Linear regulator input rectifier / capacitance multiplier - Page 1
Forum section: electronics / beginners

author: UltraRC
author_rank: Contributor
timestamp: October 04, 2020, 11:22:46 pm
content: Hello, I am currently working on a power supply project and i'm going to tackle it in three steps. First the input stage, the regulator stage and then the output stage.Logically I am starting with the input stage which is what my question(s) is about.Specifications i'm aiming for:- 1 ADC max current- Roughly 0-12 VDC variable (I'll tackle this when I get to the regulator stage)- Linear regulationThe components i'm working with are a 30VA toroidal transformer which has two output coils which I may put in series or parallel depending on how much voltage overhead I have after the input and regulator stages. I'm using a salvaged gbu6005 recitifier.Most of my question is about the ripple smoothing, i've done some spice sumulations and i'm not convinced that I like the brute-force / add more capacitance approach to smoothing so I tried a simulaton (image below) with a "capacitance multiplier" with a smaller value filter capacitor (4,400uF) on the output of the rectifier and a low pass filter / emitter follower circuit with a darlington arrangment with a BC547B and a 2n3055. The capacitance multiplier left around 70mV pk-pk which i'm sure the regulator can remove most of. I simulated the output of the transformer as 15*sqrt(2) before the rectifier.Is it worth the trouble of adding all the extra components to make the capacitance multiplier at the cost of ~2V? Or should I just add more caps? The other option is to wire the two transformer secondary coils in series and half the max output current to 0.5A as I probably don't need 1A.Thanks, Reuben.

author: SilverSolder
author_rank: Super Contributor
timestamp: October 05, 2020, 12:39:29 pm
content: I like the idea of using a capacitance multiplier there, seems worth a play! - I guess the main downside is increased complexity compared to just piling on the capacitors.There will be some losses in the pass transistor for this - but the losses would have to occur in the regulator if they didn't occur in the capacitance multiplier, so overall it doesn't immediately seem like it would be less efficient than an all-caps solution...

author: Kleinstein
author_rank: Super Contributor
timestamp: October 05, 2020, 01:02:09 pm
content: Linear regulators are usually very good at low frequency, like the 100 Hz ripple. So there is limited use to the extra capacitance multiplier. If parts of the circuit are sensitive, one may consider separate filter caps / diodes - this may allow for reduced ripple and a little more headroom for the regulator part.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: October 05, 2020, 01:18:03 pm
content: Kleinstein just beat me to it...Since you're going to regulate the voltage on the output anyway, adding a capacitance multiplier in between only adds 2 diode drops with no benefical result.If you add an opamp, which compares a reference voltage, with a voltage divider on the output and then use it to regulate the base of the first transistor of your capacitor multiplier, you've already got an adjustable power supply.An Idea for later:Think about adding a switching pre-regulator. This can be as simple as a EUR2 buck module from China, with some small modifications.Such a module is probably a lot cheaper then the big chunk of aluminium heatsink that you would need otherwise.I also applaud your modest goal of building a 0 to 12V at 1A power supply. This is plenty of power for most applications these days, and it's a lot more manageable then a 30V 3A power supply which is some default from another era when electronics were much more power hungry.If you can get the voltage a bit higher though that would increase it's usefulness. Opamps and such are often used with + and - 15V (or +/- 18V).If you build two of your power supplies (The second iteration is always better of course) then you can simply put the power supplies in series, to get a symmetrical positive and negative voltage. The main thing here is that there may only one connection between the two power supplies. So you can connect the positive output of one supply, and connect it to the GND of the other as long as it is the only connection between these two power supplies.

author: ledtester
author_rank: Super Contributor
timestamp: October 05, 2020, 01:39:03 pm
content: Quote from: Doctorandus_P on October 05, 2020, 01:18:03 pmAre you talking about one of those LM2596 modules such as:https://www.amazon.com/LM2596-Converter-Module-Supply-1-23V-30V/dp/B008BHBEE0What would the modifications be?

author: Kleinstein
author_rank: Super Contributor
timestamp: October 05, 2020, 01:56:49 pm
content: To make it a pre-regulator following the final output one would have to change the feedback part to react to the difference over the final linear stage. This often uses an additional (PNP) transistor with the emitter with series resistor towards the SMPS output and the base from the final output.A tricky part here is that this tends to add gain and one may also have an eye on the compensation.A little simpler than using a switched mode regulator would be using 2 raw voltages - this can be done with the split transformer an a single rectifier, but 2 filter caps. The cross over between the 2 voltages can be instant, like in a class G amplifier.

author: TimFox
author_rank: Super Contributor
timestamp: October 05, 2020, 03:19:15 pm
content: If the power supply is in a separate enclosure, to minimize interference from the AC mains and power transformer, then it could be reasonable to put a capacitance multiplier in the power supply box, to reduce the AC component on the voltage sent to the amplifier box, with a linear regulator in the amplifier box to stabilize the voltage at the load.

author: SilverSolder
author_rank: Super Contributor
timestamp: October 05, 2020, 07:35:45 pm
content: I guess the real trade-off with using a smaller capacitor is that the voltage has to be increased to make up for it, so the regulator has "something to work with" despite the increased ripple.With a small capacitor, higher voltage, and high ripple - will the regulator output be as quiet as the OP's idea of using a capacitance multiplier to quiet things down before giving the job to the regulator - is it really clear cut that a regulator is so good that it won't make any measurable difference?

author: S. Petrukhin
author_rank: Super Contributor
timestamp: October 05, 2020, 08:29:27 pm
content: You need to stabilize the voltage on the basis of a composite transistor, for example, put a Zener diode. The operation of the circuit with a common collector (current amplifier, voltage repeater) is characterized by the fact that the transistor automatically regulates the current flow so that the base voltage is repeated on the emitter minus the transition losses. If the base voltage is stable, then so is the emitter. You need to make a supply of input voltage from the rectifier so that at maximum load there was 2-3 volts more.

author: David Hess
author_rank: Super Contributor
timestamp: October 06, 2020, 06:55:04 pm
content: Quote from: Doctorandus_P on October 05, 2020, 01:18:03 pmIt lowers amplitude of high frequency ripple, so add directly to the total ripple rejection, but a better regulator could achieve the same thing without increasing in total dropout voltage. Sometimes linear preregulators are included before the main regulator for exactly this reason, but there are better configurations than the capacitance multiplier to implement them. An integrated regulator like a LM317 or LT1083 can have a ripple rejection of 75dB at 120 Hz so reduces a typical input ripple of say 2 volts to 350 microvolts, so there is something to be gained, but a properly designed discrete regulator should be better than this.Assuming that the ripple rejection needs to be increased with a preregulator, then what I might do is use a duplicate pass element and error amplifier to divide the voltage drop between the two pass transistors so that the power dissipation is divided evenly, lowering the thermal requirements for the pass elements. That would effectively halve the thermal resistance of the output transistor or double its power capability, and since it works by dividing the voltage, it improves secondary breakdown even more, if that was a consideration.

author: Kleinstein
author_rank: Super Contributor
timestamp: October 06, 2020, 07:04:37 pm
content: I would start with the main part and worry about ripple later. If in deed needed, one could add the capacitance multiplier, but chances are high one would not need it.If the ripple rejection turns out not good enough one can still decide between the pre-regulation and other methods.

author: SilverSolder
author_rank: Super Contributor
timestamp: October 06, 2020, 07:09:43 pm
content: How effective against ripple would it be to put two standard regulators in series - i.e. the first one drops the "dirty" voltage down to 15V, then the second one drops the 15V down to 12V? (You could choose the drops so each one dissipates the same power.) - would that approach improve ripple rejection dramatically, or only a little bit, or not at all?

author: Kleinstein
author_rank: Super Contributor
timestamp: October 06, 2020, 07:57:48 pm
content: If the layout is done correctly 2 regulators in series improve the ripple rejection a lot - just multiply the number or add the dB values, like going from some 70 dB to some 140 dB.

author: UltraRC
author_rank: Contributor
timestamp: October 06, 2020, 09:27:22 pm
content: I do like that idea of using two series pass elements, I think i'll work on the regulator part first and worry about ripple later

author: SilverSolder
author_rank: Super Contributor
timestamp: October 07, 2020, 02:34:56 am
content: So going back to the original idea of using a smaller capacitor: the "price to pay" for doing that, is having to increase the voltage (and therefore burn more power/ lose efficiency), as well as having to use more serious regulators to get to the same ripple. As usual, nothing in life is free!

author: Kleinstein
author_rank: Super Contributor
timestamp: October 07, 2020, 06:48:54 am
content: There is also a downside with a very large capacitor: the power factor is a little lower, especially with a low impedance transformer. It is not that bad with a small one like here, but it can be a problem with a large transformer. A lower power factor means more stress to the transformer (less current available) and possible more noise from the transformer due to higher peak currents.

author: David Hess
author_rank: Super Contributor
timestamp: October 08, 2020, 03:04:42 am
content: Quote from: UltraRC on October 06, 2020, 09:27:22 pmPass elements in series have a big advantage when constrained by the limits of secondary breakdown, which happens at higher voltages. For your requirements where the maximum voltage across the pass element is below 30 volts, the only advantage is better ripple rejection. Better power distribution could more easily be done with pass elements in parallel.But if you are going to use a preregulator, then might as well get the most advantage out of it.