What to do with +/-36.5V DC from +100W transformer, higher than realized - Page 1
Forum section: electronics / projects

author: lordvader88
author_rank: Frequent Contributor
timestamp: March 29, 2018, 06:12:08 am
content: I picked out all the parts for a LM317/LM337 dual tracking supply like this here at the BOTTOM of the page not the 1st onehttp://www.joebrown.org.uk/wp/?p=7294But I had forgot the DC output so high, after a FWR. The FWR and tank caps are soldered up and it just below the 37V max across each LM3x7. And at 73V rail to rail, I can't use an LM741 op-amp or the LM7808 as in that schematic for the tracking.I have a 300W AC dimmer switch of some kind very common, I also have BT136 600V 4A TRIACs. Can I just 1 to knock back the voltage 5-10V DC as I please , with a triac circuit or the dimmer ?And what side of the transformer ?

author: Smokey
author_rank: Super Contributor
timestamp: March 29, 2018, 06:47:50 am
content: Unwind some turns on the transformer secondary?

author: Zero999
author_rank: Super Contributor
timestamp: March 29, 2018, 08:12:18 am
content: Wind more turns in series, but anti-phase.Transistors can also be added to the LM317 to boost the voltage rating. You might want to select a lower voltage zener than 39V though: try 27V.

author: Kjelt
author_rank: Super Contributor
timestamp: March 29, 2018, 10:14:36 am
content: Quote from: lordvader88 on March 29, 2018, 06:12:08 amYes this has been done over tens of years by a dutch PSU company called Delta see picture.They call it a triac preregulation and it is used to keep the powertransistors cool by setting the input voltage just high enough to get the required output voltage so the linear losses will be minimized.In your case it is even simpler since you need a fixed preregulation, only downside is you probably will get some humm (audio noise).

author: mariush
author_rank: Super Contributor
timestamp: March 29, 2018, 10:31:03 am
content: A relatively simple and cheap solution would be to put a few diodes in series before the regulator, each dropping 0.5v...1v (depending on current) .. ex use some 3A diodes or bridge rectifiers.For the positive you could just use LM317HV

author: lordvader88
author_rank: Frequent Contributor
timestamp: March 29, 2018, 11:32:27 am
content: Yeah no way will I try opening this nice transformer.I could use diodes to drop the LM3x7 voltages. Just watched a video on the output of a SCR and it's not pretty, but I'll try it for now, never made a TRIAC/SCR circuit before. But still can't use the 741/7808 part over 43V rail to rail for now.The transformer/FWR+filter caps/Neon bulb is on 1 PCB. I will then clip on other PCBs as needed. I was breadboarding a series pass reg with power BJTs and LM358, but didn't get the negative reference voltage working right. I'll try that again soon too.The transformer output should drop a lot at 1A or whatever it is, but for now that's no help either, nothing I make uses over 100-200mA

author: RoGeorge
author_rank: Super Contributor
timestamp: March 29, 2018, 11:36:39 am
content: Seems ideal for a full analog power supply, or an audio power amplifier supply.

author: ch_scr
author_rank: Frequent Contributor
timestamp: March 29, 2018, 12:05:26 pm
content: Maybe try a SCR preregulator to limit DC Bus voltage? Like in Linear AN32 "High Efficiency Linear Regulators" page 3

author: madires
author_rank: Super Contributor
timestamp: March 29, 2018, 01:32:03 pm
content: I'd suggest to put the transformer aside for a nice audio amp and to get a matching one for your PSU project.

author: duak
author_rank: Super Contributor
timestamp: March 29, 2018, 04:09:50 pm
content: How much do you want to reduce the voltage by? When I was very much younger the only parts I could reasonably afford or get were from Radio Shack or salvaged from cast off TVs & the like. Transformers were never the right voltage. I found that wiring up a smaller transfomer as essentially an autotransformer in series with the main transformer I could increase the secondary voltage by say 10% but also reduce it by any value I wanted. The smaller transformer's secondary had to be rated to carry the larger transformer's primary current.I also worked out a novel SCR preregulator circuit that was far simpler than any others I'd seen. Basically, the + output of the bridge rectifier went thru an inductor to the anode of an SCR. The SCR's cathode connected to the filter capacitor's + terminal. The SCR's gate was driven by a reference voltage developed by a zener diode clamp to the - of the filter cap. A resistor from the rectifier + supplied current to the SCR gate. The circuit operates by triggering the SCR when the voltage on the filter cap is less than the zener clamp voltage.There are some things to consider:- SCRs like to have a solid gate drive pulse to turn them on solidly before they are required to pass their rated current. Better and more complex designs use driver transistors and often pulse transformers to do this.- the SCR should be a sensitive gate type rated for the peak charging current of the filter cap and maximum voltage,- the resistor has to be able to provide enough gate current for the voltage range expected.- the inductor in series with the anode should be sized to limit the peak current. I found I didn't need one because the leakage inductance of the transformer limited the peak current anyway.- the transformer made a bit more of a buzzing noise but a series inductor should control this.- one refinement I thought of to improve the SCR's gate drive is to use another SCR in a sort of darlington configuration to develop a faster, sharper pulse.Hope my description makes sense,Cheers,

author: lordvader88
author_rank: Frequent Contributor
timestamp: March 29, 2018, 07:02:44 pm
content: Quote from: madires on March 29, 2018, 01:32:03 pmThis transformer is from a salvaged 100W panasonic 5CD player, got a few nice things from that.It's now for a bench PSU. It does have 2 other control windings, all seperate. At some point I should re-make its relay circuit.Never used an SCR before so I don't really know anything about them.But I'm not spending any more money for, just using what I have, thanks

author: ArthurDent
author_rank: Super Contributor
timestamp: March 31, 2018, 01:34:54 pm
content: Where you are determined to use this transformer instead of a more suitable one, your choices are somewhat limited and none of the options are going to be ideal. It is not clear if the 36.5 VDC you measured is just from the xmfr/FWR/caps with no load, or not. If it’s with no load, then just placing a 470 ohm 5W resistor across each of the 2 caps will probably drop the voltage down near the 32 VDC range and correct the problem and this would be the simplest solution.You haven’t mentioned what the other windings on the transformer are or what the main secondary winding voltage you are using is and that could be important. Others have mentioned winding another winding on your transformer and connecting it in series with the primary out of phase as one of the best ways to correct the problem. Depending on what the other existing secondary winding voltages are you may already have an unused winding on the transformer that can be connected in series with the primary to lower the voltage on the winding you are using. The only caution is that the insulation on whatever winding you put in series with the primary should have adequate insulation to withstand line voltage. You can make sure this isn’t a problem by using a 3-wire line cord and making sure the added winding is connected in series with the neutral side of the transformer’s primary winding.

author: Zero999
author_rank: Super Contributor
timestamp: March 31, 2018, 06:10:17 pm
content: I take it that it's the op-amp, which is the limiting factor, rather than the regulators. If the power supply voltage is 36.5V worst case, it should be possible with the good old TL072. Here's a simulation.I know this isn't ideal, but there are far worse s PSU designs around. The maximum output current at lower voltages (VIN= VOUT> 15V) will be much less than 1.5V, 500mA or less, due to the reguator's safe operating area protection kicking in.