Suitable relay for delay cicuit - Page 2

tool49 · 28th May 2005, 12:54 PM

Hello again Nuuk,

My guess is that this 68ohm resistor has been placed there for this exact purpose. So yes, I think it would be safe to clamp the output to ground. To make sure of that we need to know the type of transistors you plan on using. Once we know that we'll be able to lookup their SOA for 20V, derate it a bit and figure out how much current they can put out without burning. (The output capacitor will prevent the buffer from emitting DC into the ground so even if your input has some DC component it won't overheat the transistors.)

Hope this helps!
Sébastien

Nuuk · 28th May 2005, 01:11 PM

Well this is the difference between those of us who mess around, and you experts!

I have downloaded the datasheets for the ZTX653 and BC547. I can't see SOA on the latter but for the ZTX653 the SOA graph is for collector current not output current. Hence I am now bewildered again!

I use both ZTX and BC547's in my buffers and also have some of the BFQ232's that were originally specified for this circuit (yet to try them)!

Russ White · 28th May 2005, 03:31 PM

OK, I am still a real noob, but....

If you use a resistor clamped to ground like that won't the resistance just be in parallel with the load? The load would still see some signal right? It just seems to me that breaking the circuit to the load would be better, especially if this was for a preamp.

Quote:

If you are talking about the opamp in the pre-amp then I would gusess it would depending on how quickly the relay reverts to closed mode.

In me previous post about what happens on power down I was referring to a preamp.

Nuuk · 28th May 2005, 04:25 PM

Quote:

If you use a resistor clamped to ground like that won't the resistance just be in parallel with the load? The load would still see some signal right? It just seems to me that breaking the circuit to the load would be better, especially if this was for a preamp.

The current will take the line of least resistance, ie through the shorted link (closed relay) to ground.

It is better to use this arrangement because you are not adding another switch in the signal path.

Quote:

In me previous post about what happens on power down I was referring to a preamp.

Yes, if the relay closes straight away, the offset produced when the power is withdrawn from the pre-amp circuit will be shorted to ground and you shouldn't hear those noises from your speakers. So you can see that this is quite a neat addition to our systems.

tool49 · 28th May 2005, 05:20 PM

Hi Nuuk,

The collector current is what matters here. How much current are those transistors going to have to sink and at what voltage. For the BC547 you have to be careful because it can only sink about 100mA (MAX). The ZTX643 on the other hand can sink up to 2A (MAX).

Keep in mind that these calculations are worst case scenarios.

The BFQ232 are specified for 300mA (MAX) and the 68ohm limits the output current (considering ±20V PSU) to 295mA thus in the safe zone (not mine but probably ok for the original designer). Following the SOA graph shows that for 20V, the Ic is around 150mA the transistor could potentially be damaged but a 20V swing is never going to happen so with the 68ohm resistor the transistor should survive.

Following this logic, if you use the BC547, you should increase the value of R3 to around 200ohm. This will limit the current sinking to 100mA with ±20V supply. The ZTX643 will probably survive with the current value of 68ohm.

Hope this helps!
Sébastien

Nuuk · 28th May 2005, 05:37 PM

Thanks once again Sébastien.

I can now (almost) go ahead and get the relays and give this a try. Then I will write it all up on DD!

Just one more thing. Looking at the circuit diagram at the start of this thread, it specifies a supply of 5-15 volts. Is the voltage supply for the relay adjusted by the 100-470 preset?

tool49 · 28th May 2005, 05:48 PM

No, the voltage applied to the relay coil is the power supply one. The 100-470 controls the threshold and trigger voltage. When the supply voltage will reach the set value, the output will flip, thus powering the relay.

Hope this helps!
Sébastien

Nuuk · 28th May 2005, 05:53 PM

I see so there is one PSU for the NE555 and one for the relay. Thanks again.

tool49 · 28th May 2005, 06:02 PM

Not really, they both get power from the same +5-15V. Only the relay's power is conditional to the state of pin 3 of the NE555. This pin is either low or high depending on the state of the threshold.

I think you should examine the following website:
http://www.uoguelph.ca/~antoon/gadgets/555/555.html

Hope this helps!
Sébastien

P.S. I'm trying to simulate your buffer to see what gives

tool49 · 28th May 2005, 06:16 PM

My simulation gives pretty good results. Actually with the values as is, using the BC547, even clamped to ground, the circuit would dissipate just a little heat but nowhere near any dangerous point!

The output capacitor does need to be of good quality, but it could be lowered to 1uF without too much effect on the sound.

Hope this helps!
Sébastien

28th May 2005, 01:11 PM	#12
Nuuk diyAudio Member Join Date: Feb 2003 Location: Somerset, SW England	Well this is the difference between those of us who mess around, and you experts! I have downloaded the datasheets for the ZTX653 and BC547. I can't see SOA on the latter but for the ZTX653 the SOA graph is for collector current not output current. Hence I am now bewildered again! I use both ZTX and BC547's in my buffers and also have some of the BFQ232's that were originally specified for this circuit (yet to try them)! __________________ The truth need not be veiled, for it veils itself from the eyes of the ignorant.

28th May 2005, 05:37 PM	#16
Nuuk diyAudio Member Join Date: Feb 2003 Location: Somerset, SW England	Thanks once again Sébastien. I can now (almost) go ahead and get the relays and give this a try. Then I will write it all up on DD! Just one more thing. Looking at the circuit diagram at the start of this thread, it specifies a supply of 5-15 volts. Is the voltage supply for the relay adjusted by the 100-470 preset? __________________ The truth need not be veiled, for it veils itself from the eyes of the ignorant.

28th May 2005, 05:53 PM	#18
Nuuk diyAudio Member Join Date: Feb 2003 Location: Somerset, SW England	I see so there is one PSU for the NE555 and one for the relay. Thanks again. __________________ The truth need not be veiled, for it veils itself from the eyes of the ignorant.

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28th May 2005, 12:54 PM	#11
tool49 diyAudio Member Join Date: Apr 2003 Location: Montreal	Hello again Nuuk, My guess is that this 68ohm resistor has been placed there for this exact purpose. So yes, I think it would be safe to clamp the output to ground. To make sure of that we need to know the type of transistors you plan on using. Once we know that we'll be able to lookup their SOA for 20V, derate it a bit and figure out how much current they can put out without burning. (The output capacitor will prevent the buffer from emitting DC into the ground so even if your input has some DC component it won't overheat the transistors.) Hope this helps! Sébastien

28th May 2005, 05:20 PM	#15
tool49 diyAudio Member Join Date: Apr 2003 Location: Montreal	Hi Nuuk, The collector current is what matters here. How much current are those transistors going to have to sink and at what voltage. For the BC547 you have to be careful because it can only sink about 100mA (MAX). The ZTX643 on the other hand can sink up to 2A (MAX). Keep in mind that these calculations are worst case scenarios. The BFQ232 are specified for 300mA (MAX) and the 68ohm limits the output current (considering ±20V PSU) to 295mA thus in the safe zone (not mine but probably ok for the original designer). Following the SOA graph shows that for 20V, the Ic is around 150mA the transistor could potentially be damaged but a 20V swing is never going to happen so with the 68ohm resistor the transistor should survive. Following this logic, if you use the BC547, you should increase the value of R3 to around 200ohm. This will limit the current sinking to 100mA with ±20V supply. The ZTX643 will probably survive with the current value of 68ohm. Hope this helps! Sébastien

28th May 2005, 05:48 PM	#17
tool49 diyAudio Member Join Date: Apr 2003 Location: Montreal	No, the voltage applied to the relay coil is the power supply one. The 100-470 controls the threshold and trigger voltage. When the supply voltage will reach the set value, the output will flip, thus powering the relay. Hope this helps! Sébastien

28th May 2005, 06:02 PM	#19
tool49 diyAudio Member Join Date: Apr 2003 Location: Montreal	Not really, they both get power from the same +5-15V. Only the relay's power is conditional to the state of pin 3 of the NE555. This pin is either low or high depending on the state of the threshold. I think you should examine the following website: http://www.uoguelph.ca/~antoon/gadgets/555/555.html Hope this helps! Sébastien P.S. I'm trying to simulate your buffer to see what gives

28th May 2005, 06:16 PM	#20
tool49 diyAudio Member Join Date: Apr 2003 Location: Montreal	My simulation gives pretty good results. Actually with the values as is, using the BC547, even clamped to ground, the circuit would dissipate just a little heat but nowhere near any dangerous point! The output capacitor does need to be of good quality, but it could be lowered to 1uF without too much effect on the sound. Hope this helps! Sébastien

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