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Electronics advice please !

shermans
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Posts: 1,162
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Registered: ‎07-09-2007

Electronics advice please !

I fear that this may not be the right forum as it has nothing at all to do with computers or the internet, but I am sure there must be an electronics engineer out there who could answer a simple question.  I have a 12 volt transformer (which registers 18 volts on a multi-meter) which I have used for twenty years to power a simple, standard electro-mechanical doorbell (instead of 2 x 1.5 volt batteries).  It has now gone to meet its maker and I need to replace it.  But the only modern equivalent takes a 9 volt battery.  I wondered whether using the 12 volt transormer would damage it ?  No doubt I could find another 9 volt transformer somewhere, but it would be easier to use the existing one, as it also has to power something else.
I do not really understand enough about DC.  I have little doubt that if I used the wrong voltage on AC, it would burn the solenoid out, but as DC voltages are so low, I am assuming that the problem would not be the same.  The nice thing about using 12 volts on the old 3 volt bell was that it made a really good clatter which could be heard all over the back garden !
12 REPLIES 12
paulby
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Registered: ‎26-07-2007

Re: Electronics advice please !

Did it read 18V with no load?  When loaded it was quite likely giving 12V (dependent on the load and the nature of the load) .  You would have had to measure it connected to the doorbell as you pressed the button to see what the "on load" voltage was.  The components in your old bell may also have had a wide tolerance, meaning that 18V was in their operating range.
As for 9V, personally I'd go and get a new transformer as, without knowing the tolerance of the components in your bell, it's impossible to say if 12V or 18V would damage it.
You could also design yourself a potential divider to drop the voltage down to 9V.
Paul
(PS I am an electronics design engineer!)
pierre_pierre
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Registered: ‎30-07-2007

Re: Electronics advice please !

try something Like This  its cheaper than buying more door bells Cry
Community Veteran
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Registered: ‎15-06-2007

Re: Electronics advice please !

Except it was the door bell which broke  Grin and that doesn't have a very loud ring
itsme
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Registered: ‎07-04-2007

Re: Electronics advice please !

If you are going the potential divider route, which I will not recommend. The current flowing through Z2 has to be at least 10 time greater than the current needed by the load at Zout. If not Z1 will just become a series resistor to the load.
pierre_pierre
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Registered: ‎30-07-2007

Re: Electronics advice please !

Should I have said more and more  Tongue when 18V burnt out 9 volt
Moderator
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Registered: ‎14-04-2007

Re: Electronics advice please !

I hope we are not proposing to mix AC supplies (transformer) with DC devices (electronic door bell) here Undecided

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Community Veteran
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Registered: ‎30-07-2007

Re: Electronics advice please !

its an 12V (18V unloaded) DC transformer supply.
Wheel_nut
Grafter
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Registered: ‎03-08-2007

Re: Electronics advice please !

The PSU described bu the OP sounds like a 12 V Unregulated Transformer based unit. These are usually simple transformer rectifier units which produce unsmoothed d.c. voltage of around 18 - 20 Volts unloaded but dropping to 12V at rated load. If used to drive a solenoid doorbell requiring 3V, the PSU voltage would probably drive 150% of iis rated current with the Voltage dropping to as low as 6V. However, because of the short duty cycle of a doorbell, neither the PSU nor the doorbell would fail ...... until .... a persistent caller finding you out, leaned on the doorbell push for more than a second or two! 
Now, the new doorbell requiring 9V is probably an electronic unit requiring 9V REGULATED and SMOOTHED d.c. I would advise against using the old PSU with this as it would inevitably cause failure of the components of the doorbell. A safer course would be to buy a 9V Regulated d.c. PSU as suggested by pierre_pierre. I have bought some excellent quality PSU's on eBay from powersellers who quote the full spec in the listing. 
Have a look at this item. I have bought these and also a 9V version from him and they are excellent.
http://cgi.ebay.co.uk/Switch-Mode-Power-Supply-12v-1000mA-regulated-adapters_W0QQitemZ120239363975QQ...
Just found the 9V Version http://cgi.ebay.co.uk/ws/eBayISAPI.dll?ViewItem&item=110050369181&ssPageName=MERC_VI_RSCC_Pr8_PcY_BI...
shermans
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Registered: ‎07-09-2007

Re: Electronics advice please !

Wow ! This has turned out much more interesting than BT Speedtesters, spam problems and BBYW updates !
Thanks everyone for your various answers - I have learnt a lot from the discussion, particularly the difference between loaded voltage and unloaded voltage on the multi-meter which I have never appreciated on other applications where I have been testing voltages, but now it makes sense.
I am going to see if I can find a good old fashioned Friedland bell like my last one on Ebay, but failing that, then I will buy a psu on Ebay as proposed by Wheel_nut.
I'm really grateful and impressed that the Plusnet world is so knowledgeable and helpfulCrazy
shermans
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Posts: 1,162
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Registered: ‎07-09-2007

Re: Electronics advice please !

Sorry to raise this old chestnut again, but .......
I got a new Friedland bell on E-bay (with postage, it cost as much as in the shop, but there you are).  However, it still does niot work with the transformer, so I am imagining that the transformer may be due to collect its £200 winter fuel allowance.
However, I am naive about electronics and don't really understand how you measure current rather than volts on a multi-meter.  I have tested the transformer with a 12 volt bulb, and it lights it fine as it always has done.  So it appears the transformer is giving out 12 volts, which the multi-meter confirms under load.
Nevertheless, while points means prizes, it does not mean volts means current.  Is it possible that due to anno domini the transformer could be providing enough volts, but not enough current any more to drive the bell ?  I do not want to buy another transformer unless I know for sure that this one is nicky-nocky-nooed.  As I say, electronics and electricity are not my forte toher than the purely mechanical part.
In other words, does current deteriorate with age whereas volts don't ?
TVM.
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Registered: ‎06-11-2007

Re: Electronics advice please !

Quote

The nice thing about using 12 volts on the old 3 volt bell was that it made a really good clatter which could be heard all over the back garden !

(above from your original post)
Presumably you are "assuming" that the bell works o.k..... ?  have you checked this....?  connect up batteries to make up minimum 6 volts, then connect the + and - to the bell....... dinging noise will indicate the bell works.... this will then confirm that the transformer is at fault..... (on the assumption that the wiring is still in good condition, and that a full circuit is available in the wiring..... i.e not broken anywhere.)
If you have a car, you could also use the 12 volts from the battery, via the cigar lighter socket to test the bell.... you will have  a good 12 volt supply, and definitely plenty of amps (current) to power the bell.... make sure the neighbours have been warned in advance ! ! !  Wink
itsme
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Registered: ‎07-04-2007

Re: Electronics advice please !

It's the other way round, you can't have current without volts.
Have you measured the voltage at the output of the transformer when the bell button is pressed? To measure current the multimeter has to be wired in series with the leads connected to the terminals on the meter marked current, i.e different terminal to the ones used to measure voltage, or at least one terminal different. One lead of the meter is connected to the output of the transformer and the other lead to the load. The other transformer output is connected to the load as normal.