Electrical question about AC adaptors

[optoman]

I have a pitney bowes 5lb scale that uses a 9 volt battery. It sucks them up like no tomorrow so I wanted to use an AC adaptor instead. It asks for a 9V DC 300mA. I have a 12V DC adaptor 300mA. Is it okay to use the 12V adaptor?

Thanks

 

[bmacd]

i wouldn't...just get a universal AC adapter from Kmart/radioshack/target for $15-20 and you won't have to buy a new scale.

-=bmacd=-

 

[Colt45]

a 9v 300ma is like.. $5.

 

[optoman]

Why can't I use the 12 V? Isn't voltage just potential energy.

 

[yellowperil]

It could potentially mess up your scale. You can use an adapter with a higher mA than required but not voltage.

 

[optoman]

Okay thanks guys. I just found this at Radio Shack, 9V Adaptor.

Off I go to the Shack!

 

[zephyrprime]

It would probably be ok to use the 12v but I guess it's imprudent for me to recommend it.

 

[Justin218]

Could work, would depend on how it's designed... though I wouldn't risk it if it costs more than 5-10 bucks

 

[NutBucket]

Originally posted by: optoman
Why can't I use the 12 V? Isn't voltage just potential energy.

It is, but voltage isn't like current. If your dveice is rated for 300ma, you can use an adapter that puts out say 1000ma because the adapter will only supply as much current as the device wants. Voltage, on the other hand, is not like that. If you have 12 volts of potential and only require 9, where is the other 3 going to go? One side of the supply is 12 volts higher then the other, ground. The 3 volts cannot return back to ground since ground is defined as a reference. You could rig up a voltage divider circuit to use your 12V adapter with the device but isn't it far simpler to just go get a suitable adapter?

 

[RossGr]

You might be able to use the 12V, it all depends on the circiut design. Then again since the circiuts are designed to run on 9V they WILL be drawing 30% MORE current then planned on. That might not cause any troubles at all. Maybe the circiuts will handle it just fine, might even work better, brighter display, more reliable etc. BUT you do not know that, the other (and more probable in my mind) is that the circiuts were designed for 9V and the extra current draw will over heat something which will sooner or later burn up, then you will have a dead scale.

If you wish you, can experiment and find out if it is a well designed robust circiut, or the normal cheap household applaince, plug it in and see how long it takes to let the smoke out.

I would guess that the odds are in favor of smoke. Your scale, you get to roll the die. Let us know the results of your experiment.

 

[AlienCraft]

Keep the voltage the same. You can use more ma Milliamps, but not anymore voltage. It may fry the scale if you do. Maybe not right away, but sooner or later.

 

[optoman]

I actually tried the 12 V adaptor and it worked fine. But I still went out to Radio Shack and got the 9 V battery replacement adaptor. Plugs straight into where the battery was connected. I found out that the AC adaptor connection on the scale was reversed and I didn't feel like switching it around. Thats why Pitney Bowes wants you to buy there adaptor. NO thanks, Radio Shacks was $6.91 out the door.

 

[NutBucket]

The radioshack ones ususally have a switch to reverse polarity.

 

[PowerEngineer]

I understand that the problem is already solved, but there is a way you could have used the 12 V supply safely. First find a variable resistor (maybe 0-2Kohm) and wire it in series between the 12 V supply and the scale. Start with the resistor at its maximum value. With the scale powered up and a voltmeter across the input to the scale, decrease the resistance until you have 9V across the scale. Make sure the scale is drawing its maximum power (i.e. all lights on, whatever...). Take out the variable resistor and measure its actual resistance. Go get a 1/4 watt resistor thats reasonably close to that value and solder it in permanently between the 12V supply and the scale. I used this method to hard wire my garage door opener into my car. Of course, it probably is much easier to get the 9V supply!

 

[NutBucket]

Originally posted by: PowerEngineer
I understand that the problem is already solved, but there is a way you could have used the 12 V supply safely. First find a variable resistor (maybe 0-2Kohm) and wire it in series between the 12 V supply and the scale. Start with the resistor at its maximum value. With the scale powered up and a voltmeter across the input to the scale, decrease the resistance until you have 9V across the scale. Make sure the scale is drawing its maximum power (i.e. all lights on, whatever...). Take out the variable resistor and measure its actual resistance. Go get a 1/4 watt resistor thats reasonably close to that value and solder it in permanently between the 12V supply and the scale. I used this method to hard wire my garage door opener into my car. Of course, it probably is much easier to get the 9V supply!

See above post, reference "voltage divider"

 

[thomsbrain]

i think it's interesting that some things like discmans (discmen? discii?) use two 1.5 volt batteries but have 4.5 volt AC and DC adapter.s

 

[Analog]

Originally posted by: optoman
Why can't I use the 12 V? Isn't voltage just potential energy.

cute, why don't you just go for 200V, we'll teach that scale!

 

[FrustratedUser]

Why mot go for rechargeable batteries?

 

[dman]

Originally posted by: thomsbrain
i think it's interesting that some things like discmans (discmen? discii?) use two 1.5 volt batteries but have 4.5 volt AC and DC adapter.s

Yeah, wonder why is that? And you can almost always use rechargeable batteries in them which are generally 1.2v each so, the AC adapter is putting out nearly 100% more voltage than they use with the batteries. I mean, clearly they are designed to work with 4.5v adapters and 2.4v of batteries but it just seems odd to design it that way.

 

[radioouman]

Just get a 9V Zenor diode and use the 12V supply. That way you aren't drawing current and wasting electricity with a resistor! (By the way, your scale probably only requires 6 volts, but has a zenor diode to keep the voltage at a constant 6 volts when the battery starts to go bad.)

 

[radioouman]

Originally posted by: PowerEngineer
I understand that the problem is already solved, but there is a way you could have used the 12 V supply safely. First find a variable resistor (maybe 0-2Kohm) and wire it in series between the 12 V supply and the scale. Start with the resistor at its maximum value. With the scale powered up and a voltmeter across the input to the scale, decrease the resistance until you have 9V across the scale. Make sure the scale is drawing its maximum power (i.e. all lights on, whatever...). Take out the variable resistor and measure its actual resistance. Go get a 1/4 watt resistor thats reasonably close to that value and solder it in permanently between the 12V supply and the scale. I used this method to hard wire my garage door opener into my car. Of course, it probably is much easier to get the 9V supply!

Isn't a zenor diode easier???