Vref on tube booster (pedal i designed) is ~3.5v instead of 4.5v

[diffeq]

so what can i do to fix that stuff?

Get rid of that *#$^@ relay, perhaps..??

and replace it with what?

3PDT?

[snow123]

so what can i do to fix that stuff?

Get rid of that *#$^@ relay, perhaps..??

and replace it with what?

3PDT?

nah, i still want to have the relay bypass stuff.

[snow123]

and as bartimaeus said, the issue probably isnt the relay itself, but that it isnt getting enough voltage (which drops the voltage even more), and im not sure how to fix that.

[ElectricDruid]

and if it matters, im using a KEMET EC2-5NU Relay.

Yes, it matters. The relay says it uses "<200mW". At 5V coil voltage, that's 0.2/5 = 0.04 = 40mA of current, worst case.

With 100R in the power supply with 40mA coming through it, you get (V=IR) 0.04 x 100 = 4V of voltage drop. So no wonder you're down to 5.8V.

Incidentally, your 555 circuit is running on 9V, so if the supply voltage were where it should be, it would actually be a bit hot for the relay coil.

The worst problem is what everyone has said all along - the relay is drawing a lot of current and casing a big volt drop through R100.

When the relay is off, there's still 1.2V of drop, but 0.4V goes across the diode, so there's 0.8V across R100. I=V/R, so 0.8/100 = 8mA. Is that reasonable for the rest of the circuit? I'm not sure...maybe.

so what can i do to fix that stuff?

Run the relay from the supply where it comes in, rather than from the filtered supply. The relay coil doesn't really need a heavily filtered supply, since it's not any direct part of the audio path, so noise there won't get into the audio. The whole section on the bottom-right of your schematic with the 555, the switching, and the relay coil can all be run direct from the 9V supply.

The rest of the circuit can still be run from the supply *after* the "de-humming" filter, but without the massive draw of the relay coil, you should have voltages which are much closer to what you expect.

Try it. See what happens and report back.

HTH,
Tom

[snow123]

so what can i do to fix that stuff?

Run the relay from the supply where it comes in, rather than from the filtered supply. The relay coil doesn't really need a heavily filtered supply, since it's not any direct part of the audio path, so noise there won't get into the audio. The whole section on the bottom-right of your schematic with the 555, the switching, and the relay coil can all be run direct from the 9V supply.

The rest of the circuit can still be run from the supply *after* the "de-humming" filter, but without the massive draw of the relay coil, you should have voltages which are much closer to what you expect.

Try it. See what happens and report back.

HTH,
Tom

how can i go about modifying the PCB to do that?

[bartimaeus]

the EC2-5NU is the 5V version. wonder if part of the issue is you're using a non-regulated supply to control the relay.

im assuming by "regulated supply", you mean using a voltage regulator? 'cause i was going to use one (a 78L09) for the circuit but did away with it, 'cause i thought it didnt make much sense to use one since i was already supplying the circuit with 9v.

if you are going to use a 5V relay, you really should have used a 5V regulator to power the relay section.

so what can i do to fix that stuff?

Run the relay from the supply where it comes in, rather than from the filtered supply. The relay coil doesn't really need a heavily filtered supply, since it's not any direct part of the audio path, so noise there won't get into the audio. The whole section on the bottom-right of your schematic with the 555, the switching, and the relay coil can all be run direct from the 9V supply.

The rest of the circuit can still be run from the supply *after* the "de-humming" filter, but without the massive draw of the relay coil, you should have voltages which are much closer to what you expect.

Try it. See what happens and report back.

HTH,
Tom

how can i go about modifying the PCB to do that?

-remove the connection where VA meets pin 4 and pin 8of the 555.
-remove the connection where VA meets R25.
-add a jumper from +9V to pin 4 and pin 8 of the 555.
-add a jumper from +9v to R25.

i'd also suggest jumpering the D20 by the relay, in order to reduce voltage drop. might break things though, but you can check.

but before you do any of that, i would suggest you remove R22 and see if the circuit operates normally. you may have an issue driving both a relay and a transistor from a single 555 pin. it might have been better to add a second transistor to control the relay (as you did with the LED).

[snow123]

the EC2-5NU is the 5V version. wonder if part of the issue is you're using a non-regulated supply to control the relay.

im assuming by "regulated supply", you mean using a voltage regulator? 'cause i was going to use one (a 78L09) for the circuit but did away with it, 'cause i thought it didnt make much sense to use one since i was already supplying the circuit with 9v.

if you are going to use a 5V relay, you really should have used a 5V regulator to power the relay section.

wouldnt the voltage divider (R24/R25) serve that purpose?

[antonis]

if you are going to use a 5V relay, you really should have used a 5V regulator to power the relay section.

wouldnt the voltage divider (R24/R25) serve that purpose?[/quote]

NO..!!

R24/R25 set about 4.5V with NO LOAD..
Any current, additional to 450μA flowing through R25 should further lower pin 6 threshold voltage level..

[bartimaeus]

as antonis says, your voltage divider does not provide a regulated 5V. it doesn't even provide 5V.

a regulator gives you a constant voltage regardless of the load and current draw.

a resistor voltage divider can't guarantee that, which is why it is unwise to use them for areas with high current draw. one fix is to buffer the output of the voltage divider, ideally with an extra opamp, that will give a consistent output.

but you could also try increasing the value of your voltage divider. for example, set it to provide a 6V output, so that it's closer to 5V even under a load from the relay.

[snow123]

how can i go about modifying the PCB to do that?

-remove the connection where VA meets pin 4 and pin 8of the 555.
-remove the connection where VA meets R25.
-add a jumper from +9V to pin 4 and pin 8 of the 555.
-add a jumper from +9v to R25.

i'd also suggest jumpering the D20 by the relay, in order to reduce voltage drop. might break things though, but you can check.

but before you do any of that, i would suggest you remove R22 and see if the circuit operates normally. you may have an issue driving both a relay and a transistor from a single 555 pin. it might have been better to add a second transistor to control the relay (as you did with the LED).

wouldn't it be better to wire that stuff after D100, and before R100? rather than wiring directly to +9v.

[snow123]

and honestly, at this point im thinking it would be better to re-design the PCB.

[Rob Strand]

Didn't read the thread but this comes up all the time.

The multimeter impedance (usually 1M or 10M but not always) loads down the Vref divider.

If you measure the DC voltage on the output of the opamps the multimeter will not load Vref.
(The assumption here is the at least one of the opamps is working.   If part of the circuit
is broken around an opamp then the output of *that* opamp might not be at Vref.)

[snow123]

and honestly, at this point im thinking it would be better to re-design the PCB.

would this kind of thing work?

[bartimaeus]

and honestly, at this point im thinking it would be better to re-design the PCB.

would this kind of thing work?

no. you are still powering the 555 from a 9V supply. in that schematic, you are only using the 5V supply to set the threshold and for the switch. but the output of the 555 (pin 3) will be determined by the power supply voltage (pin 8 ). that output pin is what controls the relay after all.

you need to keep the R24+R25 voltage divider you had. that divider is not powering the relay section, it sets the threshold for the 555 (note that it's connected to pin 6). see this for some explanation: http://www.555-timer-circuits.com/flip-flop.html

the more i look at your flip flop, the more i wonder if D21 and D20 are the issue. why did you include them?

i also think it would be more robust to add a second transistor to control the relay, instead of controlling it directly from the 555. though in theory the 555 can provide more than enough current for the job.

if you redesign the PCB to use a regulator, it is better to power the relay section from A (after D100), as you did in this case. but if you want to modify your existing PCB, it is better to power the relay section from before D100 to avoid the diode voltage drop.

[snow123]

and honestly, at this point im thinking it would be better to re-design the PCB.

would this kind of thing work?

the more i look at your flip flop, the more i wonder if D21 and D20 are the issue. why did you include them?

honestly i was just copying this pedalpcb relay thing: https://docs.pedalpcb.com/project/BasicRelayBypass-PedalPCB.pdf

[snow123]

and honestly, at this point im thinking it would be better to re-design the PCB.

would this kind of thing work?

no. you are still powering the 555 from a 9V supply. in that schematic, you are only using the 5V supply to set the threshold and for the switch. but the output of the 555 (pin 3) will be determined by the power supply voltage (pin 8 ). that output pin is what controls the relay after all.

you need to keep the R24+R25 voltage divider you had. that divider is not powering the relay section, it sets the threshold for the 555 (note that it's connected to pin 6). see this for some explanation: http://www.555-timer-circuits.com/flip-flop.html

the more i look at your flip flop, the more i wonder if D21 and D20 are the issue. why did you include them?

i also think it would be more robust to add a second transistor to control the relay, instead of controlling it directly from the 555. though in theory the 555 can provide more than enough current for the job.

if you redesign the PCB to use a regulator, it is better to power the relay section from A (after D100), as you did in this case. but if you want to modify your existing PCB, it is better to power the relay section from before D100 to avoid the diode voltage drop.

so what gets connected to the regulator?
im assuming it goes to pin 4 and pin 8 on the 555, correct?

[bartimaeus]

and honestly, at this point im thinking it would be better to re-design the PCB.

would this kind of thing work?

no. you are still powering the 555 from a 9V supply. in that schematic, you are only using the 5V supply to set the threshold and for the switch. but the output of the 555 (pin 3) will be determined by the power supply voltage (pin 8 ). that output pin is what controls the relay after all.

you need to keep the R24+R25 voltage divider you had. that divider is not powering the relay section, it sets the threshold for the 555 (note that it's connected to pin 6). see this for some explanation: http://www.555-timer-circuits.com/flip-flop.html

the more i look at your flip flop, the more i wonder if D21 and D20 are the issue. why did you include them?

i also think it would be more robust to add a second transistor to control the relay, instead of controlling it directly from the 555. though in theory the 555 can provide more than enough current for the job.

if you redesign the PCB to use a regulator, it is better to power the relay section from A (after D100), as you did in this case. but if you want to modify your existing PCB, it is better to power the relay section from before D100 to avoid the diode voltage drop.

so what gets connected to the regulator?
im assuming it goes to pin 4 and pin 8 on the 555, correct?

tbh, just follow the original pedalpcb schematic for the relay section, including the regulator. but you can probably jumper D20 and remove D21. those are not necessary with the relay you're using.

[snow123]

so what gets connected to the regulator?
im assuming it goes to pin 4 and pin 8 on the 555, correct?

tbh, just follow the original pedalpcb schematic for the relay section, including the regulator. but you can probably jumper D20 and remove D21. those are not necessary with the relay you're using.

im not sure about using a 9v regulator with a 9v input voltage though, so im leaning more towards using the 5v regulator.

[bartimaeus]

so what gets connected to the regulator?
im assuming it goes to pin 4 and pin 8 on the 555, correct?

tbh, just follow the original pedalpcb schematic for the relay section, including the regulator. but you can probably jumper D20 and remove D21. those are not necessary with the relay you're using.

im not sure about using a 9v regulator with a 9v input voltage though, so im leaning more towards using the 5v regulator.

the 9v regulator will provide a constant voltage to that section of the circuit despite any current draw. that holds true even if you supply it with 9V.

if i understand the circuit right, you should be able to replace the 9V regulator with a 5V regulator. but i'm not 100% sure. in theory the high output pin of the 555 should match the voltage at the power supply pin. but R23 complicates things in a way i can't understand.

[FiveseveN]

the 9v regulator will provide a constant voltage to that section of the circuit despite any current draw. that holds true even if you supply it with 9V.

Nope. See dropout voltage.