Sorry for the delay - busy day...
First - despite using a bridged amp, you don't need two relays. Just one will break the circuit.
Second - your modified diagram doesn't reflect the changes I was talking about. I suggested removing U13B (which is now called U17C on the latest version). Other changes will be required - refer back to my original schematic.
Third - add 1n4148 diodes in parallel with R1 and R5 to protect the IC from damage when it is powered down.
Forth - Q8 needs a 10K resistor in series with its base, coming from the same source as R16, which if you attend to point 2, will be C5.
Peranders makes some good points. It's worth pointing out that my circuit is well debugged, and certainly worked well in extended testing (although I haven't built the final versions, as explained on the webpage). So I don't offer any guarantee, but I'm confident enough to offer my help and encouragement at this point. If it goes wrong, just remember what my advice cost 😉
But as I say, I'm confident. I've been building these sorts of things since I was a kid (>20 years), and have seen and dealt with most issues.
- Simple is definitely best, and you can't get simpler while maintaining the functionality and reliability. I've seen cruder versions where the presence of the 12V trigger locks out the front panel switch, which is unacceptable IMO...
- Fair point about HC, although I didn't find this a problem in practice. In my app, I needed to minimse current consumption because of the mains dropper supply I used. Standard 74-series might have better noise immunity, but I'm not sure it's an issue as there are no high AC impedances anywhere. Normally I use 4000B-series, but these lack the drive capability needed here...
- Plain parts - I agree. Hence encouraging the use of bipolars rather than MOSFETs, 78L05's rather than LDO's. I don't like unusual things - not out of fear of the unknown, but I expect my things to last forever 😉
- Hopefully an off-board mains TX and relays will solve the mains worries, or at least make them the responsibilty of the end user. This is a good point, however, and something we should help Alex with.
- You mention switching spikes on the mains. A EI core TX followed by a bridge should offer much greater attenuation of these than my resistive dropper supply, and this didn't suffer from any problems. I had the tumble drier in my old workshop, and this used to put all sorts of spikes on mains when it stoped/reversed/etc. The Arcam Alpha suffered with these, but this circuit didn't.
That's my experiences, for what they're worth...
Mark
First - despite using a bridged amp, you don't need two relays. Just one will break the circuit.
Second - your modified diagram doesn't reflect the changes I was talking about. I suggested removing U13B (which is now called U17C on the latest version). Other changes will be required - refer back to my original schematic.
Third - add 1n4148 diodes in parallel with R1 and R5 to protect the IC from damage when it is powered down.
Forth - Q8 needs a 10K resistor in series with its base, coming from the same source as R16, which if you attend to point 2, will be C5.
Peranders makes some good points. It's worth pointing out that my circuit is well debugged, and certainly worked well in extended testing (although I haven't built the final versions, as explained on the webpage). So I don't offer any guarantee, but I'm confident enough to offer my help and encouragement at this point. If it goes wrong, just remember what my advice cost 😉
But as I say, I'm confident. I've been building these sorts of things since I was a kid (>20 years), and have seen and dealt with most issues.
- Simple is definitely best, and you can't get simpler while maintaining the functionality and reliability. I've seen cruder versions where the presence of the 12V trigger locks out the front panel switch, which is unacceptable IMO...
- Fair point about HC, although I didn't find this a problem in practice. In my app, I needed to minimse current consumption because of the mains dropper supply I used. Standard 74-series might have better noise immunity, but I'm not sure it's an issue as there are no high AC impedances anywhere. Normally I use 4000B-series, but these lack the drive capability needed here...
- Plain parts - I agree. Hence encouraging the use of bipolars rather than MOSFETs, 78L05's rather than LDO's. I don't like unusual things - not out of fear of the unknown, but I expect my things to last forever 😉
- Hopefully an off-board mains TX and relays will solve the mains worries, or at least make them the responsibilty of the end user. This is a good point, however, and something we should help Alex with.
- You mention switching spikes on the mains. A EI core TX followed by a bridge should offer much greater attenuation of these than my resistive dropper supply, and this didn't suffer from any problems. I had the tumble drier in my old workshop, and this used to put all sorts of spikes on mains when it stoped/reversed/etc. The Arcam Alpha suffered with these, but this circuit didn't.
That's my experiences, for what they're worth...
Mark
Are you saying I can't believe everything I read on the Internet?? I appreciate your help very much. I'm busy myself today so I'll have to process all this new information later.
I made the changes as you suggested and also the inhibit time delay back to 47uF*100kohm. My delayed output was triggering ON briefly while the mains were shut off. I don't suspect this would have been a problem but I'm not risking it. Now, what can I do with that spare inverter?
You haven't quite got the current sources right - check my original to see what I mean. Apologies for not being clearer earlier, but the 10K resistors go to the current sense resistors, and the resistors feeding the base from the reset capacitor can be increased to around 47K
Also, there's a problem with the speaker relay - it doesn't go off immediately because C7 has to discharge. A diode across R17 (pointing down) will help, alternatively your previous arrangement was probably ok.
With the spare inverter, this can drive a soft-start relay. Copy the speaker delay circuit and make the delay shorter (<0.5 seconds). People then have a choice - with small amplifiers they just need the one mains relay, but larger amps have two relays and a power resistor to limit the surge. I've used the metal-clad wire-wounds as they can be bolted safely to an earthed chassis.
I won't be able to reply until next week, but hopefully you've got some ideas to play around with. Try getting something bread-boarded - simulations only go so far when you're trying to learn and get a "feel" for this stuff 😉
Mark
Also, there's a problem with the speaker relay - it doesn't go off immediately because C7 has to discharge. A diode across R17 (pointing down) will help, alternatively your previous arrangement was probably ok.
With the spare inverter, this can drive a soft-start relay. Copy the speaker delay circuit and make the delay shorter (<0.5 seconds). People then have a choice - with small amplifiers they just need the one mains relay, but larger amps have two relays and a power resistor to limit the surge. I've used the metal-clad wire-wounds as they can be bolted safely to an earthed chassis.
I won't be able to reply until next week, but hopefully you've got some ideas to play around with. Try getting something bread-boarded - simulations only go so far when you're trying to learn and get a "feel" for this stuff 😉
Mark
I added everything as you said. It will be some time before I get to breadboard this but the circuit simulates perfectly.
I decided it would not matter if I didn't shut off the soft-start relay when the main power relay kicks on. What is a common value of resistor to use for soft-start?
Hi,
for 110/120Vac about 10r to 20r,
for 220/240Vac about 30r to 60r.
A bank of 5W resistors giving about 20W to 30W is usually suitable if switched out quickly (<=1S).
I prefer to series connect the bank of resistors because I think the surge rating of the thicker wire in a low value resistor is better i.e. more robust. (for me using 240Vac 5*10r in series =25W 50r and this works upto my largest 1kVA transformer)
for 110/120Vac about 10r to 20r,
for 220/240Vac about 30r to 60r.
A bank of 5W resistors giving about 20W to 30W is usually suitable if switched out quickly (<=1S).
I prefer to series connect the bank of resistors because I think the surge rating of the thicker wire in a low value resistor is better i.e. more robust. (for me using 240Vac 5*10r in series =25W 50r and this works upto my largest 1kVA transformer)
I've read a webpage about that (http://sound.westhost.com/project39.htm)
It would certainly help keep the BJT's cooler. I would try something like that but unfortunately I don't know the hold current required for my relays.
It would certainly help keep the BJT's cooler. I would try something like that but unfortunately I don't know the hold current required for my relays.
Hi,
similar but uses a transistor or FET to bring in the relay.
Great when you are using a main PSU with much higher voltage than even a pair of series relays.
The example shows a 24V relay. I measured the pull in and dropout voltage and the coil resistance.
similar but uses a transistor or FET to bring in the relay.
Great when you are using a main PSU with much higher voltage than even a pair of series relays.
The example shows a 24V relay. I measured the pull in and dropout voltage and the coil resistance.
This discussion comes up way too often. Makes me want to try to find a 20+ YO design that's scribbled on paper and works well. It was capable of protecting a Leach amp when I accidently reversed the two output transistors. I had no damage. Just the resistor that was supposed to pop.
First, the inputs are disconnected via a FET opto and speakers are disconnected via a relay.
Circuit is powered by an independent power supply of 12 volts.
AC line charges the power supply caps through a metal oxide flame proof resistor (about 50 ohms, I believe, four 50 V power supplies with 9600 uf of filtering each).
When all of the power supplies have reached about 2/3 of their value. This is done by the Opto LED's in series with resistor, LED and zener diode combination and connected across the appropriate supply The outputs of the optos are wired in series. The PS caps are proteced with ZNR's, otherwise the optos will pop.
If they don't make it to 2/3 of their value, the resistor pops. If a single rail fuse pops after the AMP has powered up, the resistor pops as well and the AMP shuts down.
When all the optos have signals that the power supplies are > 2/3 their final value, the speaker relay engages and the LED FET opto in series with the audio inputs ramps up exponentially. It's regulated with an LM334 current regulator at steady state.
The logic is handled with an LM324 quad op amp. I may have also used a ULN2004 relay driver IC.
First, the inputs are disconnected via a FET opto and speakers are disconnected via a relay.
Circuit is powered by an independent power supply of 12 volts.
AC line charges the power supply caps through a metal oxide flame proof resistor (about 50 ohms, I believe, four 50 V power supplies with 9600 uf of filtering each).
When all of the power supplies have reached about 2/3 of their value. This is done by the Opto LED's in series with resistor, LED and zener diode combination and connected across the appropriate supply The outputs of the optos are wired in series. The PS caps are proteced with ZNR's, otherwise the optos will pop.
If they don't make it to 2/3 of their value, the resistor pops. If a single rail fuse pops after the AMP has powered up, the resistor pops as well and the AMP shuts down.
When all the optos have signals that the power supplies are > 2/3 their final value, the speaker relay engages and the LED FET opto in series with the audio inputs ramps up exponentially. It's regulated with an LM334 current regulator at steady state.
The logic is handled with an LM324 quad op amp. I may have also used a ULN2004 relay driver IC.
the back emf from the relay dissipates the magnetic flux in the solenoid.alexcd said:
It appears that a diode alone to protect the surrounding semiconductors slows down the release of the relay.
To speed up the release one can add a resistor or a Zener in series with the diode. I chose to go with a low voltage Zener.
Running the relay on low current (at a voltage between pull in and drop out) reduces the solenoid flux and reduces the relay release delay.
AndrewT said:
This is really useful, but in this app, he's using a separate transformer which can be more closely matched to the relays. Also, IIUC, he's leaving relay selection to the kit users - in his shoes I'd be worried about the extra variables this brings.
Doug Self did some work on speeding up relay off-time with zeners, IIRC it's in his "Self on Audio" book. Also, you might be interested to know that my central heating boiler uses the resistor+cap trick for each of the 4 relays it uses 🙂
Mark
PS: Did you get my PM?
yes, two extra components and the relay runs on about 5% to 10% of it's rated coil power. That is much cooler.
We have a pump set that uses relays for logic control, one of a pair are permanently on equating to about 50% average duty, but they keep burning out the coils till we replaced with a different manufacturer. No water in our houses each time they burnt out.
That's unusual IME. In fact, the only relay coil I can recall failing was in another boiler (neighbours this time) - I assumed the heat and very thin wire were factors here (IIRC, it was a 48V coil)
We have the opposite problem to you - no pump means water in the house. Well, the cellar, but you get the idea 🙂
We have the opposite problem to you - no pump means water in the house. Well, the cellar, but you get the idea 🙂
The cap/resistor is definitely an option and somewhat negates the need for the zener. I think if there was a cost/benefit of either adding the cap or zener, the cap wins. The coils will last longer for sure. I doubt anyone wants to replace theirs after a few years. In my case, I am using a 12.5VAC trafo that I had laying around. Unfortunately this gives me around 17VDC-some loss for bridge/resistance, etc. I need to cut this down. I like the idea of using the resistor/cap to achieve pull-in and minimize average power dissipation. Obviously this throws a huge monkey wrench into the useability for most other people but I can add a disclaimer and maybe a cheat sheet of how to bypass the circuit with appropriate resistor values. Mark, what are your thoughts?
Hi Alex,
the cap and Zener do completely different jobs.
the cap gives a current pulse to pull in the relay solenoid when the transistor closes.
The Zener absorbs the back emf when the transistor opens.
The resistor is changed to accomodate different supply voltages and different relay parameters. Just use the spreadsheet to select a suitable resistor that gives a sensible range of hold in voltage balanced with power dissipation in the relay coil and resistor (paralleled if necessary).
Remember that the relays must hold in even when the mains input voltage is at minimum and the transformer is suffering maximum current draw.
For your particular transformer the cap will hold 17V waiting for the transistor to close. Then that 17V pulse will pull in the 12V relay even faster than a normal 12V supply. After the pulse is over (lasting just a fews mS) the voltage collapses (maybe just 6V or so) to the operating current/voltage modelled in the spreadsheet.
the cap and Zener do completely different jobs.
the cap gives a current pulse to pull in the relay solenoid when the transistor closes.
The Zener absorbs the back emf when the transistor opens.
The resistor is changed to accomodate different supply voltages and different relay parameters. Just use the spreadsheet to select a suitable resistor that gives a sensible range of hold in voltage balanced with power dissipation in the relay coil and resistor (paralleled if necessary).
Remember that the relays must hold in even when the mains input voltage is at minimum and the transformer is suffering maximum current draw.
For your particular transformer the cap will hold 17V waiting for the transistor to close. Then that 17V pulse will pull in the 12V relay even faster than a normal 12V supply. After the pulse is over (lasting just a fews mS) the voltage collapses (maybe just 6V or so) to the operating current/voltage modelled in the spreadsheet.
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