EEVblog Electronics Community Forum

Electronics => Beginners => Topic started by: Cliff Matthews on December 04, 2017, 12:59:19 am

Title: Fixing a small SMPS (for dual LM317 split supply)
Post by: Cliff Matthews on December 04, 2017, 12:59:19 am

Sorry for all the pictures... I want to make a split variable 317 supply of +/-24v at 700ma, so I picked-up 2 15-watt 24v SMPS's for use as pre-regulators. I wanted to test the limits of the optional adjust pot to see if I could get 30v stable. It turns out I can, but I pooched one of the units zener by briefly testing its maximum output.

Googling ZC 339D gives me nothing. Now I need to know what zener to use, or can I use a 1watt glass type?
Also, is there a definitive way to read part numbering like this? (eg: the stepped line around barrel)

(https://s33.postimg.org/loz0aby73/1_Cosel_LDA15_F-24.jpg)

(https://s33.postimg.org/r0dwv1rzj/2_Cosel_can_be_adjusted.jpg)

(https://s33.postimg.org/p8ky06qn3/3_Cosel_missing_adjust_and_caps.jpg)

(https://s33.postimg.org/5dywe114v/4_Cosel_populated.jpg)

(https://s33.postimg.org/6g92wletb/5_Cosel_at_34ohms_both_good_no_ripple.jpg)

(https://s33.postimg.org/uwr8r1sen/6_Cosel_34ohms_29.1v_with_2.20k_adj.jpg)

(https://s33.postimg.org/mehsmptlr/7_Cosel_2nd_unit_at_220ohms_ok.jpg)

(https://s33.postimg.org/j7n933gvj/8_Cosel_pot_should_not_do_any_harm.jpg)

(https://s33.postimg.org/517i7vlfz/9_Cosel_2nd_unit_croaked_at_40v.jpg)

Googling ZC 339D gives me nothing

(https://s33.postimg.org/4bopvj0bz/10_Cosel_mystery_zener_Kaput.jpg)

Title: Re: Fixing a small SMPS (for dual LM317 split supply)
Post by: mikerj on December 04, 2017, 10:02:56 am

When you say the adjustment pot is an option, I suspect you mean that you can order it with the pot fitted?  This being the case, it's almost certain that some of the fixed resistor values are changed when the pot is fitted, which gives the stated adjustment range (and prevents things blowing up when the pot is turned to minimum resistance).

Title: Re: Fixing a small SMPS (for dual LM317 split supply)
Post by: Cliff Matthews on December 04, 2017, 11:27:43 am

Thanks, makes sense one smd part underneath must be different. On the zener: since I don't need the stock 24v, does it really matter how close I get? As long as clamping doesn't kill the 35v caps, I could test some pulled parts for 30 to 33 volts, right?
By its number, ZC339D could mean 33.9v, sure wish I knew  :-//

(Note: Ideally, I'd like to switch-in 24 or 32v, then I can current limit on another 317 with a few resistors and panel switch)

Title: Re: Fixing a small SMPS (for dual LM317 split supply)
Post by: Cliff Matthews on December 05, 2017, 12:57:32 am

OK, last 2 digits were a date code and complete part # was 1ZC33. Replaced zener's with 33v 5-watt (ITT #1N5364B), and upgraded output el-caps to a more reliable 50v.  These will be switch selected to run at 24v-700ma or 32v-500ma).
(https://s33.postimg.org/f905e1ugf/1_ZC12_to_1_ZC120_Toshiba_1_W_zener.jpg)