Powering a small electronic device within another device by using a Voltage Reg

steppinthrax

Diamond Member
Jul 17, 2006
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I have a requirement of making giving a non bluetooth capable audio device those capabilities. I have a Nutone IM3303 intercom system within my home. I've traditionally been using a 3.5mm jack to connect to my phone for broadcasting it throughout my house.

I have since came up with a much better idea and believe I have it all sorted out, but want to run it by fellow Anandtech's to see if it should work out.

There are two 115v to 9V AC traditional transformers within the intercom system. One transformer goes to the Amplifier section while the other goes to the Electronics. The circuitry of the intercom system is very traditional. I was looking into installing a Bluetooth Module (board) inside the device along with powering the board with power from the transformer. The power range of the bluetooth I'm looking at is 8 - 24V DC.

Problem is the voltage coming from the transformers is obviously AC. So I looked into buying one of these.

http://www.ebay.com/itm/DC-7V-36V-A...r-/201105105103?ssPageName=ADME:X:AAQ:US:1123

I was planning to connect two additional leads from the transformer to this device, that should give me 9V DC right????

I would then run the DC out to one of these.

http://www.ebay.com/itm/TDA7492P-50...d-/171789188824?ssPageName=ADME:X:RTQ:US:1123

This should accept the 9V DC and should power up. I have all the audio wiring worked out. I also plan on putting a toggle switch on the outside of the intercom that goes before the regulator to the transformer. So it can essentially be shut off entirely.

Few issues/questions.

1. I'm having a hard time finding available amperage. I don't imagine the bluetooth module drawing too much. I can't find a white paper that describe the power req, so I'm not sure what they are etc...

2. My understanding is if you increase the voltage the amperage req should be much less. I was thinking of hooking this voltage reg board to BOTH outputs of the two 9V AC transformers making a total of 18 volts. Essentially increasing my voltage, thus decreasing my amperage req, right????? My fear is that with both connected together it might cause them to short each other or something???

3. Should the output of the voltage regulator be clean enough to run the bluetooth module. It seems it should, it's regulated and filtered with the capacitor?
 
May 11, 2008
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That bluetooth device is essentially a bluetooth audio receiver with a digital amplifier( a high frequency pulse width modulation amplifier). Is that what you had in mind ? It says 2x50W. Even if you take that with a grain of salt, and assume 2x10W, that 7-36V device will not be able to handle the needed current. Are you going to just use the blue tooth module and use some sort of line out signal of about 1Vrms or are you going to connect speakers ?
 

freeskier93

Senior member
Apr 17, 2015
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That power converter says it will supply 1.2A of current, assuming that bluetooth thing can really output a total of 100 watts it's going to be pulling at least 11 amps at 9 volts.
 

steppinthrax

Diamond Member
Jul 17, 2006
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OK thanks. So what I really need is a bt module w/o amplification function. The output goes directly to the intercom system. The intercom system has a built-in amplifier. What do you guys suggest.

I liked this board due to the voltage range and the screw connectors that pretty much eliminates soldering.
 
May 11, 2008
19,471
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OK thanks. So what I really need is a bt module w/o amplification function. The output goes directly to the intercom system. The intercom system has a built-in amplifier. What do you guys suggest.

I liked this board due to the voltage range and the screw connectors that pretty much eliminates soldering.

If it is the cheapest bluetooth module you can get and space is not an issue, you might try to find out if you can access the audio out pins of the bluetooth module. You could then just power the device without connecting speakers. Not using the amplifier itself , just the blue tooth module. And connect a 3.5mm jack plug or something to the bluetooth audio module out pins. Be sure to put capacitors in series with the bluetooth analog outputs as it may have a dc component on them.

It might be possible if you could get a datasheet or pdf with a schematic of it.

The TDA7492 is the digital amplifier :

http://www.st.com/web/en/catalog/sense_power/FM125/CL1503/SC983/PF214773

However,
when looking at the ebay pictures, it seems to have audio out.
Perhaps the audio from ther bluetooth module can be made available as a line out signal. But it is a guess.

s-l1600.jpg
 
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steppinthrax

Diamond Member
Jul 17, 2006
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I can get cheaper than that. Just that this one looks the nicest and is from China. It looks high quality.

So your saying the raw bt module on the left side has audio output pins?

Another thing I mention in the audio forum but got no answer. This will be installed within the intercom and not accessible on the reg. So I want to be able to never have to go back there and rep pair or push some magically button. Based on the YouTube videos that I have seen it looks like it works that way.

I need to check how others work?

But a power button for the whole shebang should work?

Thanks
 

freeskier93

Senior member
Apr 17, 2015
487
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I did miss the 3.5mm out on the board you linked, I would also assume it's pre-amplifier so you could hook it up to the intercom.

If you were to use the converter you also linked to then just bypass the regulator to get 9V DC. I can't tell you how clean of power it will be, the thing is pretty bare minimum. It's just 4 diodes to make the rectifier bridge and two caps, the large one for smoothing and smaller one for filtering. Everything after that is just the voltage regulator to drop it from the input voltage down to 5V, but if the input is 9V and you need 9V then just bypass the regulator.
 

steppinthrax

Diamond Member
Jul 17, 2006
3,990
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I did miss the 3.5mm out on the board you linked, I would also assume it's pre-amplifier so you could hook it up to the intercom.

If you were to use the converter you also linked to then just bypass the regulator to get 9V DC. I can't tell you how clean of power it will be, the thing is pretty bare minimum. It's just 4 diodes to make the rectifier bridge and two caps, the large one for smoothing and smaller one for filtering. Everything after that is just the voltage regulator to drop it from the input voltage down to 5V, but if the input is 9V and you need 9V then just bypass the regulator.
Thanks the voltage source is ac not DC. So it has to go through the regulator. Unless there is some other way.
 

freeskier93

Senior member
Apr 17, 2015
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Thanks the voltage source is ac not DC. So it has to go through the regulator. Unless there is some other way.

I'll try to explain a bit better...

The converter you linked to does two things, first it rectifies the AC voltage to DC voltage, which is done with nothing more than 4 diodes and 2 capacitors. The diodes perform the rectification to DC and the capacitors smooth/filter the output. After that is a fixed linear voltage regulator, it's the thing on the right attached to the heat sink, which takes some input DC voltage and drops it down to 5V DC.

s-l1600.jpg


Basically if you're inputting 9V AC to the converter and you want 9V DC out the linear voltage regulator is performing an unnecessary step. All you'd have to do is remove it and jump the two outer holes together.
 

steppinthrax

Diamond Member
Jul 17, 2006
3,990
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Thanks

Looks like I made a dumb mistake. I thought that regulator simply converted voltages, not dropped it down. Re-read the title, it drops it down to 5V!!!! Yeah you are right, I can simply solder a jumper (bypassing the reg).

It's been awhile since messing with electronics. Now I've been watching YouTube videos about Full wave Bridge Rectifiers and Buck converters!!!!!

So, I did some more research. This right here looks like the smallest bluetooth module I can get. It has a Texas Instruments small chip amplifier on it. But I believe It's very low power....

http://www.ebay.com/itm/Bluetooth-4...846575?hash=item1a0aa6b2af:g:RhIAAOSwcOFWa2CI

What do you think.
 

steppinthrax

Diamond Member
Jul 17, 2006
3,990
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Well looks like I made another dumb mistake.

The voltage coming from the transformer is not 9V but 16V at 10VA. It's specifically this transformer.

http://www.ebay.com/itm/New-Nutone-C905-Transformer-16V-10VA-/141835935989

Looks like I have a lot of amperage to play with then right?????

What is the calculation for how much available voltage when going through a Full Wave?

My understanding is 16V AC == 16V DC, but that's probably not right?
 
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freeskier93

Senior member
Apr 17, 2015
487
19
81
Thanks

Looks like I made a dumb mistake. I thought that regulator simply converted voltages, not dropped it down. Re-read the title, it drops it down to 5V!!!! Yeah you are right, I can simply solder a jumper (bypassing the reg).

It's been awhile since messing with electronics. Now I've been watching YouTube videos about Full wave Bridge Rectifiers and Buck converters!!!!!

So, I did some more research. This right here looks like the smallest bluetooth module I can get. It has a Texas Instruments small chip amplifier on it. But I believe It's very low power....

http://www.ebay.com/itm/Bluetooth-4...846575?hash=item1a0aa6b2af:g:RhIAAOSwcOFWa2CI

What do you think.

I mean, it looks like it should work, but you never know until you get it in your hands and mess around with it.

Well looks like I made another dumb mistake.

The voltage coming from the transformer is not 9V but 16V at 10VA. It's specifically this transformer.

http://www.ebay.com/itm/New-Nutone-C905-Transformer-16V-10VA-/141835935989

Looks like I have a lot of amperage to play with then right?????

What is the calculation for how much available voltage when going through a Full Wave?

My understanding is 16V AC == 16V DC, but that's probably not right?

Depends on the forward voltage drop of the diodes, also depends on how big the smoothing capacitor is. If you assume the smoothing capacitor is sufficiently large (therefore no ripple) the only loss is in the diodes, which can actually be quite large.
 

mindless1

Diamond Member
Aug 11, 2001
8,052
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Well looks like I made another dumb mistake.

The voltage coming from the transformer is not 9V but 16V at 10VA. It's specifically this transformer.

http://www.ebay.com/itm/New-Nutone-C905-Transformer-16V-10VA-/141835935989

Looks like I have a lot of amperage to play with then right?????

What is the calculation for how much available voltage when going through a Full Wave?

My understanding is 16V AC == 16V DC, but that's probably not right?

The calculation is roughly (VAC * 1.41) - (2 * Vf(rectifier diodes)) = VDC

That does not take into account the load rating of the transformer nor the load already placed on it, nevermind the additional load of the BT module, but very roughly that would be [16V * 1.41] - 1.4V = 21VDC

Must this all be plug and play instead of soldering? I ask for a few reasons, one of which is that the output of the transformer is almost certainly already being rectified and smoothed on the electronics board.

BT modules are relatively low power and it's quite possible that you could merely tap into the power circuit on the electronics board to directly power the BT module, without needing a voltage regulator nor even a bridge rectification circuit. Tap means you just solder a couple jumper wires from that section of the circuit to the BT board's power input pins.

Presumably since your setup has worked previously using headphone output from a phone, the intercom system would be okay with a line level signal, that your BT module doesn't need to have a headphone or speaker "amp" per se, just a dac's output. If we were talking high end audio there could be arguing about that, but on an intercom, the intercom itself swamps that fidelity difference.

If you can probe about and determine the voltage regulation on the intercom electronics board, it would help to know the primary power rail voltage in order to pick a directly compatible (without more voltage regulation) BT receiver. For example if it happened to be a common value like 5VDC, for $1 you can get a USB BT receiver dongle, where the USB is only the power source which could be connected to the intercom 5V if that value exists on the electronics board.

Edit: However if that is only rectified and not tightly regulated, you might end up with 120Hz hum on the audio output. Might be prudent to just plan on tossing a 50 cent linear regulator into the mix. [/edit]

There are also bare board (opposed to USB dongle) BT receivers for a little more money with voltage range specs provided instead of only the assumption it would be (5V) USB powered, but either way, most of these would be less expensive and smaller than the one you linked previously.

It's the cart before the horse though, before figuring out if you need to tap off the transformer directly and build a rectification and possibly reglation circuit, or already have a DC voltage you can use, present in the intercom.

* I am not responsible for errors above. I'm much better at hacking something like this than describing it when I don't have it and am relying on a 3rd party description and my own guesswork. *
 
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sm625

Diamond Member
May 6, 2011
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That bluetooth module linked in your OP is designed to drive speakers. That seems like a lot more than what you want. Why not just pick up a $10 android 4.2 phone and use the bluetooth in that? Or better yet, use a wifi app like soundwire or soundseeder then you dont have to mess with bluetooth at all. You can just plug the phone into a nearby outlet using a bog standard 5V adapter, and run an audio cable from the phone to the intercom.