AC to DC adapator puzzle; educate me please

Thread Starter

Phil Russell

Joined Dec 21, 2014
19
I really should have paid more attention at school but do not recall this being taught.
I have an adaptor with output stated as 13.5v Dc and 700Ma. It stopped working and as part of the search for a cause I used a multimeter across the output wires. At times, most times in fact, output was zero but at others and with a bit of wiggling of the output wires where they entered the adaptor casing, I got about 19.5v. Clearly a wiring issue but the voltage puzzled me as I have another 13.5v adaptor that gives me 13.6v across the output.
I took the casing apart to find a standard bridge rectifier of 4 IN4002 diodes and a 3300 microF 25v capacitor. There is no obvious damage anywhere. I rewired the unit to get a wiggle free system of 19.5v. I then connected it to the original equipment and tested the adaptor under load. Voltage dropped to 18v. I tried another load, this time a 12v car bulb, most likely 5w rating, and voltage dropped to 15.5v.
Now the original equipment has a voltage requirement of 12 - 18v DC according to the manual so the adaptor would appear to be OK for that use. But is it? Why am I getting a value of 19.5v DC without load with this adaptor but 13.5v (13.6v)DC with another adaptor when both state 13.5v DC on the adaptor casing, albeit a different make.

Could it be that one or more diodes are failing or failed? Could it be the capacitor?

Sorry to be asking what may seem such an elementary question but my theoretical knowledge of these matters is dismal

All advice gratefully received
Cheers, Phil
 

LesJones

Joined Jan 8, 2017
3,133
The voltage regulation of that type of power supply is very poor so with no load on it the output will be higher than with it's rated load of 700 mA. From the readings it looks like the equipment it is powering takes much less than 700 mA. There in nothing to worry about.

Les.
 

Alec_t

Joined Sep 17, 2013
12,001
If your adapter is the old-school type which includes a chunky mains transformer then you can expect the voltage to drop with increased load. The better quality and more conservatively rated the transformer the less will be the voltage drop. On the other hand, if your adapter is a modern switched mode power supply type (SMPS) then its output voltage should be fairly constant with load changes.
 

Thread Starter

Phil Russell

Joined Dec 21, 2014
19
Thank you both for your positive and swift replies. It is good to hear that all is well. I guess it would have been too much of a coincidence to have a broken cable wire as well as failed components.
The 'broken adaptor' is quite chunky (MRD ESP 1005, 13.5v 700Ma). The other one I have is a Braun make at about half the weight (13.5v 800Ma) ... no idea of internal circutry so cannot comment.
Cheers, Phil
 

ian field

Joined Oct 27, 2012
6,539
I really should have paid more attention at school but do not recall this being taught.
I have an adaptor with output stated as 13.5v Dc and 700Ma. It stopped working and as part of the search for a cause I used a multimeter across the output wires. At times, most times in fact, output was zero but at others and with a bit of wiggling of the output wires where they entered the adaptor casing, I got about 19.5v. Clearly a wiring issue but the voltage puzzled me as I have another 13.5v adaptor that gives me 13.6v across the output.
I took the casing apart to find a standard bridge rectifier of 4 IN4002 diodes and a 3300 microF 25v capacitor. There is no obvious damage anywhere. I rewired the unit to get a wiggle free system of 19.5v. I then connected it to the original equipment and tested the adaptor under load. Voltage dropped to 18v. I tried another load, this time a 12v car bulb, most likely 5w rating, and voltage dropped to 15.5v.
Now the original equipment has a voltage requirement of 12 - 18v DC according to the manual so the adaptor would appear to be OK for that use. But is it? Why am I getting a value of 19.5v DC without load with this adaptor but 13.5v (13.6v)DC with another adaptor when both state 13.5v DC on the adaptor casing, albeit a different make.

Could it be that one or more diodes are failing or failed? Could it be the capacitor?

Sorry to be asking what may seem such an elementary question but my theoretical knowledge of these matters is dismal

All advice gratefully received
Cheers, Phil
Iron cored transformers that small have lousy regulation - SMPSU wall warts are usually pretty close to the specified voltage.
 

Thread Starter

Phil Russell

Joined Dec 21, 2014
19
To add more information. The manual for the original equipt (chronoscope) states it needs a voltage of 12 -18vDC. I have heard from another person that the manufacturer of the equipt really recommends an 18vDC supply so the adaptor is about spot on.
I still have the puzzle though of why the adaptor states 13.5v DC while it is clearly not. But I can guess that it would be 13.5v DC with a specified load that is not given.
So, thanks for all your help; it is appreciated.
Cheers, Phil
 
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