Real life dilemma: Help powering an AC device with DC

[Freezer]

I'd like to see if anybody has suggestions to power a low voltage AC device with DC. Here's the story: I purchased a doorbell chime which requires 16v AC continuous power to actuate a series of chimes. The doorbell chime also requires a separate circuit to initiate the chimes when the doorbell button is pushed. In total, the doorbell chime needs 4 wires: 2 from the 16v AC transformer, and 2 from the doorbell button.

The problem is that my electrician ignored the installation instructions and installed only 2 wires in the wall (which is now drywalled and painted) that make up the doorbell button circuit, so the doorbell chime won't work because it requires power after the doorbell is pressed. Rather than tearing up drywall to run more wires directly to the transformer, I'd like to see if it is feasible to supply power to the chime with a couple of 9v batteries in series. A competing chime uses this system as an alternative to continuous 16v AC power, and it doesn't have a big inverter as far as I can see, so it can be done, I just don't know what would be required. Any suggestions? Thanks!

 

[Minder]

You may have to peek inside and do a little reverse engineering to see what the chime is activated with, if it is AC inductive components (coils etc) they are much lower impedance than the comparable DC.
M.

 

[ADRT]

I don't think that's possible with this circuit. You will need the AC to the chime. Do you have a basement? It is possible to pull more wires without removing the Sheetrock if you can go through the bottom plate of the wall or in an attic craw space through the top of the wall.
Any self respecting electrician would be able to get the other two wires to the chime without having to tear everything apart. Don't use that other guy though. Not unless hes willing to fix the mistake for free and not goof anything else up.

 

[ADRT]

If they are rectifying the AC to DC in the internal circuitry you may be able to do it. There would be a rectifier of some sorts shortly after the incoming voltage. Look for 4 diodes right together on the board or a rectifier bridge.

 

[ADRT]

Here is a photo of one type of rectifier bridge but they come in many shapes and sizes.

 

[ADRT]

If there is a bridge and you can figure out where + and - are, you could simply solder battery leads at those terminals in theory.

 

[Freezer]

If they are rectifying the AC to DC in the internal circuitry you may be able to do it. There would be a rectifier of some sorts shortly after the incoming voltage. Look for 4 diodes right together on the board or a rectifier bridge.

So if it is rectifying AC to DC, would I be able to power with DC without burning up the diodes or rectifier bridge? I'll snoop inside and post a pic of what components follow the power inputs.

 

[Gryd3]

So if it is rectifying AC to DC, would I be able to power with DC without burning up the diodes or rectifier bridge? I'll snoop inside and post a pic of what components follow the power inputs.

Please also dig around the house a little...
Most door bell installations I've seen have a 2 wire conductor run to the front door, and another 2 wire conductor attached to a power supply. The transformer is somewhere accessible... and depending on how the wires are joined, you may be able to do some magic otherwise

 

[BGB]

You may have to peek inside and do a little reverse engineering to see what the chime is activated with, if it is AC inductive components (coils etc) they are much lower impedance than the comparable DC.
M.

is that why a 24vAC contactor slams closed pretty hard with only 9v DC os so?... I had wondered about this (with a few contactors scavenged from a broken HVAC unit), namely they slam closed pretty hard and start getting rather warm well under the theoretical 24v (and seem to still work down to around 5-7 volts). contactor rating for contacts: 600v, FLA: 35, LRA: 180, they are essentially simple SPST switches, too bad not DPDT, but oh well... my guess is they could possibly be driven from 12v using a PWM driver.


but, yeah, otherwise if it is relays and solenoids, it seems like it shouldn't care too much if powered with AC or DC, apart from maybe using it at a lower voltage or similar.

 

[Freezer]

Here's a pic of the board. The pen is where the two wires from the AC transformer connect. Look like anything here to convert AC to DC?

If they are rectifying the AC to DC in the internal circuitry you may be able to do it. There would be a rectifier of some sorts shortly after the incoming voltage. Look for 4 diodes right together on the board or a rectifier bridge.

 

[Gryd3]

Lucky you!
Keep my fingers crossed... the little round black thing with the + , -, and two ~ (tilde) on it is the rectifier
If you can manage to provide a clean shot of both sides of the board, and are not afraid to poke around with a volt meter, it's looking promising.

 

[Minder]

If feeding a device with a bridge input using DC, the polarity will not matter as the DC is 'steered' by the bridge.
m.

 

[Gryd3]

If feeding a device with a bridge input using DC, the polarity will not matter as the DC is 'steered' by the bridge.
m.

I just want to make sure the AC input goes to the bridge directly, and does not branch off and power any parallel circuits.

 

[Minder]

is that why a 24vAC contactor slams closed pretty hard with only 9v DC os so?... I had wondered about this (with a few contactors scavenged from a broken HVAC unit), namely they slam closed pretty hard and start getting rather warm well under the theoretical 24v
.

An AC inductive device relies on the inductive reactance to limit current, if fed from DC the load is purely the resistance of the coil, which is very low for a AC device.
Usually it is not a good idea, even if the voltage is lowered.
The AC device current is determined by the inductive reactance effectively in series with the DC resistance.
M.

 

[Minder]

Those three coil looking symbols feed what appears to be bare copper?
They each have a BEMF diode across them so appear to be DC, strange if the copper is bare??
M.

 

[Gryd3]

Those three coil looking symbols feed what appears to be bare copper?
They each have a BEMF diode across them so appear to be DC, strange if the copper is bare??
M.

Could very well have an enamel insulation on it. I've seen it before even though I don't like it.

 

[Y2KEDDIE]

I recently repaired a similar (Sears) Chime unit. It used a synchronous motor to rotate a switch (integral to the circuit board) to play different chime melodies depending on which door switch was activated.. It was strictly AC, the solenoids were the shaded pole type and the synchronous motor is AC only.

 

[Freezer]

The AC inputs go directly to the rectifier. Does this mean that I can connect DC power to the AC inputs safely?

Lucky you!
Keep my fingers crossed... the little round black thing with the + , -, and two ~ (tilde) on it is the rectifier
If you can manage to provide a clean shot of both sides of the board, and are not afraid to poke around with a volt meter, it's looking promising.

 

[davenn]

The AC inputs go directly to the rectifier. Does this mean that I can connect DC power to the AC inputs safely?

yes and expect around 1.5 V drop through the bridge rectifier

 

[Y2KEDDIE]

Hopefully your battery has enough capacity to supply enough current to energize the solenoids..

Good luck!