Transformer buzz under load in STAMP project
KatyBri
Posts: 171
Hello,
·
I've finished a BS2 project which required a·16VAC voltage, so I bought a 120VAC bell transformer form one of the local home centers that had two secondary taps - 24VAC and 16VAC.
·
I tapped the 16VAC for the BS2 5VDC circuitry (used standard rectifier, 5VDC regulator circuitry) and also tapped the same 16VAC to connect with four doorbell buttons (with integral LEDs) for lighting. The doorbell buttons are connected in parallel across the 16VAC tap.
·
If·I turn on the power without the doorbell buttons connected, there is no transformer hum. But with all four doorbell buttons connected a moderately loud hum comes form the transformer. The doorbell buttons draw little current (only their LEDs). The total load on the 16VAC is very small, so I do not understand why the transformer hum is so loud. I've used these smaller transformers many times before without hum-seams to have something to do with the doorbell buttons. Each has an LED to light up the button. I don't know the circuitry of the LED, but it somehow lights up with only one leg of the 16VAC active. A basic schematic is attached. The BS2 circuits work fine. The problem is the hum in this transformer is just too loud.
·
I would appreciate some advise here.
·
Thanks
Post Edited (KatyBri) : 6/26/2008 1:44:12 AM GMT
·
I've finished a BS2 project which required a·16VAC voltage, so I bought a 120VAC bell transformer form one of the local home centers that had two secondary taps - 24VAC and 16VAC.
·
I tapped the 16VAC for the BS2 5VDC circuitry (used standard rectifier, 5VDC regulator circuitry) and also tapped the same 16VAC to connect with four doorbell buttons (with integral LEDs) for lighting. The doorbell buttons are connected in parallel across the 16VAC tap.
·
If·I turn on the power without the doorbell buttons connected, there is no transformer hum. But with all four doorbell buttons connected a moderately loud hum comes form the transformer. The doorbell buttons draw little current (only their LEDs). The total load on the 16VAC is very small, so I do not understand why the transformer hum is so loud. I've used these smaller transformers many times before without hum-seams to have something to do with the doorbell buttons. Each has an LED to light up the button. I don't know the circuitry of the LED, but it somehow lights up with only one leg of the 16VAC active. A basic schematic is attached. The BS2 circuits work fine. The problem is the hum in this transformer is just too loud.
·
I would appreciate some advise here.
·
Thanks
Post Edited (KatyBri) : 6/26/2008 1:44:12 AM GMT
doc
26K
Comments
Transformers don't necessarily need a high load to buzz.· Sometimes a modest load or even no load will do it.· A little bit of silicone rubber squirted here and there will often do wonders.
Post Edited (Mike Green) : 6/25/2008 10:15:59 PM GMT
Leon
▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
Amateur radio callsign: G1HSM
Suzuki SV1000S motorcycle
I'm using a Porter-Brumfield KHAU-17A11-24 relay which has a 24VAC coil. I measured the output of the new 16VAC transformer and got 20VAC no load and 19.8VAC under load. When the doorbell buttons are pressed, the relay closes fine, activating the BS2 circuit. After 24 hours, the transformer gets very warm-almost to hot to touch.
I disconnected the doorbell buttons from the circuit and found there was no hum from the relay. I only get the hum when the doorbell buttons are connected into the circuit. The only thing allowing current to flow would be the led circuitry built into the doorbell buttons.
I'm puzzled. Do I need to put something across the relay coil?
If any one else has a suggestion, I would also appreciate it.
A schematic is attached to me original thread.
Thanks.
Post Edited (KatyBri) : 6/26/2008 10:21:39 PM GMT
While it will reduce the brightness of the LEDs, try reducing the overall current in the idle circuit path by increasing the resister values for each of the LEDs and see if the issue resolves itself.
▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
There's nothing like a new idea and a warm soldering iron.
Just to expand on the previous posts, the ac relay is buzzing because you are powering it with half-wave rectified power through the LEDs any time the push-buttons are open.. As an experiment, reverse the connections on two of the push buttons. Then, the relay will see full wave voltage. As long as the LEDS have a resitor in series with them, the relay should not see enough current to operate, except when one button is pushed.
The other solution is to come off the transformer and rectifiy/filter, then use a DC relay. You will have to make sure the push button LEDs are oriented properly.
Cheers,
▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
Tom Sisk
http://www.siskconsult.com
·