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

[duck_arse]

what do you mean wait?

R100 at 100R, the volt drop shouldn't be excessive. The 555/relay should be powered from after D100 and before R100, not after it. The LED should be powered after D100 too.

R100 at 100R, the volt drop shouldn't be excessive. The 555/relay should be powered from after D100 and before R100, not after it. The LED should be powered after D100 too.

well FWIW, VA was at around 7v with R100.

Well there's your problem then. Q2, the relay, the LED... you are draining to much current through R100 which is pulling your supply down to 7V, and Vref is half that or 3.5V.

[antonis]

what do you mean wait?

It seems to me you're strung up a bit, arent' you..?? 

History teaches to wait for more than a couple (4-5 min) of snow123 consecutive posts before proceeding to any answer..

[snow123]

what do you mean wait?

nothing, its just something i sometimes say before a sentence.

[snow123]

so is the relay stuff causing the vref to be lower than it should be?

[antonis]

Is it..??
(who knows..)

The answer (apart from blowin' in the wind) seems to me impenetrably ambiguous..  

[GibsonGM]

Can you disconnect the relay and check?

[snow123]

Can you disconnect the relay and check?

well FWIW, i breadboarded it without the relay, since i didnt have enough space.

[antonis]

@snow123: Plz have some mercy and tell us about the final circuit and what's happening..

[snow123]

@snow123: Plz have some mercy and tell us about the final circuit and what's happening..

this is the final circuit:


I have the PCB fully populated, except for R11, which i have now omitted.

[snow123]

and here's what VREF & VA was/is under different conditions:

VREF was 4.5v when i breadboarded the main tube screamer circuit, without the relay bypass stuff, and VA might have been closer to 9v, not quite sure though.

on the fully populated PCB, with the pedal "off", VREF is 3.5v and VA is 7.76v.

on the fully populated PCB, with the pedal "on", VREF is around 2.8v and VA is 5.8v.

VREF didnt change with the recent modifications i did (swapping the values of C2 & R4, omitting R11).

and FWIW, i thought that the reason VREF is lower on the PCB, than it is on the breadboard, was because of the traces building up resistance (or something like that), and not the relay bypass thing drawing too much current.

and the "vref" at the relays voltage divider (R24/R25) is at 3.88v, which what it actually should be, given that VA is 7.76v.

[snow123]

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

[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.

[bartimaeus]

as you mention, your Vref seems to be scaling correctly with the VA. so the problem is definitely whatever is causing VA to drop so much.

just to confirm, you're using a latching footswitch?

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. it also looks like the relay is receiving less than 5V, which could cause it to draw excess current and bring down the supply voltage.

personally, i also prefer the EC2-5SNU version, as you do not need to provide constant power to it when the pedal is active. but be warned it is not a drop-in replacement, you'd have to use a different circuit to control it.

[antonis]

VREF didnt change with the recent modifications i did (swapping C2 & R4, omitting R11)

By swapping C2 & R4, the last thing you should  care about is VREF..

You result into misbiased IC1A and no audible signal through it 'cause you turn 16Hz HPF into a 16Hz LPF..

[snow123]

just to confirm, you're using a latching footswitch?

nope, im using a momentary footswitch.

[snow123]

VREF didnt change with the recent modifications i did (swapping C2 & R4, omitting R11)

By swapping C2 & R4, the last thing you should  care about is VREF..

You result into misbiased IC1A and no audible signal through it 'cause you turn 16Hz HPF into a 16Hz LPF..

by "swapping C2 & R4" i meant that i replaced R4 (which was a 10k) with a 100k, and replaced C2 (which was a 1uf), with a 100nf. i just replaced the values. sorry about the confusion.

[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.

[snow123]

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?

[antonis]

so what can i do to fix that stuff?

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

[snow123]

so what can i do to fix that stuff?

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

and replace it with what?