Hi,
I rarely blow anything up these days but a quick accident led to the destruction of my $150 USD Mastech power supply.
What happened was a momentary reverse polarity connection of a 12v battery to the output terminals. I heard a loud "snap" followed by zero output voltage. Needless to say, i was very disheartened after that. This only happened to me once before when i connected an expensive meter across a very large 12v battery with the meter in the current mode setting, and that was some 10 years ago. The current function of the meter stopped working even though it could still read voltages. It is interesting however that these two blow ups have something in common which i'll get to in a second.
The next day i took apart the power supply and of course looked inside for any blown up diodes or blown up bridge transistors because that is exactly what makes that kind of 'snap' noise when it gets a huge overcurrent. To my surprise, there were none to be found! Nothing blown up, no burn marks anywhere, nothing. But what bothered me the most was on the back of the case it stated that there would be a fuse, but guess what, no fuse to be found anywhere either! I was very bothered by this because they clearly state on the case that there is a fuse, somewhere, even though they dont state where it is exactly.
After examining it for a while longer, it appears that they want the user to use a line cord (like for a PC computer power supply) with a fuse BUILT INTO IT! They expect you to use a special line cord with a fuse in it. That's totally ridiculous i think. There should be a fuse holder and fuse like everything else like that.
Ok, back to troubleshooting...
I checked the bridge diodes even though they were not burnt or broken. One read good while three looked bad. Later however i found that i had not contacted the leads of the diodes correctly from the angle i had available for the two meter leads, so i found the diodes to actually look all good.
The schematic is very complex for this thing however, and some of the parts do not show the correct value, so it became very difficult to troubleshoot with a lot of guesswork.
Finally after removing the bottom panel too, i found a very small board mounted just behind the three banana jacks used for the output. Hidden behind the board was, yes, a large diode. And yes, it was burnt and broken! Ha ha! They hid the diode behind the board so it was not very visible, AND they DID NOT show this diode on the dang schematic!! That was really a b----. If they had shown this diode on the schematic that would have been one of the first things i would have checked, because it is wired in reverse polarity orientation so that if an outside source is connected backwards the diode attempts to shunt all the current to protect the power supply.
Because they did not show that diode on the schematic and hid it under a small board that looked like it was just used for mounting of the banana jacks it cost me FOUR HOURS of troubleshooting instead of 5 minutes.
In the companies defense, the diode did protect the power supply because once it was cut out of the board the power supply worked again. But i believe this was only because the reverse polarity was only applied for a very short time. They did not use a fuse with this diode as would have been the normal way to do it, so it could have taken out the whole power supply if the diode blew open instead of short.
Now the similarity to the meter...
In the meter they did use a fuse. But it was also hidden beneath one of the PC boards so i did not find it until some years later when i went back into the meter to see if i could get the current function working again.
The meter was a different brand however.
I rarely blow anything up these days but a quick accident led to the destruction of my $150 USD Mastech power supply.
What happened was a momentary reverse polarity connection of a 12v battery to the output terminals. I heard a loud "snap" followed by zero output voltage. Needless to say, i was very disheartened after that. This only happened to me once before when i connected an expensive meter across a very large 12v battery with the meter in the current mode setting, and that was some 10 years ago. The current function of the meter stopped working even though it could still read voltages. It is interesting however that these two blow ups have something in common which i'll get to in a second.
The next day i took apart the power supply and of course looked inside for any blown up diodes or blown up bridge transistors because that is exactly what makes that kind of 'snap' noise when it gets a huge overcurrent. To my surprise, there were none to be found! Nothing blown up, no burn marks anywhere, nothing. But what bothered me the most was on the back of the case it stated that there would be a fuse, but guess what, no fuse to be found anywhere either! I was very bothered by this because they clearly state on the case that there is a fuse, somewhere, even though they dont state where it is exactly.
After examining it for a while longer, it appears that they want the user to use a line cord (like for a PC computer power supply) with a fuse BUILT INTO IT! They expect you to use a special line cord with a fuse in it. That's totally ridiculous i think. There should be a fuse holder and fuse like everything else like that.
Ok, back to troubleshooting...
I checked the bridge diodes even though they were not burnt or broken. One read good while three looked bad. Later however i found that i had not contacted the leads of the diodes correctly from the angle i had available for the two meter leads, so i found the diodes to actually look all good.
The schematic is very complex for this thing however, and some of the parts do not show the correct value, so it became very difficult to troubleshoot with a lot of guesswork.
Finally after removing the bottom panel too, i found a very small board mounted just behind the three banana jacks used for the output. Hidden behind the board was, yes, a large diode. And yes, it was burnt and broken! Ha ha! They hid the diode behind the board so it was not very visible, AND they DID NOT show this diode on the dang schematic!! That was really a b----. If they had shown this diode on the schematic that would have been one of the first things i would have checked, because it is wired in reverse polarity orientation so that if an outside source is connected backwards the diode attempts to shunt all the current to protect the power supply.
Because they did not show that diode on the schematic and hid it under a small board that looked like it was just used for mounting of the banana jacks it cost me FOUR HOURS of troubleshooting instead of 5 minutes.
In the companies defense, the diode did protect the power supply because once it was cut out of the board the power supply worked again. But i believe this was only because the reverse polarity was only applied for a very short time. They did not use a fuse with this diode as would have been the normal way to do it, so it could have taken out the whole power supply if the diode blew open instead of short.
Now the similarity to the meter...
In the meter they did use a fuse. But it was also hidden beneath one of the PC boards so i did not find it until some years later when i went back into the meter to see if i could get the current function working again.
The meter was a different brand however.