Author Topic: Linear lab power supply  (Read 21841 times)

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Offline coromonadalix

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Re: Linear lab power supply
« Reply #150 on: December 14, 2018, 07:28:42 pm »
@Xavier60    Your montage is bad 

You use an aluminum plate for your montage and put it back on an other "big" heatsink with an huge thermal pad, and you say you dont put an thermal pad or any micas on the power transistor ??

You want to avoid thermal loss and you're creating it the other way  loll
 

Offline not1xor1

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Re: Linear lab power supply
« Reply #151 on: December 14, 2018, 08:38:14 pm »
@Xavier60    Your montage is bad 

You use an aluminum plate for your montage and put it back on an other "big" heatsink with an huge thermal pad, and you say you dont put an thermal pad or any micas on the power transistor ??

You want to avoid thermal loss and you're creating it the other way  loll

it is anyway better than a transistor directly mounted to the heatsink via thermal pad as in this case the thermal pad is much larger.
I would rather object regarding the single screw for the power transistor since that usually do not ensure a proper contact with the heatsink.
 
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Online xavier60

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Re: Linear lab power supply
« Reply #152 on: December 14, 2018, 09:33:35 pm »
@Xavier60   
Please delete your post. Or correct it.
« Last Edit: December 14, 2018, 09:37:20 pm by xavier60 »
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Offline JuanGg

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Re: Linear lab power supply
« Reply #153 on: December 15, 2018, 02:14:25 am »
Quote
hi,
in real life you don't need tap switching.I don't do it even for 5 amps/0-27Vdc (limits of my 2'nd power supply).So I review my own theory, just cut off your budget and pain and series the secondaries, do the regulation and you're done.don't forget measuring current on low side will put problems for a beginner more than adding 4 $ with high side sensing, the right way to do it in your case.
If you do tap switch for future developement, that's another thing.
Anyway with your current/voltage specs you are well within SOA, you can do tap switch only for echology reasons, but buying relay+diodes+... isn't echological no more, it's just like the big lie buying electrical car will do good to the ozone :)
I just tried to simplify your design and get you focused on important stuff like keep it simple and get the voltage and current measurement simply and the PS stable.tap switch implies further decisional voltage comparator, watching what happens when commuting if you forgot to remove load you'll have to think what you do with the spike when you switch.imho too complicate for almost zero result.
tap switching method is for amps not for mAmps.But what the hell, I'm talking too much, as I said it before, do as you like
pierre

You are right, I'll do away with tap switching, just wanted to try to as I had two transformers and some relays laying around, but as you said it's not worth it. Better keep it simple. But I'll have to look for a way of not getting control and fan current going through the 15V transistor, so I can have at least 500 mA output current.

Quote
The power transistor is mounted in direct contact with the heat sink bracket to reduce thermal resistance.
Although this power supply circuit responds to output short circuits much faster than most others, the LM358 takes 20us to take control of output current which peaks to 20 amps.  The TLC072 responds in 2us.
The 3 BD137 transistors from left to right are 8V series pass, Darlington 1st transistor and constant current  load.
Ill post an updated schematic after I do more testing. At this stage I have made only minor changes.

Not many people are willing to do what you are doing. Thank you. I am looking foward to seeing the revised schematic and how you are getting the 8V rail.

    Juan

Online xavier60

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Re: Linear lab power supply
« Reply #154 on: December 15, 2018, 02:11:23 pm »
The VCC1 8V regulator has a lot of components, it needs to have good performance,  mainly a small dropout voltage which is 0.8V and tolerance to high input voltage.
Regulation will understandably be lost if the unregulated supply drops below 8.8V.
 The control circuit using the TLC072 also works properly with VCC1 set to 7V.
R23 is just to allow the current to range to include zero in case the CC op-amps input offset happens to be a particular polarity.
If D3 is replaced with a silicon diode, there will be an increase in controlled overshoot current when the output terminals are short circuited.
If the preload circuit is replaced with a resistor, transient response will suffer in situations where B-E of Q5 is not forward biased.
In its present state, I estimate the circuit is safe to work up to 3A and 30V with an adequate heatsink and tap changing.
The next step will be to check the performance with paralleled power transistors. I have also ordered some Darlingtons for testing.

Any power supply design can become dysfunctional if not laid out properly.

* C8 changed to 330pF
« Last Edit: December 17, 2018, 09:24:01 am by xavier60 »
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Online xavier60

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Re: Linear lab power supply
« Reply #155 on: December 15, 2018, 04:35:02 pm »
The purpose of Q4 is to allow the CC op-amp to operate open loop until it takes control of the power supply's output.  At this point Q4 turns on, switching the compensating capacitor into circuit.
The waveform shows the shunt's voltage drop at the point the output is shorted and the current limit set to 2 amps.
The large initial spike is because of the 2us it takes the op-amp to respond. After that is an intentionally controlled overshoot.
If Q4 is jumpered out, the current will peak to about 20 amps and take hundreds of microseconds to settle.
« Last Edit: December 15, 2018, 05:04:17 pm by xavier60 »
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Offline JuanGg

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Re: Linear lab power supply
« Reply #156 on: December 16, 2018, 06:34:23 am »
Quote
The VCC1 8V regulator has a lot of components, it needs to have good performance,  mainly a small dropout voltage which is 0.8V and tolerance to high input voltage.
Regulation will understandably be lost if the unregulated supply drops below 8.8V.
 The control circuit using the TLC072 also works properly with VCC1 set to 7V. ...

Ok. Thank you. I will make a bill of materials and order the exact same parts, including the op-amp, so I can have a go myself. Any part number for the voltage reference? I'll order some ina196s just in case we want to try out high side sensing. Any other component suggestions that may come in handy are welcome.
I suppose adjusting the current range is a matter of resistor values.
I was thinking of mounting the perfboard as attached (not many options given the space I have to work with) Rectifiers and filter caps will go between the transformers, no tap switching.
Will a well-laid out perfboard do? If not I can make a pcb.
Also, will the 8 V rail be able to handle a couple hundred milliamps, or would it be better to have a separate regulator for the fan, displays and arduino?

    Thanks again, Juan
« Last Edit: December 16, 2018, 08:01:57 am by JuanGg »
 

Online Kleinstein

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Re: Linear lab power supply
« Reply #157 on: December 16, 2018, 08:43:06 am »
The reference could be something like TL431, at least the circuit looks like it is made for this reference. Changing the reference part is likely one of the easier parts. Another interesting (low noise) reference would be the LM329.

It may be tricky to use the VCC1 also use for other uses, as the current would also flow through the shunt. So about the only part one might consider running from here is something like an DAC if needed.  One may even consider a different supply (negative side) for the OP.

Alone from the noise and interference, I would use a separate regulator for the µC and display.

Perf board should be OK, if one takes some care with star ground and similar. But this is also important for a board.
If the filter caps are separate, it may take an extra small capacitor on the perf board. 

The shunt should of cause higher value for the lower current at some 500 mA.
 

Offline JuanGg

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Re: Linear lab power supply
« Reply #158 on: December 16, 2018, 09:55:43 am »
The reference could be something like TL431, at least the circuit looks like it is made for this reference. Changing the reference part is likely one of the easier parts. Another interesting (low noise) reference would be the LM329.

It may be tricky to use the VCC1 also use for other uses, as the current would also flow through the shunt. So about the only part one might consider running from here is something like an DAC if needed.  One may even consider a different supply (negative side) for the OP.

Alone from the noise and interference, I would use a separate regulator for the µC and display.

Perf board should be OK, if one takes some care with star ground and similar. But this is also important for a board.
If the filter caps are separate, it may take an extra small capacitor on the perf board. 

The shunt should of cause higher value for the lower current at some 500 mA.


Thank you. That's what I was thinking, a PWM driven fan and multiplexed 7-segments would be no good for the control rail. I'll add a separate regulator, before the shunt. The micro still has to be referenced after the shunt I'm afraid. Displays take 5V, clk and data, I don't know if they can be driven with the micro referenced to the other side of the shunt...will have to test it.
I have some 0R1 resistors around, I can parallel more or less to get different shunt values. Still looking at high side current sensing, which would certainly make things simpler.

    Juan

Offline nick_d

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Re: Linear lab power supply
« Reply #159 on: December 16, 2018, 10:57:23 am »
Regarding obsolete parts available on Ebay, many are fakes. Do not trust them. If you have a WORKING circuit and can drop in the chip then fine, it will work or not (but may prove unreliable over time). On the other hand if you are developing then you can end up implementing elaborate workarounds for non-problems due to weird characteristics of the fake chips and wasting enormous development time. Don't do it.
cheers, Nick
 

Online xavier60

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Re: Linear lab power supply
« Reply #160 on: December 16, 2018, 12:14:27 pm »
The reference could be something like TL431, at least the circuit looks like it is made for this reference. Changing the reference part is likely one of the easier parts. Another interesting (low noise) reference would be the LM329.

It may be tricky to use the VCC1 also use for other uses, as the current would also flow through the shunt. So about the only part one might consider running from here is something like an DAC if needed.  One may even consider a different supply (negative side) for the OP.

Alone from the noise and interference, I would use a separate regulator for the µC and display.

Perf board should be OK, if one takes some care with star ground and similar. But this is also important for a board.
If the filter caps are separate, it may take an extra small capacitor on the perf board. 

The shunt should of cause higher value for the lower current at some 500 mA.


Thank you. That's what I was thinking, a PWM driven fan and multiplexed 7-segments would be no good for the control rail. I'll add a separate regulator, before the shunt. The micro still has to be referenced after the shunt I'm afraid. Displays take 5V, clk and data, I don't know if they can be driven with the micro referenced to the other side of the shunt...will have to test it.
I have some 0R1 resistors around, I can parallel more or less to get different shunt values. Still looking at high side current sensing, which would certainly make things simpler.

    Juan
Having circuitry on both sides of the shunt could be made to work with a lot of thought because the shunt voltage can be kept low enough to not affect even TTL, but transients should be expected that would need to be filtered out which means using LC filtering. It gets messy.
An independent supply  from a separate winding or transformer is best.
I have tested the design with a 0R1 shunt if needed, no problem. I would prefer 0R05 or less.
Perf board will be fine if the guideline in post #126 is followed. The most important detail is the single connection between the - output and the ground of the control circuit. I have represented this in the schematic also.
There is an advantage to using the same type parts in the schematic, mainly the transistors because it has all been tested.
 The voltage divider and current set resistor,R12,  can be changed to suit.
« Last Edit: December 16, 2018, 12:19:36 pm by xavier60 »
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Online xavier60

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Re: Linear lab power supply
« Reply #161 on: December 16, 2018, 12:29:18 pm »
The position of the shunt resistors on my PCB is a compromise. Ideally they should be in the middle of the PCB, but they would have obscured access to the transistors mounted on the heat sink bracket.
Yes, U2 is a TL431CLP. Ill update the schematic in post #154.
« Last Edit: December 16, 2018, 12:53:23 pm by xavier60 »
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Offline JuanGg

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Re: Linear lab power supply
« Reply #162 on: December 17, 2018, 03:29:13 am »
Quote
Having circuitry on both sides of the shunt could be made to work with a lot of thought because the shunt voltage can be kept low enough to not affect even TTL, but transients should be expected that would need to be filtered out which means using LC filtering. It gets messy.
An independent supply  from a separate winding or transformer is best.
I have tested the design with a 0R1 shunt if needed, no problem. I would prefer 0R05 or less.
Perf board will be fine if the guideline in post #126 is followed. The most important detail is the single connection between the - output and the ground of the control circuit. I have represented this in the schematic also.
There is an advantage to using the same type parts in the schematic, mainly the transistors because it has all been tested.
 The voltage divider and current set resistor,R12,  can be changed to suit.

Ok, I'll try to look for more transformers to power the micro, fans and displays.
I have attached a bill of materials for your chematic, I hope I haven't missed anything. Couldn't find some of the transistors, added some similar ones: BD135 instead of BD137 and BD559 instead of BD558, I've gone through the datasheets and the characteristics are almost the same. I'll order this tonight, as well as some smd to dip adapters and misc stuff. It's going to take at least a week or two, I hope I can get something done this christmas.
I'll prototype the thing on a perfboard, but at the prices PCB services are running at, it may be an option to have some made later on (as I'm planing on having two channels... we'll see)
    Juan
« Last Edit: December 18, 2018, 02:30:20 am by JuanGg »
 

Online xavier60

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Re: Linear lab power supply
« Reply #163 on: December 17, 2018, 08:49:17 am »

Ok, I'll try to look for more transformers to power the micro, fans and displays.
I have attached a bill of materials for your chematic, I hope I haven't missed anything. Couldn't find some of the transistors, added some similar ones: BD135 instead of BD137 and BD559 instead of BD558, I've gone through the datasheets and the characteristics are almost the same. I'll order this tonight, as well as some smd to dip adapters and misc stuff. It's going to take at least a week or two, I hope I can get something done this christmas.
I'll prototype the thing on a perfboard, but at the prices PCB services are running at, it may be an option to have some made later on (as I'm planing on having two channels... we'll see)
    Juan
Although the BD135 will be fine, try to keep all transistors above 60V where necessary, Is BD139 available instead?
There should be no BD558, only BC558, maybe a misprint. Although I haven't used the transistor types consistently, use BC546, BC556 where the voltage is high and BC548,  Bc558 for low voltage positions or just about any BC series at hand.
The BC546, BC556, BC547, BC557, BC548, BC558 are very commonly used general purpose signal transistors. You should take the time to get familiar with the data sheets. Take note of the hFE or Current Gain grading. If you get the ungraded versions, they should be tested but usually have plenty of current gain.
I have ordered more TIP35C transistors on Aliexpress, very cheap. Ill be checking to see if they are fakes.
I tested with an actual mains transformer last night. There was a few millivolts of ripple at the output. I have changed C8 to 330pF
« Last Edit: December 17, 2018, 09:00:59 am by xavier60 »
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Online xavier60

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Re: Linear lab power supply
« Reply #164 on: December 17, 2018, 11:19:21 pm »

I have attached a bill of materials for your chematic, I hope I haven't missed anything. Couldn't find some of the transistors,
[/quote]
The XLsX file won't open for me, trying 3 different viewers.
Can you upload a text file?
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Offline JuanGg

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Re: Linear lab power supply
« Reply #165 on: December 18, 2018, 02:44:21 am »

Ok, I'll try to look for more transformers to power the micro, fans and displays.
I have attached a bill of materials for your chematic, I hope I haven't missed anything. Couldn't find some of the transistors, added some similar ones: BD135 instead of BD137 and BD559 instead of BD558, I've gone through the datasheets and the characteristics are almost the same. I'll order this tonight, as well as some smd to dip adapters and misc stuff. It's going to take at least a week or two, I hope I can get something done this christmas.
I'll prototype the thing on a perfboard, but at the prices PCB services are running at, it may be an option to have some made later on (as I'm planing on having two channels... we'll see)
    Juan
Although the BD135 will be fine, try to keep all transistors above 60V where necessary, Is BD139 available instead?
There should be no BD558, only BC558, maybe a misprint. Although I haven't used the transistor types consistently, use BC546, BC556 where the voltage is high and BC548,  Bc558 for low voltage positions or just about any BC series at hand.
The BC546, BC556, BC547, BC557, BC548, BC558 are very commonly used general purpose signal transistors. You should take the time to get familiar with the data sheets. Take note of the hFE or Current Gain grading. If you get the ungraded versions, they should be tested but usually have plenty of current gain.
I have ordered more TIP35C transistors on Aliexpress, very cheap. Ill be checking to see if they are fakes.
I tested with an actual mains transformer last night. There was a few millivolts of ripple at the output. I have changed C8 to 330pF

I ordered already, from "LCSC", let's see how it works out. Yes, BD139 was available  :palm:. Oh well. And yes, BD558 was a typing error.
I have some BC547 and BC557 transistors around. Anyway, I got ten signal transistors of each kind (minimum order quantity) and four TIP3455C so I should have plenty to play with. I'll eventually end up using them I suppose. While they arrive (at least a week or two), I'll see if I can finish the transistor labs on "Learning the art of electronics" and go through the datasheets so I can get a better understanding.

Quote
I have attached a bill of materials for your chematic, I hope I haven't missed anything. Couldn't find some of the transistors,
The XLsX file won't open for me, trying 3 different viewers.
Can you upload a text file?
Uploaded it to the original post. :palm: I should have thought of that.
    Juan

Online xavier60

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Re: Linear lab power supply
« Reply #166 on: December 18, 2018, 04:40:17 am »
I get no search results for " TIP3455C "
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Offline JuanGg

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Re: Linear lab power supply
« Reply #167 on: December 18, 2018, 05:27:00 am »
I get no search results for " TIP3455C "

This thing manages to mess everything up. Here is a screenshot and a pdf. Sorry for the inconvenience, I should be the one dealing with this stuff.

    Juan

Online xavier60

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Re: Linear lab power supply
« Reply #168 on: December 18, 2018, 03:15:58 pm »
The top trace represents a 0 to 6 amp ramped load on the regulator's output.
The bottom trace was at the Base of Q2. The transconductance is very linear.
The gm of 40 is a bit higher than I would have preferred.
I did notice a problem, some voltage fluctuation that is present all the way up to the Base of the TIP35C, but not at the Emitter.
It could mean that the transistor is failing. It is a spare that was left over from a repair job.
I bought them on Aliexpress and are ST branded. I hope the ones on order are ok.
« Last Edit: December 18, 2018, 03:48:27 pm by xavier60 »
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Offline JuanGg

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Re: Linear lab power supply
« Reply #169 on: December 18, 2018, 06:14:15 pm »
The top trace represents a 0 to 6 amp ramped load on the regulator's output.
The bottom trace was at the Base of Q2. The transconductance is very linear.
The gm of 40 is a bit higher than I would have preferred.
I did notice a problem, some voltage fluctuation that is present all the way up to the Base of the TIP35C, but not at the Emitter.
It could mean that the transistor is failing. It is a spare that was left over from a repair job.
I bought them on Aliexpress and are ST branded. I hope the ones on order are ok.

Are you putting 6A through this thing?
The ones I ordered are ST as well.
    Juan

Offline nick_d

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Re: Linear lab power supply
« Reply #170 on: December 18, 2018, 06:21:09 pm »
No way the AliXpress ones are actual ST. They will be fakes definitely. That does not mean they are no good. Likely they will have similar characteristics to what they are marked as, but not always. See my earlier post and do not waste valuable development time tracking down possible weird characteristics of the fakes. If it works great -- if it doesn't, swap in the Digikey or other genuine part before proceeding.
cheers, Nick
PS I know this from bitter experience... Hello fake LM386...
 

Online xavier60

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Re: Linear lab power supply
« Reply #171 on: December 18, 2018, 08:19:35 pm »
The TIP35C transistors I have ordered are supposed to be the newer TO-247 types. It would be possible to test the transistor's spec somehow, but how does one check for poor quality bonding?
The control circuit  can easily drive the power transistor to  high currents, but the transistor needs to be kept cool and in its Safe Operating Area.
I have increased the current carrying capacity  of the PCB by placing 0.8mm solid core copper wire along the ground track.
The BD137, TIP35C Darlington has a current gain of over 10,000. There is only about 0.7ma through R4.
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« Last Edit: December 18, 2018, 09:30:35 pm by xavier60 »
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Re: Linear lab power supply
« Reply #173 on: December 20, 2018, 08:29:27 pm »
The top trace represents a 0 to 6 amp ramped load on the regulator's output.
The bottom trace was at the Base of Q2. The transconductance is very linear.
The gm of 40 is a bit higher than I would have preferred.
I did notice a problem, some voltage fluctuation that is present all the way up to the Base of the TIP35C, but not at the Emitter.
It could mean that the transistor is failing. It is a spare that was left over from a repair job.
I bought them on Aliexpress and are ST branded. I hope the ones on order are ok.

Are you putting 6A through this thing?
The ones I ordered are ST as well.
    Juan
Let me know when you have received the parts, I will update the schematic.
I have made a few minor changes. The gm has been reduced to 20.
There are reasons for the current overshoot being allowed to occur when the output is shorted.
I have added a method for easily setting the overshoot.
I have not introduced any new part values.
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Offline JuanGg

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Re: Linear lab power supply
« Reply #174 on: December 21, 2018, 02:13:52 am »
The top trace represents a 0 to 6 amp ramped load on the regulator's output.
The bottom trace was at the Base of Q2. The transconductance is very linear.
The gm of 40 is a bit higher than I would have preferred.
I did notice a problem, some voltage fluctuation that is present all the way up to the Base of the TIP35C, but not at the Emitter.
It could mean that the transistor is failing. It is a spare that was left over from a repair job.
I bought them on Aliexpress and are ST branded. I hope the ones on order are ok.

Are you putting 6A through this thing?
The ones I ordered are ST as well.
    Juan
Let me know when you have received the parts, I will update the schematic.
I have made a few minor changes. The gm has been reduced to 20.
There are reasons for the current overshoot being allowed to occur when the output is shorted.
I have added a method for easily setting the overshoot.
I have not introduced any new part values.

I should receive them within a week from now. I'll let you know. Thanks again. I bought a few of each just in case.

I'll start winter break tomorrow, so I'll have more time. I can start planning the perfboard layout or continue working on the case. I'll also look for transformers for the micro and displays.
    Juan
« Last Edit: December 21, 2018, 02:28:04 am by JuanGg »
 


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