Author Topic: Hall Effect to Trigger Light Switch  (Read 20658 times)

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Offline ceamicloverTopic starter

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Re: Hall Effect to Trigger Light Switch
« Reply #25 on: September 12, 2014, 10:17:50 am »

Also, the relay requires 71.4 mA. This is fine for the 2n3904 to handle, but you do need to provide enough base current (which is controlled by your 10k resistor) for it to work well. From the datasheet, the current gain of the 2n3904 is only guaranteed to be at least 30 when collector current is upwards of 50 mA, so you want 71.4 mA / 30 = 2.38 mA base current. That means the resistor should be at most (5 - 0.7)V / 2.38 mA, so at most 1.8k (anything down to 1k is fine if you don't have a big selection to work with). You could use a N-channel mosfet instead of the transistor, in which case at 10k (or more) resistor would be fine, so your circuit would consume less standby power (for zero standby power, use a reed relay).


That, I think, alleviated some confusion.  The set-up worked on my breadboard, but not when hooked up on my PCB.

When I jumped ground to the relay, it tripped, but the transistor was not sufficiently outputting.  I will try dropping the resistance.  Otherwise, would a 2nd transistor suffice?

I am open to trying a MosFet down the line, as everyone keeps steering me to them.  I would just need to order them.

With respect to the quenching diode, that means that I should have it across 85/86 with band pointing toward 5v, correct?
I have tried actually dioding the output of the transistor, but it seems the voltage drop may be part of my problem.


Is there a decent resource for PCB best practices?  I am just shoving braided wire through right now, and struggling to make great connections, especially with that relay, the pins of which need to be bent to barely fit into the board.
 

Offline rs20

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Re: Hall Effect to Trigger Light Switch
« Reply #26 on: September 12, 2014, 12:23:55 pm »
That, I think, alleviated some confusion.  The set-up worked on my breadboard, but not when hooked up on my PCB.

When I jumped ground to the relay, it tripped, but the transistor was not sufficiently outputting.  I will try dropping the resistance.  Otherwise, would a 2nd transistor suffice?

Two transistors in parallel don't have a higher current gain overall -- the current from the resistor would be split in half between the bases of the two resistors, each halved base current is multiplied by the DC current gain, and that's added together again. Overall, that achieves nothing, except each transistor won't get as hot (transistor overheating is not at all a concern with your numbers). You could use a darlington arrangement, but that doubles the voltage drop across the transistors. It's just much easier and better in pretty much every way to just use a smaller resistor (if you only have 10k resistors to hand, put 5 of them in parallel.)

I am open to trying a MosFet down the line, as everyone keeps steering me to them.  I would just need to order them.

With respect to the quenching diode, that means that I should have it across 85/86 with band pointing toward 5v, correct?
I have tried actually dioding the output of the transistor, but it seems the voltage drop may be part of my problem.

Is there a decent resource for PCB best practices?  I am just shoving braided wire through right now, and struggling to make great connections, especially with that relay, the pins of which need to be bent to barely fit into the board.

Are you designing a PCB and getting it made professionally / making your own PCB with etchant / using veroboard or similar? Judging by the "barely fit into the board" comment, I'm assuming you're using veroboard. It's much better to use a drill to put extra holes where they need to be, rather than bending and stressing pins (the relay will never sit flush this way in any case). Solid-core (not multi-stranded) wire is preferred for rigid assemblies like a PCB, but having said that multi-stranded wire should solder fine. There are plenty of videos on the internet about how to solder.

About the quenching diode, yes, that's the right way to do it.

( Minor niggle: you're not putting your replies after the close quote tag, or you're deleting opening tags without deleting closing tags or something, so it looks like I'm saying what you're saying -- look at your own message above )
 

Offline ceamicloverTopic starter

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Re: Hall Effect to Trigger Light Switch
« Reply #27 on: September 15, 2014, 01:08:57 pm »
http://www.ebay.com/itm/2PCS-6CMx8CM-Double-side-Protoboard-Circuit-Fibre-Glass-DIY-Prototype-PCB-Board-/221543786602?pt=LH_DefaultDomain_0&hash=item339509e86a

This is pretty much what I used.  I will try drilling holes.  I am familiar with soldering wire-to-wire from automotive.  Translating that to circuit board is the challenge now.  I will try to find more videos and solid core wire.

I have 1k resistors, and I will try that.  Thank you.
 

Offline ceamicloverTopic starter

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Re: Hall Effect to Trigger Light Switch
« Reply #28 on: September 16, 2014, 05:45:01 pm »
I rigged it up and it works perfectly.
I have 4 AA batteries as the 5-6v power supply.

Now I am wondering how this type of set-up could be more efficient.  Would replacing the relay with a triac cut power consumption?

What is the simplest or best way to switch 120VAC with a low current, DC sensor?
 

Offline rs20

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Re: Hall Effect to Trigger Light Switch
« Reply #29 on: September 17, 2014, 01:35:12 am »
I rigged it up and it works perfectly.
I have 4 AA batteries as the 5-6v power supply.

Now I am wondering how this type of set-up could be more efficient.  Would replacing the relay with a triac cut power consumption?

What is the simplest or best way to switch 120VAC with a low current, DC sensor?

Circuits like these have different power consumptions when the light is on vs when it is off:

When the light is off, you have a 1k resistor, with the low side of it pulled down to ground by the hall sensor. That's 5mA running through the resistor and the hall sensor, which will run down a 2000mAh AA battery (each battery is contributing a full 5mA, even though the whole stack is outputting 5mA -- the stack compounds voltage, not current) in 2000mAh / 5 mA = 400 hours = ~17 days. The solution (besides using reed switches) is to use a MOSFET, with which a 100k resistor (~1700 days) would work fine.

I forgot to account for the hall effect sensor, it consumes about 5mA on its own -- really not ideal for battery operation. Again, that's ~17 days battery life on its own, even if you had a MOSFET and a 1000000k resistor. A reed switch is just an open circuit when it's open, it consumes zero power.

So everything above is talking about the magnetic sensing side of the circuit, we haven't even thought about the relay yet. Once the switch is activated, you've got ~80 mA of current running through the relay coil, which works out to about 24 hours of light output. A solid state relay or optotriac would consume less.
 

Offline ceamicloverTopic starter

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Re: Hall Effect to Trigger Light Switch
« Reply #30 on: September 17, 2014, 02:02:59 pm »
At this point, I am going beyond my little project and am basically trying to get better at controlling higher current/voltage loads with low current/voltage sensors.

I got some reed switches in the mail, and would like to get some cheap Mosfets, triacs, and scrs.  What specific part numbers would you guys recommend for similar applications (under 10A, 12vdc or 120VAC).


And to clarify, the set-up people were recommending was:
Reed switch triggers Mosfet or Triac, which connects the two poles of the light switch, right?
Where would you need diodes in this circuit, and what current rating?

Thanks for all your help.  I am trying to self-teach and have not mastered transistors, which I feel has been a bottleneck.
 

Offline rs20

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Re: Hall Effect to Trigger Light Switch
« Reply #31 on: September 18, 2014, 12:22:40 am »
At this point, I am going beyond my little project and am basically trying to get better at controlling higher current/voltage loads with low current/voltage sensors.

I got some reed switches in the mail, and would like to get some cheap Mosfets, triacs, and scrs.  What specific part numbers would you guys recommend for similar applications (under 10A, 12vdc or 120VAC).


And to clarify, the set-up people were recommending was:
Reed switch triggers Mosfet or Triac, which connects the two poles of the light switch, right?
Where would you need diodes in this circuit, and what current rating?

Thanks for all your help.  I am trying to self-teach and have not mastered transistors, which I feel has been a bottleneck.

There are two completely separate routes you can go down here:

A. Safe isolation between detection and switching sides.

Here is just one of many ways to achieve this with acceptable battery life: check out figure 10 of this optotriac datasheet. It shows how simple it is to use an optotriac (the MOC3063) to trigger a more powerful triac. The "LOAD" is your light bulb. On the left hand side of the circuit, they're using a NAND gate to drive the optotriac; ignore that, you can use your reed switch there instead. Connect earth to the primary side somewhere for safety too. The bad thing about this is that it is battery powered, so it will run out at some point (although it will last years), the good thing is that it is isolated and low-voltage, so it's safe to develop it more later, e.g. add a microcontroller/555 timer that keeps the light on for 10 seconds after the door is closed, etc.

Where are you getting parts from? Local shop, eBay, or somewhere fancy like element14/digikey? The latter is more expensive, but they have excellent range and easy-to-search websites.

B. No isolation

In this case, you'd just use a reed switch entirely on its own (if it's rated to handle the current), or a reed switch hooked up to a TRIAC. (An SCR is no good for switching AC because it only passes current in one direction). All exposed metal in the system is zap-if-you-touch-it.
 

Offline ceamicloverTopic starter

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Re: Hall Effect to Trigger Light Switch
« Reply #32 on: September 18, 2014, 11:57:15 am »
I have been getting most of this stuff on ebay, and mainly from china so that I can get 2-3 times as many for the same price.  If that is ill-advised, I am unaware.

Where do you recommend getting the "fairchild semiconductors"? 
Are these also called triacs/Mosfets?
Thank you for indulging my lack of education.

I already have resistors and capacitors.
the caps i have are:
http://www.ebay.com/itm/351059190131?_trksid=p2059210.m2749.l2649&ssPageName=STRK%3AMEBIDX%3AIT

http://www.ebay.com/itm/161117336372?_trksid=p2059210.m2749.l2649&ssPageName=STRK%3AMEBIDX%3AIT

I haven't used them yet, so I am not particularly familiar with them.

I got them so that I could regulate voltage with LM317T.
I was thinking of using these with a 9v battery to drop down to 5v for my sensors.
 

Offline rs20

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Re: Hall Effect to Trigger Light Switch
« Reply #33 on: September 18, 2014, 12:57:43 pm »
I tend to pay (5 times) full price from the big distributors, but many people get their parts from eBay and seem to get by just fine. The optotriac datasheet I linked to is for the MOC3063; you can do an eBay search for MOC3063 and see a bunch there. An eBay search for triac also brings up a bunch of stuff, BTA16-600 looks fine but probably lots of other things are fine too.

You should Google around to find pages explaining MOSFETs, SCRs, TRIACs, SSRs. Mosfets can only control DC current (so a fine suggestion for driving a relay coil, not suitable for AC power switching), SCRs also only work in DC but stay on once triggered, TRIACs are like SCRs but work with AC power, etc, etc. They're all different things. Opto-triacs are just like triacs, except you trigger them on using light (from the LED that is sitting in the same package) instead of a directly connected terminal. This gives them isolation and safety. Opto- refers to this optical isolation, you can get opto-BJTs too (except they're actually called optocouplers, but think opto-BJT).

TRIACs have ~1V drop across them when on, multiply that by the 1A that a 100W bulb consumes because of the silly 110 VAC power supply in your country (100W bulbs only consume 0.4A in sensible 240V countries :P) and your TRIAC will be dissipating 1W of power. Multiply by 60 degrees C/W, the TRIAC will be getting very warm (85 degrees C or so at junction). This isn't strictly true, because TRIACs have a nice property that they have a lower voltage drop when they get hot (see the datasheet for the BTA16-600). so it won't get quite this hot. One more reason not to go poking your circuit, in any case.

Draw up a schematic with part numbers and values included and we'll see how it looks.

Please be careful when messing around with mains power; turn off and remove the fuse from the fuse box before touching anything. Do not poke around in live circuitry. Use an isolation transformer if the lid is off your project. Etc.
« Last Edit: September 18, 2014, 01:08:19 pm by rs20 »
 

Offline WarSim

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Re: Hall Effect to Trigger Light Switch
« Reply #34 on: September 20, 2014, 11:54:17 pm »
Yes a MOSFET is a unipolar device but I can be used to control AC within a bridge.  Which has been done for decades.  It is not a new unproven method, as suggested by others.  (Not here, other sites, which I will not name).


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Offline ceamicloverTopic starter

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Re: Hall Effect to Trigger Light Switch
« Reply #35 on: October 02, 2014, 03:54:58 pm »
I just got 10 BTA16-600B and 10 MOC3063 DIP-6 Optoisolators in the mail and am researching them and trying to figure out my circuit with reference to figure 10 in your link.

Would 9v w/ 1800 Ohm resistor work for VCC and Rin?
     How will this be affected as battery voltage drops, and should I pick a lower resistance to account for the 2 conductor ~3 meter run to the reed switch?
Do I need to modify resistances after opto-isolator to account for 110VAC?
Do I need the 39 ohm resistor and .01 uF capacitor for "snubbing?"  I don't know what snubbing is.
     Does this cause some current to flow through the device at all times when you connect the hot to the switch-leg through that resistor and capacitor?
     

Again, thanks for your help
 

Offline rs20

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Re: Hall Effect to Trigger Light Switch
« Reply #36 on: October 03, 2014, 09:35:15 am »
Would 9v w/ 1800 Ohm resistor work for VCC and Rin?
     How will this be affected as battery voltage drops, and should I pick a lower resistance to account for the 2 conductor ~3 meter run to the reed switch?

Always design for the worst case -- so, measure the resistance of the cable (I can't imagine that it'll contribute much compared to the 1.8k resistor, but who knows), and assume your 9V battery has discharged to around 6 V. The LED inside the MOC3063 has a forward voltage drop of 1.5V worst case, so that's 4.5V across our resistor. The trigger current is 5mA minimum, so we want at least that much. Resistor value = 4.5V / 5mA = 900 ohm. Subtract your cable resistance from this (or forget it if the cable resistance is less than 50 ohms), and that's the value to use.

Do I need to modify resistances after opto-isolator to account for 110VAC?
Good question; I think those values are fine as is.

Do I need the 39 ohm resistor and .01 uF capacitor for "snubbing?"  I don't know what snubbing is.
http://en.wikipedia.org/wiki/Snubber, look under the "Thyristor" section (since a TRIAC is a bidirectional thyristor). It seems like you might not need it; since it seems to be for inductive loads like motors.

     Does this cause some current to flow through the device at all times when you connect the hot to the switch-leg through that resistor and capacitor?
Yes, although 0.01uF is quite a small value, so not very much. You can calculate the reactance ("resistance" in a loose sense, see the link) using the formula 1 / (2*pi*f*C). That's 300 kiloohms at 50 Hz, which works out to 0.34mA of current, 40mW of power. 40mW isn't much, but it might be 40mW more than necessary. Leave space on your circuitboard for a snubber, but I'd probably leave it out, at least at first? If you need it, the symptom of leaving it out is that the light turns on when not commanded to.
 

Offline eneuro

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Re: Hall Effect to Trigger Light Switch
« Reply #37 on: October 03, 2014, 01:28:17 pm »
Mosfets can only control DC current (so a fine suggestion for driving a relay coil, not suitable for AC power switching)
Maybe I have diffrent mosfets  but they switch AC 230VAC 50Hz  :-DD
This AC mosfets (2xBUZ78) switch is designed for 230VAC 30W light bulb soft start and dimming using PWM with inductor.

Will see how long this light bulb will last-I guess forever? For sure much longer than without any soft starts.
There is no high start currents while soft start is implemented and additionaly rectifier and caps can be used while I hate 100Hz light flickering @ 50Hz mains in the case of light bulb connected directly to mains and still for somereason need additional heat source so it is quite nice artifical Sun now without any flickering  8)

BTW: AC mosfets switch can switch DC too, but of course its series RDSON and body diode conduction after some power level  have to be taken into account.
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Offline WarSim

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Re: Hall Effect to Trigger Light Switch
« Reply #38 on: October 03, 2014, 02:19:23 pm »
It is a bad idea to depend on the characteristics of the body diode as a cct element.  I am repeating myself now, so obviously you are not heading my advice.  Good luck on your project. 


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Offline rs20

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Re: Hall Effect to Trigger Light Switch
« Reply #39 on: October 03, 2014, 02:57:35 pm »
Maybe I have diffrent mosfets  but they switch AC 230VAC 50Hz  :-DD
This AC mosfets (2xBUZ78) switch is designed for 230VAC 30W light bulb soft start and dimming using PWM with inductor.

I looked up the BUZ78, and it's just a standard N channel FET? It's true that MOSFETs can be placed in anti-series to successfully switch AC; but if used to switch mains, the source pins are flapping around at mains voltages, so to drive the gate, you need isolated circuitry there. Unlike a thyristor, which has a clean, safe opto-isolated solution to this problem (the optotriac), I'm not sure which method you have in mind. Unlike a thyristor, which stays switched on for the rest of the cycle by itself, the MOSFET Vgs must be held positive despite the FETs essentially shorting out their own power supply. (Some?) solid state relays work by using a small solar panel to drive the gate voltage, which gives a hint at how difficult this is to do elegantly. Of course, another option is to have a battery-powered solution referenced to the source pin, flapping around at mains voltage, but that seems like a bad idea (TM) (care to change the battery that's at line voltage?). It's unsurprising that your average dimmer still uses thyristors, despite the disadvantage of a nearly 2V drop.

BTW: AC mosfets switch can switch DC too, but of course its series RDSON and body diode conduction after some power level  have to be taken into account.

So, what's an AC mosfet? Because, as above, the BUZ78 is just a standard N fet. Don't confuse a device that can't block AC (e.g. MOSFET) with a circuit topology that protects DC devices from the true AC nature of the load (e.g, single MOSFET behind bridge rectifier, anti-series MOSFETS...)  If you're switching DC into a non-inductive load, you don't need to think about the body diode at all...

Will see how long this light bulb will last-I guess forever? For sure much longer than without any soft starts.

Interesting question; although it can't be any better (in terms of total on time) than just leaving a light on all the time. I assume it's well established how switching a bulb on and off detracts from that ideal case. According to wikipedia, a 5% reduction in voltage will more than double the lifetime of the bulb (while reducing light output by only 20%), which might be a more successful approach.

There is no high start currents while soft start is implemented and additionaly rectifier and caps can be used while I hate 100Hz light flickering @ 50Hz mains in the case of light bulb connected directly to mains and still for somereason need additional heat source so it is quite nice artifical Sun now without any flickering 

Interesting idea, but rectification and filtering can be performed regardless of AC switch technique, so this is a rather orthogonal discussion (except if you rectify first, in which case, hey, we have the single MOSFET behind a bridge rectifier solution). Without PFC, your power company might be unhappy though...

 

Offline eneuro

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Re: Hall Effect to Trigger Light Switch
« Reply #40 on: October 03, 2014, 05:38:26 pm »
I looked up the BUZ78, and it's just a standard N channel FET? It's true that MOSFETs can be placed in anti-series to successfully switch AC;
In previous post you wrote that mosfets can NOT switch AC, so I've shown PCB with two mosfets which does it easy  ;)
Single BUZ78 mosfet can't, but on my prototype PCB are two mosfets (I've shown schematic in other post) and when 9.6V from this fresh battery is connected to VGS which has additionaly 100k resistor to discharge gate and ensure switch is OFF when there is no external gate drive voltage, than you can put multimeter and I got 12 Ohm resistance, while this switch will be used for <30W light bulbs.
Typical BUZ78 RDSON is about 6.5 Ohm, so in series without load we get 2x more bigger.
Nice thing with this AC mosfets switch is it does not need any minimal load current to triger it on like in the case of triac , so I was able to measure its RDSON when is ON without any load.
To keep this switch ON for long time only about 0.1mA will be drown from battery while 100k resistor will try discharge this AC mosfets switch gates  8)

Light bulb lifespan extending and driving  it using AC/DC mosfets switch is another story and small off topic, so maybe in other thread will be better discus it, but nothing fancy-just trying to keep constant voltage on this light source to prevent flickering @ 230VAC 50Hz mains  8)
« Last Edit: October 03, 2014, 05:46:52 pm by eneuro »
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Offline ceamicloverTopic starter

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Re: Hall Effect to Trigger Light Switch
« Reply #41 on: October 03, 2014, 09:24:21 pm »

 The "LOAD" is your light bulb. On the left hand side of the circuit, they're using a NAND gate to drive the optotriac; ignore that, you can use your reed switch there instead. Connect earth to the primary side somewhere for safety too.

Looking at the Opto-isolator:
I am confused about how to trigger the moc363 with the reed switch.  I had been using 5v at the reed switch to trip the relay, and keeping the relay off while the reed switch was grounding it.

In the diagram, it looks like pin 1 has voltage.  3 and 5 are open.  Is it controlled by supplying ground to Pin 2?
In my case, should I do constant ground at 2, and use resistance and/or a transistor to get the right voltage at 1 while the reed switch is open?

Basically, should I control the opto-isolator with power or ground, and does that modify the resistance from the 900 ohms from the previous post? 
 

Offline ceamicloverTopic starter

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Re: Hall Effect to Trigger Light Switch
« Reply #42 on: October 03, 2014, 11:35:59 pm »
I got the breadboard to work by with:
opto-isolator
pin 1 to 9v through 900 ohm.
Reed switch pulled up with 100k to 9v, down to ground when closed.
    this trips transistor, which supplies ground to Pin 2 of opto-isolator.

That seems to reliably work.  I measured current and found that the higher voltage power source, the more power consumption.
At 9v, it consumes .18mA off, and 9.3mA on.

Does that all sound like I am on the right track?  That doesn't seem low enough to last for years, but it should be better than the 3 days the last one worked.

Should I have been using a N.C. reed switch the whole time?
« Last Edit: October 03, 2014, 11:40:34 pm by ceamiclover »
 

Offline rs20

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Re: Hall Effect to Trigger Light Switch
« Reply #43 on: October 04, 2014, 12:50:04 am »
I got the breadboard to work by with:
opto-isolator
pin 1 to 9v through 900 ohm.
Reed switch pulled up with 100k to 9v, down to ground when closed.
    this trips transistor, which supplies ground to Pin 2 of opto-isolator.

That seems to reliably work.  I measured current and found that the higher voltage power source, the more power consumption.
At 9v, it consumes .18mA off, and 9.3mA on.

Does that all sound like I am on the right track?  That doesn't seem low enough to last for years, but it should be better than the 3 days the last one worked.

Should I have been using a N.C. reed switch the whole time?

You don't need the 100k pull-up (if the reed switch is open-circuit, we want the optotriac emitter to be off -- and it's impossible for any current to flow throw the optotriac emitter if the switch is open, so that's fine as it is). How are you seeing any current at all when the reed switch is open circuit? The whole point of the reed switch is that it mechanically disconnects the battery when open.

Because we designed for a worst case battery voltage of 6V, and you're testing with 9V, we should be expecting more than our target current of 5mA -- so 9.3mA sounds right. 2000mAh / 9.3mA = 200 hours continuous on time, or over a year if the light is on for half an hour every day. You can try feeding in 6V supply, and then progressively increasing the resistance until it stops working, and then back off a bit. This means you're tuning the circuit to your particular optotriac.

Single BUZ78 mosfet can't, but on my prototype PCB are two mosfets (I've shown schematic in other post) and when 9.6V from this fresh battery is connected to VGS which has additionaly 100k resistor to discharge gate and ensure switch is OFF when there is no external gate drive voltage, than you can put multimeter and I got 12 Ohm resistance, while this switch will be used for <30W light bulbs.

Very good, but if you put this solution into real service, how do you propose to make changing that battery a non life-threatening activity? "Turn the entire circuit off with an upstream switch" is not an acceptable answer for any kind of real-life scenario.

Typical BUZ78 RDSON is about 6.5 Ohm, so in series without load we get 2x more bigger.
Nice thing with this AC mosfets switch is it does not need any minimal load current to triger it on like in the case of triac , so I was able to measure its RDSON when is ON without any load.
To keep this switch ON for long time only about 0.1mA will be drown from battery while 100k resistor will try discharge this AC mosfets switch gates  8)

12 ohm * 166 mA is 2V, so for anything more than 166mA (20W in 120V countries, 40W in 240V countries), the thyristor will be burning less power, while being much easier to safely drive. I'm sure you could find better FETs though.

This would all be very interesting if your solution didn't involve a battery connected to mains. If we were to throw out the requirement that parts of the circuit be safely isolated, then we wouldn't need a battery at all and we'd just use the reed switch directly (or, use the reed switch to trigger the thyristor directly, no isolation).

 

Offline ceamicloverTopic starter

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Re: Hall Effect to Trigger Light Switch
« Reply #44 on: October 04, 2014, 01:01:26 am »

You don't need the 100k pull-up (if the reed switch is open-circuit, we want the optotriac emitter to be off -- and it's impossible for any current to flow throw the optotriac emitter if the switch is open, so that's fine as it is). How are you seeing any current at all when the reed switch is open circuit? The whole point of the reed switch is that it mechanically disconnects the battery when open.


I am apparently confused about how to turn on and off the opto-isolator.

The reed switch is normally open.  When the door is closed, there is a magnet closing the reed switch.  I probably should have been using a n.c. switch, which would open the circuit when the door is closed.  That didn't occur to me because I have never used reed switches before and had not jumped through the hoops of wiring it backwards like I have been.


My pullup resistor is used to close the circuit, feeding voltage to the opto-isolator, which is resting at ground.

Again, thank you for all of your input.

and Warsim, thank you for your input.  It sounds like I need to familiarize myself with MosFets as well, escpecially if they can be useful in automotive, which is my profession.

 

Offline rs20

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Re: Hall Effect to Trigger Light Switch
« Reply #45 on: October 04, 2014, 01:16:29 am »
I am apparently confused about how to turn on and off the opto-isolator.

If you look at the datasheet, you'll see there's an LED hidden inside the opto-triac. The opto-triac is on if you make that LED shine. If you remove the optotriac, and replace it with a normal red LED, you can use that to understand when the circuit would trigger the triac.

The reed switch is normally open.  When the door is closed, there is a magnet closing the reed switch.  I probably should have been using a n.c. switch, which would open the circuit when the door is closed.  That didn't occur to me because I have never used reed switches before and had not jumped through the hoops of wiring it backwards like I have been.

You definitely want it arranged so that when the light should be on, the reed switch is closed. If the light should be on when the door is open, then I think, yes, that means an n.c. switch.

My pullup resistor is used to close the circuit, feeding voltage to the opto-isolator, which is resting at ground.

You should have four components, arranged in a loop: battery, resistor, reed switch, optoisolator emitter. These can be in any order you like, as long as the optoisolator emitter is connected the right way around with respect to the battery. When the reed switch is on, current flows from the battery to the optoisolator emitter, limited by the resistor. When the reed switch is off, no current can flow, end of story. I'll need to see a circuit diagram if you have further questions.

(btw, I think Warsim's comment was directed at eneuro)
 

Offline eneuro

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Re: Hall Effect to Trigger Light Switch
« Reply #46 on: October 04, 2014, 07:59:16 am »
12 ohm * 166 mA is 2V, so for anything more than 166mA (20W in 120V countries, 40W in 240V countries), the thyristor will be burning less power, while being much easier to safely drive. I'm sure you could find better FETs though.
This AC switch with cheap 2xBUZ78 can be used for 30W @ 230VAC easy and in this case I was not looking for anything since I had such two mosfets and a few quick calculations showed that power losses when this switch is ON will be around 0.25W in whole thing, so it is nothing when will compare it with switched 30W power.

How much power (resistive not inductive) do we need to switch and it is only ON/OFF no dimming etc?
Why battery, while small transformers costs a few $es and we have mains power available?

I think that one image or schematics is equivalent to hundreds written words, so simply draw something.
« Last Edit: October 04, 2014, 08:01:47 am by eneuro »
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Offline ceamicloverTopic starter

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Re: Hall Effect to Trigger Light Switch
« Reply #47 on: October 04, 2014, 11:49:20 am »
I will order some normally closed reed switches.

General questions on triacs:

What is the opto-isolator outputting to trigger AC current through the gates? 

Can you use triacs for 12vDC applications, or is a MosFet more appropriate?
 

Offline rs20

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Re: Hall Effect to Trigger Light Switch
« Reply #48 on: October 04, 2014, 12:30:06 pm »
What is the opto-isolator outputting to trigger AC current through the gates? 
Firstly, opto-isolators and opto-triacs are different things. At least, that's how I think most people use the terms. The MOC3063 is an opto-triac, not an opto-isolator.

The output side of the opto-triac has two pins which are normally open circuit. When the opto-triac is triggered the two pins are connected together (and stay connected until the current drops below holding current). It's just like a triac (because that's what it is, an optically triggered triac). The diagram in the datasheet is less mysterious than it seems, you can see the LED and the Triac in the package there.

( Opto-isolators, on the other hand, have outputs that look like a BJT transistor (again, just as it looks like in the circuit symbol). )

Quote
Can you use triacs for 12vDC applications, or is a MosFet more appropriate?
Thyristors have quite a large voltage drop (around 2 V), which is an insignificant fraction of mains voltage, but a significant fraction of 12V. Also, Thyristors kinda need the zero-crossings in AC; without a zero-crossing, a thyristor might never turn off. MOSFETs would be a most appropriate choice for 12V DC, and a digital isolator is a nice easy way to drive a MOSFET in an isolated way (although in most cases, you won't need isolation for a 12V system).
 

Offline WarSim

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Re: Hall Effect to Trigger Light Switch
« Reply #49 on: October 04, 2014, 04:38:14 pm »
For those that are interested.
Opto and Triac are component descriptors. 
Isolator is a function descriptor. 

Opto means optometricly isolated.  Opto term is used to define it primary method of isolation.  Yes a device could use more than one.  Other types of devices use inductive and galvanic etc.  Many energy activated devices maybe integrated into an opto-isolator.  BJTs, MOSFETs, TRIACs, SCRs etc.

When the simplified phrase of opto-isolator is used it will refer to the most common type at the time of use.  If used today it means opto-isolated BJT.  Opto-BJT is another simplified phrase. 

If you use an opto-TRIAC read the datasheet.  Typically the trigger energy requirements are higher.  Various methods are used to compensate.  Multiple LED elements and increased gate surface are common are two common methods.  Multiple LED elements will increase the isolated trigger burden.  The second will slow the TRIAC down and require more heat dissipation.  Turn off is also slowed but the effect is trivial at mains frequencies. 


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