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Other Vintage Household Electrical or Electromechanical Items For discussions about other vintage (over 25 years old) electrical and electromechanical household items. See the sticky thread for details.

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Old 9th Jan 2021, 11:11 am   #1
BigClick
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Default 7.5 volts into 6 volt coil

Push pull solenoid for a door bell was originally powered in the 1960s by 4x1.5 batteries (Size C).

Thinking of using a dc supply of 7.5 volts I have handy.

Is it worth dividing the voltage as it’s just the occasional “ding dong” during operation?
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Old 9th Jan 2021, 12:15 pm   #2
Peter.N.
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Default Re: 7.5 volts into 6 volt coil

I would think a bell duty cycle would cope with 7.5 volts fine. If it uses the old fashioned bell wire it could well loose 1.5 volts on the way.

Peter
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Old 9th Jan 2021, 12:35 pm   #3
G6Tanuki
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Default Re: 7.5 volts into 6 volt coil

I wouldn't worry - as mentioned the duty-cycle is so low...

In the past I remember 'bell transformers' usually had several different voltage taps on the secondary - between 6 and 12 Volts. You generally selected the tap that gave the required loudness of ring/ding-dong/buzz rather than worrying about the precise voltage!
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Old 9th Jan 2021, 12:44 pm   #4
mickm3for
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Default Re: 7.5 volts into 6 volt coil

Hi it will be fine had a shop door bell 6v type ran for over 20 yrs on 9v just louder as the hammer hit that bit harder Mick
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Old 9th Jan 2021, 12:49 pm   #5
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Default Re: 7.5 volts into 6 volt coil

Stick a couple of 1N5404's in series if you're really worried about it!
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Old 9th Jan 2021, 3:04 pm   #6
The Philpott
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Default Re: 7.5 volts into 6 volt coil

....On a general note just worth bearing in mind that the cheaper unregulated type of DC wall-warts (if this indeed a wall wart) of 7.5v rating will be packing at least 11v off load- so it's max current rating should be compared to the current taken by the appliance it is intended to power.

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Old 9th Jan 2021, 3:11 pm   #7
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Default Re: 7.5 volts into 6 volt coil

Thank you friends for your posts ����✅
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Old 9th Jan 2021, 3:56 pm   #8
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Default Re: 7.5 volts into 6 volt coil

My Freidland bell transformer here is 8VAC, they aren't generally fussy about AC or DC either (unless its got a chip in it!)
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Old 10th Jan 2021, 8:59 am   #9
Peter.N.
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Default Re: 7.5 volts into 6 volt coil

3 - 5 - or 8v I seem to remember the old bell transformers were.

Peter
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Old 10th Jan 2021, 9:00 am   #10
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Default Re: 7.5 volts into 6 volt coil

Quote:
Originally Posted by McMurdo View Post
My freidland bell transformer here is 8VAC, they aren't generally fussy about ac or dc either (unless its got a chip in it!)
No chips here lol. I was going to replace the thing because it was corroded solid with the core / shaft in a right state. I used WD40 to release it and the shaft cleaned up really well. The metal frame for the coil basically split apart so I made a “C” bracket from a small shelf bracket

The return gong is a bit weak. I think this is because the original spring is corroded and doesn’t have the same elasticity it once had. I am thinking about making a new spring but it’s conical and I’m not sure how to start making one.
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Old 10th Jan 2021, 1:54 pm   #11
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Default Re: 7.5 volts into 6 volt coil

The latest CPC catalogue lists, as new items, inexpensive replacement conical springs for battery holders in two sizes (one size for AA and AAA, the other for C and D cells). Nickel-plated steel, about £1 and £2 respectively (plus VAT) for a pack of 10. They are called "spring contacts" in the catalogue.
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Old 11th Jan 2021, 1:45 am   #12
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Default Re: 7.5 volts into 6 volt coil

In my last-house-but-one the Freidland ding-dong stopped working and the lamp in the bell-push was out too. Took me a while to find the transformer - it was behind the panel in the fusebox - and it had an open circuit primary. No label or anything on it so I replaced it with a small 240v-9v transformer - which heated up quite rapidly after only one ding and still the lamp in the bell-push was still out. But a light did come on in my head - ripping the top off the bell-push I saw that button was still pushed in. A judicious tap freed the button, a'dong' sounded and the light came on. Just shorting the wires with my screwdriver made the required noises but every press of the bell-push jammed the contacts closed - it looked like some heavy handed-insurance salesman had really leaned on the button and deformed the flat spring - a tweak with needle-nosed pliers fixed the problem. I fitted the transformer to the front of the fuse panel in a plastic see-through box with a 1amp fuse in the 240v side - much nicer than round the back with those three brass buss-bars.....
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Old 11th Jan 2021, 10:41 am   #13
Peter.N.
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Default Re: 7.5 volts into 6 volt coil

I once called on a customer who had no bell push just two wires sticking out of the door frame, I connected them together and the bell rang, customer opened the door and let me in, never mentioned the 'bell push'

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Old 26th Jan 2021, 1:54 pm   #14
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Default Re: 7.5 volts into 6 volt coil

As many others have said, the increased voltage should be fine. It is however a larger increase than it looks.
The four dry cells would be unlikely to exceed 5 volts under load. 7.5 volts is a 50% increase on this.
Still almost certainly fine in view of the short duty cycles.

My slight concern would be a stuck bell push or a short circuit in the wires thereto. That could energise the coil indefinatly at 150% of the rated voltage. Slight fire risk ? Remembering that the battery voltage is not only lower, but also of limited duration as the batteries would run down quickly.

It might be prudent to fit a small fuse, rated at a bit LESS than normal operating current. This fuse will withstand a brief overload during normal operation, but blow if the current be continued.
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