Opposite of a Transistor

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George Sephton

Senior Member
What is the opposite of a transisitor?
ie a transistor makes the circuit with an input but what sort of thing breaks the circuit with an input. I have a handheld device that is run on batteries and I want it to run off the power source when it is plugged into a dock, so the dock's power will run the handheld and stop the power from the batteries being used and therefore wasted.

Thanks,
George S.
 
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Dippy

Moderator
Whats the opposite of a transistor?
A santristor.

How about 2 transistors?

Do a sketch for yourself George see how to invert the function of a transistor.

Or maybe if you do a schematic someone can see an easier solution.
And it sounds like you need to read up on a bit of real basic transistor theory ;)
 
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gengis

New Member
You'd probably get a more meaningful answer if you post what you expect to do.

One answer might be two transistors - the common emitter mode switch (the most common kind) uses an NPN to pull the collector low - to do that the base has to be high - so the base is always 180 degrees out from the collector. The second NPN is biased on as long as the first transistor isn't grounding its base drive.

Another answer is a depletion mode mosfet - normally conducting until you drive the gate with enough voltage to "pinch off" the electron flow see: [ulr]flow.http://www.answers.com/enhancement%20mode[/url]

They are more scarce than enhancment mode mosfets
 
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MartinM57

Moderator
If you use something like Rapid 20-0970 as your power connector then it's dead easy - it has a pair of normally closed contacts that you use to connect the battery to your circuit...

...and when you plug in the external supply, it breaks those contacts leaving an open connection to your battery and feeding the supply from the plug to the circuit.

Simples.
 
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Jeremy Leach

Senior Member
Well, you've stated what you want ...
I have a handheld device that is run on batteries and I want it to run off the power source when it is plugged into a dock, so the dock's power will run the handheld and stop the power from the batteries being used and therefore wasted."
Click to expand...
So you're looking for a way of instantly disconnecting the batteries when 'docked'. I suppose a relay might give a solution here. If the relay detects the power source then it energises and disconnects (breaks) the battery supply. You could do the same with transistors.

It does sound like you need to brush up on some basics though, and I'm a bit worried about your 'power source'. What is it? Also, you probably need to watch you don't allow both your batteries and power source to be connected at the same time, even momentarily. Although it all depends on the details and your circuit.
 
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Wrenow

Senior Member
George Sephton said:
I have a handheld device that is run on batteries and I want it to run off the power source when it is plugged into a dock, so the dock's power will run the handheld and stop the power from the batteries being used and therefore wasted.

Thanks,
George S.
Click to expand...
Are you sure it doesn't already do so?

Most devices like you describe that I have seen that have docks and rechargeable batteries tned to charge them on the dock and use the power from the dock at the same time.

The ones that use disposable batteries tend to not allow recharge but disable the onboard, battery, power, as you are indicating you want to do.

Alternatively, I would guess they could be designed so that they will draw from the dock as long as the voltage is higher than the battery output with little, if any drain on the batteris (as I understand it).

Cheers,

Wreno
 
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George Sephton

Senior Member
Thanks for all the questions. Just to say the batteries are 4 rechargebles, ie 4.8V and the power source is 5V. It's basically a SPDT swtich but electronically controlled, hence why transistors. I'm building the whole thing, the handheld and the dock and the dock is simply to power it when it's sat there instead of wasting battery power. But thanks for the 2 transistor and the link to build a NOT gate.
Thanks very much.
George S.
 
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MartinM57

Moderator
George Sephton said:
It's basically a SPDT swtich but electronically controlled, hence why transistors.
Click to expand...
...as opposed to my solution in post #4 which is basically a SPDT switch but mechanically controlled.

Unless there's some other unstated requirement, I can't see why you need anything more complex. Up to you of course....
 
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ylp88

Senior Member
George Sephton said:
Thanks for all the questions. Just to say the batteries are 4 rechargebles, ie 4.8V and the power source is 5V
Click to expand...
If your device can stand running from only around 4.2V (or maybe even 4.5V if you can use a low power Schottky diode) then you can probably just use a forward biassed diode in series with the batteries with the powerpack connector hardwired:

Case 1: With the plugpack connected, the diode is reversed biassed and thus the system is powered directly from the plugpack with the batteries effectively isolated.

Case 2: With no plugpack, the diode is forward biassed and the system is powered from the batteries via the series diode.

If you're really smart, you might even be able to use a similar method to trickle charge the rechargable batteries when the plugpack is connected!

ylp88
 
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manuka

Senior Member
Virtually all cheap solar garden lamps contain a low forward voltage drop 1N5817/9 style Schottky diode.
 
premelec

premelec

Senior Member
Yes... however some panels have a diode built in etc... put a milliameter between youor panel and battery and see what happens at night [with flashlight on the ameter :)]
 
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Dippy

Moderator
Is the suggestion therefore to buy a cheap solar garden lamp and take the diode out?
Is that cheaper than just buying a diode?

Or, and possibly far more sinister, is Stan suggesting you steal a neighbour's solar lamp for the diode. And to turn a profit, ransom the eviscerated lamp back to the owner....
"Five quid in used notes or the lamp gets it!"

I'm surprised no-one has mentioned magnets and reed switches... or have they? I haven't got the energy to look back.
 
hippy

hippy

Ex-Staff (retired)
I'd go for simply diode mixing the batteries and external power supply. As long as the external power supply is a higher voltage than the battery then there will be no current drained from the battery.

You need a diode for the external power supply to prevent the batteries discharging into that when its power is off.

There are IC's which will will provide battery / PSU management if you need something more complicated.
 
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manuka

Senior Member
My Schottky hint related to their anticipated parts box absence. Rather than drop everything & zip into town, just grab one off a gutted solar garden lamp. I've got a lamps parts stash that'd been previously robbed for NiCds, white LEDs , inductors & small PVs etc.
 
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Dippy

Moderator
I know. I was teasing, Stan.
I think George has gone to make his circuit....

Now, the dilemma.
Do I drive into town to buy a cheap solar garden lamp (1 gall of petrol and £1 carpark ticket) - or do I order a schottky from Farnell?
Sorry, teasing again :)

Seriously though, I undestand the point (where practical) about recycling components - perfectly sensible thing to do.
I just hope George has got some cheap garden solar lamps.. sorry, I'm doing it again.
 
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Dippy

Moderator
Sorry, George. That won't work properly.
I take it your transistor theory is a little rusty.

Just try a couple of diodes as outlined above.
 
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George Sephton

Senior Member
I've looked at ics and I can't say there's much good stuff, ie they're all QFN and require MOFSETs (something that im sure is useful but i have no need to learn about now)
So how can I used diodes, I've re-looked over the last post on P1 but don't really understand the bias stuff. I can't say my knowledge of diodes is much better than my knowledge of transistors.
Thanks.
 
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BeanieBots

Moderator
Simply put a diode in series with each supply you have.
You can have as many supplies as you like.

Connect all grounds together. (PICAXE, battery & wall-wart)
Diode between battery positive and PICAXE positive.
Diode between wall-wart positive and PICAXE positive.
Pointy end (cathode identified with a bar) in both cases to the PICAXE.

The battery will not discharge into an unpowered wall-wart because of the diode in series with the wall-wart's output.
The wall-wart will not try to charge the batteries because of the diode in series with the battery.

Edit:
Missed an important part.
The wall-wart supply MUST be a slightly higher voltage than the batteries.
Whichever supply has the higher voltage "wins", ie gets to power the circuit.
 
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Dippy

Moderator
Just think of them as one-way valves or a non-return valve, like the valve in your Dad's car tyre.

Once you have grasped that then we can move onto trickier stuff.
 
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eclectic

Moderator
George Sephton said:
I've looked at ics and I can't say there's much good stuff, ie they're all QFN and require MOFSETs (something that im sure is useful but i have no need to learn about now)
So how can I used diodes, I've re-looked over the last post on P1 but don't really understand the bias stuff. I can't say my knowledge of diodes is much better than my knowledge of transistors.
Thanks.
Click to expand...
@George.

Please have a look at this excellent reference.

Diodes:
http://www.kpsec.freeuk.com/components/diode.htm

And then, as a Summer Holiday task, browse the
whole site.

There is so much learning in there.

e
 
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George Sephton

Senior Member
Sorry I'm being really thick here but reading your description I came up with this, minus the PICAXE bit, hence the 3 floating wires. Am I kinda close to what your saying or like way off.

EDIT: I know a diode allows current only in 1 directions, that's one of few things they didn't take out of GCSE Physics.

EDIT 2: If I were to move onto Power Management ICs (although im looking into MAX712 to charge the batteries) would this circuit be sufficient and remotely easy to use: http://media.maxim-ic.com/images/appnotes/947/DI284Fig01.gif.

Thanks all.
 

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B

BeanieBots

Moderator
No need for the third diode, just the two on the left of your diagram.

What is your ACTUAL battery voltage? (no such thing as a 5v battery).
4.5v (eg 3 X 1.5v) would work if the other supply is 5v.
A 6v battery would continue to drain if the other is 5v.
 
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BeanieBots

Moderator
Fully charged, 4 X NiMh will be about 5.6v.
1.2v per cell is actually quite close a flat battery.

So, if your 'mains' supply is 5v, your battery will continue to power the circuit until it discharges to 1.25v per cell.
You could prevent this by adding a second diode in series with the battery pack, thus giving TWO diode volt drops. (see the link posted by eclectic).
 
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