Rectify 12v ac for relay supply

[KarenColumbo]

bit of scrap PCB I had - I put the dropping resistor at the end of the line - I hope that's no problem. Had it done yesterday before the voltage drop discussion

[antonis]

I know I'm going too far in relevance with original issue but what about input current of BC549..??

[KarenColumbo]

I know I'm going too far in relevance with original issue but what about input current of BC549..??

I must admit I just took Master Elliott at face value without trying to calculate something I couldn't possibly grasp. Here's the article: https://sound-au.com/articles/relays.htm I think I remember reading it, thinking a bit and deciding: "That's alright, probably works." But you're right, of course, I should peruse this bit of information once more ...

[GibsonGM]

That Elliot circuit is so very much fancier than I would devise, LOL! 

Good point Antonis - what will you be 'feeding' the BC549, Andreas?  If you power it from a B+ HT suppy, you would need to reduce that (of course!) ha ha.

[KarenColumbo]

The BC will be fed by the rectified and filtered and voltage-dropped 12vac (now 12vdc) from the ps transformer secondary. I hope he survives the ordeal.

[antonis]

I think you're confused a bit about BC549 feed/bias, Andreas.. 
(Elliot's scheme is based on PWM trigger which doesn't deal with Base Voltage/Current)

BJT Collector should be held at almost GND (actually, a little higher due to V_CEsat but let it be..) for the whole 12VDC voltage drop across the relay coil..
That said, BJT has to pass through its Collector about 44mA curent (12V/270R)..
(that should be OK for a 100mA max collector current & 500mW BJT - as far as it is saturated 'cause if it should work from +24VDC PS, meaning it should have a VCE of 12V, it should overcome its power rating -> smoke..!!)
Depending on specific BC549 h_FE classification (A min 110, B min 200, C min 420) you'll have to estimate Base current and doube it, for peace of mind reasons..
(e.g. for a BC549B you'll need a Base current of about 500μA..)
That current should come from somewhere..
(Input in your scheme, which actually is the switch activator..)
So you'll have to place a kind of switching device driver between some Voltage/Current source and BC549 1k Base resistor..
For a 500mV voltage drop on 1k resistor (500μA X 1000R) & another 600 - 700 mV V_BE you'lll have to drive Base from a source of at least 1.2V (1.5V should be a convenient value) capable to provide base current in excess..
Of course, higher voltage source should also be OK but proper resizing of 1k Base resistor should be advisable..

[KarenColumbo]

I think you're confused a bit about BC549 feed/bias, Andreas.. 
(Elliot's scheme is based on PWM trigger which doesn't deal with Base Voltage/Current)

BJT Collector should be held at almost GND (actually, a little higher due to V_CEsat but let it be..) for the whole 12VDC voltage drop across the relay coil..
That said, BJT has to pass through its Collector about 44mA curent (12V/270R)..
(that should be OK for a 100mA max collector current & 500mW BJT - as far as it is saturated 'cause if it should work from +24VDC PS, meaning it should have a VCE of 12V, it should overcome its power rating -> smoke..!!)
Depending on specific BC549 h_FE classification (A min 110, B min 200, C min 420) you'll have to estimate Base current and doube it, for peace of mind reasons..
(e.g. for a BC549B you'll need a Base current of about 500μA..)
That current should come from somewhere..
(Input in your scheme, which actually is the switch activator..)
So you'll have to place a kind of switching device driver between some Voltage/Current source and BC549 1k Base resistor..
For a 500mV voltage drop on 1k resistor (500μA X 1000R) & another 600 - 700 mV V_BE you'lll have to drive Base from a source of at least 1.2V (1.5V should be a convenient value) capable to provide base current in excess..
Of course, higher voltage source should also be OK but proper resizing of 1k Base resistor should be advisable..

Yeah, now I see. And yes, I've been confused about this ever since. I thought if I feed 12v through a switch to the transistor I will activate the coil. if there's nothing at the input (0V, switch "off") the relay is deactivated.
So I got to tone down those 12V at the "input" to 1.5V? Would a bigger resistance instead of the 1k in the schematic help?

I thought if worked like this:

[Ben N]

(- I stick 100n, 100uF and 220uF in series, just for the heck of it)

Sorry so late to the party, but why in series? The point is to shunt ripple to ground; also to have more capacitance, not less, so parallel. (Caps work in the opposite manner from resistors--in parallel they add up.)

[Ben N]

The ancient greeks had 100uF capacitors??  WOW! 

They discovered Ohm's law, dontchaknow. Ohm was a "booteek" theorist who ripped it off from a Greek DIY forum and claimed it as his own. (Just my hypothesis--I need Antonis to confirm or deny.)

[antonis]

Sorry so late to the party, but why in series?

Its never too late for a good party..!!

I presume Andreas meant one cap after each other..

They discovered Ohm's law, dontchaknow. Ohm was a "booteek" theorist who ripped it off from a Greek DIY forum and claimed it as his own. (Just my hypothesis--I need Antonis to confirm or deny.)

Nahhhh... Ancient Greeks simply considered "Earth" as a rather big non-polarized cap..

[antonis]

So I got to tone down those 12V at the "input" to 1.5V? Would a bigger resistance instead of the 1k in the schematic help?
I thought if worked like this:

So you want us to make the maths for Ohm's law & KVL..?? 

12V - 0.7V - (I_B X R_B) = 0 => 11.3 - (0.0005 X R_B) = 0 => R_B = 11.3 / 0.0005 => R_B = 22k

So, be careful to use a BC549B or be able to make calculations for A or C types..
(which, by the way are 12k for A & 47k for C)

P.S.
On a second thought, just throw a 15k resistor instead of 1k, don't bother about BC classification and pray..   

[KarenColumbo]

(- I stick 100n, 100uF and 220uF in series, just for the heck of it)

Sorry so late to the party, but why in series? The point is to shunt ripple to ground; also to have more capacitance, not less, so parallel. (Caps work in the opposite manner from resistors--in parallel they add up.)

theyre not in series, I wrote bollocks, sorry.

[KarenColumbo]

So I got to tone down those 12V at the "input" to 1.5V? Would a bigger resistance instead of the 1k in the schematic help?
I thought if worked like this:

So you want us to make the maths for Ohm's law & KVL..?? 

12V - 0.7V - (I_B X R_B) = 0 => 11.3 - (0.0005 X R_B) = 0 => R_B = 11.3 / 0.0005 => R_B = 22k

So, be careful to use a BC549B or be able to make calculations for A or C types..
(which, by the way are 12k for A & 47k for C)

P.S.
On a second thought, just throw a 15k resistor instead of 1k, don't bother about BC classification and pray..   

Thank you very much!! This I will memorize!

[PRR]

So...... a switch to drive a transistor to drive a relay which is a switch...... Rube Goldberg?

Why won't this do your job?

[antonis]

Nahhhh... I was ready to propose a switch to activate the switch which activates the BJT which activates the relay..

BTW, 2 cascoded Darlingtons could be used for a finger touch latching switch..

[KarenColumbo]

So...... a switch to drive a transistor to drive a relay which is a switch...... Rube Goldberg?

Why won't this do your job?

I'm not sure, really. I think I overthought the whole thing. In my case there are two switching points: a gain stage in/out and cap pairs in the tone stack. A third would be a toggle between biasing methods at the power stage - but I wouldn't switch this "live". But it was the thought of having quite a lot of voltage and current on a dpdt switch that probably could produce sparks and so on. When I scratched the idea of toggling high voltage (there wasn't really the need to do so) the relay thing remained.

In the beginning I wanted latching relays switched by a momentary sexy little button. Couldn't find the right ones, so I bought "normal" relays. And i do have this 12V secondary which otherwise would hang loose. AND I wanted to build a relay circuit - so I can tell myself I have build a relay circuit.

AND I thought about a remote foot switch thing - which probably works better via relay than looping the dignal through 10 meters of regular guitar cable.

[KarenColumbo]

I'm feeling quite moronic at the moment. Seems I unlearned how to use my brain 

But your hints and calculations will help me should I go yet one step further and actuate those relays with a Pi zero or similar high tech application.