Linear lab power supply

[spec]

I would not be put off by the general statements about circuit design, which sound very learned but often do not take in the whole picture. A circuit needs to be analyzed in it's entirety to reach a proper conclusion.  One example is K's treatise where he warns of the danger of varying the feed back voltage rather than the reference voltage. Well, perhaps the millions of three terminal regulators in use ought to be withdrawn and redesigned. You simply cannot design by platitudes,

Hi
Are you referring to Kleinstein's message? 
Sorry... but he is right and you are wrong.
Platitudes???
It doesn't matter if a theory is fancy of fashionable, but if it is confirmed by the facts.
And if you change the loop gain the phase margin does change while it generally doesn't if you just change the input signal level (in this case the voltage reference).

Regarding 3 terminal regulators I guess you are referring to the ubiquitous LM317 ?!?!
Well its feedback is fixed, not variable.
You just program a fixed current via the resistor between output and adjust which sets the output voltage via the resistor between adjust and the ground reference (just the plain, old, boring and unfashionable Ohm Law  ).

For that reason you can add a 10µF or even greater capacitor in parallel to that resistor (adj-ground) without affecting stability, while the output capacitance may cause ringing or oscillations.

It is not a case of right or wrong it is a matter of usefulness. You are doing the same thing- going on about theory in isolation without looking at the overall picture. There are many power supplies that use both kinds of feedback.

By the way, not that it matters, but are you sure about the LM317 feedback being fixed?

Can you explain your mention of capacitors- I don't understand what you are saying?

[Kleinstein]

The Forum software seems to have a small bug: depending on the settings (Show most recent posts at the top. under Forum Look and layout), the numbering of the answers is off by 1.  This causes the confusion with the numbering.

The circuit under Reply 37 (with recent post at bottom)  has the output stage in between a low impedance (voltage setting) and high impedance (current setting). These are the 2 main classes I spoke off. The low impedance output stages are like the conventional voltage regulators, while the current setting stages are the low drop ones.  Generally these two types use a different type of compensation, which might make the still missing compensation a real challenge.  Getting the compensation right is about the main difficulty in a lab supply.

The voltage adjustment in the feedback divider is another problem. This changes the loop gain depending on the set voltage, so the regulation would be fast at low set voltage and slower at a higher voltage. There are other supplies working this way, but it's still a weak point. It is relatively easy to solve by having a fixed divider and adjusting the voltage on the other side of the OP.

For a precision current regulation the OPs should get a separate negative supply from the reference: the current through the "GND / ADJ" Pin of the -12 V regulator also flows through the current shunt and depending on the regulator this current depends on the load current and thus the controlling current from the OPs. Besides precision this might also effect stability of the current regulation.

As shown the ripple rejection could be a problem, because the collector side of Q2 would be heavily influenced by the raw voltage. It depends on the details of the compensation however.

The high loop gain due to the transistor Q2 could be a real problem: the OPs kind of need some extra gain, as there supply is smaller than the output range. This also makes the compensation really tricky.

The current regulator works with the OP at its upper supply edge. This can be tricky with some RR OPs. So one has to be careful with the choice of OP here. Especially at some 1-2 V below the upper limit RR OPs may behave a little odd.  The voltage regulator also works at it upper supply - though this might change anyway.

For the relatively low power level in question here (e.g. 0.5 A and up to about 30-35 V raw voltage) I don't think one would need switching of the transformer tap. It could be a good idea with higher power (e.g. more than 1-2 A) though.
At some 15 W of worst case power loss, cooling it not that difficult either and one could getaway without a fan.

[nemail2]

Take a look at my Lab PSU - so far everything working as intended. Tried to make it oscillate but wasn't successful yet.
https://github.com/mamama1/LabPSU_Darlington/blob/master/Hardware/schematics.pdf

It uses a DAC to drive a voltage control opamp at a rather hilarious gain (17.14something) which controls the bases of two BD139/TIP3055 Darlingtons.
The voltage control loop is coupled to the current control loop through a diode and does high-side current measuring with a 100mOhm shunt and a MAX4080 current measuring IC. the current control loop is controlled via a DAC as well. Because the DAC can only output 1,195V at max, I had to up that a bit using another opamp and a gain of 1.713something.

In the end, this PSU goes from 50mV to 20.48V in 5mV steps and from ~10mA (that's pretty much what the fixed minimum load which is at the output consumes) to 4.096A in 1mA steps. Precision is about 0.1% over the whole range, however I didn't verify that thoroughly.

I really like the idea of the high side shunt + the MAX4080, took that from Daves µSupply so credits to him.

[JuanGg]

I can't keep up with the pace this is going at!

With a lab supply designed essentially from scratch it is a good idea to use a simulation (e.g. LTspice) to do the first tests and adjustment if the loop. It is a really powerful tool for this.

Learning LTSpice has been in my to do list for a while, I guess it's time to do it now.

I would not worry about controlling a PSU with a negative supply- it is dead easy and not an issue. ...

Ok. Good to know. Between the lm723 and the custom approach, if the custom one provides more flexibility and better performance, I' ll go for it. I can give the lm723 a try if the other doesn't work out.

JuanGg

The PSU in answer #36 may be suitable for your needs: https://www.eevblog.com/forum/beginners/help-me-design-a-psu/msg1971227/#msg1971227

It is certainly the cheapest and simplest approach.

I could very well start by doing something like that, as I dont have any experience on power supplies, but it doesn't have adjustable current limit and I don't know how would it perform. As I am planing on spending quite some time making a decent case, adding micro-controlled voltage and current displays, I would like to do something more sophisticated. However, I can always upgrade it later.

There is a topic which I have been meaning to mention.

With the transformers arrangement you have with 10V and 15V windings it may be an advantage to have a range switch which selects just one of the windings for PSU low output voltages and both windings for high output voltages. This is common practice on many linear supplies, either manually or automatically. On my linear lab PSUs the switching is automatic by a relay.

The reason for this switching is to reduce the power dissipation in the output power transistors.

I mentioned that in my first post, maybe a relay driven by a comparator or even the microcontroller would do. At a maximum power disipation of around 15W/channel I don't know if it's worth it, but it wouldn't be hard to implement.

Other thing I was thinking of is that I am going to need a +12V rail to power the fans, a possible relay and the Arduino. One way I found to do this is as in the schematic attached (https://electronics.stackexchange.com/questions/153399/using-all-the-taps-of-the-transformer-at-the-same-time), using the two transformers instead of a center tapped one, and a 7812 regulator. This way the fan and anything else would draw current from the 10 V 2 A transformer. A 10V rail would do, getting regulated 12V from 10 V ac does't seem easy.

Take a look at my Lab PSU - so far everything working as intended. Tried to make it oscillate but wasn't successful yet.
https://github.com/mamama1/LabPSU_Darlington/blob/master/Hardware/schematics.pdf

That is a bit complicated-looking. I will have a read of the schematics and documentation when I am able to.

I fould a bridge rectifier I took out of a broken power supply a while ago. I will start making the case and try the raw power supply soon.
Juan

[nemail2]

Take a look at my Lab PSU - so far everything working as intended. Tried to make it oscillate but wasn't successful yet.
https://github.com/mamama1/LabPSU_Darlington/blob/master/Hardware/schematics.pdf

That is a bit complicated-looking. I will have a read of the schematics and documentation when I am able to.

In fact, it is as simple as it gets. I have to admin, however, that the readability of the schematics isn't too good because I didn't draw it very well.
But the concept is really easy and basic. It is a microcontroller controllable PSU with constant voltage and constant current mode. Whether it is very precise or not, simply depends on the parts chosen (high precision resistors for shunt, voltage dividers, feedback loop and high precision opamps).

[spec]

UPDATED: 2018_11_18

The circuit under Reply 37 (with recent post at bottom)  has the output stage in between a low impedance (voltage setting) and high impedance (current setting). These are the 2 main classes I spoke off. The low impedance output stages are like the conventional voltage regulators, while the current setting stages are the low drop ones.  Generally these two types use a different type of compensation, which might make the still missing compensation a real challenge.  Getting the compensation right is about the main difficulty in a lab supply.

I have no idea what you mean here. The voltage and current opamps drive Q2 emitter, and hence the output darlington transistor in  the same way for both voltage and current sense modes. Of course you have to compensate a feedback circuit. That is a standard procedure for all PSUs, in fact all feedback systems.

Incidentally, the handover from the constant voltage mode to the constant current mode is done by a diode AND gate and uses the reducing gm technique to help prevent the two control loops fighting with each other. The way that this is done is one of the features of this architecture.

The voltage adjustment in the feedback divider is another problem. This changes the loop gain depending on the set voltage, so the regulation would be fast at low set voltage and slower at a higher voltage. There are other supplies working this way, but it's still a weak point. It is relatively easy to solve by having a fixed divider and adjusting the voltage on the other side of the OP.

That is not true. The open loop voltage gain will be around 110dB (when the voltage gain of Q2 is reduced to around 1 in the future) whereas the change in feedback ratio amounts to a mere 2.5dB. You would get that much gain change between individual opamps of the same type, and with temperature changes. You also seem to be missing the point that the current through the 12K resistor is constant at 1mA.

I would like to see your calculations that prove that the voltage stabilization would be inadequate. I suggest that you would find the voltage stabilization would be highly accurate, way better than the average designs you see on the net etc.

Incidentally, PSU #27 has the same feedback arrangement, yet you said not a word about that.

For a precision current regulation the OPs should get a separate negative supply from the reference: the current through the "GND / ADJ" Pin of the -12 V regulator also flows through the current shunt and depending on the regulator this current depends on the load current and thus the controlling current from the OPs. Besides precision this might also effect stability of the current regulation.

The three terminal regulator is not specified so how can you make any comments about its performance. Besides which, you greatly exaggerate the situation in a negative way. There are three terminal regulators with a ground current of 10uA. Anyway you are talking in terms of a precision voltage calibration supply, not a lab supply.

As shown the ripple rejection could be a problem, because the collector side of Q2 would be heavily influenced by the raw voltage. It depends on the details of the compensation however.

Of course, just the same as millions of other PSUs that use that architecture. Remember this is not intended to be a precision reference PSU. It is a general purpose lab PSU. Once again can I stress that this is not a complete optimized design. You must know that it is a relatively simple matter to reduce the effects of ripple, if necessary.

But I do agree that this area could be improved.

The high loop gain due to the transistor Q2 could be a real problem: the OPs kind of need some extra gain, as there supply is smaller than the output range. This also makes the compensation really tricky.

Everything is a problem with you. But what you are talking about are normal design procedures to solve so called problems. Besides which, the gain controlling elements are not included in the circuit- it is an outline circuit.

The current regulator works with the OP at its upper supply edge. This can be tricky with some RR OPs. So one has to be careful with the choice of OP here. Especially at some 1-2 V below the upper limit RR OPs may behave a little odd.  The voltage regulator also works at it upper supply - though this might change anyway.

This is just general lecturing. The opamp that is specified, OPA191, is quite happy working at VCC. Incidentally both the current and voltage  opamps are working at VCC. Besides, it would be a simple matter to change the opamp for one of the many over the rail types.

You do not know what the regulator is, so how can you make such a statement. Besides which, it is not a problem that can't be easily fixed. But you are not correct even if you are assuming an LM337, which has a maximum Vin to Vout limit of 40V.

The maximum voltage possible on the negative regulator input is -1* ((25V * 1.414) - (1V +1V)) = -33.35V DC. That in itself is withing the 40V ipV/opV rating of an LM337. But as the LM337 output voltage would be -12V, this means the voltage across the LM337 would be -1 *(33.35V -12V) = -21.35V. So well within an LM337 ipV/opV limit.

For the relatively low power level in question here (e.g. 0.5 A and up to about 30-35 V raw voltage) I don't think one would need switching of the transformer tap. It could be a good idea with higher power (e.g. more than 1-2 A) though.
At some 15 W of worst case power loss, cooling it not that difficult either and one could getaway without a fan.

I am suggesting going for 1A. Besides which, if the facility is there, why not be aware of it as a possibility.

I find your comments completely negative and general. You could make the same sort of critisisems about practically any PSU circuit. And none of your claims are backed by any calculations or figures- just sweeping statements.

You are also picking holes in a circuit which has been categorically stated as work in progress and for demonstrating the principle only.

But as I said before, as you have all this knowledge about PSU design, please lets see your design for his application. Or, if not at least recommend a PSU circuit that meets your criteria and the OPs.

Finally, on a general point, not only are you wasting my time, which could be spent on something more constructive than warding of your unwarranted criticisms of the circuit but, worse sill you will be discouraging the OP who may not have the technical knowledge to establish the degree and relevance of your list of problems as you put it.

After all that, I do appreciate you posting all the above information. It does give me a feel for how you think. And I am sure that your comments are well intended.

[spec]

I can't keep up with the pace this is going at!

Neither can I 

I would not worry about controlling a PSU with a negative supply- it is dead easy and not an issue. ...

Ok. Good to know. Between the lm723 and the custom approach, if the custom one provides more flexibility and better performance, I' ll go for it. I can give the lm723 a try if the other doesn't work out.

Very wise. As mentioned before, once you have the chassis and raw supplies built, tested and the performance established you can pretty much go anyway that you want

The PSU in answer #36 may be suitable for your needs: https://www.eevblog.com/forum/beginners/help-me-design-a-psu/msg1971227/#msg1971227

It is certainly the cheapest and simplest approach.

I could very well start by doing something like that, as I dont have any experience on power supplies, but it doesn't have adjustable current limit and I don't know how would it perform. As I am planing on spending quite some time making a decent case, adding micro-controlled voltage and current displays, I would like to do something more sophisticated. However, I can always upgrade it later.

As you say you have no experience of building PSUs, this simple approach may be a good idea. By the way, you will need a practical circuit with decoupling capacitors etc. You know that even simple power supplies can be problematic. And even three terminal regulators can play up. You would not believe the the things that can go wrong.

There is a topic which I have been meaning to mention.

With the transformers arrangement you have with 10V and 15V windings it may be an advantage to have a range switch which selects just one of the windings for PSU low output voltages and both windings for high output voltages. This is common practice on many linear supplies, either manually or automatically. On my linear lab PSUs the switching is automatic by a relay.

The reason for this switching is to reduce the power dissipation in the output power transistors.

I mentioned that in my first post, maybe a relay driven by a comparator or even the microcontroller would do. At a maximum power dissipation of around 15W/channel I don't know if it's worth it, but it wouldn't be hard to implement.

Other thing I was thinking of is that I am going to need a +12V rail to power the fans, a possible relay and the Arduino. One way I found to do this is as in the schematic attached (https://electronics.stackexchange.com/questions/153399/using-all-the-taps-of-the-transformer-at-the-same-time), using the two transformers instead of a center tapped one, and a 7812 regulator. This way the fan and anything else would draw current from the 10 V 2 A transformer. A 10V rail would do, getting regulated 12V from 10 V ac doesn't seem easy.

Ahh missed that!

Another nice touch, that is easy to do, would be to have the fans temperature controlled, or have you already mentioned that too.

Getting any DC voltage converted to any other DC voltage, within reason, is dirt cheap and easy these days.
But you could probably just use a bridge rectifier and reservoir cap off the 10V 2A winding. If necessary, you could put a power resistor in series with the fan to get the right current.

I am hoping to see the test results of your raw supply and also some pictures

[ArthurDent]

I think we've seen now that there are as many ways to build a power supply as there are posters here. The circuit I found and posted in #27 is an old design but should work fine and replacing the 4.7V zener in the negative supply with a 7905 as Wolfgang suggested should improve stability slightly. The circuit spec designed in #37 with some fine tuning should be better yet (NOTE-a slight oversight, D8 was drawn backwards). 

As much as I love to try to continually improve any circuit, if the power supply you make is to allow you to test other circuits you are more interested in, I'd say decide on one circuit you think is adequate for your needs and after you've built it, get on to constructing the other stuff you want to play with that will be powered by this supply.

[fsr]

Several years ago, i built the "5A Constant Voltage/Constant Current Regulator" circuit in the LM317 datasheet, with some modifications. Of course the 5A and 35v input are bullshit, i went with far lower input voltage and current limits. But it seems to work fairly ok (didn't test it seriously, however). There are probably better, simpler circuits posted here, anyways.

But what i was going to, is that i searched for some power transistors with low thermal resistance and found the MJ15003/MJ15004 complementary pair. They have crazy specs. Thermal resistance 0.70 degrees C Junction-Case. Nice stuff.

[spec]

(NOTE-a slight oversight, D8 was drawn backwards).  [PSU # 37]

Well spotted AD: will correct

[spec]

But what i was going to, is that i searched for some power transistors with low thermal resistance and found the MJ15003/MJ15004 complementary pair. They have crazy specs. Thermal resistance 0.70 degrees C Junction-Case. Nice stuff.

One of my all-time favorites too.

ON (Motorola) Do some pretty good power devices in general. They second source a few of the Toshiba beauties, like the MJL3281A/MJL1302A, which were no longer available at one time.

[JuanGg]

I think we've seen now that there are as many ways to build a power supply as there are posters here. ...

I think I am going with the circuit in #37. I am building this as a tool, but as a learning project as well. When I have something that works for me, I'll use it until I have the need to upgrade it.

Just noticed that the fans on the heatsinks blow air into them, instead of out. If they are inside the case, this would heat everything up (not good). I am thinking on taking the fans out  and flip them so they blow air from vents on the sides/front, across the supply and out of the case, cooling everything. It's a pity because they are nicely fitted to the heatsinks... Or I could have the heatsinks sit at the back of the case, using thermal pads (switching taps to reduce power dissipation). Any ideas on thermal design etc?
Juan

[ArthurDent]

In the 'Beginners' section there is a thread called "Help me design a PSU" and post #17 has another power supply design that has all the basic building blocks we are discussing here. It appears that the idea for the negative supply using 2 caps and 2 diodes was gleaned from the same old schematic I had found and posted here but the rest is different and might be of interest in the power supply you are thinking of building. There aren't a lot of parts values shown but the circuit uses a different approach than what has been discussing. It's always good to see as many variations as you can before making a decision.

https://www.eevblog.com/forum/beginners/help-me-design-a-psu/?action=dlattach;attach=571409;image

[spec]

It appears that the idea for the negative supply using 2 caps and 2 diodes was gleaned from the same old schematic
https://www.eevblog.com/forum/beginners/help-me-design-a-psu/?action=dlattach;attach=571409;image

That technique goes back much further than that... 1920s I would guess 

[ArthurDent]

JuanGg - "Just noticed that the fans on the heatsinks blow air into them, instead of out."

The way a processor heat sink fan is used is it removes the heat from the immediate area of the processor heatsink inside the PC case then they have a large fan or two on the back of the case to remove the heat from the case. If you can design your power supply so the top of the fan/heatsink you're using is against a hole in the case you're using or you have something like a piece of duct work pipe to go from the fan to a hole in the case, that would work to force in/remove the heat from the case. I would look for an aluminum can the correct diameter and cut that to the correct length and attach that if you need small duct work round pipe.

As far as sucking air in or blowing air out I have wrestled with this question for years and concluded it probably doesn't make much difference. One piece of equipment I have, an Array 3711A a max 360V/30A/300W electronic load, solved the problem by using 4 fans, 2 sucking in and 2 blowing out.   

[spec]

I think we've seen now that there are as many ways to build a power supply as there are posters here. ...

I think I am going with the circuit in #37. I am building this as a tool, but as a learning project as well. When I have something that works for me, I'll use it until I have the need to upgrade it.

Just noticed that the fans on the heatsinks blow air into them, instead of out. If they are inside the case, this would heat everything up (not good). I am thinking on taking the fans out  and flip them so they blow air from vents on the sides/front, across the supply and out of the case, cooling everything. It's a pity because they are nicely fitted to the heatsinks... Or I could have the heatsinks sit at the back of the case, using thermal pads (switching taps to reduce power dissipation). Any ideas on thermal design etc?
Juan

The heatsink is fundamental to the success of all power supplies, even very simple low-power types.

I did a rough thermal budget for your PSU, assuming 0V to 20V and 0A to 1A and based on that you will need a heatsink of 2 degC/W or lower if you are using a single output power transistor.

You can do a very simple test to establish the actual thermal resistance of a heatsink. Clamp one of those gold-colored  high-power, wire-wound resistors directly to your heat sink where the transistor will fit. Then dissipate, say 10W in the resistor.

After no less than 15 minutes, measure the room temperature and measure the temperature of the heatsink as close to the resistor clamp area as possible. With the three parameters:  power dissipation, heatsink temperature, room temperature (ambient), we can calculate the thermal resistance of your heatsink.

[spec]

 Online JuanGg,

Just a quick question: on PSU #37 do you want to control the voltage and current by MCU, or just voltage?

I take it that the control signal will be a positive going analog signal originating at 0V: 0V to 5V or 0V to 2V5 say.

[spec]

I think we've seen now that there are as many ways to build a power supply as there are posters here. ...

I think I am going with the circuit in #37. I am building this as a tool, but as a learning project as well. When I have something that works for me, I'll use it until I have the need to upgrade it.

Just noticed that the fans on the heatsinks blow air into them, instead of out. If they are inside the case, this would heat everything up (not good). I am thinking on taking the fans out  and flip them so they blow air from vents on the sides/front, across the supply and out of the case, cooling everything. It's a pity because they are nicely fitted to the heatsinks... Or I could have the heatsinks sit at the back of the case, using thermal pads (switching taps to reduce power dissipation). Any ideas on thermal design etc?
Juan

The heatsink is fundamental to the success of all power supplies, even very simple low-power types.

I did a fag packet thermal budget for your PSU, assuming 0V to 20V and 0A to 1A and based on that you will need a heatsink of 2 degC/W or lower if you are using a single output power transistor.

You can do a very simple test to establish the actual thermal resistance of a heatsink. Clamp one of those gold-colored  high-power, wire-wound resistors directly to your heat sink where the transistor will fit. Then dissipate, say 10W in the resistor.

After no less than 15 minutes, measure the room temperature and measure the temperature of the heatsink as close to the resistor clamp area as possible. With the three parameters:  power dissipation, heatsink temperature, room temperature (ambient), we can calculate the thermal resistance of your heatsink.

As you say, suck or blow, makes little difference, provided the hot air is expelled from the equipment case, the cooling air impinges on the heatsink, and there are no stagnant areas around the heatsink

[JuanGg]

The way a processor heat sink fan is used is it removes the heat from the immediate area of the processor heatsink inside the PC case then they have a large fan or two on the back of the case to remove the heat from the case. If you can design your power supply so the top of the fan/heatsink you're using...

As you say, suck or blow, makes little difference, provided the hot air is expelled from the equipment case, the cooling air impinges on the heatsink, and there are no stagnant areas around the heatsink

Will post a screenshot of the CAD when I have it in a decent state. I am thinking both heatsinks at the back, two holes for the fans directly on the back panel and some vents.

The heatsink is fundamental to the success of all power supplies, even very simple low-power types.

I did a rough thermal budget for your PSU, assuming 0V to 20V and 0A to 1A and based on that you will need a heatsink of 2 degC/W or lower if you are using a single output power transistor.

You can do a very simple test to establish the actual thermal resistance of a heatsink. Clamp one of those gold-colored  high-power, wire-wound resistors directly to your heat sink where the transistor will fit. Then dissipate, say 10W in the resistor.

After no less than 15 minutes, measure the room temperature and measure the temperature of the heatsink as close to the resistor clamp area as possible. With the three parameters:  power dissipation, heatsink temperature, room temperature (ambient), we can calculate the thermal resistance of your heatsink.

Thanks again for the effort!. Not sure about going to 1 A with the 1 A transformer (although it was on a hermetically sealed plastic case, so current rating would be a bit more I suspect...). I don't have any of those resistors nor a way to acurately measure temperature (I have some k thermocouples around, but my DMM does not support it  ). What I do have is a 3d printer with a heated bed that i can bring up to 95 ºC. That may work.

However, I found an Intel thermal desing guide recomending this heatsink for aplications requiring about 0.6 ºC /W case to ambient. (here it is: http://download.intel.com/design/intarch/designgd/27370403.pdf , there is a graph in page 28 and a table with the heatsink on the following page, assuming ambient temperature of 42 ºC). (With the fan running I would assume). It should be more than adecuate.

Online JuanGg,

Just a quick question: on PSU #37 do you want to control the voltage and current by MCU, or just voltage?

I take it that the control signal will be a positive going analog signal originating at 0V: 0V to 5V or 0V to 2V5 say.

I would like to control both if posible, so I can set both say using an encoder/keypad. I intended to use a filtered 12 bit PWM, from 0 to 5 V. I did this on my electronic load and it worked quite well (more details here: https://www.eevblog.com/forum/projects/arduino-based-electronic-load). If I need to, maybe an i2c DAC with a voltage reference.

Juan

[spec]

All sounds good. You have a handle on the heatsink.

You may be correct about the transformer, but on paper it should do 20V at 1A and that is what I suggest we should aim for. If the transformer won't cut it, that is just bad luck, but nothing lost.

Thanks for info about the V and I control inputs.

Just to restate then, the signal will be analog and ground referenced. 0V input will be zero volts and zero amps output.

I don't care too much about the maximum input voltage (that can be changed at any time in the future) but I will work with 5V in both cases.

We have not discussed input resistance. Would around 100k Ohms be suitable?

[JuanGg]

All sounds good. You have a handle on the heatsink.

You may be correct about the transformer, ...

Perfect.

We have not discussed input resistance. Would around 100k Ohms be suitable?

No idea on input resistance on power supplies. Looked for it on the internet but no luck. How can it be calculated?
Juan

[spec]

We have not discussed input resistance. Would around 100k Ohms be suitable?

No idea on input resistance on power supplies. Looked for it on the internet but no luck. How can it be calculated?
Juan

Think of the input resistance of a scope (1M Ohm), Audio power amplifier (10k Ohm), Muiltimeter  set to 10V DC range (1G Ohm). The only criteria is, will your analog signal drive the input resistance.

Don't worry about it, 100K will be fine. And if not it can be changed anyway. 

[JuanGg]

Think of the input resistance of a scope (1M Ohm), Audio power amplifier (10k Ohm), Muiltimeter  set to 10V DC range (1G Ohm). The only criteria is, will your analog signal drive the input resistance.

Don't worry about it, 100K will be fine. And if not it can be changed anyway.

Ok. Sorry, just realized now. Input resistance of the control signals . Alright. As they will probably be driven from an op amp buffer, 100k should be more than enough.

[spec]

Think of the input resistance of a scope (1M Ohm), Audio power amplifier (10k Ohm), Muiltimeter  set to 10V DC range (1G Ohm). The only criteria is, will your analog signal drive the input resistance.

Don't worry about it, 100K will be fine. And if not it can be changed anyway.

Ok. Sorry, just realized now. Input resistance of the control signals . Alright. As they will probably be driven from an op amp buffer, 100k should be more than enough.

All good

[JuanGg]

Haven't been able to do much since last time I posted (exam week!, and more to come), here is what I have on the case so far. Hopefully I will finish designing the back panel and print it this weekend. I will leave the front panel to last, when meters and binding posts arrive. Attached are a couple of screenshots, one showing the airflow. Excuse the crudity of the pictures. There is yet a lot of work to be done.