use a transformer for multiple regulators - Page 1
Forum section: electronics / beginners

author: christos
author_rank: Regular Contributor
timestamp: January 14, 2014, 09:56:59 pm
content: in my last topic i wanted to know how many amps my transformer gives on output..i made a test and at 5 amps its gets a bit warm,not hot..it has output 2 secondaries from 26 volts each..can i use voltage divider to power up +- 05,08 and 12 volts?

author: Rudane
author_rank: Regular Contributor
timestamp: January 14, 2014, 10:09:43 pm
content: You could, but remember that is not the best way to produce those voltages, because as soon as you connect those voltage rails to their respective circuits you will load the rail and probably change the voltage value (think that you are paralleling the voltage divider network with the circuit it powers). Now, if you want to do a voltage divider that feeds regulators to produce those voltages, you'll need to pick some regulators and see what their input impedance are before you can determine if the voltage values will change. I would think the voltage divider feeding the regulator network would work, but there are better ways to do this.I would get a 12 Volt regulator. Feed the 26 into it and produce the 12 Volts. Take that 12 Volts and use a regulator on it to get the 8 Volts, and finally regulate the 8 V down to 5 V. I'm not sure what kind of power or current each of your rails would require. I only suggest this method because I design low power mostly, and this is easy to do. If you require higher power, maybe another member can chime in on a better method.You could do the same thing with negative regulators to get the other voltages.

author: christos
author_rank: Regular Contributor
timestamp: January 14, 2014, 10:19:02 pm
content: Quote from: Rudane on January 14, 2014, 10:09:43 pmi can make it your way and i think its better but if i want to use both 12v and 5 volts fused with 1 amp ,i may burn the fuses..but with a program called proteus isis...i made this circuit ,and when i use a load on the output of one regulator,all the voltage drops from all the regulators..shoud this work or is the program not so good, and if it works ,shoud i use 5watt res or higher?

author: Simon123
author_rank: Regular Contributor
timestamp: January 14, 2014, 10:30:34 pm
content: You need capacitor on REG output, otherwise voltage drops.Check cap value on voltage regulators datasheets.Why would you use 5W resistors?

author: christos
author_rank: Regular Contributor
timestamp: January 14, 2014, 10:51:55 pm
content: Quote from: Simon123 on January 14, 2014, 10:30:34 pmhight currenr maybe ..im thinking of making 3 voltage dividers..one main that will lower the voltage and the other 2 for the voltages that the regulators can use..i was thinking to use a 5W resistor for the main divider

author: christos
author_rank: Regular Contributor
timestamp: January 14, 2014, 10:53:24 pm
content: or just use 3 transformers with 2 secondaries

author: rdl
author_rank: Super Contributor
timestamp: January 14, 2014, 11:04:43 pm
content: Your transformer has two secondaries of 26 volts each. This voltage is really too high for any of the regulated DC voltages you want to get out from it. It will work but you're going to be burning off a lot of voltage as heat if you try to draw much current. I don't know if your 26 volts is peak or RMS, but you will be dissipating well over 10 watts per amp with 12 volts out and even more at the lower voltages.

author: Rudane
author_rank: Regular Contributor
timestamp: January 14, 2014, 11:32:46 pm
content: I was thinking, if you don't care about a floating ground reference, you could leave one secondary at 26 Volts and regulate the other channel between 14 V and 26 V. Taken differentially the voltage would span 12 Volts.Something like I've attached. This is only an idea, and would need more work to be a viable option. Maybe the farthest left regulator would be a powerful regulator while the others would be not so power hungry.EDIT: After reading that the RMS voltage is 37 Volts, I agree with later posters that this method is not preferred for this application.

author: christos
author_rank: Regular Contributor
timestamp: January 15, 2014, 12:58:52 pm
content: Quote from: rdl on January 14, 2014, 11:04:43 pmits rms ,,, 37 volts each..74v both peak

author: christos
author_rank: Regular Contributor
timestamp: January 15, 2014, 01:30:52 pm
content: Quote from: Rudane on January 14, 2014, 11:32:46 pmi had the same idea but,if i use 2 or 3 regulators at the same time..the first regulator will draw much current and burn the fuse cuz i think all the regulators are at 1 amp..or use a 2n3055 to maximize the current..the output of the regilator to the base of the 2n3055 ,collector to ground and emitter to output..would it work?

author: mariush
author_rank: Super Contributor
timestamp: January 15, 2014, 01:56:32 pm
content: No, it won't work.The voltage of each secondary winding is too big for linear regulators. The difference between input voltage and output voltage is wasted as heat on the regulator.You're too much of a beginner to try to make something out of transistors and resistors and the parts will cost you as much as a proper dc-dc converter chip.Go on eBay and get one of those LM2576 or LM2596 based buck regulators (switching dc-dc converter) which can convert from up to around 35-60v in down to whatever you want :http://www.ebay.com/itm/LM2576HV-LM2576-DC-DC-Step-Down-Adjustable-Power-Supply-5V-60V-to-1-25V-30V-/370983238824?pt=LH_DefaultDomain_0&hash=item566052fca8http://www.ebay.com/itm/LM2576HV-LM2576-DC-DC-Step-Down-CC-CV-Adjustable-Power-Supply-Module-Over-LM2596-/180976351390?pt=LH_DefaultDomain_0&hash=item2a2307f89ehttp://www.ebay.com/itm/LM2576HV-LM2576-DC-DC-Step-Down-Adjustable-Power-Supply-5V-60V-to-1-25V-30V-/130940844577?pt=LH_DefaultDomain_0&hash=item1e7caeba21If you want to do it yourself, for 1A of power the MC34063 chip will work and Dave had a video explaining the formulas (for the boost mode) but there are online calculators out there which can tell you what parts are neededhttp://www.ebay.com/itm/10-x-MC34063API-34063API-IC-ICs-DIP-8-pin-High-Reliability-Integrated-Circuit-/280835749417?pt=AU_B_I_Electrical_Test_Equipment&hash=item41631d7629http://uk.farnell.com/texas-instruments/mc34063ap/ic-dc-dc-controller-34063-dip8/dp/1053579http://uk.farnell.com/on-semiconductor/mc34063ap1g/ic-dc-dc-controller-34063-dip8/dp/1191818You'll need an inductor, a schottky diode and some resistors and capacitors and you can make it yourself. The inductor and diode are the most important, the rest are easy to find.

author: rdl
author_rank: Super Contributor
timestamp: January 15, 2014, 02:14:05 pm
content: Yeah, with 37 volts out you would have to dissipate 25 watts of heat for 1 amp at 12 volts. That's not going to work. You would need a huge heat sink. I agree with the buck converter idea. If you still want to use the linear regulators for lower noise and ripple, you could use the buck converters to drop the input to about 3 volts above the regulators output. This would still have fairly high power dissipation, but not too bad. Certainly in the range of what 78xx regulators can handle with an adequate heat sink.

author: christos
author_rank: Regular Contributor
timestamp: January 15, 2014, 03:56:03 pm
content: thx ..the transformer just sits there..i got no problem buying a new one with 6 secondaries or 3 transformers with 2 secondaries

author: mariush
author_rank: Super Contributor
timestamp: January 15, 2014, 04:49:40 pm
content: You don't need to buy three transformers to get +/- 5v , +/-8v and +/- 12v. It's possible to use just only one transformer, or two transformers, one for +/- 12v and one for +/-5v and +/-8v.How big the transformer will be, it depends on how much current you need on each of those voltages.Figure out how much current you want on each voltage, because that matters when selecting the transformer voltage and power rating.For example, let's say 5v @ 0.5 A , 8v @ 0.5 A , 12v @ 1 ANext, you should know by now that a linear regulator will need about 1-2 volts above the output voltage to get a stable output voltage, so for 12v DC you will need at least 14v at the input.So you have transformer - > bridge rectifier -> capacitor - > 14v Input for the 12v linear regulatorNow if you go back towards the transformer. You know you'll have a capacitor that will smooth out the output of a transformer after the bridge rectifier so depending on how big the capacitor and the current you want there's going to be a fluctuation of voltage.There's a simple approximation formula which can tell you how much voltage will fluctuate between a minimum and maximum ... let's ignore the 5v and the 8v for the moment and focus on just the 12v... which means we work with 1 Amp.Capacitance = Current / 2 x AC Frequency x VoltageIn your case, capacitance = 1 A / 2 x 50 Hz (Europe) x Voltage. Let's say you're going to use a 3300uF capacitor or in other words 0.0033 F ===> 0.0033 F = 1A/ 100xVoltage ===> 0.33 Voltage = 1 , so Voltage = 1 / 0.33 = 3 volts.So with a 3300uF capacitor, you have transformer - > bridge rectifier - > capacitor (14-17v) -> linear regulator , maybe lm317 or 7812 -> 12vNow you know that pretty much every bridge rectifier loses about 0.8-1.4 volts so you should have a peak voltage of 17v + 1.4v before the bridge rectifier or about 18.4 volts at the transformer.The peak voltage of a transformer is determined with the formula Vpeak = Vtransformer x 1.4142 so 18.4 = Vtransf x 1.414 therefore Vtransf = 18.4/1.4142 = 13 volts AC.So your transformer should have a secondary winding of 13v AC or more to be able to get a stable 12v @ 1 A after going through bridge rectifier, capacitor, linear regulator. If you increase the capacitance to decrease that 3v of ripple to maybe 1-2v, you could work with a 12v AC transformer, which is more standardized than 13v AC or more.Or, instead of trying to keep the voltage after capacitor to a minimum of 14v by using large capacitors (3300uF 25-35v capacitors are large and expensive), you could just use a transformer with higher secondary voltage, like maybe 15v AC, which will give you a peak voltage of 20v so you can have 14-20v DC with smaller capacitors, like maybe 1000uF 25v for 1A of current...With a 12v AC transformer, if you want to get 12v @ 1A from it, you have the formula that sort of approximates it : VA rating = VA DC / 0.62 = (12v x 1A)/0.62 = 19.3 VA ... let's say 20 VA.If you want +/- 12v @ 1A, you can get a center tap 24v AC transformer, and you'll have two windings with 12v AC and you're in business... a 24v center tap, 40 VA transformer. 50 VA would be easier to find.Now all the talk above is for just +/- 12v 1A.If you want to use the same transformer to get 8v @ 0.5A and 5v @ 0.5 A, that means you need a transformer that can also handle 1A of current for these two other voltages. You can use a linear regulator to like 7808 or LM317 to output 8v @ 1A , and then use a 7805 or LM317 to get 0.5 A from the output of the 8v regulator.This means that the regulators after the bridge rectifier and capacitor need to have at least 14v @ 2A to do everything so you can adjust the math above (increase capacitor size using the formula and using 2A for current in the formula) and you'd also need a larger transformer, rated for around 75-100VA.

author: Rudane
author_rank: Regular Contributor
timestamp: January 15, 2014, 05:37:44 pm
content: Just to chime in, I now agree that if the RMS Voltage is 37 Volts, you definitely need to be looking at a switching power supply to get the voltages you require IF you want to use that transformer. They're not that complicated. Look for information about BUCK converters.

author: christos
author_rank: Regular Contributor
timestamp: January 16, 2014, 12:57:30 pm
content: i bought 2 transformers one with 15v and one with 12 volts at 1 amp and its working very good..thx for the help

author: Rudane
author_rank: Regular Contributor
timestamp: January 16, 2014, 06:12:44 pm
content: Glad to hear it. Have fun!