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Troubleshooting a diode voltage drop

Lawrenciumbc

Nov 2, 2015
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I am a building project (DSO138 oscilloscope) I bought just to help me practice soldering, circuit building etc... to assist my electronics study, but during troubleshooting after the build, there appears to be a mystery voltage drop accross a diode I cannot explain.

The diode in question (attached picture - diode labelled D2) is pretty much the first component after the 9V power supply and is a 1N4004 diode. The schematic says that the voltage pre-diode should be around 9.39V (I am measuring -9.33V for some reason but at least a voltage is there?), but only around 200mV after the diode.

I've tested the forward voltage drop of the diode on the board and it is measuring around 0.5V which I believe to be correct as the next test point after the diode on the schematic is meant to be 8.6V which would approximately work out correct.

As an inexperienced electronics hobbyist, I don't understand how the diode performance can measure 'correctly' (or at least as expected according to the schematic) when measured on its own, yet appears to drop significantly more voltage in reality?

Is this a common scenario that can happen? what should be my next port of call for checking be? I am thinking of replacing the diode but as I say, the diode appears to be fine (according to the diode measurement mode on the multimeter) and so wondered if I should look at something else first.
 

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Alec_t

Jul 7, 2015
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If the forward voltage drop is about 0.5V to 0.7V then the diode is almost certainly ok. I don't see a problem and wouldn't replace the diode.
Measuring components in circuit can be misleading, because of the possible loading effect of other components connected to them.
 

Lawrenciumbc

Nov 2, 2015
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yeah that was my thinking, I was reluctant to change it because of it's reading. With regards to loading, that was something I thought about but wondered what the best way to tackle this would be?

Between the first and second test points, there is a capacitor that goes to ground. I checked the capacitance value before putting into the circuit and the component appeared fine (100 microfarad). Is working through every component, even after the second test point, the right way to go about this?
 

Alec_t

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Is working through every component, even after the second test point, the right way to go about this?
Not the most efficient way. It helps to identify first a few key points in the circuit. Can you post the schematic?
You said "The schematic says that the voltage pre-diode should be around 9.39V (I am measuring -9.33V for some reason but at least a voltage is there?), but only around 200mV after the diode.". Check that you have the correct polarity supply. Sounds like it's reversed. Not all wall-warts have the centre pin positive on the connector.
 

Lawrenciumbc

Nov 2, 2015
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Ok no problem, the schematic is actually downloadable from here : https://s3-ap-northeast-1.amazonaws.com/sain-amzn/20/20-010-214/20-010-214DSO-EN%2Bupdata.zip

For the power supply, there wasn't one supplied so I am just using a 9V supply that I use for a guitar pedal, it can deliver up to 1.7A (current requirements of the circuit being at least 200mA). I will check the polarity of it, if it is delivering the voltage incorrectly (as could be indicated by my measurements), does this mean that I could well be forcing 9.3V the wrong way through the diode and the small voltage I measure is the leaked voltage that the diode can't hold back? or does it not work like that? (i.e. is a diode forward voltage of 0.5V regardless of a + or - supply?)
 

Alec_t

Jul 7, 2015
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does this mean that I could well be forcing 9.3V the wrong way through the diode
Not unless the diode is faulty. The peak reverse voltage rating for a 1N4004 is 400V :D.

Edit: I'm wary about downloading a zip file. Can you post a plain image of the schematic?
 

Lawrenciumbc

Nov 2, 2015
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that's good news then as measuring the diode in reverse on the multimeter just returns open loop so that's fine!
 

Lawrenciumbc

Nov 2, 2015
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I have checked the polarities of the power supplies I have and they are all negatively polarised so I bodged a temporary connection with a 9V battery and all voltages are correct and the scope is working fine! I'm glad it wasn't a build error as I spent a long time making sure everything was correct beforehand.

Thanks for your help and direction towards the power supply polarity, I learnt something new there! (despite it sounding fairly obvious in hindsight!)
 

CDRIVE

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You'll be pleased to know that when I'm proclaimed King of the world my first decree will be that ALL coax power plugs shall have a ( + ) center pin. Failure to comply with this standard will require forfeiture of a testicle! :D

Chris
 

CDRIVE

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Hey, I forgot to mention that the Diode saved your a$$. If it weren't for that 'Reverse Polarity Protection Diode' you'd have fried the board!

Chris
 

Lawrenciumbc

Nov 2, 2015
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That's lucky then! thanks for letting me know, I'll make sure I know exactly what the supply is before I do it again!

It's strange that the polarity isn't printed on the two supplies I have (both adjustable voltage general use power supplies). Unless of course standard product protection techniques by manufacturers involve adding a diode such as this for this very scenario?!

Haha, I think your suggested forfeit of a testicle for people not complying is probably fair.
 

CDRIVE

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It may be worth noting that protection diodes aren't or weren't always wired in series with the supply input. Things may have changed since I had a land mobile / marine electronics shop but back then the protection diodes were wired like this...

The concept was that a reverse polarity would blow the fuse. Unfortunately the manufacturers didn't contemplate that the average user would think ...."Hey I just blew the fuse! I wonder if a 20A or 30A fuse would be OK were a 6A fuse used to be?" Consequently before the 20A fuse blew it would literally explode the diode open, leaving the reverse polarity to destroy everything it could find in its path! This usually included the expensive finals.

I hated servicing those buggers! :eek:

Chris
 

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Lawrenciumbc

Nov 2, 2015
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Interesting, so putting a diode in is a normal safety mechanism. To me it seems the old/alternative method you mention sounds like a more over-engineered solution than just sticking a diode in series in. Surely an in-series diode would be logically thought of first?
 

hevans1944

Hop - AC8NS
Jun 21, 2012
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Surely an in-series diode would be logically thought of first?
Well, in your case it was. And you still managed to hook up the external power wrong without realizing what the real problem was!

BTW, you are very lucky. Most manufactures do not include a "reverse polarity protection" diode. Save a penny, earn a penny.

I have a mixed bag of used wall-warts of various voltage and current capabilities. Some of them have + on the center conductor of the barrel connector, and some of them have - on the center conductor. Some don't provide any DC at all, they just provide low-voltage AC (probably... I hope... from a transformer) for rectification by whatever they plug into. Gotta know what you are working with before putting it to work.

... As an inexperienced electronics hobbyist...
And now you have even more valuable experience: always check the power supply voltage and polarity and current rating before connecting power supply to the load.:rolleyes:
 

Kiwi

Jan 28, 2013
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I used to fit a diode as a crowbar to protect against reverse polarity in the modules I build at work.

I changed to fitting in series diodes after getting a couple of units back with blown diodes because the "highly trained intelligent operators" had put the batteries in reversed.
I also fit in series diodes to some inputs.
The diodes protect the electronics. The load is usually just fused, but won't get powered up if the electronic circuit is not operating due to reversed voltage supply.

So far I haven't had any more failed units back.
 

CDRIVE

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Interesting, so putting a diode in is a normal safety mechanism. To me it seems the old/alternative method you mention sounds like a more over-engineered solution than just sticking a diode in series in. Surely an in-series diode would be logically thought of first?
Actually that's a very good question. Doing so would have required a Diode with a higher 'continuous' current rating. Other than that... I don't know. I can tell you that it was standard OEM design and used on all models and manufacturers.

Chris
 

Chemelec

Jul 12, 2016
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Put a Full Wave Bridge on the DC Input.
Than it doesn't matter about Input Polarity, it will be Self Correcting.
 
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