Powering AD826 opamp with two 9V batteries question

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Colin151

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Sorry for this very basic question (I am an electronics novice!), but I want to power an AD826 opamp (in an active crossover) with two 9 V batteries. I've read that you can configure the batteries as a split supply by wiring them in series (wiring one +ve of one battery to the -ve of the other battery), then taking the spare -ve on one battery and connecting to the negative supply pin on the opamp, taking the spare +ve on the other battery and wiring that to the positive supply on the opamp. A ground is supposed to come off the middle wire between the two batteries, but I am not sure where I am supposed to connect this to, or if its supposed to be connected at all.

If anyone had any advice on this it would be really appreciated!

Cheers,
Colin
 
rayma

rayma

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Joined 2011
Colin151 said:
I want to power an AD826 opamp (in an active crossover) with two 9 V batteries. I've read that you can configure the batteries as a split supply by wiring them in series (wiring one +ve of one battery to the -ve of the other battery), then taking the spare -ve on one battery and connecting to the negative supply pin on the opamp, taking the spare +ve on the other battery and wiring that to the positive supply on the opamp. A ground is supposed to come off the middle wire between the two batteries, but I am not sure where I am supposed to connect this to, or if its supposed to be connected at all.
Click to expand...

Right, the center battery connection is used the same way as any other power supply "ground," as a circuit common.
Use large capacitors across the batteries as well. Be sure to use fresh identical batteries, with a switch in each leg before the capacitors, to turn them off.
 
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Colin151

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richie00boy said:
You have it correct. The middle connection is called ground in reference to the circuit it is being used with. It's not necessary to connect to mains earth or any other ground.
Click to expand...

Thanks. So I just connect the middle wire between the two batteries (so they are in series) and it's then not necessary to connect that middle wire to anything else?

Sorry I am learning v slowly...

Cheers,
Colin
 
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Colin151

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Joined 2007
rayma said:
Right, the center battery connection is used the same way as any other power supply "ground," as a circuit common.
Use large capacitors across the batteries as well. Be sure to use fresh identical batteries, with a switch in each leg before the capacitors, to turn them off.
Click to expand...

Thanks for the info. The question I have is do I connect that middle wire (running between the two batteries) to anything else too?

Thanks for the tips on the caps. Any ideas what values to use? When you say wire them "across the batteries" how should I connect them up exactly?!

Cheers,
Colin
 
rayma

rayma

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Joined 2011
Colin151 said:
do I connect that middle wire (running between the two batteries) to anything else too? Any ideas what values to use? When you say wire them "across the batteries" how should I connect them up exactly?!
Click to expand...

The center connection of the two batteries goes to circuit "ground". All the other grounds in your circuit go there as well.
Use 100uF/16VDC or larger caps across the batteries, with + end of cap #1 to + battery, and - end of cap #2 to - battery,
and other ends of the two caps connected directly to the center common connection of the batteries.

Connect the capacitors with the two switches between them and the batteries, so the capacitor leakage doesn't drain the batteries
unnecessarily over a long period of time.

Here's a drawing: http://www.dummies.com/how-to/content/electronics-components-what-are-operational-amplif.html
 
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Colin151

Member
Joined 2007
Thanks for the clarification on the ground. I see I will connect to the circuit ground.

As for the caps thanks for the info on how to wire them. Is it a major problem though if I miss them out?!

Cheers,
Colin
rayma said:
The center connection of the two batteries goes to circuit "ground". All the other grounds in your circuit go there as well.
Use 100uF/16VDC or larger caps across the batteries, with + end of cap #1 to + battery, and - end of cap #2 to - battery,
and other ends of the two caps connected directly to the center common connection of the batteries.

Connect the capacitors with the two switches between them and the batteries, so the capacitor leakage doesn't drain the batteries
unnecessarily over a long period of time.

Here's a drawing: Electronics Components: What are Operational Amplifiers? - For Dummies
Click to expand...
 
Mooly

Mooly

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Unless I've missed something... that circuit shows a power amp type device driving speakers.

Opamps and power chip amps need the correct decoupling. For the opamps there are several options. A single small (say around 10uf) electrolytic across the supply pins close to the opamp covers 99% of what you can achieve. You could also add 0.1uf from each supply pin to ground as well.

The chip power amp needs correct decoupling in accordance with the manufacturers recommends but typically something in the 100 to 470uf region from each supply pin to ground, again bypassed with 0.1uf caps.
 
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Colin151

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Thanks a lot for the tips.
Sorry for the confusion in the digram. There is going to be a dedicated hifi power amplifier in between the circuit and the speaker drive unit (just using the bottom half of the diagram).

So when you say put a 10uF cap across the opamp suply pins, do you mean the pins connected to the battery?

Cheers,
Colin
Mooly said:
Unless I've missed something... that circuit shows a power amp type device driving speakers.

Opamps and power chip amps need the correct decoupling. For the opamps there are several options. A single small (say around 10uf) electrolytic across the supply pins close to the opamp covers 99% of what you can achieve. You could also add 0.1uf from each supply pin to ground as well.

The chip power amp needs correct decoupling in accordance with the manufacturers recommends but typically something in the 100 to 470uf region from each supply pin to ground, again bypassed with 0.1uf caps.
Click to expand...
 
Mooly

Mooly

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Joined 2007
Paid Member
Colin151 said:
Thanks a lot for the tips.
Sorry for the confusion in the digram. There is going to be a dedicated hifi power amplifier in between the circuit and the speaker drive unit (just using the bottom half of the diagram).

So when you say put a 10uF cap across the opamp suply pins, do you mean the pins connected to the battery?

Cheers,
Colin
Click to expand...

Yes, very close to the opamp pins.

(As this is a battery supply then perhaps raymas' suggestion of two caps is better. The impedance of batteries increases as they discharge and so each battery should also have a cap across it to counteract that and maintain a low impedance)
 
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