yup, 47K, 43K,39K - whatever you have in drawer, trimpot in series will cover that
P1 Trimpot to max = clockwise 25 turns = output ≈ 5k ohm?
Not sure about how to connect the trimpot (not p1) in series? In-out is easy but where do I connect pin 3 / gnd?
Not sure about how to connect the trimpot (not p1) in series? In-out is easy but where do I connect pin 3 / gnd?
be inventive - find a way to check with ohmmeter
I can't remember and I don't care much about CW/CCW even on my own pcbs, always trusting only what DMM is showing
sorry, I wasn't precise - I meant P1, not additional trimpot
just correct 10K position, as already told
Last edited:
Thank you ZM, good advices as always. Measure output I would have anyway [emoji1]
It would/will be SO cool if baby-F4 comes to life after just this issue!
[emoji1317] david
I‘m a bit in type-before-thinking-mode, most of the advices become clear just after i go „huh?“... [emoji51]
It would/will be SO cool if baby-F4 comes to life after just this issue!
[emoji1317] david
I‘m a bit in type-before-thinking-mode, most of the advices become clear just after i go „huh?“... [emoji51]
R8 is the bummer: 22.1r instead of 22.1K
now comes what I hate: ordering 2 pieces worth $1, $20 shipping. So have to invent a new project to get to the required amount for saving shipping. (Which brings an advantage back, haha)
Well, at least you've found your problem.
While you're at it, order some other values, like 27.4K in the unlikely event that
22k isn't enough.
While you're at it, order some other values, like 27.4K in the unlikely event that
22k isn't enough.
now it's closing in...
Heya!
Got a spare hour to try doing the correction.
Replaced R8 (22.1K) on first channel, switched on (first with light-bulb, then fully) and here's BIAS! instant 0.13mA, offset between 0.00 and 0.03 mA.
Next question: Why does it not get warm? The channel has been on for ~15 minutes, no real temp-rise.... (heatsink and device are both room-temp, 18°C)
😕
Heya!
Got a spare hour to try doing the correction.
Replaced R8 (22.1K) on first channel, switched on (first with light-bulb, then fully) and here's BIAS! instant 0.13mA, offset between 0.00 and 0.03 mA.
Next question: Why does it not get warm? The channel has been on for ~15 minutes, no real temp-rise.... (heatsink and device are both room-temp, 18°C)
😕
Your units are funny. Please reconfirm your measurements.
Your dc offset is a voltage. So what is it in mV?
Is your bias really 0.13mA? If so it's hardly on. So please confirm your voltage across
any of the source resistors (R16-R21).
Your dc offset is a voltage. So what is it in mV?
Is your bias really 0.13mA? If so it's hardly on. So please confirm your voltage across
any of the source resistors (R16-R21).
Argh. was too nervous and didn‘t see it !
Bias is at 0.1 V, trimpot to max
Dc offset is now a very jumpy +- 0.15 mV
Need another round of resistor-swap and will report back.
Bias is at 0.1 V, trimpot to max
Dc offset is now a very jumpy +- 0.15 mV
Need another round of resistor-swap and will report back.
This time it looks good!
R9 is 5K and I get 200 mV bias (slowly growing, me gently keeping it below 200 mV), still jumpy offset close to 0 mV.
It‘s warming up and I am very confident!
Thank you for chiming in so quickly, dennis!
[emoji4]
R9 is 5K and I get 200 mV bias (slowly growing, me gently keeping it below 200 mV), still jumpy offset close to 0 mV.
It‘s warming up and I am very confident!
Thank you for chiming in so quickly, dennis!
[emoji4]
Sounds good. 
Just take your time, monitor and let it warm up with the lid on and trim offset and bias if needed.
Just take your time, monitor and let it warm up with the lid on and trim offset and bias if needed.
My turn to feel embarasssed [emoji51]
But it’s on it‘s way. I connected both channels and threw it on which instantly blew a 2A fuse. On a 230V line it is running btw...
Having a break until ... thursday if I‘m lucky.
But it’s on it‘s way. I connected both channels and threw it on which instantly blew a 2A fuse. On a 230V line it is running btw...
Having a break until ... thursday if I‘m lucky.
When I was in need of a little distraction, I went to the chassis and tried my mechanic skills, making a prototype of the LED-indicator:
Since I have dozens of rectangular leds, I'm using them for the front. Cutting out a hole 5 x 2 mm is a bit of a challenge though. (As one can barely see, the LED sits snugly, but the cutout got half a mm too long). I luckily made it on a spare part.
Next try: round the led, make the hole 5 x 2 mm with round ends...
edit: That yellowish stuff is the masking tape protecting the anodized aluminium.
Since I have dozens of rectangular leds, I'm using them for the front. Cutting out a hole 5 x 2 mm is a bit of a challenge though. (As one can barely see, the LED sits snugly, but the cutout got half a mm too long). I luckily made it on a spare part.
Next try: round the led, make the hole 5 x 2 mm with round ends...
edit: That yellowish stuff is the masking tape protecting the anodized aluminium.
Attachments
Did you set P1 to max value before you power up the other channel. With R9=5K
(and I'm assuming R8=22.1k) the TL431 will give fairly high Vgs voltages even with P1
set to max (~5k) and you may already get a healthy amount of current
flowing through the mosfets. And if P1 was close to zero that current will be very
high.
(and I'm assuming R8=22.1k) the TL431 will give fairly high Vgs voltages even with P1
set to max (~5k) and you may already get a healthy amount of current
flowing through the mosfets. And if P1 was close to zero that current will be very
high.