Homemade Bench Supply V2 - Page 1
Forum section: electronics / projects

author: EpicIntelGamer
author_rank: Regular Contributor
timestamp: July 30, 2013, 08:49:45 pm
content: Hello everyone!In this thread I am going to make a log of me building my second homemade Bench PSU! I'll be asking a lot of questions along the way.In fact, I'm starting off with a question, If I have a transformer with a center tapped 24v (no load about 34v) winding can I use the center tap as one leg and connect the other two sides of the winding together as the other leg to get twice the amperage? If not, how else can I get maximum amperage from this?Thanks!

author: mariush
author_rank: Super Contributor
timestamp: July 30, 2013, 08:56:56 pm
content: See below:

author: EpicIntelGamer
author_rank: Regular Contributor
timestamp: July 30, 2013, 09:00:26 pm
content: I'm sorry but could you explain that a bit, all of those formulas and what not seem to go right over my head

author: mariush
author_rank: Super Contributor
timestamp: July 30, 2013, 10:10:36 pm
content: They're not formulas...The page shows several methods of connecting a transformer, depending on how you want to use the energy in the secondary. Below each drawing, it tells you what the dc voltage coming out would be and the maximum current. You have V DC peak which is peak voltage , V DC avg which is average DC voltage and I DC tells you the maximum current.A transformer outputs AC voltage.AC voltage by design is a sinusoidal wave, meaning the voltages goes up to a certain point gradually, then slowly goes down to the same value, but negative. This gets repeated 50 times a second if you're in Europe, 60 times a second if you're in US:Seehttps://en.wikipedia.org/wiki/Alternating_current#Mathematics_of_AC_voltagesThis is one cycle out of 50-60 cycles a second ... in each cycle the voltage goes up to a value, in the picture that's1 voltand then goes down to-1 volt.So an AC voltage like the one in this picture has a peak voltage of 1 volt, a peak-to-peak voltage of 2 volts (from -1v to 1v)...But you see there that you have a sine wave which goes up and down and so on ... it only stays at the peak voltage for a small amount of time, so the fact that you know the peak voltage of 1 volt doesn't help you much.So here comes the notion of RMS voltage.. which is a sort of average voltage for that AC cycle.Think of it like this... if you have a tiny toy car and you connect it to the AC for half an AC cycle (up to the peak of 1v and back down to 0) the car will run forward a small distance. Now you reverse the wires of the motor to keep the car going forward and let it go for the other half of the cycle (up to -1v and then down to 0.. the toy car will probably travel the same distance.But the car didn't travel the whole distance at the same speed - it started slowly and gradually accelerated until it got to 1v peak then slowly went down until it stopped at 0v then again started to accelerate and then slow down.The RMS value basically is a sort of equivalent voltage... what voltage would you need to get the car running at constant speed the same distance with the same energy you would have in that AC cycle .... and they came up with the formula V rms = 0.707 x V peak ... or Vpeak = 1.414 x VrmsSo if you have a transformer that says 24V RMS, that basically means that it outputs a sinusoidal wave, with a peak voltage of 1.414 x 24v = 34 V. 1.41 is often used instead of 1.414.Ok, now there are several uses for the output of a transformer. For some applications, you can work directly with this sinusoidal wave, for example with an incandescent light bulb.. a classic light bulb won't care about polarity, the filament will light up no matter in which direction the energy goes.You need to get the energy from the transformer to power something that requires DC voltage. DC voltage is basically only positive, or only negative, it's not a sinusoidal wave going up to 1v, then down to -1v and so on.So you have to figure out a way to get that sine wave to never go below 0v.Diodes basically only allow current to flow in one direction. So if you put one diode on one wire coming out of the transformer, then the current will only flow on one side of the cycle , either the positive or the negative one :https://en.wikipedia.org/wiki/Rectifier#Half-wave_rectificationIn the above picture, the diode was put on the positive side so now each time the ac cycle goes from 0 to 1 and back down, current goes through. But when AC cycle was supposed to go down to -1 and then back up to 0, the diode blocks that. So now you have a DC voltage but with large holes where those half of ac cycles were blocked by the diode.This is the Half Wave Resistive Load in the first picture I posted. If you add a capacitor, the capacitor will charge with energy when the voltage slowly goes up on that half of AC cycle and then when the voltage goes down or is completely gone for the other half of AC cycle, the capacitor can deliver the energy it stored to the circuit. The bigger the capacitor, the more steady the output voltage will be.Hence, you have the second graph there Half Wave Capacitor Load and the "formulas" below tell you... the peak voltage will be 1.41 x Secondary Voltage - let's say 34v if you have 24v AC rms transformer - but the average DC voltage will only be 0.9 x Secondary Voltage.This rectification sucks because you lose half a cycle just blocking it with a diode, so that's where full bridge rectification comes up, which basically adds 3 other diodes and changes the half of ac cycle that was negative in such a way that it shows up on top. Now instead of 60 small bumps of voltage you have 120 bumps of voltage, and now you the current you can use increases a lot - see the full wave bridge Capacitor input load in the first picture orWith this arrangement, since you have 120 times a second those bumps of energy compared to 60 with large gaps between before, you now only need smaller capacitors to store energy and fill in the gaps between the bumps of energy and get a stable dc voltage.https://en.wikipedia.org/wiki/Rectifier#Full-wave_rectification

author: Unknown
author_rank: Unknown
timestamp: July 30, 2013, 10:24:18 pm
content: Quote from: mariush on July 30, 2013, 10:10:36 pmNice explanation!Quote from: mariush on July 30, 2013, 10:10:36 pmIf the cap is sufficiently large, then the average DC voltage will be identical to the peak voltage, 1.41 times Vac. No idea what the average and peak voltage of a DC voltage is supposed to mean.But to answer the initial question: yes, by connecting the two windings in parallel you get twice the current. Although the maximum DC current will be less than the maximum AC current (and the voltage will be higher), for the reasons described in the two posts above this one.The voltage increases by approximately 1.4, so the current will be reduced by at least a factor 1.4 (can't get more power out of the rectifier than went in). The 0.62 number that Hammond quotes is less than 1/1.4 because the capacitors will only be charged at the peaks of the input. My guess is that they calculated it for a certain capacitor size and then applied a fudge factor to end up at 0.62, but 0.6 sounds about right to me.

author: EpicIntelGamer
author_rank: Regular Contributor
timestamp: July 30, 2013, 11:09:33 pm
content: Quote from: mariush on July 30, 2013, 10:10:36 pmThanks for putting all that time into your post, you explained all of that very well!I'm curious about the last picture in your post. For the most part all I know about is half and full wave rectification. I usually just use full wave bridge rectifiers and I've never really seen the circuit portrayed in the last picture. Does that circuit in the last picture do full wave rectification and if so, how exactly?Also I would like to say I'd prefer to use a normal bridge rectifier in this PSU but I guess I could switch it up if it is beneficial enough. Oh and also, I don't have very large uF value caps.

author: Paul Price
author_rank: Super Contributor
timestamp: July 30, 2013, 11:22:54 pm
content: Aim says"But to answer the initial question: yes, by connecting the two windings in parallel you get twice the current. Although the maximum DC current will be less than the maximum AC current (and the voltage will be higher), for the reasons described in the two posts above this one."This is a center-tapped transformer you've described and connecting the center tap as one output connection and connecting the other two windings(wires) in parallel will just short out the transformer an burn it up in about a few seconds or minutes.

author: EpicIntelGamer
author_rank: Regular Contributor
timestamp: July 30, 2013, 11:25:09 pm
content: Quote from: Paul Price on July 30, 2013, 11:22:54 pmI dont think it will burn it.Center tapped winding is like two individual windings, just both are connected, all I suggest is connect the other side.Now if I did what I said then connected the center tap to the other two ends of the winding that are connected together, then that would short it.EDIT: I did some reasearch and actually it looks like you are right. I can not really figure out why though.

author: EpicIntelGamer
author_rank: Regular Contributor
timestamp: July 30, 2013, 11:32:42 pm
content: What if I was to stick a bridge rectifier on both sides of the center tapped winding and then connect the outputs of the bridge rectifiers in parallel?

author: SArepairman
author_rank: Frequent Contributor
timestamp: July 30, 2013, 11:36:19 pm
content: What is the choke full wave rectifier used for? I have never seen that.Nor the bridge choke.

author: Unknown
author_rank: Unknown
timestamp: July 30, 2013, 11:39:17 pm
content: Quote from: Paul Price on July 30, 2013, 11:22:54 pmYou're correct if it's a center-tapped transformer. I was implicitly assuming a transformer with dual secondaries, since these are much more common, but doing this to a real center tapped transformer would indeed be a bad idea.Quote from: SArepairman on July 30, 2013, 11:36:19 pmA choke provides additional smoothing. It was common in the tube days when a big choke was cheaper than a regulated power supply. These days you'd either add a regulator or add extra capacitance to reduce the ripple.

author: EpicIntelGamer
author_rank: Regular Contributor
timestamp: July 30, 2013, 11:40:23 pm
content: Quote from: alm on July 30, 2013, 11:39:17 pmYeah this is a center tapped winding were talking about.I looked it up and apparently its because its 180 degrees out of phase that it would not work.

author: Paul Price
author_rank: Super Contributor
timestamp: July 30, 2013, 11:50:58 pm
content: Aim says, "If the cap is sufficiently large, then the average DC voltage will be identical to the peak voltage, 1.41 times Vac.The voltage increases by approximately 1.4, so the current will be reduced by at least a factor 1.4 (can't get more power out of the rectifier than went in)"The rectified voltage fed to a filter capacitor will be approximate 1.4 x 24VAC, but only with no load, with no current delivered to any output device demanding any amount of current. The current is not reduced by a factor of 1.4.Aim says, "you can't get more power.."With no load and peak voltage output, the "power" is not an issue, since no power is being delivered to the load.As power is delivered to a current drawing circuit connected to the output the power delivered to the load causes the resistance of the primary winding to drop the voltage delivered to the secondary winding.This in effect lowers the effective voltage appearing across the primary winding which in turn reduces the voltage on the secondary winding, so the output voltage drops with increasing load.Because the rectifiers conduct only when the transformer secondary voltage is greater than the output rectified voltage, this effect is not linear with increasing load because of the shape of a sinewave power cycle and so the time the diodes conduct to deliver power to the load increases as the power supply's load resistance decreases to demand increased current.The real power capability of a transformer is limited by heating of the primary and secondary windings due to copper losses and finally by transformer saturation.The primary and secondary winding wire resistances causes heat and heat is what can cause the transformer to overheat and this heating increases dramatically if the transformer becomes saturated by being asked to deliver more than its rated current.

author: EpicIntelGamer
author_rank: Regular Contributor
timestamp: July 31, 2013, 12:09:04 am
content: So how can I hook this transformer up.Will a two bridge rectifiers from center tap to the ends of the winding then with the output of the bridge rectifiers hooked in parallel work?

author: Paul Price
author_rank: Super Contributor
timestamp: July 31, 2013, 12:09:41 am
content: EpicIntelGamer asks,"What if I was to stick a bridge rectifier on both sides of the center tapped winding and then connect the outputs of the bridge rectifiers in parallel?"And the answer is: Yes it would work, but you would get 12V DC rectified with large filter caps, but still, you cannot expect to exceed the current rating of the transformer.If you try to demand more current than the transformer is rated to deliver, it will overheat and burn out.For example if you have a 1.2Amp 24V AC RMS transformer and you try to use two bridge rectifiers to attempt to get more than 1.2 amps of current output, you can still burn out the transformer.If you try by any rectifier configuration to increase the output load current much over 1.2A (as in this transformer example) you risk overheating and burning out the transformer.

author: EpicIntelGamer
author_rank: Regular Contributor
timestamp: July 31, 2013, 12:21:20 am
content: Quote from: Paul Price on July 31, 2013, 12:09:41 amI don't know exactly what this transformer is rated for. It was in a kit from the 1980's for a power supply and I vaguely remember .5amp rating but I don't know if that rating was on this winding or how it was wired in that kit.So really there's no possible way for me to pull more than .5 amp if its only rated for .5 amp? I thought maybe each side of the center tapped winding was rated .5 and then combining them would make 1 amp.

author: Paul Price
author_rank: Super Contributor
timestamp: July 31, 2013, 12:25:05 am
content: A good strategy to maximize the power capability of a power transformer like this is first to decide what voltage max you need. If you need more than 12V at full transformer rated load then you might consider just using a single bridge rectifier across the ends of the 24V transformer output and not use the center tap at all. This will give you approx 34-36 volts peak at the filter capacitor. You then use this output to feed as power input to variable voltage output switching regulator to give you any voltage you want up to about 20V at full load and up to approx 30-32V at light loads.

author: EpicIntelGamer
author_rank: Regular Contributor
timestamp: July 31, 2013, 12:29:05 am
content: Quote from: Paul Price on July 31, 2013, 12:25:05 amHow can I figure out what this transformer is rated for? Is there a way to measure saturation and then just load it up until it saturates?

author: Unknown
author_rank: Unknown
timestamp: July 31, 2013, 12:29:40 am
content: Quote from: Paul Price on July 30, 2013, 11:50:58 pmThe peak voltage will still be 1.4 * Vac (minus diode drops) under load. The minimum voltage will be the peak voltage minus ripple, which should only be a few volts with a decent amount of capacitance. Drawing the rated AC current in DC current will overload the transformer, however, otherwise you could draw 1.4 times more DC power from the rectifier than the AC power going in. If the voltage goes up by a factor 1.4, then the current has to go down by at least a factor 1.4, whether this current draw is 1 A through a power transistor or some tiny leakage current through the cap.Quote from: Paul Price on July 30, 2013, 11:50:58 pmThe rated voltage of the transformer is determined at max AC current, though, so you should be able to draw the rated current (less DC because of the non-sinusoidal current draw) while maintaining Vdc = ~1.4 * Vac (given enough capacitance). Unloaded voltage will be higher, about 10% for larger transformers, but sometimes 40% above nominal for small mains transformers.Quote from: Paul Price on July 31, 2013, 12:09:41 amSo if you have a 24 Vac / 1.2 Adc transformer providing its maximum current driving a load (cap + resistor) that draws 12 Vdc / 1.2 Adc or even 12 Vdc / .72 Adc (since you claim that the maximum current does not increase), where is that other 12 VA going? The power factor of a full wave bridge rectifier + caps is fairly close to 1. Are you claiming that the full wave center tapped configuration as shown by mariush does not work? The maximum current might be slightly lower because the lower duty cycle of the current draw, not the factor 2 that you're implying.

author: Paul Price
author_rank: Super Contributor
timestamp: July 31, 2013, 12:33:02 am
content: You can test what the maximum current capability of the transformer you have by increasing the load until the filtered output voltage drops to approx 24V. If the transformer remains warm but not to hot as to lay upon it your fingers, you can increase the load even further to you reach over several minutes that critical "Ow, Owee this thing is burning my hand" digital thermometer temperature. Any more than that and your transformer will soon fail. At that point, you have the max available current output capability of the transformer.

author: Paul Price
author_rank: Super Contributor
timestamp: July 31, 2013, 12:36:50 am
content: Alm says,"however, otherwiseyou could draw 1.4 times more DC power from the rectifier than the AC power going in.If the voltage goes up by a factor 1.4, then the current has to go down by at least a factor 1.4,"That is totally wrong.You cannot get more power out than the power going in. Also, the voltage to current is not even approximately related by your suggested 1.4 x factor.

author: Paul Price
author_rank: Super Contributor
timestamp: July 31, 2013, 12:41:23 am
content: Alm says,"Drawing the rated AC current in DC current will overload the transformer, "That is totally wrong.The transformer rated AC current is equal to the rated DC output rectified current.

author: EpicIntelGamer
author_rank: Regular Contributor
timestamp: July 31, 2013, 12:43:15 am
content: Quote from: Paul Price on July 31, 2013, 12:33:02 amI'll go until its warm but I don't think I want to go until its hot lol.And really the thing is, How can I know for sure that the output is supposed to be 24v? I could be remembering it incorrectly from the instructions of the 80's supply.

author: Paul Price
author_rank: Super Contributor
timestamp: July 31, 2013, 12:48:06 am
content: Alm asks, " o if you have a 24 Vac / 1.2 Adc transformer providing its maximum current driving a load (cap + resistor) that draws 12 Vdc / 1.2 Adc or even 12 Vdc / .72 Adc (since you claim that the maximum current does not increase),where is that other 12 VA going? "Because he is suggesting using the center-tap and two bridge rectifiers,the output voltage must be halved at full load,or with any other rectifier configuration.

author: Paul Price
author_rank: Super Contributor
timestamp: July 31, 2013, 12:57:45 am
content: EpicIntelGamer asks,"How can I know for sure that the output is supposed to be 24v? I could be remembering it incorrectly from the instructions of the 80's supply?"Power up the transformer and measure the voltage at the secondary. This will be the RMS unloaded peak output voltage rating of the transformer. For a 24V AC transformer it might read as high as 30-34 V for a high-resistance winding small transformer with a low output current capability. For a larger transformer it will be closer to 24 -26VIf it is a small, cheap transformer, load the power transformer with resistors or a dummy load or automotive 12V taillight lamps in series until the voltage drops to approx .7 of the unloaded value. That voltage is the RMS voltage rating of the transformer.If it is a higher quality transformer, the unloaded output RMS voltage read on your voltmeter is the transformer rated voltage.How to know if you have a high quality transformer? Measure the 120VAC primary resistance with an ohm meter and if the resistance is <5 ohms it is high quality. If it is >100 ohms then expect the open-circuit output voltage to be higher than its full-load current delivered rated AC output voltage by a considerable (up to 30-50%) amount.Transformers usually are sold at common output voltages such as 6.3 VAC, 12.6, 18Vac, 24Vac, etc. Yours should fall into one of these categories.

author: Paul Price
author_rank: Super Contributor
timestamp: July 31, 2013, 01:03:20 am
content: Any cheaper transformer is designed to get warm to hot at rated output, so don't be afraid that it heats up. It just shouldn't heat up to get hot enough to burn out and that means that if it's not Ow Owee quite hot to the steady finger touch, then this operating temperature is ok.

author: EpicIntelGamer
author_rank: Regular Contributor
timestamp: July 31, 2013, 01:06:38 am
content: Quote from: Paul Price on July 31, 2013, 12:57:45 amUnloaded the winding jumps between 35vac and 39vac because my DMM is terrible.What range of values resistors will work?

author: Paul Price
author_rank: Super Contributor
timestamp: July 31, 2013, 01:14:29 am
content: Your DVM may be terrible, but unless it is broke or your fingers are touching the measuring probe leads, the voltage will be steady!From what you readings indicate, the transformer in question is a cheap, low current (<.25Amp) 24VAC RMS transformer.Try a 100-ohm 1-watt (you can parallel a bunch of same low-watt resistors to compound their watt capability.)You only need to connect the meter connected across the load resistor for a second to get a steady reading, so a small valued resistor may not have time to burn out but just get damn hot.

author: Unknown
author_rank: Unknown
timestamp: July 31, 2013, 01:16:45 am
content: Quote from: Paul Price on July 31, 2013, 12:36:50 amI love your well reasoned arguments! You could make your statements even stronger by increasing the font size.Quote from: Paul Price on July 31, 2013, 12:41:23 amThis is beyond nitpicking. Now your just making stuff up to have an argument. Either that or you're seriously misguided. Good luck with your perpetual motion machine. I'm out of here.

author: EpicIntelGamer
author_rank: Regular Contributor
timestamp: July 31, 2013, 01:20:48 am
content: Quote from: Paul Price on July 31, 2013, 01:14:29 amI can assure you the voltage is fluctuating, the leads are currently some banana plug to alligator clips and I'm not touching them. The meter is just really that bad.Its cheap and small but I know it can output at least .5amps, i remember that, but that might've been on the other winding (it has two, the big center tapped one, and another without a center tap which reads about 11v).Why am I going to connect a meter across the resistor?

author: Paul Price
author_rank: Super Contributor
timestamp: July 31, 2013, 01:25:00 am
content: You connect a meter across the resistor to measure the loaded output voltage of the transformer.

author: mariush
author_rank: Super Contributor
timestamp: July 31, 2013, 01:26:51 am
content: It would be cheaper to buy some 12v light bulbs meant for cars .. just put 2-3 in series so they would handle the voltage without burning up.You can use your multimeter's current measurement function to determine how much current the lights consume - just put the meter in series with the lightbulbs.If you plan to use resistors ... you have ohm's law which is... well, the law. Voltage = current x resistance ( V = I x R ) ... so assuming your voltage is 34v and you have a 100 ohm resistor, then current = 34/100 = 0.34 AThe power dissipated in the resistor is determined with the formula Power = Current x Current x Resistance = 0.34 x 0.34 x 100 = 11.56 watts. So your 100 ohm resistor should be rated for at least 12 watts but if you don't want it to get smoking hot, the resistor should be rated for at least 15 watts.To work with cheaper resistors, you can start from the other end. 7 watt resistors are cheap ... those will dissipate 5 watts with no problems. So now you have Power = 5 watts = Current x Current x Resistance so you can pick a resistor value which would fit. For example maybe 220 ohm ... 34/220 = 0.154 ... p = 0.154*0.154*220 = 5.25wSo a 220ohm 7w resistor would waste 5.25w ... now you can put in parallel 10 of them and waste 52 watts ... provided the voltage remains 34v ... as you add a load, the transformer would slowly get closer to its rated output voltage so the voltage would drop.

author: Paul Price
author_rank: Super Contributor
timestamp: July 31, 2013, 01:30:36 am
content: As I said you only need to hold the load with the meter across it on the transformer output until you get a reading, no longer, only a second or two should be sufficient, so things can hot but not smoke with a quickee with smaller wattage resistors.

author: Paul Price
author_rank: Super Contributor
timestamp: July 31, 2013, 01:33:48 am
content: AcHMeg99 ask, "What do you mean by that?"If a transformer is rated at some RMS Secondary Winding AC current, then this is also the maximum DC steady-state DC current load current output to a load that will not overload the transformer.

author: Paul Price
author_rank: Super Contributor
timestamp: July 31, 2013, 01:49:06 am
content: Alm says, "The rated voltage of the transformer is determined at max AC current, though, so you should be able to draw the rated current (less DC because of the non-sinusoidal current draw) while maintaining Vdc = ~1.4 * Vac (given enough capacitance). "This is totally wrong and confused.

author: Paul Price
author_rank: Super Contributor
timestamp: July 31, 2013, 01:57:35 am
content: ArcHameg99,Your diagram doesn't relate to any real-world spec sheet for a transformer delivering its rated RMS current or voltage and it also doesn't begin to address the important spec of max. Degree C temp rise at rated RMS current.BTW the original poster last indicated he would use two bridge rectifiers, one to each CT and their + outputs + paralled to a common large valued filter cap. What is that power factor equal to?

author: EpicIntelGamer
author_rank: Regular Contributor
timestamp: July 31, 2013, 01:59:49 am
content: I tried a few different resistors and found a quarter watt 22ohm resistor and it takes the transformer down to about 20vac.Don't know if this helps...

author: GK
author_rank: Super Contributor
timestamp: July 31, 2013, 02:10:42 am
content: Quote from: Paul Price on July 31, 2013, 01:49:06 amSomeone is certainly confused.If I have an unregulated linear supply delivering 20V DC (just for example) into a 20 ohm load, the power delivered to the load by the supply will be 20W. The DC current will be 1A.The rms current drawn from the secondary of the transformerwillbe greater than 1A as the rms voltage is significantly less than 20V. The power drawn from the transformer secondary cannot be less than the power delivered into the load!

author: EpicIntelGamer
author_rank: Regular Contributor
timestamp: July 31, 2013, 02:18:23 am
content: So a quarter watt 22ohm resistor and it takes the transformer down to about 20vac and an 4ohm speaker takes it to 6vac.

author: GK
author_rank: Super Contributor
timestamp: July 31, 2013, 02:41:26 am
content: Quote from: alm on July 30, 2013, 11:39:17 pmActually, the real reason is that tube rectifiers could only reliably handle a limited value of filter capacitance due to their limited surge/peak current handling abilities. The amount of capacitance permitted was generally not great enough for adequate ripple reduction/filtering, so LC filters were required. I've never seen an all-tube eletronically regulated supply that did not use chokes for ripple filtering after the rectifier.

author: Paul Price
author_rank: Super Contributor
timestamp: July 31, 2013, 02:46:39 am
content: Ok, taking into account your DVM is just plain silly and guesstimating with a touch of optimism from its readings, it looks like we've got the answer to your question..you got something like a .5A to 1A RMS 24V transformer that might deliver up to an amp at 20V..will that work for you to make a power supply?

author: EpicIntelGamer
author_rank: Regular Contributor
timestamp: July 31, 2013, 02:59:55 am
content: Quote from: Paul Price on July 31, 2013, 02:46:39 amI suppose I could hook this up to my oscilloscope if we need some more accurate readings.I guess it'll work, my homemade v1 only outputs ~3-15VDC at ~.6A and 33VAC at .3A so I guess this will do.I still need to figure out what I want for this in terms of outputs.

author: Paul Price
author_rank: Super Contributor
timestamp: July 31, 2013, 03:07:49 am
content: I have seen such a thing, some designers are not that so chocked-up on chokes.Perhaps by inductive reasoning they could resist using chokes.See the attached .corrected link and .pdf schematic for well-known tube amp mfg.http://www.ampslab.com/vintage_dynaco_6bq5.htm

author: Paul Price
author_rank: Super Contributor
timestamp: July 31, 2013, 03:16:59 am
content: GK says, "The rms current drawn from the secondary of the transformer will be greater than 1A as the rms voltage is significantly less than 20V.Have you in fact measured the AC RMS voltage under this situation?If we should talk about the RMS current and voltage spec for a transformer, then the transformer rated RMS current delivered by the secondary should be equal but greater than the DC load current to not overload the transformer.Perhaps a VA rating for the transformer gives a better product(I * E) for the product being used.

author: GK
author_rank: Super Contributor
timestamp: July 31, 2013, 03:20:07 am
content: Quote from: Paul Price on July 31, 2013, 03:07:49 amThat tube amp you linked to uses silicon diode rectifiers, not tube rectifiers.

author: GK
author_rank: Super Contributor
timestamp: July 31, 2013, 03:21:45 am
content: Quote from: Paul Price on July 31, 2013, 03:16:59 am

author: EpicIntelGamer
author_rank: Regular Contributor
timestamp: July 31, 2013, 03:41:26 am
content: Since everyone seems reluctant to answer the transformer questions I'll introduce another question.Does anyone have any suggestions for a good box to house this PSU?My previous one was made from a VCR turned backwards (the back panel became the PSU's front panel).

author: Paul Price
author_rank: Super Contributor
timestamp: July 31, 2013, 04:03:41 am
content: I am not reluctant to answer the transformer question!A project box with a metal case or high-temperature plastic with a metal front might be a good start.I built my first bench power supply on a very shiny steel coated with tin, easy to drill into and solder to and just as easy to get from the local supermarket,a loaf-sized tin bread baking pan.It's a little funky but easy to get the job done kinda chassis.In it I build a 4 separate outputs 0-24V 1A supply using a 5-KG multi-secondary output power supply transformer from a retired custom NASA test set I bought for a few bucks from an electronics surplus store. The test set also had power transistors, heatsinks, large valued high-quality electrolytics, over a dozen C&K panel switches, indicators, several 10-turn dial indicating potentiometers, and lots of nice white Teflon insulated wire to work with.You can get the heatsinks free from discarded stereo audio amplifiers or a Desktop PC power supply.

author: Paul Price
author_rank: Super Contributor
timestamp: July 31, 2013, 04:05:13 am
content: I corrected the link and schematic for the non-choke tube high power audio amps using rectifier tubes, sorry about the wrong link.

author: EpicIntelGamer
author_rank: Regular Contributor
timestamp: July 31, 2013, 04:12:10 am
content: Quote from: Paul Price on July 31, 2013, 04:03:41 amWhat would I use for the bottom or top then?I've got plenty of little components at my disposal, many puny transistors, some larger mosfets, huge diodes, and lots of different heatsinks.I do actually have two PC PSU's that are in my junk pile that I'm slowly stripping components off of. As my name implys I do a lot with computers.My current PSU uses a LM7805 for the 5-15VDC supply. I don't exactly remember the circuit but theres on you can make to turn it into a variable regulator instead of just 5v.I currently have a brand new +12VDC and -12VDC regulator as well as three salvaged LM7805's and as I said plenty of switches and diodes, 1/8watt resistors, and transistors and mosfets, not many potentiometers though.

author: GK
author_rank: Super Contributor
timestamp: July 31, 2013, 04:14:22 am
content: Quote from: Paul Price on July 31, 2013, 04:05:13 amLOL. And its power supply isn't electronically regulated, so what is your point?

author: Paul Price
author_rank: Super Contributor
timestamp: July 31, 2013, 04:19:13 am
content: EpicIntelGamer,The bread pan already has a top, you can easily use single or double sided PCB stock for the bottom..add a couple rubber feet.I mounted the heatsinks on the exterior rear of the pan. I mounted black and red Banana Jacks/binding posts for output connections.See the attached. Later, I mounted some DIY LED Amp/voltmeters upon the front just supporting the PCB and MCU etc on short standoffs. I really liked the finished result because it was original one-of-a-kind and it works well.Have you looked at the schematics I've designed in my replay a few replies back?

author: EpicIntelGamer
author_rank: Regular Contributor
timestamp: July 31, 2013, 04:26:04 am
content: Quote from: Paul Price on July 31, 2013, 04:19:13 amI guess if I go that route I'll use some plywood for the bottom. Are the sides on those pans completely vertical or are they angled?I did take a look at those schematics but unfortunately they look far too complicated for the amount of money (I'd like to keep this free or as close to it as I can) and effort I want to put into this PSU.One day I'd like to get a HUGE transformer and put a lot of time to make it be built like a tank and very precise.

author: Paul Price
author_rank: Super Contributor
timestamp: July 31, 2013, 04:36:14 am
content: Some are slope-sided some are not, who'd wanta square loaf, anyway. The slope sided looks nice and presents a easy viewing angle for indicators and displays attached to the front.Do you really think my two schematics posted here were complicated?

author: EpicIntelGamer
author_rank: Regular Contributor
timestamp: July 31, 2013, 04:44:05 am
content: Quote from: Paul Price on July 31, 2013, 04:36:14 amThey are more complicated than smacking a linear regulator on a board with some smoothing caps.They aren't really complicated but they also require a lot of stuff I don't have, namely those IC's.

author: EpicIntelGamer
author_rank: Regular Contributor
timestamp: July 31, 2013, 05:02:02 am
content: I was looking at more stuff I have and I noticed I have 3x of these chips.They are the REF02 "5V Precision Voltage Reference/Temperature Transducer" from Analog DevicesIf I'm thinking correctly, these can be used to make a Voltage Reference to calibrate DMM's and what not? Does anyone know how I can do that?Maybe I'll make a thread for it if I decide to use one.

author: megajocke
author_rank: Regular Contributor
timestamp: July 31, 2013, 10:13:16 am
content: Quote from: Paul Price on July 31, 2013, 03:16:59 amI have measured this, and the last time was using a transformer with twin 20V 6A secondaries. With a DC load of 3A after capacitive filtering (per winding) I measured 6A RMS draw (per winding) from the transformer, which did agree with SPICE simulations previously done using a model of the transformer including winding resistance.The average rectified value (http://en.wikipedia.org/wiki/Average_rectified_value) of the transformer current is obviously the same as the output DC current. But because the current drawn by the rectifier is not square, the form factor (http://en.wikipedia.org/wiki/Form_factor_%28electronics%29) is higher than one, and the RMS current is higher than the output DC current (which is equal to the winding ARV current).Typical values are in the range of the RMS current being 1.5 to 2 times the DC current when the transformer is of typical construction and size, capacitors are large and the transformer is operated at its RMS current limit.A rectifier with a large inductor before the first capacitor will have a form factor of close to 1 and DC output current and transformer RMS current will have the same value.

author: EpicIntelGamer
author_rank: Regular Contributor
timestamp: July 31, 2013, 07:21:28 pm
content: I slapped a bridge rectifier and 1000uf cap on it along with a small 12v DC fan on the 36v center tapped winding ignoring the center tap.I'm drawing .4ADC and getting 27VDC and the transformer seems to get pretty warm and after a while it does get a bit hotter than I'd like it to.

author: Paul Price
author_rank: Super Contributor
timestamp: July 31, 2013, 07:26:22 pm
content: Point the fan towards the transformer.

author: EpicIntelGamer
author_rank: Regular Contributor
timestamp: July 31, 2013, 07:31:12 pm
content: Quote from: Paul Price on July 31, 2013, 07:26:22 pmlolThe thing is, I want this to be somewhat reliable, and I don't want to have to fully load the transformer with a fan to be able to keep the transformer running, its like a vicious circle.Looks like .4ADC is too much in this configuration.

author: Paul Price
author_rank: Super Contributor
timestamp: July 31, 2013, 08:14:56 pm
content: It seems to me that you might try connecting the transformer to just two rectifier diodes to create a full wave rectifier at half the voltage output.It might be that running a 12V fan at 27V might actually be drawing more surge current than you think you are measuring.And It might just be that this transformer seems to be like those I've seen that would be used for light duty, just supply enough to power an alarm clock radio type of project and and not required to deliver more than a few hundred mA.Often, if you build for yourself a custom power supply, you might find it handy to have an isolated output on it that can deliver over 32V, even it only at small currents, and this transformer might just fit the bill as an accessory power supply, while another bigger transformer might be required to deliver the high currents for regulator circuits to power lower voltage projects.There must be so many other more Amp capable transformers out there just waiting to become the heavyweight star performer of your custom power supply. Some may lie waiting free of charge in discarded audio equipment, some on the shelves of Radio Shack stores ready for you to empty your wallet and take home.Save some money by finding a nice used transformer and save up to buy yourself a new multimeter.BTW, I was just kidding about the fan.

author: Paul Price
author_rank: Super Contributor
timestamp: July 31, 2013, 08:22:57 pm
content: If I needed to create a power supply in a hurry, I would find a discarded PC laptop power supply. They are easy enough to find in my area. Most are capable of supplying 18 to 21V at up to 4.5 amps and are short-circuit protected and well regulated. Perfect for stepping down to any voltage required for your bench breadboard projects using linear or switching power supply chips. Could be the immediate perfect solution for you to power your custom power supply project.

author: EpicIntelGamer
author_rank: Regular Contributor
timestamp: July 31, 2013, 08:24:54 pm
content: Quote from: Paul Price on July 31, 2013, 08:14:56 pmNo I am actually using the little 4 pin full wave bridge rectifier diode packages like what you find in computer psu's. I'm positive it's providing full wave rectification.I'm also sure the fan is pulling .4adc because I'm measuring it, not going off of its rating.Hopefully someday I'll salvage a beefy transformer.I am still keeping my eyes peeled for a cheap used good DMM and also a cheap bench MM.

author: Paul Price
author_rank: Super Contributor
timestamp: July 31, 2013, 08:44:03 pm
content: If you want to build your own regulator circuits using the transistors you have and those REF-02 references, see the attached link, full of very well explained, easy to understand knowledge about all sorts of things electronics, and some very helpful info on making power supplies.http://www.introni.it/pdf/GE%20-%20Transistor%20Manual%201964.pdf

author: EpicIntelGamer
author_rank: Regular Contributor
timestamp: August 01, 2013, 01:00:43 am
content: A bit of an update, I was testing the second output winding on the little transformer, this shows about 14VDC after rectification and smoothing. I found that I could run a decent size DC motor off of that which was pulling 1.7ADC and pulling the voltage down to 4VDC. It took about two minutes for the transformer to get really warm while drawing that much power.So it looks like the lower voltage winding has potential!

author: EpicIntelGamer
author_rank: Regular Contributor
timestamp: August 01, 2013, 06:33:48 am
content: Maybe I'll make a couple low amp adjustable outputs and one fixed high amp output.Still not completely sure though...

author: EpicIntelGamer
author_rank: Regular Contributor
timestamp: August 02, 2013, 01:12:04 am
content: Quote from: EpicIntelGamer on August 01, 2013, 06:33:48 amAny ideas guys?

author: EpicIntelGamer
author_rank: Regular Contributor
timestamp: August 04, 2013, 04:21:40 am
content: I customized an ammeter for this project. Here's the thread asking about how to mod it:https://www.eevblog.com/forum/beginners/change-analog-ameter-range/I'll post pics when its completely done.Oh and I think I have a bit of a plan.I have a single LM317 which I think I'll use for constant current, then use a LM7805 with a pot to give me variable voltage. That's for at least one of the outputs, don't know what to do with the other two though.

author: c4757p
author_rank: Super Contributor
timestamp: August 04, 2013, 04:24:31 am
content: Quote from: EpicIntelGamer on August 04, 2013, 04:21:40 amA 7805 for variable voltage? It works... but why?

author: EpicIntelGamer
author_rank: Regular Contributor
timestamp: August 04, 2013, 04:38:12 am
content: Quote from: c4757p on August 04, 2013, 04:24:31 amBecause I have one on hand and I currently use one for variable voltage in my current Power supply.

author: pickle9000
author_rank: Super Contributor
timestamp: August 04, 2013, 04:47:05 am
content: Just curious, Are you varying the ground reference on the 7805? I think Radio Electronics magazine had an article called the "Junkbox Special Power Supply". Early 80's I think.

author: EpicIntelGamer
author_rank: Regular Contributor
timestamp: August 04, 2013, 05:08:26 am
content: Quote from: pickle9000 on August 04, 2013, 04:47:05 amYep, I plan to put a pot on the ground pin pretty much.

author: EpicIntelGamer
author_rank: Regular Contributor
timestamp: August 07, 2013, 04:14:51 am
content: So now I need to know what is the lowest voltage that a LM317 configured for CC will run on, and also what is the lowest voltage a LM7805 that is hacked for adjustable voltage will run on.Thanks!

author: ElectroIrradiator
author_rank: Frequent Contributor
timestamp: August 07, 2013, 08:25:44 am
content: You need a minimum 3V input-to-output differential in both cases, if they need to supply the full output current.In case of the CCS you would then need 3V plus the 1.23V across the CC resistor, for a grand total of 4.23V.

author: ptricks
author_rank: Frequent Contributor
timestamp: August 07, 2013, 10:42:04 am
content: One thing you can do to find out the safe capacity for a transformer is to peel back the outer insulation and look at the size of the wire used in the winding, most will use coated copper wire which has insulation that can withstand fairly high temps. The size of the wire will give you an idea of the current capability safety margins, it does not give you the transformers capacity though ,a transformer can saturate long before it reaches the point the wire fails, at that point you are just producing heat.For regulation, 78xx and LM3xx parts are nice if you have them, but don't overlook simple methods made from basic components. Things like zener diodes and resistors work well at under 2A and don't have a high dropout voltage.

author: EpicIntelGamer
author_rank: Regular Contributor
timestamp: August 07, 2013, 02:57:54 pm
content: Quote from: ElectroIrradiator on August 07, 2013, 08:25:44 amGood, this is exactly what I wanted to hear.Thank you for your informationQuote from: ptricks on August 07, 2013, 10:42:04 amI don't think seeing the wire will help me estimate much so I'm going to leave the transformer alone.I figure since I am just lucky enough to have the exact regulators I need, I'll use them.Thanks for the information!

author: ptricks
author_rank: Frequent Contributor
timestamp: August 08, 2013, 06:54:08 pm
content: Quote from: EpicIntelGamer on August 07, 2013, 02:57:54 pmI should have given you more information. If you know the wire size and measure the resistance you can get a close idea of a transformers rating and number of turns on a winding.http://wiki.xtronics.com/index.php/Wire-Gauge_Ampacity