Need advise with schottky voltage drop - diyAudio
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Old 18th January 2012, 03:11 PM   #1
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Default Need advise with schottky voltage drop

hi, im a newbie diy'er and am making a power supply from a kit.

schematic for the rectifier and filter part of the kit is attached.

the kit is fairly prehistoric and i cant find any references to it on the web. its also fairly short on description. the schematic is pretty much the only thing in english.

when i connect a 13.8V AC (measured) center tapped output to the circuit, the rectified voltage drops to around 9V DC (measured at the filter caps)

when i connect a 16 V AC measured CT transformer, i get about 11 V DC at the caps.

the kit is supposed to give me a variable regulated (via lm723) output of 0 to 15 VDC @ 5 or 10 A.

with the 13.8VAC input, im getting about 8.8 VDC regulated output. and with the 16 VAC input, i get about 10.8 VDC regulated output.

i tried applying a bench 14VDC supply to the circuit and the voltage at the caps was about 13.8VDC.

the kit is so old that i cant find any specs on the diodes.

what im wondering is:
1. is such a drastic voltage drop in the rectified output normal?
2. what i want is a 12 to 13 VDC output, so should I use a 17 or 18 VAC transformer? the filter caps are rated at 16V.
3. the diodes are 2-pin TO-220 and share a heatsink that is soldered to the positive rail because of where the heat sink pcb holes are. the kit shipped with mica insulating pads for the diodes. i put heat sink paste on both sides of the mica and used the supplied nylon washer to screw the diodes to the heat sinks. given the general lack of instructions with the kit, did i do soemthing wrong?

all measurements were taken with a modern RMS multimeter.

Thanks!!
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Old 18th January 2012, 04:31 PM   #2
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No, normally a Schotky diode must have about .2 to .4V voltage drop. What may be happing is that electrolytic´s are fulminated although outside are well viewed. I work with power industrial electronics and is very common that electrolytics fault inside, but outside are pretty good. Try to add some in parallel to see if the voltage in them approaches 1.4 VCA´.
Good luck !!!
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Old 18th January 2012, 07:46 PM   #3
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The kit expects a 24 to 27 volt center tapped transformer. The "x 2" means two coils of 12 to 13.5 volts that would give you 24 to 27 volts across both coils.
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Old 18th January 2012, 09:11 PM   #4
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hi,
thanks!
i ended up trying both suggestions. connecting a different set of known good caps did not change the result atall.

a 24vct transformer allows me to vary the regulated output voltage between 0 and about 17VDC. my only concern is that i see slightly over 18 V across the caps and they are rated for 16v. will this drastically shorten the life of these caps? the power supply will be used for up to 6 or 7 A @ 12 VDC.

as a precaution, should i add a fuse between the secondary and the pcb if the caps blow and short?

thanks again!
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Old 18th January 2012, 10:07 PM   #5
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Quote:
Originally Posted by joystick View Post
my only concern is that i see slightly over 18 V across the caps and they are rated for 16v. will this drastically shorten the life of these caps?
Yes. They'll last a week or so. You need 25v caps there.
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Old 19th January 2012, 11:16 AM   #6
AndrewT is offline AndrewT  Scotland
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Fitting a fuse between the secondary and the smoothing capacitors achieves almost nothing in respect of increased Safety.

The Primary (Mains) fuse should be rated to suit the maximum output of the equipment.
If you overload the transformer then this Primary fuse should blow if the overload is sustained. It should not blow every other time you switch ON.

The secondary fuse after the smoothing should be rated to suit the average current draw during all operating conditions. Again this will blow if an overload is sustained.

A secondary fuse between the secondary and the smoothing caps will need to be rated to suit the switch on surge, that is quite prolonged, in charging up the capacitors and supplying the downstream equipment during the power up phase.
This fuse would need to be so highly rated that it will rarely if ever blow due to sustained overloading.

A shorted cap is more likely to blow the Primary fuse if you have fitted the correct fuse for the duty you have imposed.
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Old 19th January 2012, 01:45 PM   #7
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thanks Andrew.

I keep blowing 2 A mains fuses from powering just the transformer even when the secondaries are not connected. the transformer is a 117 VAC to 24 VACT at 10 A creature. do i need to use a slow-blow fuse or is 2 A not enough?

I'm planning on putting a 10 A fuse for the regulated output. if that also tends to blow during cap charging of the powered device, should i put a slow-blow type thing there too?

thanks a tonne!
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Old 19th January 2012, 01:49 PM   #8
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thanks. im going to replace these with 25v caps soon.
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Old 19th January 2012, 02:07 PM   #9
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You'll need slow blow fuses as transformers have an inrush current surge at turn on. Also, the large caps add more surge current as they are essentially a short circuit when first charged at turn on.
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Old 21st January 2012, 12:07 AM   #10
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Quote:
Originally Posted by djQUAN View Post
You'll need slow blow fuses as transformers have an inrush current surge at turn on. Also, the large caps add more surge current as they are essentially a short circuit when first charged at turn on.
thanks!
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