common? — Parallax Forums

# common?

Posts: 200
edited 2005-01-10 19:14
I'm looking at this ultrasonic sensor, and it says:

Pin1 is "Power Supply; MUSTa8v-12v requires 8-16VDC regulated with 30mA current capacity

Pin2: Common; Return for DC power supply, TTL outputs and clock signals

Now, how would I regulate a 9v battery to 30mA current? And does Common mean to attach the negative side of the battery to it?
·

• Posts: 27
edited 2005-01-09 23:16
You require a regulated voltage between 8 and 12V DC.
The unit will draw 30mA +- within that voltage range
A 9 volt battery is pretty well regulated unless you have other things connected to it at the same time.
tma
• Posts: 200
edited 2005-01-09 23:45
How will the 9v battery react if multiple things are connected to it at once?
• Posts: 359
edited 2005-01-10 00:05
You can't really answer that with a direct answer. It depends on the condition of the battery, how many things you are connecting, and what kind of current each item draws. I don't know off-hand how much current a 9 volt battery can provide, or what it's milliamp/hour rating is, but you certainly can connect several 50-100ma load items for a while, but only one 5 amp load for a very brief time.

Batteries have a finite current they can produce, and a finite time for which they can provide a given current. The current over time is expressed as Amp/hours, or in the case of a small battery milliamp/hours. This is an indication of how much current a battery can provide over an extended duration of time. In theory, if a battery has a 500 milliamp/hour rating you should be able to connect a 50 milliamp load to the battery for 10 hours, or 2 50 millamp loads for 5 hours, etc., before the battery is flat. It is more complex than that as the battery rating is derived at a specific amperage of load and different amp loads will result in shorter or longer battery life than the rated load yields.

If the current draw of the items exceeds that battery's ability to provide that current, the voltage sags. And, of course, as you delete the battery the voltage sags.

So I guess that real answer to your question is "It depends."

Jim
• Posts: 200
edited 2005-01-10 04:25
Wow, thank you very much for your explaination, Jim.

But one question, is 'common' the same as 'negative'?
• Posts: 12,537
edited 2005-01-10 10:26
Common is a 'context sensitive' term.· I would say that usually Negative is common in DC applications because a 'common ground plane' is preferred for noise isolation.·

But if you look at an AC transformer's three wire output, it becomes obvious that Common means the SHARED lead and has no relationship to positive or negative until you insert either one diode, two diodes or a four diode bridges.· The results of what becomes negative and what positive are entirely dependent upon your choice of design.

ALSO - Those 9V batteries don't have much of a life (at least the rechargible ones don't). I have two that are rated at 160mAH and two rated at 200mAH. They tend to need recharging daily when in use.

I posted an idea of a 9 Volt pre-regulator for Basic Stamp projects, but the same 7809 run from a wall wart of 9volts or greater will run forever on AC and puts out a very clean 1000mAH.

If you want a great battery source, the Lithium 8.4V R/C model airplane batteries are rated at 2000mAH. That at least gives you a few days of operation and maybe a week or more.

I still like the 9 Volt battery for convience and size.· They allow me to built something very quickly and I don't have to recharge 4, 5, or 6 batteries to get up and running.

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G. Herzog in Taiwan

Post Edited (Kramer) : 1/10/2005 10:34:38 AM GMT
• Posts: 6,351
edited 2005-01-10 19:14
In your case common is ground (negative).·In situations where this is not always the case it is expressed as "common anode" and "common cathode" such as 7 segment LEDs.

Current regulation it most easily accomplished by placing a resistor in series with the battery and load. But this presents many problems, first, the voltage seen by your load drops, and if you drop below the specified rating it may not work, second it will decrease the useful lifetime of your battery. Power regulation in the form of current regulation is actually more difficult than it sounds·and ussually requires a special circuit like a switched capacitor regulator where the number of times a capacitor is charged/discharged dictates what the current flow is. But in rexamination of your quoted specs I don't think you have anything to worry about. It says "capacity" of·30mA. If you want to be safe, take an multimeter and measure the resistance between pin 1 and pin 2. Calculate I=V/R if this is in the ballpark of 30mA·your good to go (parts with a supply range will list the max current draw over that range so I would use 16V in the calculation (but not the application)·to be on the safe side.

If you have access to a bench power supply, switch it to constant voltage and set the current limit to 30mA and test it. You'll likely find it works just fine.

Post Edited (Paul Baker) : 1/10/2005 7:37:27 PM GMT