35V X 2 power supply - Page 1
Forum section: electronics / projects

author: neo
author_rank: Super Contributor
timestamp: October 31, 2016, 03:31:44 am
content: Before i say anything i feel fit to mention that this power supplies construction is a dangerous process that can easily kill or maim you, so if you try to follow in my footsteps and it maims or kills your i take no responsibility. The same goes double for your cat!This entire thing was a thought experiment built around the lm 338k (2 of them) to give me 35 volts at 5 amps technically this is two supplies in one but could easily be used in series to give me up to 70 volts. The thought experiment part is that everything i needed to build this was found in 5 minutes or less. Besides the fact i had the regulators and wanted a more powerful power supply i built this out of boredom while waiting on parts from russia. Despite being slightly harem scarem construction it works and i like it, the chrome gauge measures ua but has a special output and is normally turned off.

author: neo
author_rank: Super Contributor
timestamp: October 31, 2016, 03:50:23 am
content: the power cord this thing (visible in far right of picture) is an old 12000 watt welding cord

author: tautech
author_rank: Super Contributor
timestamp: October 31, 2016, 04:40:19 am
content: Yeah, looks rough and ready.Where's the supply transformer/s?You need something reasonably decent to handle 30V @ 5A continuous x 2.LM338 is what I used for my first PSU too and you need a good bit of heatsinking for the bridge and LM338's if you're going to run all anywhere near full capacity.

author: neo
author_rank: Super Contributor
timestamp: October 31, 2016, 05:37:45 am
content: I knew i forgot something! thanks for reminding me the regulators are on a heatsink the size of my smallest transformer (which is still huge) the metal cans are specced at 10 amps at 30 volts a piece im running then at 16 (different tap) and in series and the one for the main supply is rated for 8 amps at 30 volts says so on it as for the bridge rectifiers im using the case as the heatsink they all have thermal pasted and are drastically overspecced, the main supply bridge is 200v at 25 amp and the other one is 15 amp at 100 volts, two in series because even im not crazy enough to try wiring ac in series for anything other than a test. and on that ammeter i have a shunt i made it for 5 amps full scale but the metal case halves it.i should note the values i give for the transformers were for 110 volts so im getting a couple more volts out just enough to be max voltage on the output of the regulator, also the tape is for labels i have yet to write, the supply only weighs 30 pounds ('only' being sarcastic)One thing is special its all old stuff made in america except one voltmeter i threw on as an after thought

author: neo
author_rank: Super Contributor
timestamp: October 31, 2016, 06:26:16 am
content: Quote from: tautech on October 31, 2016, 04:40:19 amone important consideration is i built this to test old stuff that likes high voltage or high amperage its not going to be just maxed out constantly most of my uses wont even use one let alone two but there is always something more power hungry and i had the space on the case so i put two. i drift off point it wont be 100% for very much time at all and if i do need it to be there is always options like fans

author: tautech
author_rank: Super Contributor
timestamp: October 31, 2016, 06:59:19 am
content: 30 lbs is not too heavy for a linear supply of those specs, especially as you're using 2 transformers. For example the linear SPD3303X, 2 x 32V @ 3.2A plus a 3.2A @ 2.5/3.3/5V supply weighs in at 8 Kg (17.6 lb).You can join the AC secondariesIFyou get the phasing correct, easy enough to confirm with a DMM.IIRC LM338 is internally thermally protected but check the temps when giving it a good test, no point running it at meltdown levels.

author: neo
author_rank: Super Contributor
timestamp: October 31, 2016, 02:46:59 pm
content: i know i can but its a bit of a headache, and my version of a test is to run it if i think its too hot ill probably add a fan, and also i know its not heavy but its still noticable, 10 pounds a transformer the rule of thumb is 20 watts to pound ive found in my experiance its more like 30 watts a pound, i have a few transformers that confirm this theory like the 8 amp one on this

author: Kleinstein
author_rank: Super Contributor
timestamp: October 31, 2016, 04:49:51 pm
content: Keep in mind that a transformer specified for 10 A AC gives you about 6 A of DC current after rectifier and filter capacitor. This is because the AC current is RMS and the power factor is rather poor, due to pulsed current. Also the voltage will be higher than the AC rating. So a 30 V AC Winding will give you about 42 V DC minus loss at the rectifier. This might be already too much for the LM338 in case of a short at the output. So 35 V output is not realistic with an LM338. Maybe 30 V if you are brave.To reduce the power loss at the regulator it would be a good idea to switch between transformer taps. Ideally something like a 12 V and 22 V tap. Otherwise the current will be limited to much lower values (e.g. 3 A) at low voltages like 5 V, as there is also a power limit for the LM338.

author: BravoV
author_rank: Super Contributor
timestamp: October 31, 2016, 05:04:56 pm
content: For further efficiency, replace the rectifier and look for LT4320, example success story from one of the member ->HEREor just read the whole thread.

author: neo
author_rank: Super Contributor
timestamp: October 31, 2016, 05:10:55 pm
content: it works and i played my numbers right to get it right near perfect, as for the power loss thats why i have 10 amp transformers, i dont plan to keep it at 5 amps but i wanted ti to be able to do it if for nothing else short pulses, im not a complete idiot the lm338k has 5 amp RMS or 7.5 amp peak, the key point is i didnt design it for 5 amp RMS at any voltage i just designed it for variable voltage with 5 amp peak conservative limit and besides that i didnt design it to be fancy or perfect just functional which it is and now if i want i can come back and add other features or abilities on its back

author: ZeTeX
author_rank: Frequent Contributor
timestamp: October 31, 2016, 08:59:44 pm
content: Quote from: Kleinstein on October 31, 2016, 04:49:51 pmHow did you calculate this? just dividing by 1.666? (10/6=1.66) what's the formula?

author: neo
author_rank: Super Contributor
timestamp: October 31, 2016, 10:50:02 pm
content: i have one question it says input out put differential is 35v but the measured voltage is 38 volt primary and 48 secondary no load, if that was the case i wouldnt be able to go down to 1 but it lets me, so much question is why/how? or should i just go with the logic of the rest of this, it works so leave it alone

author: Kleinstein
author_rank: Super Contributor
timestamp: November 01, 2016, 06:37:27 pm
content: The limit for the LM338 or 317 is the difference from input to output, as it does not see the 0 at all. For normal operation this is not a problem, even with a 48 V (or even higher) supply. However this can be a problem in case of overload / short. Than the output voltage will drop and it might blow the regulator. So the supply will not be short circuit save and over-temperature protection is also not really effective.To be on the save side the input side should be 35 V maximum, at least with a little load (e.g. 0.5 A).P.s. The ratio of AC current to DC current is a rule of thumb number. Depending on the impedance of the transformer and size of filter caps. The ratio is usually between about 0.7 and 0.5, with the higher numbers for smaller transformers / less filter cap. In extreme cases (e.g. very large transformer and cap) the numbers can go down even more - very small transformers with high output impedance can be a little higher than 0.7. If you just take into account that the filter cap will give up to the peak voltage (1.4 time the RMS value) it is clear that the DC current must be smaller than the RMS one by at least the factor of 0.7 = 1/1.4 .

author: neo
author_rank: Super Contributor
timestamp: November 02, 2016, 01:20:29 am
content: the primary is 36 volts under half amp load so i guess ill just fix the secondary

author: Kleinstein
author_rank: Super Contributor
timestamp: November 02, 2016, 03:35:07 pm
content: 36 V of AC voltage is just too much. It will end up somewhere around 48 V, just as measured. One way to reduce it would be a series inductor to do power factor improvement, but the inductor can be quite bulky. The no load voltage would stay the same, but under load the voltage would go down.Otherwise one should not go much above 24 V AC and thus accept a maximum DC output and thus accept a maximum DC output somewhere around 27 V. That is about the highest one can get reliable.

author: neo
author_rank: Super Contributor
timestamp: November 03, 2016, 05:02:50 am
content: thats 36v from the capacitor aka dc

author: neo
author_rank: Super Contributor
timestamp: November 04, 2016, 10:58:22 pm
content: ok after redoing it im getting 38/37 volts dc no load