Soft start & Soft Switch circuit: can anybody help? - diyAudio
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Old 12th May 2005, 06:11 PM   #1
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Question Soft start & Soft Switch circuit: can anybody help?

I have just bought the attached circuit from a German firm named THEL. The quality is fantastic, maybe a little expensive, but this circuit is just what I needed... at least this what I thought!

Depending on the position of the J1 switch, this circuit behaves in the following ways:

Pin 2-3 shorted: simple softstart. The soft start duration is determined by a trimmer

Pin 3-4 shorted: the circuit is activated by an external signal (0.2 mV to 2mV, sensitivity set by trimmer), cycles through the soft start time and then stays on. After an adjustable period from the last signal received (1-10 minutes, set by a trimmer), the circuit turns off.

Pin 1-2 shorted: the circuit behaves like a soft switch activated by the S1 switch. Therefore, when S1 is closed the soft start cycle starts and the circuit is activated until S1 is opened again.

I have a problem with the last way of working: I didn't understand that the circuit operated based on a switch (i.e. permanent contact), I thought it operated based on a button (i.e. a momentary contact turns on, another momentary contact turns it off).

I definitely need the circuit to work with a button. Can anybody tell me if there is a way to modify the circuit in order to implement this feature? Help please!
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Old 12th May 2005, 06:42 PM   #2
AndrewT is offline AndrewT  Scotland
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Hi,
you need a latch.
Add a third relay in parallel to K1.
After the button is pressed both K1 & K2 come on. K1 starts the timing period and things work as designed. K3 output contacts are wired in parallel to S1. K3 will now hold BC879 on and maintain voltage to the timer even after you release S1.
A comment on the design if I may. K1 & 2 are wired to hold in on full voltage for the whole listening period. They will run pretty warm and the extra K3 will also do likewise. Select a suitable relay and make sure the enclosure is well ventilated. Even better if you could modify the circuit so that each relay receives a higher voltage pulse & then holds on with a reduced voltage, usually about half to two thirds rated voltage.
Check my logic before you implement!!!!
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Old 12th May 2005, 09:21 PM   #3
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I understand the latch principle, but how do you turn the whole thing off? Can it be done by pressing again the same button?

I'll check the temperature of the relays after a few hours, and I'll let you know.
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Old 13th May 2005, 12:53 PM   #4
AndrewT is offline AndrewT  Scotland
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Hi,
try to find the recent thread (maybe 4 or 5 weeks ago) that showed a schematic for latching and unlatching using a push to make unlatched button.
I'm going to do a search under my name, see who finds it first.
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Old 13th May 2005, 01:02 PM   #5
AndrewT is offline AndrewT  Scotland
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Hi,
found it. A lot older than 5 weeks.
You might want to read the whole thread.
http://www.diyaudio.com/forums/attac...amp=1108675104
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Old 13th May 2005, 01:20 PM   #6
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somebody posted this circuit here - I cannot remember who, apologies for not giving proper credit. you could build the flip flop portion on a small proto board and connect the output to the end of the switch that connects to the base of the BC879.
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Old 21st May 2005, 04:29 PM   #7
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In the end, I've made the circuit Kristjan designed (see attachment), but I can't get the relay to switch.

Everything seems to work fine, when I connect a LED instead of the relay it works fine, the tension measured at the poles of the relay (without the relay in place) is 8.3V

I wonder if the transistor can't provide enough current for exciting the relay. If this is right, how can I solve the issue?
Please bear in mind that all the components work fine, I've already changed them at least once!

Thanks for any help!
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Old 21st May 2005, 04:54 PM   #8
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perhaps a silly question, but is the relay rated for 9V operation?

How much current does it draw? If it is within the limits of the pass transistor, you might try reducing the 10K resistor to permit more base current and therefore more current to the relay. You may also want to try connecting two of the pass transistors as a darlington pair for more current.

What happens to the 9v supply voltage when you try to activate the relay?

What voltage do you get at the relay terminals when you try to operate it?
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Old 21st May 2005, 05:13 PM   #9
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The supply is oversized, I've operated the relay by tapping it directly into the regulator's output without any problems. In this configuration it draws 50mA approx.
Based on what I know about the transistor, it shoud be able to operate it without any problems. However, when I measure the voltage with the relay in place and the circuit activated I find only 2.5V.
I've also tried to measure th current flowing to it and it's 16mA approx., way too low.

I've tried lowering the resistor feeding into the transistor, but with 5K (instead of the original 10K) I get only 19mA

Can I put 2 transistors in parallel? Alternatively, how much can I dare lowering the resistor?
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Old 21st May 2005, 05:18 PM   #10
SY is offline SY  United States
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Put the relay in the collector circuit, not the emitter.
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