Simple Universal Speaker Delay Using A Triac - Page 2 - diyAudio
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Old 4th December 2012, 06:56 PM   #11
Elvee is online now Elvee  Belgium
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Quote:
Originally Posted by AndrewT View Post
The supply simply decays to zero volts to allow the triac to switch off.
Is this last statement more correct?
The triac will turn off when the current falls below Ihold, which generally has a value comparable to It.
The remainder of the stored energy (IČL/2) will create a voltage spike, in practice the first half-cycle of a ringing because of the winding's capacitance.
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Old 4th December 2012, 07:26 PM   #12
Mooly is online now Mooly  United Kingdom
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Quote:
Originally Posted by AndrewT View Post
That a signal diode that has a reverse current limiting resistor is a reliability issue. It may well last decades worth of power downs.
That is OK by me.

I was just confused that you and Tony were discarding the need for the diode or diode+zener that everyone else uses so flippantly.

It may be that the triac cannot be turned off that saves the semiconductors around it.
The supply simply decays to zero volts to allow the triac to switch off.
Is this last statement more correct?
Elvee mentioning the series resistor is a valid point. When the triac turns on it can be thought of as putting the 1N914 diode across the charged up cap. Its not an instant nano second switch on but is it quick enough to take the diode over its limits.

-----------------------------------------------------------------------------------------

So I've just looked at the data sheet for the diode,

non-repetitive peak forward current square wave; Tj = 25 °C prior to
surge; see Fig.4
t = 1 us - 4 A
t = 1 ms - 1 A
t = 1 s - 0.5

In practice and again I don't think there is an issue seeing those figures. To swap the diode for somthing like a 1N4004 would surely remove all doubt if anyone were concerned about it.

The back emf... I'll see what shows in the real circuit.
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Old 5th December 2012, 11:45 AM   #13
Mooly is online now Mooly  United Kingdom
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The scope test was interesting. Measuring on "top" of the relay i.e. on C3 and all is clean as expected. Measuring on the bottom end and this is the result...

On switch on "noise" occurs as the triac comes into conduction.

On switch off a high positive transient is seen. Highest amplitude I could discern during many off cycles was around 115v volts.

So this is one real issue with the design as it stands and the fix is simple. A higher voltage diode (the P.I.V. rating) should be used for D3. A common 1N4007 would be suitable.

I looked at ways of reducing the "noise" and found that adding a small cap across base and collector of Q1 worked well. Anything from 100pf to 0.47uf stopped the switch on "noise". Also observed was how this cap reduced the high voltage transient at switch off, the larger the cap the less the transient. Values around 470pf to 2200pf seem a good compromise. With 470pf the transient is around 40 volts peak and with a much slower rise time. Going too large in value slows the rate at which the voltage across the boost cap builds. There must be some conduction via the triac gate for this effect to occur, not enough to trigger the device but enough to impact on the charge time of the cap.

So to summarise up to now. I think fitting a 1N4007 or similar is sensible for D3, and adding a small cap in the 470 to 2200pf range across B and C of the transistor as a refinement.

Edit
Scope on 50 volts/div
Attached Images
File Type: jpg Switch On Noise.JPG (72.4 KB, 263 views)
File Type: jpg Switch Off Transient.JPG (73.2 KB, 244 views)
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Old 5th December 2012, 12:55 PM   #14
Mooly is online now Mooly  United Kingdom
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For a bit of fun I replaced the 500 ohm relay resistor in the simulation with a 1000mH inductor of 500 ohm resistance. How close that inductance value is to a typical relay I have no idea but the simulation now shows an 80 volt positive going spike at switch off.

The cap mentioned in the above post added between B and C of the transistor also seems to work in simulation by reducing the transient. The noise produced as the triac comes into conduction eludes simulation and doesn't show.

So here we have with a capacitor of 2200 pf and without.
Attached Images
File Type: jpg Switch Off In LTspice.JPG (89.0 KB, 237 views)
File Type: jpg With Capacitor.JPG (92.0 KB, 231 views)
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Old 5th December 2012, 03:54 PM   #15
Elvee is online now Elvee  Belgium
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Quote:
Originally Posted by Mooly View Post
t = 1 us - 4 A
t = 1 ms - 1 A
t = 1 s - 0.5

In practice and again I don't think there is an issue seeing those figures. To swap the diode for somthing like a 1N4004 would surely remove all doubt if anyone were concerned about it..
Did you attempt to compute, or at least simulate the actual current in the diode? I think you would be surprised.

Quote:
So this is one real issue with the design as it stands and the fix is simple. A higher voltage diode (the P.I.V. rating) should be used for D3
Told you so

Quote:
The cap mentioned in the above post added between B and C of the transistor also seems to work in simulation by reducing the transient.
Using a transistor as a snubber to dissipate transients may not be the best of practices....

Anyway, making oscilloscope measurements on an equipment having all of its silicon on the mains side is completely off-limits.
This thread should urgently be closed, but where are the moderators?
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Old 5th December 2012, 05:08 PM   #16
Mooly is online now Mooly  United Kingdom
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Quote:
Originally Posted by Elvee View Post
Did you attempt to compute, or at least simulate the actual current in the diode? I think you would be surprised.
Neither actually boss... I'll see if I can come up with anything useful

Quote:
Originally Posted by Elvee View Post

Using a transistor as a snubber to dissipate transients may not be the best of practices....
The cap across B and C moves it out of "transient spike" territory.

Quote:
Originally Posted by Elvee View Post
Anyway, making oscilloscope measurements on an equipment having all of its silicon on the mains side is completely off-limits.
This thread should urgently be closed, but where are the moderators?
Absolutely, you just can't get staff
(Just report any you see and the offending thread will be swiftly dealt with... honest... but make sure they are (offending) first )
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Old 5th December 2012, 05:20 PM   #17
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Quote:
<snip>Anyway, making oscilloscope measurements on an equipment having all of its silicon on the mains side is completely off-limits.
This thread should urgently be closed, but where are the moderators?
Elvee, it's been stated in this thread that the circuit in question is not operated off of the mains, and operates off an available winding that is fully up at the time the timing delay starts.
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Old 5th December 2012, 06:07 PM   #18
Elvee is online now Elvee  Belgium
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Mea culpa, I missed that
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Old 5th December 2012, 07:13 PM   #19
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This illustrates Elvee's point about the transient current in D3. How much It sure looks scary in simulation and its obviously a very valid point. I'm going to stick my neck on the block and say I don't believe it will approach anything like this in practice but it will still be high, possible a spike of a "few" amps. So the fix for that is a series resistor for D3. Value anything in the 47 to 100 ohm range. Go higher and the timing cap starts to take a more noticeable time to discharge meaning the rapid reset ability on power off is reduced. Its all relative though and we are talking fractions of a second for values below 1k

So the new data and circuit is as follows.

Transient spike is now 25 volts.
Peak current in D3 is now below 300 ma
Peak emitter current is below 8 ma
Timing cap voltage take around 15 ms to reset (still very quick)
Attached Images
File Type: jpg Circuit Revision 1.JPG (138.8 KB, 206 views)
File Type: jpg Transient Current.JPG (91.1 KB, 109 views)
File Type: jpg Revision 1.JPG (172.0 KB, 38 views)
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Old 6th December 2012, 09:37 AM   #20
AndrewT is offline AndrewT  Scotland
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Why are you not considering the conventional back emf solution: the diode across the coil?

I prefer the diode + zener after D.Self and others told us that it allows faster opening.
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