Fuse /Power supply troubleshooting

The supply pictured is a switchmode supply, and the fuse has definately gone bang.

As jw0752 said it has lethal voltages and the first stage they rectify the mains voltage, filter it and then chop it to arrive at the final voltage, or voltages.

Replacing the fuse is the least of your problems, you'll need to test some other parts to find died and took out the fuse.

I was shown a really good trick for testing switch mode supplies.

if you wire the mains through a 100-150W lamp, if there is a short, it will light the lamp, rather than creating minature explosions and causing more damage.

Obviously some care is required, but the idea is the lamp is in series with the phase and can be achieved using a simple box with a lead and plug at one end, and cord socket on the other end.

Mark

Ok so not as simple as I would have liked it.

Thank you everyone for you advise. I not sure I'm going to use this power supply again because I still don't trust my self yet with lethal voltages. I am in the USA so its 120v but still kind of scary.

I would like to continue testing it even if I don't plan to power it up since I have already learned some much from just the one post. Sorry I'm very new.

from what I'm reading I need to test most of the parts out of circuit. So before I start desoldering and getting confused I would like help identify parts because I'm still struggling with that a little.

Triangle = spym 13009 mx38 is written on it. google says its a switch mode transistor is that how you guys are able to identify that the power supply is a switch mode one.

square= ? not sure what this is.

rectangle= says cv 400 b275jq on it

oval= what is this goo its also near the caps on the left

pentagon= has four pins + AC - is this the bridge rectifier?

circle = not sure.

Hi Giovanni,

Let's start with in circuit testing. Do you have an analog multimeter? I ask as it is easier to visually make these tests watching a needle than trying to interpret a digital display that jumps around. Set your Ohm meter to Rx1

The circle is very hardy and will not likely be damaged. It is a common mode inductor which filters out interference.

The Pentagon is a bridge rectifier. It is possibly damaged. There are four diodes in this package and for each diode the meter should show continuity when the leads are hooked up one way and no continuity if they are hooked up in reverse polarity. Note there is a + and - which are the outputs and 2 side ways S's. This is the input from the AC power. The diodes are laid out between the + and the S's and between the - and the S's. You may see some resistance between the + and the - as well as between the two ss but you should not see a short.

The oval under the glue is unknown but probably not the problem.

The rectangle is a capacitor and probably not the problem as they can tolerate quite high voltages

The Square is a switching transformer that is used to drop the high input voltage to the lower output voltage and not likely the problem.

The Triagles are likely power MOSFETS. Looking at them from the front labeled side their leads are likely left to right Gate - Drain - Source. This is the most common configuration. You want to make sure that there is not a short between the Drain and Source on each one. Put you meter probes on the Drain and Source and then note the reading . Switch the probes and repeat. Even if these are not shorted they may have failed open so further testing may be necessary out of circuit. There is another device above the transformer that looks like a MOSFET but it is a Double Diode and used to turn the power from the switching transformer back into DC current. It is most likely ok but you can test it just like you did the bridge. From the center pin to each outer pins you should get a reading on the meter in one direction and not in the other.

Try these tests and let us know what you find out. Do not plug in the unit until we have talked a bit more.

John

Thank you John for taking the time to explain this to those of us who don't know the ins and outs of power supplies

Kas

Dab I agree probably easier to scrap it and buy a new one from all the post here and knowing my skill level I probably won't be able to fix it. this is more a learning thing at this point and boy have I learned a lot from these post.

John thank you so much for the explanation of the parts that helps me put proper names for me to do more research and learn more about those components. I have a digital multimeter still learning how to use it.

I removed the bridge rectifier and tested it in diode mode with the black probe on the minus pin i get nothing touching the red probe to each pin. when I put the red probe on the minus pin I get around .63 and .62 on the middle ac pins and .97 on the plus pin.

the part behind the transformer says spym 30100 the it has a picture of two arrows going into each other with parallel lines in the middle.

all this power supply has been sitting for about two months unplugged and unused I understand there is a risk to what I'm doing and i'm trying tot red lightly. I don't plan to power this unit unless I feel extremely safe.

Thank you

Giovanni

A little follow up.

The white glue is just for stabilizing components that do not have good mechanical attachments. The power supply may be used in an environment with lots of vibration and this keeps the heavy components from breaking loose.

The Triangle components SPYM 13009 are not MOSFETs as I said before but switching NPN transistors as you mentioned. Incidentally these are similar transistors to the ones that are commonly used in the switching driver circuits of the common CFL (compact Florescent lights). The configuration of their leads from left to right are likely: Base - Collector - Emitter. Check between the Collector and Emitters for Shorts.

Take general sequence of the circuit begins in the lower right corner where the AC power attaches and then works it way around the edge clockwise until the output attaches to the screw terminal at the upper right of the board. The circuit likely has current protection on the output so the most likely place for failure will be in the circuit along the lower and left edge. Once we get to the transformer and after failures will probably not blow the fuse but just shut down the switch. This is speculation as I can not do more than guess at the actual circuit without seeing it.

John

The bridge out of the circuit should give OL on the connection of the meter between the s and s. Between the plus and each s with the black probe on the plus you should get OL. If you put the red probe on the minus you should get OL to each of the s s. This assumes that the test polarity of your ohmmeter  circuit is like most of the meters I have seen.

When you read your meter make sure you are aware of the "K" or "M" that may be displayed as these can drastically change what the numbers indicate.

.63 is basically a short.  .63K is 630 ohms and .63M is 630,000 ohms. If you indeed got .63 ohms when you tested across the AC (ss) terminals then the bridge is bad and probably caused the fuse to blow.

John

Thank you I was wondering about the glue that makes sense now

I attached some pictures of readings and the bridge rectifier mine doesn't have a tilde (side ways S) it just says AC which makes sense since that is the symbol for AC.

my multimeter isn't auto ranging and after today I'm considering upgrading it.

Giovanni is using the meter on the diode setting, so the reading is the forward voltage.

Giovanni Laucella wrote:

 

Thank you everyone for you advise. I not sure I'm going to use this power supply again because I still don't trust my self yet with lethal voltages. I am in the USA so its 120v but still kind of scary.

Hi Giovanni,

Just for reference, even though you are in the US with "only" 120V, if you were working on an AC/DC converter that included power factor correction the nature of how the PFC works usually involves the voltage being boosted to much higher, so you could end up with voltages in the range of 400V on large capacitors so you are very right to be cautious when working on this type of circuit.

Best Regards,

Rachael

Hi Jon,

Good catch. After I turned off the computer and walked away it dawned on me that he was using the diode test and not the Ohmmeter. Always difficult to trouble shoot with limited access to info.

John

Hi Giovanni,

Based on your pictures it looks like the Bridge is OK. Just be sure you can get approximately .62 Volts from the plus to each of the AC and from the minus to each of the AC. The information I gave you before was based on the assumption that you were testing the diodes with the Ohm meter set on Rx1.

John

Hi Giovanni,

Based on your pictures it looks like the Bridge is OK. Just be sure you can get approximately .62 Volts from the plus to each of the AC and from the minus to each of the AC. The information I gave you before was based on the assumption that you were testing the diodes with the Ohm meter set on Rx1.

John

Thank you everyone I will continue to test.

Rachael I didn't know that, I will look more into PFC they sound interesting. this power supply has been sitting for a few months so that should help. Do you have any recomedations of what precations I should take or look out for.

Giovanni Laucella wrote:

 

Rachael I didn't know that, I will look more into PFC they sound interesting. this power supply has been sitting for a few months so that should help. Do you have any recommendations of what precautions I should take or look out for.

Hi Giovanni,

There are quite a lot of interesting things to be learning about power supplies. Its a fascinating topic when you get into it Unfortunately it's an area which is considered by a lot of people as a black art and they shy away from it, but when you start to look further into it, there's lots of fun to be had!

If you are working on an PSU that is powered off, just make sure all the capacitors have been discharged so there can be no retained high voltages before you handle it and you should be fine. If in the future you are considering debugging a misbehaving power supply "live" then that's entirely different and you'll need more test equipment such as an isolation transformer (this helps with safety), an oscilloscope with high voltage differential probes, and possibly an AC/DC current probe. I'd not recommend attempting this until you are very familiar with how these power supplies work and know where all the dangerous bits are and have the knowledge and experience to be able tackle this more safely.

Best Regards,

Rachael