Electronics > Beginners
Looking for 30V 5A schematic
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edalsan:
That's a good option to repurpose a second hand power supply updating it's circuit b_force, but as Kleinstein pointed out I'm already the happy owner of those two transformers, and as far as I can remember they were not so cheap.

For the power supply seen in last Dave's video, there's a version capable of delivering 50V 5A, called DPS5005, which costs around 25€ in aliexpress (there's even one version with USB connection to computer). So, as it's a DC-DC converter I will need to set up the bridge rectifier and filter capacitors before powering this little module. I've been checking the ripple and specs and people are pretty impressed with the performance of the power supply. I'm considering that option as a good one.

Have been checking also for 48V 5A power supplies (the switching metal enclosed ones) and they cost around 18-20€... so I'm not either sure about throwing my fellow transformers or go ahead with them. But so far provide them with a rectifier bridge + caps and add the 50V5A power supply, seems a good option. What do you think about series/paralleling two of those 50V 5A power supplies? I think that if each one uses it's own transformer there will be no problem.



Zero999:

--- Quote from: edalsan on October 06, 2017, 05:08:22 pm ---By usin a 3 pins regulator I know I will lose current limiting and I think I wanna have it to control and check any flaws in circuit designs and avoid burning them. LM317 built around center tapped transformer looks good, but I was looking maybe for something bit more "regulated".

--- End quote ---
Adding current limiting is fairly easy. This one effectively drives the pass transistors with a multiple of the LM317's current, it will reduce the current, when the IC current limits. The voltage across R3 is dependant on the current flowing into the LM317. It sets the voltage on Tr1's base, which acts as a common emitter amplifier. The voltage across R5 will be equal to the voltage across R3 minus the base-emitter drop of Tr1. The voltage across R4 is roughly double, the voltage across R5 and sets the voltage on Tr2's base which acts as a current source. The higher the base voltage on Tr2, the higher the voltage across RE, thus the current. In short the current through R3, controls the current through RE.



The centre tapped transformer design is regulated. Only the input voltage to the LM317 is changed, depending on the output voltage.
Does the current limit need to be adjustable? If so, how accurate does it need to be? The LM723 only has rough current limiting.
edalsan:

--- Quote from: Hero999 on October 06, 2017, 11:38:07 pm ---Does the current limit need to be adjustable? If so, how accurate does it need to be? The LM723 only has rough current limiting.

--- End quote ---

Yes, I would like to adjust the current limit, the schematic looks great, but I'm not sure if that adjust limit could be done in your schematic as both resistors are dependant.

Regarding to accuracy I'll be happy moving between 0.1A steps, but if 0.01A is possible it would definetely be much better. The LM723 seems capable of providing that, that's why I thought about it at the beginning, but now looking into those 50V 5A modules it makes me wonder if wouldn't be better to just get one of those off the shelf solutions + rectifier bridge + filter cap.
Kleinstein:
A linear lab supply and those cheap  SMPS modules will have different properties. So it depends on what you need it for.

The SMPS modules have noticeable higher frequency ripple, though it does not seem to be bad for a SMPS it is still much higher than with a linear regulator (like 50 mV (at a few 100 kHZ) compared to maybe 50 µV at 50/100 Hz).

The regulation of a SMPS is usually slower than with a linear regulator. So things like the voltage drop on transients will be larger ( longer. Still those SMPS modules look like they are reasonably well tuned - so only moderate overshoot for the CC-CV transition. With a self build linear supply the tricky part can be finding the right balance between fast reaction and not to much overshoot / ringing.

The SMPS modules can deliver more current. At the full voltage there will be not much difference - so the 5A (Ac) transformer would set a limit at about 3 A (DC), but at lower voltage SMPS version could deliver more current as the input side current will be lower. So one could get the 5 A from about 20 V down. The linear supply is always limited by the transformer current. It's only a version with tab switching that might deliver a little more current (e.g. 5A) at lower voltage, like below about 15 V, depending on how it is made.

The Linear regulator will need a large heat sink and likely a fan. So it is not a small solution.

The linear regulator is usually a good idea for low power and sensitive circuits. The SMPS version is more something for high power loads, that are not sensitive (e.g. motor, lights, computer that has a switched more regulator anyway).

It can still make sense to use a transformer with one of these SMPS modules. The switched mode supplies (e.g. for 48 V , or maybe better a little lower as Chinese specs on such low cost parts are often stretching the envelope to the corners) have extra common mode noise. They are either grounded (thus the supply will not be isolated from ground) or have common mode caps (class Y) and thus have a voltage relative to ground. The conventional transformer provides a better isolation and suppression of common mode noise.

The current limit in the basic 723 circuit is only crude and usually not very temperature stable. Its adjustable, but depending on how it is done it gets less useful below something like 1-10% of the full range.

The LM317 based circuits have a few additional problems: the stability with a difficult load can be tricky. The drop out is rather high (thus getting 30 V out might need larger filter caps and thus lower power factor and more load to the transformer). When combining with external power transistors the 723 is usually the better solution than the LM317, at least it is made for this purpose.
Zero999:

--- Quote from: edalsan on October 07, 2017, 12:38:56 am ---
--- Quote from: Hero999 on October 06, 2017, 11:38:07 pm ---Does the current limit need to be adjustable? If so, how accurate does it need to be? The LM723 only has rough current limiting.

--- End quote ---

Yes, I would like to adjust the current limit, the schematic looks great, but I'm not sure if that adjust limit could be done in your schematic as both resistors are dependant.
--- End quote ---
It's possible to have adjustable current limiting by adding a low value (10R to 50R) potentiometer across R3 and connecting the base to the wiper, but the minimum current limit will be determined by the LM317's internal circuitry, which will be more than 1.5A. The LM317L could be used to take it down to 100mA, but it could still be easily double that. Still no good, if you want to limit the current to under 200mA.


--- Quote ---Regarding to accuracy I'll be happy moving between 0.1A steps, but if 0.01A is possible it would definetely be much better. The LM723 seems capable of providing that, that's why I thought about it at the beginning, but now looking into those 50V 5A modules it makes me wonder if wouldn't be better to just get one of those off the shelf solutions + rectifier bridge + filter cap.

--- End quote ---
The LM723 and LM317 are analogue solutions, so they have a theoretically indefinite number of steps. The trouble is, the current limit is not accurately controlled and is purely there to protect the driver transistor from overheating. It will depend other factors, such as the temperature and input-output voltage differential.

I have no personal experience with those modules and haven't seen any videos. If cost is a consideration, then it might be better to buy a switched mode power supply and power it off that.
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