# Control voltage and relays **URL:** \ **Category:** Factual Questions\ **Created:** [October 11, 2013, 3:55pm UTC](https://boards.straightdope.com/t/control-voltage-and-relays/670990 "2013-10-11T15:55:30Z")\ **Posts on this page:** 11\ **Page:** 1
**Author:** ![KCB615](https://avatars.discourse-cdn.com/v4/letter/k/ea666f/32.png) [@KCB615](https://boards.straightdope.com/u/KCB615)\ **Post date:** [October 11, 2013, 3:55pm UTC](https://boards.straightdope.com/t/control-voltage-and-relays/670990/1 "2013-10-11T15:55:30Z")
I have a hard-wired doorbell in my house that runs at 16 volts. As with most electromechanical doorbells, when the doorbell button is pressed, it sends power to the bell itself and it goes “ding dong.” I’m good with that, and it works just fine. I am attempting to connect my doorbell to an antique fire alarm gong, the gong will strike once with every press of the doorbell button. The striker mechanism for the gong does not release with power, it releases when power is cut. Furthermore, it only runs at less than 12 volts at a maximum of 100 milliamps, else it burns out the coils in the gong. That power requirement is at odds with the doorbell. My intent is to use a standard 5-terminal relay to switch the gong circuit “off” when the doorbell circuit is powered by the button. My problem comes in with the relays. The relays I can find seem to be either 12VDC or 24VDC on the control side. I am guessing the 16VDC doorbell will be too much for the 12VDC relay. Will the 16VDC doorbell be sufficient to trip the 24VDC relay, or will I be changing transformers and doorbells to make everything operate a one common voltage? ---
**Author:** ![beowulff](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/beowulff/32/542_2.png) [@beowulff](https://boards.straightdope.com/u/beowulff)\ **Post date:** [October 11, 2013, 4:13pm UTC](https://boards.straightdope.com/t/control-voltage-and-relays/670990/2 "2013-10-11T16:13:37Z")
I’ve never heard of a 16VDC doorbell. If that’s truly the case, just figure out the current though the coil of your 12v relay, and then pick a dropping resistor of value (16-12)/(coil current). But, I don’t think your setup will work as you want. I think you will need a one-shot circuit made with a large-value capacitor. ---
**Author:** ![friedo](https://avatars.discourse-cdn.com/v4/letter/f/8edcca/32.png) [@friedo](https://boards.straightdope.com/u/friedo)\ **Post date:** [October 11, 2013, 4:16pm UTC](https://boards.straightdope.com/t/control-voltage-and-relays/670990/3 "2013-10-11T16:16:32Z")
In general, the voltage rating listed on the relay is the minimum required to actuate it. Putting some more voltage on it won’t damage it, unless it’s a _lot_ more. The data sheet for your relay should have maximum voltage/current ratings for the coil. ---
**Author:** ![KCB615](https://avatars.discourse-cdn.com/v4/letter/k/ea666f/32.png) [@KCB615](https://boards.straightdope.com/u/KCB615)\ **Post date:** [October 11, 2013, 4:17pm UTC](https://boards.straightdope.com/t/control-voltage-and-relays/670990/4 "2013-10-11T16:17:43Z")
> [@](#): > > I’ve never heard of a 16VDC doorbell. And if I had proofread my post I would have caught the typo - the doorbell is 16VAC, not DC. My bad. Does that change the need for a resistor prior to the relay, or will the AC trigger the coil? ---
**Author:** ![Crafter\_Man](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/crafter_man/32/458_2.png) [@Crafter\_Man](https://boards.straightdope.com/u/Crafter_Man)\ **Post date:** [October 11, 2013, 4:21pm UTC](https://boards.straightdope.com/t/control-voltage-and-relays/670990/5 "2013-10-11T16:21:43Z")
> [@KCB615](#): > > I have a hard-wired doorbell in my house that runs at 16 volts? Do you know the current required by the door bell, and the max current the power supply can source? > [@KCB615](#): > > My intent is to use a standard 5-terminal relay to switch the gong circuit “off” when the doorbell circuit is powered by the button. The power for the gong… where will it be coming from? Will it be coming from the power supply that is currently used in the doorbell circuit, or from some other power supply? ---
**Author:** ![Amateur\_Barbarian](https://avatars.discourse-cdn.com/v4/letter/a/59ef9b/32.png) [@Amateur\_Barbarian](https://boards.straightdope.com/u/Amateur_Barbarian)\ **Post date:** [October 11, 2013, 4:25pm UTC](https://boards.straightdope.com/t/control-voltage-and-relays/670990/6 "2013-10-11T16:25:43Z")
I would start by rectifying the 16VAC to DC, then limiting or regulating that voltage to power a simple one-shot circuit and the gong. As beowulff said, the simplest possible one-shot circuit for this application would be a big capacitor (I’d guess about a 25,000 uF electrolytic to start, depending on how much striking current the gong needs) and a simple charging resistor. So: two power diodes or a bridge rectifier, a big cap, a 5W resistor, and (ideally) an SPDT switch so that the cap is switched from charging to discharge, but an SPST-NO would work, too. Need a diagram? Know the best striking current of the gong? ---
**Author:** ![Crafter\_Man](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/crafter_man/32/458_2.png) [@Crafter\_Man](https://boards.straightdope.com/u/Crafter_Man)\ **Post date:** [October 11, 2013, 4:29pm UTC](https://boards.straightdope.com/t/control-voltage-and-relays/670990/7 "2013-10-11T16:29:24Z")
And does the gong run on AC or DC? ---
**Author:** ![Uber\_the\_Goober](https://avatars.discourse-cdn.com/v4/letter/u/58956e/32.png) [@Uber\_the\_Goober](https://boards.straightdope.com/u/Uber_the_Goober)\ **Post date:** [October 11, 2013, 4:37pm UTC](https://boards.straightdope.com/t/control-voltage-and-relays/670990/8 "2013-10-11T16:37:18Z")
Would it be reasonable to wire it so that the press of the doorbell raises the gong, and the releasing of the doorbell button drops the gong, so it’s not being held up under a load all the time like that? Whenever I wire something up, I try to minimize the possibility of unintended voltage transference 🙂 So having the firebell not be “live” constantly might help with that. ETA: You should probably just do whatever **Crafter\_Man** says, he’s pretty smart with electrical. Give him the information, and I’m sure he’ll steer you straight. I didn’t see all the replies that came in while I was away from my desk with the reply window open. ---
**Author:** ![KCB615](https://avatars.discourse-cdn.com/v4/letter/k/ea666f/32.png) [@KCB615](https://boards.straightdope.com/u/KCB615)\ **Post date:** [October 11, 2013, 4:40pm UTC](https://boards.straightdope.com/t/control-voltage-and-relays/670990/9 "2013-10-11T16:40:56Z")
The doorbell requires 16VAC from a 10VA transformer. The power supply (transformer) for the doorbell is a Trine 125 transformer (8, 16, or 24 VAC depending on how you wire it), wired to the 16VAC/10VA terminals. The fire alarm gong, with other components, is on its own 6 volt DC power supply, entirely separate from the doorbell circuit. ---
**Author:** ![Amateur\_Barbarian](https://avatars.discourse-cdn.com/v4/letter/a/59ef9b/32.png) [@Amateur\_Barbarian](https://boards.straightdope.com/u/Amateur_Barbarian)\ **Post date:** [October 11, 2013, 4:45pm UTC](https://boards.straightdope.com/t/control-voltage-and-relays/670990/10 "2013-10-11T16:45:50Z")
Then I’d use the same one-shot circuit, powered from the 6VDC (although you could build a charging circuit from the 16VAC to keep it simple), and use a 16VAC SPDT relay to fire the gong. ---
**Author:** ![Crafter\_Man](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/crafter_man/32/458_2.png) [@Crafter\_Man](https://boards.straightdope.com/u/Crafter_Man)\ **Post date:** [October 11, 2013, 5:30pm UTC](https://boards.straightdope.com/t/control-voltage-and-relays/670990/11 "2013-10-11T17:30:24Z")
So if I understand correctly, a relay with N.O. contacts – where the coil is powered by the existing doorbell power supply and the contacts are connected to the gong’s power supply – would work for you? If so, I think **beowulff’s** idea would be the simplest, i.e. use a relay with 12 VAC coil and with an appropriate resistor in series with the coil. For even better voltage regulation you could use two back-to-back 1N5333 zener diodes in lieu of the resistor. A few things to keep in mind if you use this approach: 1. Make sure the doorbell’s power supply can provide the current necessary to drive both the doorbell and relay coil (plus series resistor). If the load is too great for the power supply, the voltage could droop to an unacceptable level. I doubt this would be a problem, since I doubt need to use a big 'ol heavy duty relay. But if this is an issue you may have to go with a SS relay. 2. The relay contacts should be rated for the maximum DC current in the gong’s power supply. 3. Connect a 1N4007 diode across the gong to act as a snubber. This will help minimize arcing across the relay contacts. > [@Uber\_the\_Goober](#): > > You should probably just do whatever **Crafter\_Man** says, he’s pretty smart with electrical. Give him the information, and I’m sure he’ll steer you straight. Thanks, but there are others on here who know a lot more than me about this stuff. And I can show you a few threads where I gave some pretty dumb advice on electrical/electronic questions. :o So buyer beware. 😛