat trying to either regulate the power to a lower voltage (has 
an internal voltage regulator) or by operating at the higher voltage 
(Note that CPU's that operate at higher voltages are faster AND require 
more cooling while the lower voltage CPU's which are cooler and require 
less cooling; try comparing Pentium III vs. Celeron, same frequency and 
class, but different Voltages).

If you can, check the power supplies for the amount of "ripple" they 
have at DC. The lower the ripple, the lower the noise.

Most cheap "wall warts" are extremely noisy with high ripple, frequently 
due to a cheap or too low value output capacitor (called a "low 
frequency filter").

If ultra low noise is required and you have to have an AC power supply, 
a gimmick used is to run a power supply to a battery pack (charging the 
battery pack) and the battery pack is used to drive the device. There is 
a reason the battery is represented by multiple polarized capacitors in 
series... it is a capacitor! Most of these designs use lead acid 
batteries so you have to be careful about instantaneous current draw and 
other such issues if you use some other type, like maybe using a 
capacitor in parallel with the battery pack. Based on the number of 
milli amps per hour and the charge equations for capacitors, the battery 
is typically in the Farads, whereas the typical capacitor is measured in 
micro Farads. Ripple is nearly non-existent with these set ups.

Alvin



Fred James wrote:
> OT battery substitute
> This may seem contrary, but I have a couple of devices (land line 
> phones) that require 4 AA batteries each and I want to substitute AC 
> adapters for those batteries.
>
> Figuring 4 AA 1.5V DC batteries equals 6V DC, I tried a 6V AC to DC 
> adapter and found two undesired effects
>    (a) a light that is meant to blink when there is or has been a new 
> or unanswered call, simply glowed steadily - as in all the time 
> regardless of call activity
>    (b) there was an annoying steady buzz in the receiver that 
> continued throughout any and all calls - incoming and outgoing
>
> Now I think we can know that the device does not require a constant 6V 
> DC because of course DC batteries do run down and while that is 
> happening the devise continues to function properly.
>    (a) I was told by a representative from a battery supplier that 
> most 6V DC devices will continue to function well down to 5V DC
>    (b) I had been running them on rechargeable batteries, which are 
> rated at 1.2V DC each, or a total of 4.8V DC, without any issues
>
> My question may be moot because I have already ordered a pair of 5V DC 
> adapters, but ...
>
> So to me, it seems possible that the 6V DC adapter is just too much 
> power (volts or amps?), and I was wondering if there might be someway 
> of mitigating the output from the adapter?
>
> By the way, I did measure the adapter and it shows a good 6.1V DC.
> Thanks
> Regards
> Fred James
>
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