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Old 20th June 2013, 09:24 AM   #1
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Default Transformer overvoltage?

Hi guys,

i brought a transformer which was marked as 18-0-18 @ 8 ampere current for powering 6 sets of tda2050.
little did i know, as i came home and connected the transformer, i found the values coming out to be outrageously high! unrectified value was 51VAC between the two rails ie. having output of 25-0-25!

i have no clue how to get rid of extra voltage as i realized the fact that this transformer will SAG to 18v when loaded at 8 ampere :\

Experts, please guide me on how to go about this? will adding high current genral purpose diodes in series to the rails be right ? 25-0-25! i wish i had brought the transformer first rather than at the last.. couldve gone for tda 7293 .

as of now, guide me please if the circuit will be safe to use/should i get rid of the voltages/how to do that?
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Old 20th June 2013, 09:33 AM   #2
canvas is offline canvas  Taiwan
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Why not return it? The spec is obviously wrong. Otherwise, use 7824/7924 or some voltage regulator is an option. You may parallel more regulators to gain more current.

Last edited by canvas; 20th June 2013 at 09:37 AM.
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Old 20th June 2013, 09:41 AM   #3
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Quote:
Originally Posted by canvas View Post
Why not return it? The spec is obviously wrong. Otherwise, use 7824/7924 or some voltage regulator is an option. You may parallel more regulators to gain more current.
i cannot return it, it was custom made on order. if the specification is wrong, i might try to talk it out with the fellow who sold it to me i specifically pointed it out, that the voltage shouldnt sag. i guess thats why he made it to 25v. nevertheless, it is too huge for tda2050
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Old 20th June 2013, 10:14 AM   #4
Johno is offline Johno  Australia
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In my opinion you have been cheated.

Good transformers with an adequate gauge on the secondary wire typically have an output voltage variation between 5-10% no load to full load. You have 40% variation. They also have a specification for temperature rise at full load after "x" hours from an ambient of 25C which is used to determine the safe load current.

You appear to have been given a 25V unit only capable of 1 or 2 amps full load. The risk of fire is huge and such a transformer would never receive approvals or certifications to be sold/used commercially in Taiwan for an 8 amp load. Talk to the guy yes but if he plays hard explain how social media and word of mouth etc can be used to alert all other potential customers to his deception.
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Old 20th June 2013, 10:23 AM   #5
AJT is offline AJT  Philippines
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at what primary voltage did you specify? and what is your wall outlet voltage? without these information, then it is hard to say with certainty..
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Old 20th June 2013, 02:38 PM   #6
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@AJT
I specified 220v Which is common domestic voltage in house. i am right now getting 235v i admit it might sound a bit high but cant be a reason for this high voltage.
@john, yes i think i might have been cheated but the transformer in no sense seems weak, i crosed checked with other transformers, the variation was very less in others. it has comparatively high gauge wire size and huge bobbin here is the comparison with different transformers. one on the right is
13-0-13 @2 amp
beside it is the ordered 18-0-18 which is showing wrong readings
above it is 25-0-25 @ 3 amp( yes this was also custom made, apart from very less no load hum noise, there is no problem with this piece and the voltage is 27 at open circuit)

Click the image to open in full size.

Last edited by godslayer; 20th June 2013 at 02:38 PM. Reason: Forgot to tag the image
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Old 20th June 2013, 02:55 PM   #7
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And that is a NEW custom made transformer. I've seen better in scrap bins. I think you've been HAD.
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Old 20th June 2013, 03:42 PM   #8
AndrewT is offline AndrewT  Scotland
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Send it back and tell him to wind it properly to your specification.
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Old 20th June 2013, 04:12 PM   #9
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I never said NEW! the two transformers came for 25 usd FLAT both 25 volt and 18 volt ones. can you beat that for a price?
My project is not complete junkyard build, but considering my budget and being a student these are the best things i can afford for the price. as for the voltages, i am sure the guy can open up the secondary windings and unwind a few turns very easily for no charge.

Last edited by godslayer; 20th June 2013 at 04:15 PM.
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Old 20th June 2013, 09:10 PM   #10
Johno is offline Johno  Australia
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Sorry I thought you meant professionally made custom unit.

Recycling is fine, I do it too. However the basic problem with this transformer is all the copper losses plus leakage inductance causing the voltage sag. At the full 8A load you will have too much temperature rise and it WILL cause a fire.

It is simply not safe to use at that current and all your good work will go up in brown smell if you try.
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