Testing a power transistor.

We all start somewhere
Post Reply
User avatar
pre65
Amstrad Tower of Power
Posts: 21373
Joined: Wed Aug 22, 2007 11:13 pm
Location: North Essex/Suffolk border.

#1 Testing a power transistor.

Post by pre65 »

I have a Maplins 13.8v regulated power supply that blows the fuse in the mains plug (2A or 5A) but not the internal 500mA fuse.

I have had a look inside, and probed with the DMM but can find nothing obvious. :(

There is a 2N3055 on the rear panel (to regulate I presume) and I wondered if there was a way to test it with a DMM to see if it's shagged, or not ?
The only thing necessary for the triumph of evil is for good men to do nothing.

Edmund Burke

G-Popz THE easy listening connoisseur. (Philip)
Cressy Snr
Amstrad Tower of Power
Posts: 10552
Joined: Wed May 30, 2007 12:25 am
Location: South Yorks.

#2 Re: Testing a power transistor.

Post by Cressy Snr »

pre65 wrote:I have a Maplins 13.8v regulated power supply that blows the fuse in the mains plug (2A or 5A) but not the internal 500mA fuse.
Have you tested the mains lead Phil ?

If the plug fuse is blowing, then there is a dead short on the AC mains side of the transformer; either live to neutral or live to ground. The lead or the transformer primary is faulty.
Sgt. Baker started talkin’ with a Bullhorn in his hand.
User avatar
pre65
Amstrad Tower of Power
Posts: 21373
Joined: Wed Aug 22, 2007 11:13 pm
Location: North Essex/Suffolk border.

#3

Post by pre65 »

Thanks Steve.

The resistance between live and neutral (measured at the plug) with the unit switched on is 0.9 ohms.

So that should be the wire and the mains transformer primary.
The only thing necessary for the triumph of evil is for good men to do nothing.

Edmund Burke

G-Popz THE easy listening connoisseur. (Philip)
Cressy Snr
Amstrad Tower of Power
Posts: 10552
Joined: Wed May 30, 2007 12:25 am
Location: South Yorks.

#4

Post by Cressy Snr »

pre65 wrote:Thanks Steve.

The resistance between live and neutral (measured at the plug) with the unit switched on is 0.9 ohms.

So that should be the wire and the mains transformer primary.
That's a fault condition.
Have you tried measuring with the unit switched off? Resistance across live and neutral/live and earth, at the plug, should be infinite.
If it is not then the lead is faulty, that's assuming it is not the switch itself. If the resistance is infinite, then you have a transformer or ground fault within the unit on the mains side. As to the cause, you'd have to investigate further.

As a comparison with two of my 230V mains toroids, the resistance of the primary is around 6-7 ohms on both of them. Also, I get the same 6 and a bit ohms on my two Danbury mains transformer primaries when configured for 230V.
Sgt. Baker started talkin’ with a Bullhorn in his hand.
User avatar
pre65
Amstrad Tower of Power
Posts: 21373
Joined: Wed Aug 22, 2007 11:13 pm
Location: North Essex/Suffolk border.

#5

Post by pre65 »

Thanks again Steve.

Tis deffo the transformer. :(

There are no markings on the item to denote secondary voltage so I'll recycle the case and bin the rest.

Ebay here I come for a replacement. :wink:
The only thing necessary for the triumph of evil is for good men to do nothing.

Edmund Burke

G-Popz THE easy listening connoisseur. (Philip)
User avatar
pre65
Amstrad Tower of Power
Posts: 21373
Joined: Wed Aug 22, 2007 11:13 pm
Location: North Essex/Suffolk border.

#6

Post by pre65 »

Whilst rooting through a box of old tut I came across an transformer wombled from a TDK woofer box.

I'd tested the secondary as 17 - 0 - 17 with no load, and as it was a similar size to the failed one I wired it into the old case.

It's now giving a steady 13.6v DC which is near enough to the 14v I was after.

So, I'll leave it running for a while to check the temperature than if that's OK it will power my Beresford DAC.

The DAC should run at 12v but Stan has stated that up to 14v may sound better. :D :D
The only thing necessary for the triumph of evil is for good men to do nothing.

Edmund Burke

G-Popz THE easy listening connoisseur. (Philip)
Cressy Snr
Amstrad Tower of Power
Posts: 10552
Joined: Wed May 30, 2007 12:25 am
Location: South Yorks.

#7

Post by Cressy Snr »

A result then Phil. :)
Sgt. Baker started talkin’ with a Bullhorn in his hand.
User avatar
pre65
Amstrad Tower of Power
Posts: 21373
Joined: Wed Aug 22, 2007 11:13 pm
Location: North Essex/Suffolk border.

#8

Post by pre65 »

Certainly is Steve. :)

One of those rare times when something we thought was ****ed is brought back to full working order. :wink:
The only thing necessary for the triumph of evil is for good men to do nothing.

Edmund Burke

G-Popz THE easy listening connoisseur. (Philip)
Post Reply