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Switching circuit and footswitch for new amp
Switching circuit and footswitch for new amp
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Old 20th September 2019, 06:23 AM   #21
Gnobuddy is offline Gnobuddy  Canada
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Quote:
Originally Posted by Audio1Man View Post
You can always add a resistor in series with the foot switch LED's
Duke
I had the same idea too. You and I might share part of our brains or something. I don't suppose you left part of your neocortex on the north side of the US border?

I used to live in Los Angeles until a few years ago, and I've been in Thousand Oaks several times. Maybe I stole some of your brain cells one day on a drive through Thousand Oaks?

Adding extra (small) resistors in series with the LEDs will help to stabilize current through the LED a bit. But also increase the voltage drop across the (LED+resistor) combination a bit. (That might not be a problem, as long as you still have enough voltage left for the relays to operate reliably.)

I also tried to think of a way to add a couple of small-signal transistors to act as current-sinks in series with the LEDs, stabilizing LED current. Or to add a transistor in parallel with the LED/resistor pair, in a way that clamps the voltage across the LED to a constant value. So far, I haven't come up with a way to do either of these things. I'm zero for two.


-Gnobuddy

Last edited by Gnobuddy; 20th September 2019 at 06:28 AM.
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Old 20th September 2019, 04:20 PM   #22
Audio1Man is offline Audio1Man  United States
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Hi Gnobuddy
Relay pickup voltage is HI, while the HOLD VOLTAGE is much lower. If you add a BIG CAP across the LED the relay will latch and the when the cap charges the LED will light. I talked about this in an early post.
Duke
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Old 20th September 2019, 06:05 PM   #23
Gnobuddy is offline Gnobuddy  Canada
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Quote:
Originally Posted by Audio1Man View Post
If you add a BIG CAP across the LED the relay will latch and the when the cap charges the LED will light.
That's a great idea!

A possible downside: the LED won't go out immediately when you hit the footswitch, because of the stored charge in the cap. This may not be a problem if the cap is sized just right: big enough to latch the relay, small enough to let the LED go out so that the guitarist can be confident the LEDs actually show the current state of the relays.

-Gnobuddy
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Old 21st September 2019, 02:44 AM   #24
PRR is offline PRR  United States
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> the LED won't go out immediately

It will get a lot dimmer very quickly due to low dynamic impedance.

It may have a long tail of very-dim. But unless the cap value is obscene (far more than needed for relay-kick) I doubt this will be noticed.
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Old 21st September 2019, 07:05 AM   #25
akkar is offline akkar  Italy
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Sorry but I'm a bit confused now...

Quote:
Originally Posted by Audio1Man View Post
Hi Gnobuddy
Relay pickup voltage is HI, while the HOLD VOLTAGE is much lower. If you add a BIG CAP across the LED the relay will latch and the when the cap charges the LED will light. I talked about this in an early post.
Duke
  1. In the datasheet of my relay here (page 2, standard 12V), "Must operate voltage" is the hold voltage or the pickup voltage?
  2. we are talking about the two leds inside the footswitch (D3) right?

    Click the image to open in full size.
  3. so you are suggesting to add a resistor in series with the led AND a cap in parallel? (should I also keep the reverse parallel diode in this eventuality?)
  4. if so, what range of values do you suggest for resistor and cap?

thanks everyone for all the help
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Old 22nd September 2019, 03:16 AM   #26
Gnobuddy is offline Gnobuddy  Canada
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Quote:
Originally Posted by akkar View Post
In the datasheet of my relay here (page 2, standard 12V), "Must operate voltage" is the hold voltage or the pickup voltage?
Akkar, let's keep it simple: it takes more voltage to close an already open relay (because the moving metal part is further away from the electromagnet when the switch is open.)

So if your relay is currently open, it's guaranteed to close if 75% of the nominal voltage is applied. 9 volts, in the case of the 12V relays you're considering.

Once the relay has closed, the movable magnetic part is in contact with the electromagnet. The magnetic field is much stronger at this very short distance, so you can decrease the current through the relay coil a lot before the magnetic field finally becomes so weak that the relay opens. According to your datasheet, the relay is only guaranteed to open once the voltage across it has dropped to 5% of the rated voltage (0.6 volts, for a 12 V relay.)
Quote:
Originally Posted by akkar View Post
We are talking about the two leds inside the footswitch (D3) right?
Yes.
Quote:
Originally Posted by akkar View Post
...So you are suggesting to add a resistor in series with the led AND a cap in parallel? (should I also keep the reverse parallel diode in this eventuality?)
I suggest that you first build the circuit exactly the way you drew it. You may need to tweak R3 a little bit (by trial and error) to make it work well. Keep it simple, see if it works!

IF - and only if - you find that you just cannot get the LED brightness right, then you might consider the additional complications we've been discussing here. I think some experimenting will be called for, as I can't think of a way to calculate exactly what additional resistors or capacitors you might need.

In any case, I suggest you always have those protection diodes wired directly in reverse-parallel with the LEDs. Most LEDs will break down and be damaged with just a few volts of reverse voltage, and a relay coil can generate short spikes of hundreds of volts at turn-off without clamping diodes. So I think it's cheap insurance to use those protection diodes - all four of them, two across the actual relay coils, two across the two LEDs, exactly as you have them in your original schematic.

-Gnobuddy
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Old 22nd September 2019, 06:20 PM   #27
Audio1Man is offline Audio1Man  United States
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I have to assume some things:
Relay pickup 20ms? C=T/R, 0.02ms (relay pickup)/300 (Ohms relay)= 66uf, I would use 47uf/6.3v .I would place the cap on R3. The Relay flyback pulse is clamped by D? (across the relay). You just need to try it and see if it work for you.
Duke
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Old 23rd September 2019, 12:42 AM   #28
PRR is offline PRR  United States
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Quote:
Originally Posted by Audio1Man View Post
I have to assume some things:... .... ....= 66uf...use 47uf
OK. Without messing with numbers I was thinking 100uFd (depending).

Tomato tomato. But not a 1uFd pea or a 5,000uFd watermelon.
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