Author Topic: First problems with the ST  (Read 3570 times)

38ac

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First problems with the ST
« on: June 10, 2013, 12:37:26 PM »
We have some machinery in the shop that pulls more power than my sub-panel can supply so we help it out with the 1115/15 KW ST. The loads are separated, we are not tying to the grid. Anyway we had the very first problem with the ST head when the 220 voltage dropped to about 70. Took about 15 minutes with the meter to figure out the bridge rectifier was bad.  This was with the help of the manual Tom Osborn has on the web page. Luckily we had heeded the good advise on the forum and had spares, actually 20 of them,  ;D on hand and had the ST back up and going in 30 minutes. I like the ST better now than I did before, I am no electric 'spert and still didn't take long to diagnose and repair a problem.
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mike90045

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Re: First problems with the ST
« Reply #1 on: June 10, 2013, 02:45:34 PM »
Question about the failed Diode Bridge,  was it Chinese, or a replaced one ?

38ac

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Re: First problems with the ST
« Reply #2 on: June 10, 2013, 03:15:31 PM »
It was the original, one of these  



 I replaced it with one I got on ebay,bought 10 of them for like $10 including shipping.  I kept tabs on the temp because the new one doesn't have the aluminum heat sink. barely got warm pulling 30 Amps on 220 it is attached to the gang box for a heat sink. The new one is like this.
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BruceM

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Re: First problems with the ST
« Reply #3 on: June 10, 2013, 04:00:42 PM »
That type of metal case bridge (my favorite also)  is intended to be bolted to a metal case or heatsink, with some thermal goop to improve heat transfer.  No heatsink is required if the current is low enough but I always error on the side of caution with cooling.


38ac

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Re: First problems with the ST
« Reply #4 on: June 10, 2013, 04:23:29 PM »
Bruce, I have it bolted to the side of my junction box for heat sink but couldn't find the heat sink goop I bought, >:(  Side of the box just barely got warm to the touch.

How warm is warm for the rectifier?
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contaucreek

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Re: First problems with the ST
« Reply #5 on: June 11, 2013, 12:45:17 AM »
You could probably buy a small heat sink to act as a back up on the outside of the box. Sooo.... you know how how to use the diode setting on the meter now huh  ;)
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38ac

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Re: First problems with the ST
« Reply #6 on: June 11, 2013, 03:33:03 AM »
From me old Declo SI rebuilding days I know how to check diodes with a meter. What I had to sit down and think about was how to check them when in a bridge. I do OK with basic 'electrics but become lost quickly once past the elementary.
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BruceM

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Re: First problems with the ST
« Reply #7 on: June 11, 2013, 07:12:56 AM »
165F is where you can touch without burning but not hold for more than 3 seconds with your finger.
If you can hold your finger on it right after a full load run then you're good.

The reason I like the metal case bridge diodes is that there is so much thermal contact, it's hard to wrong with them.  In a pinch you could use silicone caulk as thermal grease/goop.


ronmar

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Re: First problems with the ST
« Reply #8 on: June 11, 2013, 03:11:22 PM »
The field current on a ST-5 at full load is not all that great, that bridge should handle it just fine. bolted to a metal case/box it should be bulletproof:)  The hands on test is a good gauge of heat.  120F is about the highest temp the human skin can sustain without sustaining permanent damage.  Above that, the time you can maintain contact is measured in seconds.  Personally if i can stand it for about 4-5 seconds it is around 150F

As for testing a bridge rectifyer, it has 4 terminals.  There is a diode connected between each of the 4 leads(4 diodes).  With a volt/ohm meter, you measure between any two leads(1-2, 2-3, 3-4, 4-1) then reverse the leads and repeate the measurements. Each terminal pair should have low resistance in one direction(meter forward biasing the diode) and high resistance measured in the opposite direction/lead polarity(meter reverse biasing the diode)...
PS 6/1 - ST-5.