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Topic: Computer controlled PS/function gen/multimeter (Read 229243 times) previous topic - next topic

Re: Computer controlled PS/function gen/multimeter

Reply #180
[quote author="alm"]
[quote author="tayken"]
- I liked the idea of using precise voltage sources which was suggested by arhi. We might also use the PGA idea suggested by him.
[/quote]
For the ADC? Sure, as long as it's significantly better than the built-in reference in the PIC and the rest of the circuitry makes the extra accuracy worthwhile. Not much point in using a voltage reference much more accurate than the voltage divider. PGA sounds like a good idea, as long as the voltages are within the PGA's input range (although we could put a fixed divider in front of the PGA), and it's not too expensive. Isn't this PGA programmed by connecting various pins to ground/Vcc? But I'm sure we can hook PIC GPIO pins to the programming pins. It would save us (relative) precision resistive dividers, if it's indeed accurate enough.
[/quote]
This might be used for calibration purposes. A few jumpers can put the unit to calibration mode. Or we can build some test points for a calibration card which can be used to calibrate the unit from time to time. Just a thought.

[quote author="alm"]
[quote author="tayken"]
- wrt negative voltage SMPS idea before: I first thought of suggesting a voltage inverter but there might be current issues related to them. But MC34063 can be used to generate negative voltages, so we can design the linear regulator stage with + and - supplies and outputs. We can also route these positive and negative supplies to Vref pins of the ADC and get signed measurements. Any objections to this idea?
[/quote]
I don't understand how this fits in with the current idea. Do you mean the symmetric power supply for the op-amps? Or the input for the negative regulator? The former idea would work fine, I guess, as long as PSRR of the op-amps is enough to reject the noise. But I'm not sure if it's necessary, since we already have a supply of opposite polarity for the other regulator. A boost converter for the positive supply above the output voltage (for positive regulator) might be a good idea, but I'd prefer a solution with lower parts count (something like diode, capacitor and linear regulator). These are low-current supplies (only op-amps and base currents, probably <<100mA or so), so simplicity and parts count are the most important, and efficiency is irrelevant.
[/quote]
Yes, symmetric power supply for op-amps if we have use a DC input. I, too, prefer something with a lower parts count but only solution if we use DC input. It is probably not necessary for AC input the way I see from your post.

Re: Computer controlled PS/function gen/multimeter

Reply #181
[quote author="tayken"]
This might be used for calibration purposes. A few jumpers can put the unit to calibration mode. Or we can build some test points for a calibration card which can be used to calibrate the unit from time to time. Just a thought.
[/quote]
I think the money and effort is better spent on improving intrinsic accuracy (use more accurate reference for ADC, more accurate dividers/PGA, better filtering for signal / power supply, better ADC). If we have more accurate/stable parts, use these in the main circuit instead of using them once a year for calibration. People can probably use the lab power supply and a DMM for calibration: connect power supply to ADC input and DMM, adjust until DMM reads exactly 1.000V, click calibrate, repeat for 10.00V, 100.0mV and 10.00mV. DMM's are pretty accurate and ubiquitous. Use the same voltage for 100% of lower range and 10% of next range to save time. Calibration constants can be stored in EEPROM.

[quote author="tayken"]
Yes, symmetric power supply for op-amps if we have use a DC input. I, too, prefer something with a lower parts count but only solution if we use DC input. It is probably not necessary for AC input the way I see from your post.
[/quote]
Agreed, we can't do the peak voltage trick with DC. But we would need an inverting SMPS anyway for the negative input voltage, unless we use the dreaded two wall warts or one wall wart with symmetrical output (if such a beast is cheap and readily available). Can't do voltage doubler from DC, and I don't know any other way of splitting the voltage without dissipating lots of power or building an SMPS. If we use an SMPS for the negative voltage, we might evaluate if we skip the linear post-regulator all together (depending on parts count and noise). My current idea pretty much requires an AC output (transformer-only) wall wart with enough power (rough guess: 18Vrms/5A or so if we want 2x15V, 2x1.5A output?). Output voltage and current upper limit can be lowered as far as I'm concerned.

Re: Computer controlled PS/function gen/multimeter

Reply #182
wrt refference - they are "very cheap" and they don't require any "additional circuitry" - just a 3 pin device Vdd, Vss and output. The output you connect to pic as vref and that's about how complex it is ?!?!? donno what you are talking about when you mention "complexity". You might want to add few capacitors to have more stable output but that's it, 1uF is more then enough. This way ADC gives you result in relationship to known value (voltage reference) and not in relationship with unknown value (Vdd)

wrt pga, the ones I used (from microchip) have SPI bus that you use to "program the gate" ... no jumpers needed here

The idea of using voltage reference and pga's is to have "known" values you compare your input from (so you know exactly what aplification op-amp will have and exactly what voltage is read by adc). There's no other way to have this values "known" (as your 7805 or 7803 or whatever regulator you use will always be +-0.3V or even worse, depending what regulator and who made it giving you 0.5V error converting ADC value to voltage - minimum if you use Vdd as Vref, but with voltage reference as Vref you get this to under 0.001V)

wrt calibration - you don't use this once a year to calibrate - you use it ALL THE TIME !!! ... the pic have Vref pin and you hook up the reference to this pin and configure adc to use it and that's about it - it is "auto calibrating all the time" .. you use the PGA-OPAMP instead of normal OPAMP + digital trimmer so instead of changing resistors defining the amplification factor, you bloody tell the opamp what amplification factor you want .. Idon't get the "use better parts" - THESE ARE THE BETTER PARTS ?!?!

Re: Computer controlled PS/function gen/multimeter

Reply #183
I agree with "arhi."  Any design based on ADC should have a voltage reference, not just a mere voltage regulator.  Some ADC chips have a built-in voltage reference, others require an external one.

The present design I am working on uses the Texas Instruments $2.72 REF5025AID, which is pin-compatible with the Maxim MAX6192AESA+  They're available in various voltages like 2.048 V, 2.5 V, 3 V, 4.096 V, 4.5 V and 5 V.  To that common list, Texas Instruments adds 10 V, while Maxim adds 1.25 V.

Re: Computer controlled PS/function gen/multimeter

Reply #184
[quote author="rsdio"]
I agree with "arhi."  Any design based on ADC should have a voltage reference, not just a mere voltage regulator.  Some ADC chips have a built-in voltage reference, others require an external one.
[/quote]
Can't remember anyone disagreeing with that, as long as the improvement over the internal reference (not much point in replacing a 0.5% internal reference by a 0.1% reference if you use 1% resistors as divider). I only disagreed with adding extra references for calibration purposes, I would rather use that money to buy a better reference for the ADC (or better ADC/PGA/whatever). Plenty of better references if you want to spend money, all the way up to the LTZ1000 which probably costs more than the total BOM of this project and wouldn't make sense at all.

I don't really understand the point arhi's latest post. Apart for correcting me on the SPI bus (thanks for that, I must have been confused with other PGA's), I don't see what he's responding to. I don't think anyone suggested using a 780x for ADC reference, and I see no need to shout about the PGA. I thought both I and tayken responded pretty positive.

By the way, don't the ADC channels need some sort of differential (ideally floating) inputs? Apologies if this was already decided, I haven't payed much attention to the ADC inputs yet. What's the current idea about that? I assume the power supply for the PIC/ADC is connected to the common output of the lab supply, since we don't want yet another wall wart. We want to measure the voltage between any two points (eg. over a high-side shunt), so connecting all negative sides together wouldn't work. Can the PGA's accept differential inputs? We have symmetrical supplies for the lab supply (if ADC ground is connected to lab supply ground), so that would give us a common mode range of -15V to +15V or so, which together with a fixed divider should do, if offset is low enough that it won't compromise accuracy at low ranges.

What about the isolation, how do we do that? A separate PIC/FT232 fed from USB power (will be turned off when USB is not plugged in) and isolated UART/SPI to the dsPIC? Or do we isolate the dsPIC from the lab power supply and ADC inputs somehow? Where would the dsPIC get its power from? USB? Requiring it to be connected to USB may be a fair trade-off, since you can't monitor or control it any other way.

Re: Computer controlled PS/function gen/multimeter

Reply #185
For isolation - it's not great for speed, but it would be easy to use the PIC UART to a FT232 chip and isolate there. FT232 is powered by the USB, PIC is powered from a local regulator.
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Re: Computer controlled PS/function gen/multimeter

Reply #186
Hi guys, sorry I've been away from this thread for so long... I'm still interested though.

alm said:
Quote
I think I found an error in Fen's schematic (he didn't present it as a finished product), Q1 was a Darlington in the Agilent design (not indicated in the schematic), and I don't see how a regular MJE3055 will have enough gain for the base current for R2 to be enough. I'm also not a fan of the 'constant current source' provided by (Vin-Vb(Q1))/R2, which will vary with output voltage.

Thanks for looking at the design in detail. I was afraid nobody would go so far! Indeed, I didn't "select" the pass transistor, I just took one for which I had a SPICE model (I forgot HP used a darlington). You're right, it probably only has a hFE of 15-20 at Ic=1A. You're also right that the bias resistor is probably inadequate in a large number of conditions.

Just to clarify some of the features of the linear stage I posted some time ago ( http://dangerousprototypes.com/forum/in ... 87#msg9187 ) :
1- it will regulate down to 0 volts, without requiring a negative supply, if the op-amps are well chosen (LM358 should work, I can't say how well though)
2- op-amps U1, U2, and their compensation network (absent on the SPICE schem I drew), are critical. To repeat myself once more : it's farily easy to mess up the component choice, and have a supply that sorta-works but will be unstable. Unstable to the point that you can get substantial oscillation on the output. Or just terrible transient load change response. It's a great topology, and very flexible, but more delicate than a single LM317.

Misc thoughts:
- having two symmetrical regulators is nice but a bit restrictive for those times when you would really need a 12V supply and a 5V supply. Sure you can set one supply to 7V and the other to -5V, but it's still not great. The best is two floating regulator outputs.

- for an initial release, I think it should be most basic. Straight DC-input with polarity protection is most important, and the .3 or 0.4V drop caused by a series diode is insignificant. Or use a "crowbar" type circuit with an input fuse (useful in any case) with a diode to ground.

- I don't like AC-input with voltage-doublers: how do you accomodate the unknown range of voltages that a user might use ? Supplying 1A (or 1A + 1A) will put a lot of strain on capacitors (high ripple current). Also, if you don't want huge 60Hz ripple you want large capacitorvalues, which are expensive and physically large.

-whether to float the output needs to be decided. One thing is for sure : the very desirable flexibility automatically means it will be more complex & more expensive. Is it worth it ? In my experience, it's a nice feature, certainly worth a few extra $. I'll also vaguely quote someone on this thread (I think it was rsdio) concerning the basic utility of a lab supply :
Quote
If I need to use a lab supply, it's either because A) I don't have a supply built yet (unknown requirements) or B) I'm troubleshooting and suspecting the regulator

-I really see no need for a dsPIC. I think almost everyone now agrees that we're not going to kludge up a software feedback loop, therefore the requirements for the regulating uC would be:
 *a bunch of ADC channels for metering the output (10-bit is plenty enough, this is not a 5-digit calibrated multimeter);
 *either a couple DAC channels or PWM outputs that are capacitor-filtered to give variable DC.
I would tend to prefer a DAC solution here since filtered PWM inherently has ripple, and possibly a slow response time.
Now here's the catch. The most basic USB-enabled PIC is about 3$. Mchip lists the new 18F13K50 @ 1.30$ in large quantities. But at that price you don't get a DAC!
If you want USB + DAC, I actually haven't found any PIC that had more than 2 DAC channels. In any case it would be expensive, like 6-10$ , and wouldn't easily be opto-isolated.
That leads me to the next point :

-if a DAC chip is used in addition to the USB-speaking chip (PIC or FT232), it becomes easy(er) to add opto-isolation and have fully floatable outputs. But then you want to meter the output, so you also need ADCs on the isolated side ! At this point it would then sound justified to add a small, very basic PIC on the isolated side, which would serve only to monitor and steer the regulator. I like this idea, but one thing to consider is how much cost overhead it adds if we need to program 2 or 3 MCUs. Ian ?

To summarize : floating outputs are nice to have, but significantly more complex and expensive. I would opt for a single-channel, single-output version 01 with modest, but solid features. I'm attaching a symbolic diagram of what I have in mind.

Re: Computer controlled PS/function gen/multimeter

Reply #187
In general the cost of each small component and programming is pretty insignificant (depending on part cost, obviously) compared to the setup fee.

My preference would probably be an FT232 chip on the PC side, and whatever PIC on the power supply side. If a DAC is used to set the supply, then an external DAC chip is probably best for cost and to stay in SPEC (the PIC DAC audio-only discussion from earlier).

Floating outputs are great, but if it adds a lot of trickery to isolate it could be a further drag on getting a real v1 out.

Quote
1- it will regulate down to 0 volts, without requiring a negative supply, if the op-amps are well chosen (LM358 should work, I can't say how well though)

I didn't realize this. I think we've all be discussing it from the perspective that negative voltage is required for 0v-0.6v output.

Quote
2- op-amps U1, U2, and their compensation network (absent on the SPICE schem I drew), are critical.


Could anyone following this thread please contribute a little more about this. A discussion or napkin sketch could be enough to help me work it into the schematic.
Got a question? Please ask in the forum for the fastest answers.

Re: Computer controlled PS/function gen/multimeter

Reply #188
Hi,

[quote author="ian"]
My preference would probably be an FT232 chip on the PC side, and whatever PIC on the power supply side.

Floating outputs are great, but if it adds a lot of trickery to isolate it could be a further drag on getting a real v1 out.

Quote
2- op-amps U1, U2, and their compensation network (absent on the SPICE schem I drew), are critical.


Could anyone following this thread please contribute a little more about this. A discussion or napkin sketch could be enough to help me work it into the schematic.
[/quote]

Re FT232 : do we have a usable PIC18-compatible USB stack ? I.e. can the BP4 stack be adapted to this project ? The same command-response structure could be used, or we could implement our own vendor requests/replies , with interrupt transfers. As I mentioned devices like the 18F13K* are quite nice, and a lot cheaper than an FT232. Plus they have an internal 1.024 / 2.048 / 4.096V reference for the ADC !

I think we should really keep floating outputs for a v2... Besides v1 will still be quite usable for a lot of people in a lot of typical situations.

As for the compensation networks, this is really at the limit of my knowledge, and will ultimately depend on the pass transistor, the drive trans, the output capacitor, and the op-amp type. If you recall the HP schematic, the op-amps all had small capacitors around them. In my SPICE schem I left them out except for a ~100p cap near the current-limiter. It slowed the response of the limiter, but consequently also added a bit of overshoot (the limiter set to 1A would still allow a short period of >1A output before cutting in).

Re: Computer controlled PS/function gen/multimeter

Reply #189
Quote
do we have a usable PIC18-compatible USB stack ? I.e. can the BP4 stack be adapted to this project ?

We can use the Microchip stack without distributing the source. There is also the zygote of an opensource USB stack, but it is still a work in progress. It's in the Bus Pirate dev-forum.
Got a question? Please ask in the forum for the fastest answers.

Re: Computer controlled PS/function gen/multimeter

Reply #190
Just some thoughts and questions:
- How about negative output? Or is the initial release will be positive output only? I don't require any negative output but I think some people were talking about it.
- DC input only is a good choice for simplicity. We might even put some pads in a square design so that people might change center positive to center negative with a quick de-solder & re-solder process.
- No dsPIC for sure, just a regular PIC for control purposes. No internal DAC as the only ones I could find were for audio applications, they were not recommended for control applications. I would suggest that we use FT232 for USB isolation purposes and for the initial design. Later we might choose to put another PIC for USB communication purposes.
- For the initial release we might go for PWM + caps instead of a seperate ADC IC. Lets see its performance, if it is really poor, then we can go for the ADC IC in the real design.

Re: Computer controlled PS/function gen/multimeter

Reply #191
[quote author="alm"]
I don't really understand the point arhi's latest post. Apart for correcting me on the SPI bus (thanks for that, I must have been confused with other PGA's), I don't see what he's responding to. I don't think anyone suggested using a 780x for ADC reference, and I see no need to shout about the PGA. I thought both I and tayken responded pretty positive.
[/quote]

I most probably misunderstood your post...

anyhow, the whole adc/pga is only important for one segment, the segment that will be generating the voltage needed is much more important (and the part I know ziltch about)... I would like to see 3-5A more then 1-2A .. but .. maybe it would be cool to make the project out of 2 segments where one segment does the read/write and the other segment does the "power" stuff so one can easily change the power segment to get 5A or 10A if needed :) :) :)

Re: Computer controlled PS/function gen/multimeter

Reply #192
[quote author="fenugrec"]
Thanks for looking at the design in detail. I was afraid nobody would go so far! Indeed, I didn't "select" the pass transistor, I just took one for which I had a SPICE model (I forgot HP used a darlington). You're right, it probably only has a hFE of 15-20 at Ic=1A. You're also right that the bias resistor is probably inadequate in a large number of conditions.
[/quote]
Nothing wrong with the pass transistor, but it would require a second transistor to make it a Darlington. I also wouldn't trust the spice model. If the hFE is so inaccurate, what else did they get wrong?

About the R2, I think a transistor/JFET constant current source would be better, since HP used a resistor to a constant voltage above Ve(Q1). No need for great accuracy, it just should be better than between 14V/R2 (almost zero output voltage) and 2V/R2 or so (max output voltage). Something like -50%/+100% should be fine.

[quote author="fenugrec"]
Just to clarify some of the features of the linear stage I posted some time ago ( http://dangerousprototypes.com/forum/in ... 87#msg9187 ) :
1- it will regulate down to 0 volts, without requiring a negative supply, if the op-amps are well chosen (LM358 should work, I can't say how well though)
2- op-amps U1, U2, and their compensation network (absent on the SPICE schem I drew), are critical. To repeat myself once more : it's farily easy to mess up the component choice, and have a supply that sorta-works but will be unstable. Unstable to the point that you can get substantial oscillation on the output. Or just terrible transient load change response. It's a great topology, and very flexible, but more delicate than a single LM317.
[/quote]

I have some doubts about your design, even though you appear to know more about this:
1. How's the stability of Q3 without emitter resistor? Isn't it very sensitive to tiny Vbe and temperature variations, making it hard to keep stable?
2. The HP design of Q1/Q6 appears be temperature compensated, since the Vb(Q6) is at about the same level as Ve(Q1). Your design isn't, which may make the job of the op-amps harder.
3. Why isn't this topology used in other lab supplies? All commercial designs I'm aware of use a negative and positive voltage relative to the output voltage. Even in a commercial design, this has to add some extra costs, why do they use it if they could easily omit it? There has to be some downside, it's not like an LM324 is a recent invention.
4. In general, the major changes compared to the original design makes me somewhat uncomfortable without doing extensive analysis and testing. Hence my preference for something closer to the HP design if we can (doesn't add much costs with the symmetrical design).

If you're convinced that it will work fine, I'm willing to believe that. But if I would have to build it, I would go for a more conservative design.

[quote author="fenugrec"]
- having two symmetrical regulators is nice but a bit restrictive for those times when you would really need a 12V supply and a 5V supply. Sure you can set one supply to 7V and the other to -5V, but it's still not great. The best is two floating regulator outputs.
[/quote]
Agreed, but not possible without two floating inputs (two wall warts).

[quote author="fenugrec"]
- for an initial release, I think it should be most basic. Straight DC-input with polarity protection is most important, and the .3 or 0.4V drop caused by a series diode is insignificant. Or use a "crowbar" type circuit with an input fuse (useful in any case) with a diode to ground.
- I don't like AC-input with voltage-doublers: how do you accomodate the unknown range of voltages that a user might use ? Supplying 1A (or 1A + 1A) will put a lot of strain on capacitors (high ripple current). Also, if you don't want huge 60Hz ripple you want large capacitorvalues, which are expensive and physically large.
[/quote]
I agree with the downsides of AC input. Input voltage could be compensated by two wall warts (110V and 230V version). About 4700uF/25V or so per side was required according to my spice simulations. Not enormous, but not small or very cheap either. It is the only way I see to offer symmetrical output with a linear supply and one wall wart, however, which is nice. Another issue is that I wouldn't trust all cheap DC wall warts to be stable and safe for delicate electronics (like the leakage issue and wall wart suicide issue I referred to earlier). Designing/manufacturing an AC wall wart is much harder to screw up.

[quote author="fenugrec"]
-whether to float the output needs to be decided. One thing is for sure : the very desirable flexibility automatically means it will be more complex & more expensive. Is it worth it ? In my experience, it's a nice feature, certainly worth a few extra $. I'll also vaguely quote someone on this thread (I think it was rsdio) concerning the basic utility of a lab supply :
Quote
If I need to use a lab supply, it's either because A) I don't have a supply built yet (unknown requirements) or B) I'm troubleshooting and suspecting the regulator
[/quote]
I don't think extra parts will be too bad, some opto's and an extra PIC/FT232. Supply for the grounded part can be USB.

[quote author="fenugrec"]
-I really see no need for a dsPIC. I think almost everyone now agrees that we're not going to kludge up a software feedback loop, therefore the requirements for the regulating uC would be:
 *a bunch of ADC channels for metering the output (10-bit is plenty enough, this is not a 5-digit calibrated multimeter);
 *either a couple DAC channels or PWM outputs that are capacitor-filtered to give variable DC.
I would tend to prefer a DAC solution here since filtered PWM inherently has ripple, and possibly a slow response time.
[/quote]
Agreed.

[quote author="fenugrec"]
-if a DAC chip is used in addition to the USB-speaking chip (PIC or FT232), it becomes easy(er) to add opto-isolation and have fully floatable outputs. But then you want to meter the output, so you also need ADCs on the isolated side ! At this point it would then sound justified to add a small, very basic PIC on the isolated side, which would serve only to monitor and steer the regulator. I like this idea, but one thing to consider is how much cost overhead it adds if we need to program 2 or 3 MCUs. Ian ?
[/quote]
The reason for considering an FT232 was that we don't really need more than an USB interface on the grounded side. We could isolate the UART connection and connect the PIC to lab supply ground. I've used the small MCU per output on the isolated side in the past, because an SPI/I2C ADC is not much cheaper, and the MCU can also control the DAC's and switch some other stuff (I had relays for tap switching). With multiple channels, we would need more intelligence on the grounded side, since we would need to control two outputs with one UART. Using an FTDI device with dual UART is ugly in my opinion, and probably even more expensive.

[quote author="fenugrec"]
To summarize : floating outputs are nice to have, but significantly more complex and expensive. I would opt for a single-channel, single-output version 01 with modest, but solid features. I'm attaching a symbolic diagram of what I have in mind.
[/quote]
Not sure if the extra complexity for a single channel is that bad. What's wrong with a single FT232 (or USB PIC doing something similar) and two opto's? Another advantage of an isolated power supply is that you get protection of your USB port from connecting the output to something like mains for free. The choice of single vs. dual output depends on our choice of input. If we want two fully floating outputs, we would need two wall warts, so it would basically be a copy/paste job. Only the grounded side would be more complex (USB pic or FT232 + PIC with two UART/SPI outputs).

[quote author="tayken"]
- For the initial release we might go for PWM + caps instead of a seperate ADC IC. Lets see its performance, if it is really poor, then we can go for the ADC IC in the real design.
[/quote]
I would offer both options in that case, so we can compare (populate one board with DAC and one with PWM, or switch with a jumper).

[quote author="arhi"]
I would like to see 3-5A more then 1-2A .. but .. maybe it would be cool to make the project out of 2 segments where one segment does the read/write and the other segment does the "power" stuff so one can easily change the power segment to get 5A or 10A if needed :) :) :)
[/quote]
That's way outside the scope of this project. The design is scalable, but you would need to replace the current shunt, add extra pass transistors with emitter resistors, and probably add an extra transistor stage to provide enough pass transistor base current. You would also need much more cooling. Also much fatter traces for the high-current part. Not sure if it's worth it, it would probably be better to make your own PCB, possibly based on this schematic. I would also seriously consider SMPS for 10A, especially if low noise is not a requirement (eg. motors).

Re: Computer controlled PS/function gen/multimeter

Reply #193
[quote author="alm"]
[quote author="arhi"]
I would like to see 3-5A more then 1-2A .. but .. maybe it would be cool to make the project out of 2 segments where one segment does the read/write and the other segment does the "power" stuff so one can easily change the power segment to get 5A or 10A if needed :) :) :)
[/quote]
That's way outside the scope of this project. The design is scalable, but you would need to replace the current shunt, add extra pass transistors with emitter resistors, and probably add an extra transistor stage to provide enough pass transistor base current. You would also need much more cooling. Also much fatter traces for the high-current part. Not sure if it's worth it, it would probably be better to make your own PCB, possibly based on this schematic. I would also seriously consider SMPS for 10A, especially if low noise is not a requirement (eg. motors).
[/quote]

Yes I know it's out of scope, just said "I'd like" :) ... anyhow I do feel that 2 stage pcb (motherboard and daughterboard) would not make design much more complex and then one can design different daughter board with different power consumption ..

Wrt smps - I never, ever made a single one work, I never even managed to modify existing smps... so that's out of my area of expertise / interest - and to be honest I'm looking at few interesting lab psu's for months now deciding to go or not to go for them (they will have to wait now for another few months as I just spent non planned 400E for a new lcd as the old one died on me few days ago) ... that is the only reason I like standard transistor design :D

but I'm going too far away off topic .. I already gave all my ideas (rv and pga's) and now all I can do is wait and see what smarter guy's then me can make :), you lost me somewhere around second page :D, but I must admit I like where the project is going :)

Re: Computer controlled PS/function gen/multimeter

Reply #194
[quote author="arhi"]
I just spent non planned 400E for a new lcd as the old one died on me few days ago
[/quote]
Did you check the power supply components, mostly the electrolytic capacitors or some of them have dried. If so, just change those to new, better models and you are good to go :) I did this once, the parts cost was about 4.5€.