LM723 bench supply with seperate control voltage ? - Page 1
Forum section: electronics / beginners

author: lordvader88
author_rank: Frequent Contributor
timestamp: October 24, 2018, 09:43:32 am
content: I have a perfect stereo transformer I salvaged with 3 windings. The main winding is 52Vp unloaded for 36V @ 1.5A, 1 is 16Vp for 10.3V @1.2A, the 3rd is only 6.2VpSo I should be able to run the LM723 and any LCD dials/etc off the 16V , it won't be loading it very much at all tho. So that would have it's own FWRThen can I use the 52V with its own FWR, and 2n3055 pass transistors for the main output ?That would mean I'd have to tie both negative terminals of the FWR's together, and be using the 16V supply to drive the base(s) of the 2n3055 ?? Is that allowed ?And I'd be sampling the output from the 52V supply, with a voltage divider such that it never goes over the max input voltage of the error amp.

author: lordvader88
author_rank: Frequent Contributor
timestamp: October 24, 2018, 10:01:22 am
content: It should be no different that using separate batteries, with a common ground, electrons are just returning to their respective sources, but I just want to make sure since all the LM723 circuits I've seen use only 1 source. But it's max input voltage is 40V

author: not1xor1
author_rank: Frequent Contributor
timestamp: October 25, 2018, 09:06:44 am
content: Quote from: lordvader88 on October 24, 2018, 09:43:32 amif the transformer sec. is 52V RMS unloaded you CANNOT use 2N3055 as they are limited to 60Vin any case check the SOA on the datasheet because you have to take into account the secondary breakdown too

author: spec
author_rank: Frequent Contributor
timestamp: October 25, 2018, 09:55:56 am
content: + lordvader88Yes, the architecture you outline is OK, providing that the three secondary windings of the transformer are not connected together.But, as not1xor1 says, a 2N3055 will not handle the rectified voltage from the 52V winding, which will be around 71.5V (52 *1.414 -2) off load, as the 2N3055 has a maximum voltage, collector/emitter (VCE) of 60V:On the other hand, the 71.5v supply would probably drop quite a bit when sourcing current, so if you can ensure sufficient load current you may be able to get away with a 2N3055.There is another aspect though. Under operating conditions the voltage across the 2N3055 (VCE) will be the voltage from the bridge rectifier minus the stabilized output voltage from the 2N3055 so, unless the 2N3055 is producing a very low output voltage, the VCE may be within the 60 VCE limit. But you would need to ensure that under all operating conditions, especially turn on, that the 2N3055's maximum VCE is not exceeded.Also, as not1xor1 says, you will need to check the 2N3055 safe operating area, which is quite restrictivehttp://www.onsemi.com/pub/Collateral/2N3055-D.PDF

author: Wolfgang
author_rank: Super Contributor
timestamp: October 25, 2018, 02:40:16 pm
content: Quote from: lordvader88 on October 24, 2018, 09:43:32 amSome comments here:- the max continuous voltage of an LM723 is 40V, so a direct hookup to the 52Vpp max is not possible.- a 2N3055 is NOT capable as a pass transistor because its breakdown voltage and SOAR. You need something better than that. 2N3773 or better is recommended.- I would recommend a floating design with the 723 powered by the 16V winding. If you rectify and stabilize this before the LM723, a very low ripple design ispossible. A lot of better lab power supply stuff does it this way.Some hints:https://electronicprojectsforfun.wordpress.com/power-supplies/a-collection-of-proper-design-practices-using-the-lm723-ic-regulator/and a floating design (not with LM723, but showing the principle):https://electronicprojectsforfun.wordpress.com/power-supplies/high-voltage-lab-power-supplies/a-250v-150ma-variable-linear-power-supply/Another (awkward) solution just came to my mind: There is an (obsolete) variety of the LM723 that can take 80V. Its the L146CB. With this one, you dont needany floating design, but just the classic 723 one.much funWolfgangHave funWolfgang