Replacing a 9V Battery with a AC Wall Adapter

Weldo

Weldo

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Hey all, I've got a quick question. Electrical stuff is not my expertise so I'm seeking some advice.

I've got a small electrical device that runs off a 9V battery. Specifically it's this thing...

1588166649673.png

The Saramonic SmartMixer+ is a small mixer device that allows two separate microphones to be input and mixed and sent to a tablet/smartphone for recording.

The thing runs on a single 9V battery and while I haven't actually used it yet, I've heard tell that battery life is around 2 hours of use. I'm gonna test it out myself before I go too far with this idea but I'm planning for the future.

I want to power this thing with an AC wall adapter so that I can eliminate the need for batteries. I've got a thing like this...

1588166949196.png

A typical AC adapter with output of 9V and 300mA.

I can solder on a connector like this to the wall adapter...

1588167054472.png

Which I can then use to power the device.

When I plug in the wall adapter it measures 14.7 volts. As far as I know this is normal since there's no load on the transformer. Can I be sure that the 14.7 volts will not damage the device? I'm not sure how the circuits are laid out in the device but isn't it possible that supplying 14.7V into a 9V device can damage it?

Second part. I have also an old Sony Li-ion battery charger from an old digital camera. It has the normal input of 120V AC but the output is rated at 8.36VDC and 600mA. I plugged it in and measured the positive and negative terminals and I actually got 8.36VDC.

Would the battery charger be a better choice for powering this 9V device? I can solder a wire with a 9V connector to the battery charger just like I would with the AC wall adapter. There is a third terminal on the charger that interfaces with a tab on the respective battery for some kind of temperature sensing. The charger seems to output the 8.36VDC even without anything connected to that third terminal. But the temperature sensing feature may somehow be a road block to the use of this charger as a power supply.

Anyone have any thoughts of this?

My last resort is to just get two rechargeable 9V batteries and rotate them. It will save me from buying lots of batteries but there's an inconvenience associated with always checking/swapping batteries.
 
I would think that the circuitry in that adapter needs some load to regulate voltage. A no load measurement won't do a ton for you.

I wouldn't hesitate to just hook it as long as the label says 9V (the attached pic does) and your load won't draw more than 300mA.
 
I agree with Macardoso however, I would be prudent to put a load across the power supply and double check the output. 14.7v into a 9v device will probably kill it.
Robert
 
Weldo said:
Hey all, I've got a quick question. Electrical stuff is not my expertise so I'm seeking some advice.

I've got a small electrical device that runs off a 9V battery. Specifically it's this thing...

View attachment 322625

The Saramonic SmartMixer+ is a small mixer device that allows two separate microphones to be input and mixed and sent to a tablet/smartphone for recording.

The thing runs on a single 9V battery and while I haven't actually used it yet, I've heard tell that battery life is around 2 hours of use. I'm gonna test it out myself before I go too far with this idea but I'm planning for the future.

I want to power this thing with an AC wall adapter so that I can eliminate the need for batteries. I've got a thing like this...

View attachment 322628

A typical AC adapter with output of 9V and 300mA.

I can solder on a connector like this to the wall adapter...

View attachment 322629

Which I can then use to power the device.

When I plug in the wall adapter it measures 14.7 volts. As far as I know this is normal since there's no load on the transformer. Can I be sure that the 14.7 volts will not damage the device? I'm not sure how the circuits are laid out in the device but isn't it possible that supplying 14.7V into a 9V device can damage it?

Second part. I have also an old Sony Li-ion battery charger from an old digital camera. It has the normal input of 120V AC but the output is rated at 8.36VDC and 600mA. I plugged it in and measured the positive and negative terminals and I actually got 8.36VDC.

Would the battery charger be a better choice for powering this 9V device? I can solder a wire with a 9V connector to the battery charger just like I would with the AC wall adapter. There is a third terminal on the charger that interfaces with a tab on the respective battery for some kind of temperature sensing. The charger seems to output the 8.36VDC even without anything connected to that third terminal. But the temperature sensing feature may somehow be a road block to the use of this charger as a power supply.

Anyone have any thoughts of this?

My last resort is to just get two rechargeable 9V batteries and rotate them. It will save me from buying lots of batteries but there's an inconvenience associated with always checking/swapping batteries.
Click to expand...

Wall warts come in two flavors. Linear and switching. Linear supplies have a transformer, usually detectable by the heavier weight and larger size. Linear supplies meant for charging are designed to deliver the nominal voltage at the nominal current. With no load, the voltage is generally higher to account for looses when current is drawn. The devices that they are servicing usually have some form of internal regulation.

OTOH, a switching supply is a regulated supplu that generally delivers the stated voltage for any current up to the maximum. There has been a tendency to change over to switching supplies as they are now less expensive to make.

Without knowing how your device is constructed, I would stay away from the linear supplies as the excess voltage could damage the circuitry. of your device. Based on your photo and measured voltage, I would say that you have a linear supply.
 
I'd use the Sony unit although it provides much more current than you probably need, and you might hear switching noise with your mike mixer.
Or you could use the Hon Kwang (great name) with a small regulator IC like a 7809 that would provide a nice quiet source of 9 volts.
protosupplies.com

7809 Voltage Regulator 9V / 1A - ProtoSupplies

The 7809 is a fixed voltage linear regulator that can output 9V at up to 1A current with an input voltage range of 11 – 35V.
protosupplies.com protosupplies.com
 
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Since this is an audio device there is also a good chance of getting a 60Hz hum added to the output. A battery gives a nice perfect DC power with no ripple.

A typical 9V battery is around 500 mAh and if you can expect 2 hours out of a battery that means you are drawing an average of around 250 mA. That is getting close to the 300mA rating of the wall wart.

A 36 ohm resistor across the wall wart will draw 250mA. Do not leave it connected long as it would require a minimum of a 2.5W resistor to handle the heat But you can measure the voltage with the load to get a better idea of how far it drops under load. But if the regulation is that bad that it is up to 14V with no load. It will probably not be a real clean DC output, You may need to add a filter capacitor to smooth it out.

Or just get something like this that you know is a switching power supply rated well above your needs and still cheap enough.

I would then see if I could add a connector somewhere on your device that matches the one on the power supply .

If you are going to solder the battery connector to the power supply, watch your polarity. red should be Negative and black should be positive.
 
Thanks for all the info guys!

@RJSakowski, I'd assume my AC adapter is the cheapest of the cheap variety, so probably linear. It seems like the battery charger I mentioned may be a better choice. Since it outputs the exact indicated voltage of 8.36VDC even with no load, it's probably a switching type power supply, huh? And since it is meant to charge batteries, could I assume it has a cleaner DC waveform than the linear AC adapter? It has capacitors all over it. I'l try to grab a pic or two shortly.

Flyinfool said:
Since this is an audio device there is also a good chance of getting a 60Hz hum added to the output. A battery gives a nice perfect DC power with no ripple.
Click to expand...

This is something I had not considered. This might be reason enough to abandon the idea.

Maybe I'll look around the house for some 9V device I can experiment with. I paid about $90 for that mixer thing and I don't want to fry it while trying to make it "better".

After reading all your responses it's probably the best option, at least for now, to just buy two rechargeable 9V batteries and rotate them as needed.
 
Linear supplies can be regulated. Most linear wall warts aren't.
 
You can effectivly charge a bettery with sloppy half wave rectified power. Some batteries are a bit more fussy. Most battery chargers will vary the output depending on the load for proper charging of the battery as it gets full. Depending on what type of battery it is for, Some vary the voltage, some vary the amps and some vary both. I would not use a charger as a power supply unless you are certain of what you have and its characteristics.
 
Sounds like rechargeable batteries it is!

Thanks again to everyone for the suggestions. I'd like to make this thing hassle free by plugging it into the wall but even more than that I don't want it to become destroyed!

I'll look up some NiMH 9V offerings.
 
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