Current Sensing Slave Power Switch
Switch on one unit, and everything else you need turns on automatically. This can save the tedium of turning on half a dozen different things, when one should be enough! This is another project created purely from necessity. In my case, it was to switch on all my computer peripherals when the PC was turned on, but I shall be building another very shortly to do the same with my Hi-Fi equipment.
When a system consists of a number of components, there is the ritual of ... switch on the preamp, power amp(s), subwoofer, CD player, and maybe a crossover or equaliser. It's not hard, but it is tedious! Then, one must remember to switch everything off again - this is not always easy after a nice long listening session, beautifully accompanied by a lovely Cabernet Shiraz that was just pleading to be demolished :-)
No longer must you suffer so. When a load current is sensed from the "master" unit, this circuit will automatically switch on everything connected to the "slave" mains output. The only thing you need to do now is turn on the preamp (for example), and all the other units 'sproing' into life, and equally, gracefully fall asleep again when the preamp is turned off.
WARNING: This circuit is directly connected to and controls household mains voltages, and must be built with extreme care to ensure the safety of you and your loved ones. All mains wiring must be segregated from low voltage wiring, and in many countries, mains wiring must be performed only by suitably qualified persons.
Please ensure that you heed the above warning. One small mistake could mean the end of you (or someone else!). I have colour coded the wiring in the diagrams (according to the International standards) so that the mains is easily recognised. Never use ordinary hook up wire for mains voltage connections, and ensure that all solder joints are secure before soldering and are insulated against accidental contact.
NOTES:
This project is not intended for beginners, or anyone who is not completely comfortable with mains wiring. It is potentially lethal if you make a mistake. Never work on the circuit with power applied.
The active ("hot") lead is Brown, and neutral is Blue. Earth is Green/Yellow
U.S. readers will most likely use black (active or "hot") and white (neutral) wiring. Although many new leads are now using the International standard, there will still be many that use the old US standard.
This project is an updated version of that shown in Project 40, and is recommended over the original. The previous circuit will (does) work, but this is a better and more versatile unit.
Description
The heart of the circuit is TR1, a completely ordinary small low output voltage transformer. In this case, it is connected in reverse, and used as a current transformer. A power resistor shunts a good proportion of the current away from the transformer, since most small transformers have an excessively high secondary resistance. Never use the circuit without the shunt resistor, as the output voltage may reach dangerous levels without it.
I used a 5VA, 240V to 18V transformer (because I had one), but anything with 9V to 15V secondaries will work just as well. The resistor should be a 10W wirewound type - make sure that the terminals are well insulated against contact, and keep all wiring well clear of the resistor. Under normal circumstances it will get quite warm at full rated power - if you use my recommendations, maximum dissipation will be 4W at rated current. Feel free to reduce the value of the resistor if higher currents are expected, or ...
The resistor may also be reduced if a lower secondary voltage transformer is used. Remember that the transformer is now a step-up unit - the small voltage across R1 is stepped up by the turns ratio of the transformer. A voltage of 1V across the secondary winding will create a much higher voltage on the "primary", so some care is needed.
220 - 240V Operation 110V Operation
TR1 12V secondary
220-240V primary TR1 6V Secondary
110V primary
R1 1 Ohm R1 0.5 Ohm
With the values shown above, the maximum load current of the master unit should be limited to about 2A (4A for 110V), and this will be more than enough for most preamps, computers, etc. If the master unit draws more than this, reduce the value of R1 to obtain a maximum voltage of 2V RMS at full load. In theory, this will give an RMS voltage of about 30V on the primary, but in reality it will be typically somewhat less than this. It is a good idea to check the actual voltage obtained from the transformer. If it exceeds about 30V, then reduce the value of R1 appropriately.
VR1 is used to set the sensitivity of the circuit. This may not be needed in some cases, but is highly recommended. The sensitivity should be set to ensure that the relay(s) activate reliably when the master unit is switched on and off. I had to include the sensitivity control in my unit, because the PC draws a small quiescent current (it is one of the "soft-switching" types) which was sufficient to activate the relays as soon as the lead was connected. (I was not amused, since I then had to dismantle everything and modify the unit. Grrr.)
Warning: If the sensitivity is set too low, Q2 may overheat! This is because it will not be saturated, and the full voltage does not appear across the relay. The transistor will have the relay current flowing through it, and some voltage across it - this equates to power, and power means heat. Such a condition will ensure that the transistor will fail at some time in the future (probably after you have forgotten how you wired the circuit, and at the least convenient moment - as always).
Figure 1 shows the schematic of the main controller. Remember that TR1 is connected with the secondary (low voltage side) in parallel with R1 as shown. The input and master connections cannot be reversed! If you accidentally wire the circuit with them interchanged, once powered on, it will stay on, with the peripherals maintaining the current flow through TR1.
The main relay (RL1) must be rated for the full AC load current and voltage that you will be controlling. Never use a relay that is not suitably rated and designed for mains switching, as either electrical breakdown of the insulation or contact failure (or both) may result. As shown, the control electronics are earthed to the mains ground.
NOTE: Do not use the unit with the control circuit "floating", as any internal insulation failure will render the unit extremely unsafe, and may cause risk of electrocution or fire (or both).
RL2 is optional, and is needed only if the 12V (nominal) DC supply is used - in my case, it operates a small preamp and headphone amp for my PC sound system. Power is supplied from an external plugpack, mainly because I had one handy (it was actually being used to power the preamp anyway). All diodes are 1N4004 or similar, and capacitors should be rated at 25V.
I suggest that you do not use a double pole relay, with one pole for the 12V supply (instead of two relays). Although mains rated relays are quite safe in this configuration, there is too much risk of mixing up the terminals, and sending deadly mains voltages out the auxiliary connector. It is also much harder to ensure that the wiring is properly segregated.
If the Auxiliary DC is not needed, then you can omit RL2, J2, D2 and C2, and run the unit from an internal supply (as shown in Figure 2) or from a 12V DC plug pack external supply. J1 is only needed if an external supply is used, otherwise, it too can be omitted.
A suitable internal supply is shown in Figure 2. The transformer is used in the normal manner - do not get the two transformers mixed up when wiring! This may be surprisingly easy to do, since the transformers may be identical (the use of two identical transformers is perfectly acceptable). I very strongly recommend that both transformers be fitted with thermal fuses if possible, or at the very least, use a fuse in the input of the auxiliary supply as shown. Small transformers are not often renowned for their build quality, and a labour saving circuit that burns down your house is not entirely satisfactory.
The supply shown will give a nominal 12V, and may be low power, since the only real load is the relay(s). If the auxiliary DC is not required, C1 may be reduced to about 470uF (which is still better than most plug pack supplies!). Diodes are 1N4004 or similar, and C1 should be rated at 25V. A 5VA to 10VA transformer will be more than adequate.
In case you were wondering ... Why did I choose a transformer rather than an opto-isolator? Quite simple, really. Opto-isolators are almost ideal, but need a PCB. Veroboard and similar strip boards are extremely unsafe with mains voltages, and I wanted this in a hurry (as usual!). Never use strip board with mains voltages - it is just not designed for this!
To use an isolator, I would still need the power resistor, but its value would need to be higher (more power loss, more heat) to generate enough potential to operate the LED in the isolator. Or, I could use diodes to get the voltage drop needed. These would need to be high current types to handle the inrush current of even relatively small "master" appliances. Either way, I would (and therefore so would you) need to obtain the needed bits, where the design shown can be made with parts from the "junk box" - a transformer from an old plug pack supply, a few very common diodes and transistors, and a relay or two.
There is also the problem of limiting the LED current for overloads or even normal operation, and this can get quite irksome. A balancing act between sensitivity and ease of construction, safety and cost. Overall, the method shown is safe, easy to implement, and very satisfactory. Since almost any mains transformer can be used (for example, I have some 110V transformers I could have used - they are no use to me in Australia for anything else). The answer to the next question is "because I couldn't find one immediately" :-)
Apart from anything else, most constructors will not have "odd" voltage transformers lying about, so will have to use those intended for their normal mains voltage, so it was better (for you, anyway) that I used a standard transformer for my testing. This also ensures that the insulation is rated to the actual voltage in your country, a not entirely unimportant consideration.
Please do not be tempted to use anything other than a mains transformer, unless it is rated for the full mains voltage. Typically, mains rated transformers will have an insulation breakdown voltage of 2kV or more between windings. This is your safety barrier, and as such it is very important that it really is a barrier.
You might also wonder why I didn't just use the output of the transformer to power the relay (via a diode and capacitor). The answer is again very simple. If your preamp draws (for example) 400mA from the mains, only a small part of this goes through the transformer winding. The output voltage will be considerably higher, but at even lower current. With a carefully selected relay and transformer, this is possible, but generally, the power available will be too low - hence the amplifier circuit.
In short - this is a very basic circuit that works well and reliably with a wide range of controlling appliances, and it has a nice "techo" touch to it as well.
good luck i made one works nice