good afternoon i bulid a circuit to convert 220v ac to 12v dc so i can power a 12v dc relay. once i attach the output to the relay the voltage drop to 2v once i disconnect it and attach the positive 12v to the relay it work but then i have the disconnect it again if i want to powr it again.
Electrical – 220v to 12v circuit problem
12v
Related Solutions
The solution you already have is the best trade-off in the given conditions and let me explain:
It's a bad idea to have a switched mode power supply near an audio device. They're noisy.
But a home made switched mode power supply is worse. You won't get the low level EMI that can be reached (and imposed by regulations) on a commercial product.
In the same time I guess that form the same battery you will supply other devices to. Most likely with the signal ground tied to 12V ground.
But without looking inside the mixer box you don't know how is the AC power input related with the signal ground so your AC supply must be insulated from 12V battery/signal ground otherwise you might short or overload some internal circuits inside the mixer. Just think what happens if you have a half-wave rectifier inside and you connect the ground to the "hot" wire.
A pure analog solution like a 50Hz generator followed by an amplifier and then a 50Hz transformer to raise the voltage from 12V peak to peak (bridge configuration) to 17V peak to peak has a very low efficiency and requires a hard to find 12v to 17V 1A transformer.
Modifying an inverter from 230VAC to 12VAC is almost impossible and certainly will alter the EMI compliance.
What you can do is to find the smallest inverter, the smallest I could find was 50W so you won't have cooling issues, and use-it with a 12V transformer which you already did.
It's the best option from the size, noise ,time spent and cost.
The next best is to disassemble the mixer and see the internal power supply schematic, this can save some money and space with much simple solutions.
Your question is very short on detail, and you should think about WhatRoughBeat's questions in comments, and come up with some specifications.
However, when you are starting, it's difficult to do that in the abstract, so here's an example circuit, a straw man, to get you thinking about why specifications are needed. There are many ways to implement your circuit, this one is convenient.
simulate this circuit – Schematic created using CircuitLab
I've shown the TLV431 as an NPN transistor for 2 reasons.
1) There isn't a symbol for TLV431 in this schematic editor.
2) The way we are using it, as a voltage comparator, it behaves like an ideal, a very precise, NPN transistor, with a 1.24v VBE which is more or less insensitive to device temperature, and with near zero base current. Just think of it as a switch that's off with VBE <1.24v, and on with VBE > 1.24v. In fact this is the way the 431 is usually used in its intended application, monitoring the output voltage of a power converter, turning an opto-iosolator on when its voltage exceeds a set point.
Questions
1) Can the grounds of the input voltage and 12v be connected as shown?
2) This draws about 1mA from the monitor point. Is that too much? It's got to be enough for the TLV431, which can use an order of magnitude or two less, but there are components which take much less input current than that.
3) Do you want the divider to be adjustable like this, or built without adjustment? What range of adjustment? This is plenty to trim out the tolerances, but a larger R3 ratio is needed to adjust the input voltages significantly. Even with adjustment, you may still want to consider the temperature coefficients.
4) If built without adjustment, what tolerance is permissible? The TLV device comes in initial tolerance grades from 0.2% to 2%, though there is a little tempco to add. Resistors of 1% and 200ppm/C are commonplace, 0.1% and <50ppm/C are fairly obtainable. +/- 2% is +/- 1.6v, is that too much? Without adjustment, you will need a higher R1/2/3 current, as you need to allow for the input current tolerance of the 431.
5) This device has no hysteresis. If this is monitoring either the charge or discharge endpoint of the 80v battery, then as soon as the charge or load stops, the voltage will change, and the charge or load will start again. This is rarley what's wanted, and so hysteresis is added to the voltage comparator, so that (for instance) it turns off at 84v, but back on at 82v. You can probably work out for yourself some sort of feedback from the relay driver or relay that changes the reference voltage or divider to acheive this.
6) This device has no noise filtering, a spike could trigger it. It would be normal to put a capacitor across the input of the comparator. What time constant would you need? Too big, it slows down operation. Too small, it lets through too much noise. You have to specify a compromise.
7) It is always live to the battery like this, or is it a one-shot, that has to be manually reset? That would need the addition of some sort of memory element.
In practice, battery systems are popular because they are usable, and usable means meeting charge/discharge endpoint accuracy with 'reasonable' consumer grade circuits. A battery system that needed calibration-lab performance to use would not survive in the mass market. Which means a few % is probably all the tolerance you need.
If you want a system that switches off the load when the battery falls below a set voltage, you might want to consider a modification of this, which draws no power when off, and works as the on/off load switch as well.
Best Answer
Although your circuit may produce 12 V for your relay in principle, I suspect that the impedance of your R-C network is too high at your load current. I have no calculations to back that up, though.
The issue that I see is danger, in the case of a component failure. In your circuit, a diode failing to short-circuit could put mains onto your low-voltage relay and risk a fire. Unless it is critical to your application to save space or parts cost, I would strongly recommend using a step-down mains transformer and bridge rectifier instead.
As a component, the reliability of a transformer will be extremely high and it will ensure that your load circuit cannot receive a dangerously high voltage. Otherwise you are avoiding a fire or similar damage only if every component works well for the lifetime of the circuit. The cost savings will look very unimportant in that light.
As I imagine you know, electronics and circuit design are only a part of engineering. Electrical safety and reliability are just as important, as are cost, environmental safety, suitability for test, suitability for servicing and a good few more.
I have designed equipment for approval to worldwide electrical standards, and before CE marking made it much easier. We used to export my designs of office equipment into 45 countries. To meet many of these approvals, our mains circuitry (PSUs, cabling) had to stay safe and protected under Single Point Of Failure (SPOF) conditions. This means that an engineer from an approving body would visit and open-circuit any single PSU/mains component or solder a short-circuit across it. The equipment would be switched on and operated and was required to not suffer a fire or present any safety hazard to the user.
In that environment, designing mains circuitry with SPOF protection becomes a lifelong discipline that you always follow. Unfortunately, your circuit is far from safe under such testing. I hope this information helps.