To facilitate my arduino experimentation I purchased a Mastech HY3005F-3 power supply. It typically works fine but maybe 50%-75% it trips a 20A breaker with nothing else connected to it. Reset the breaker and then it's usually ok. I know inrush current could cause this issue however it would seem the designer would have built the circuit to limit that. So whatever is used to control the inrush could have problems. Any tips on what to look for when I troubleshoot it?
The power supply: http://www.mastechpowersupplies.com/mastech-adjustable-power-supply-hy3005f-3.html
Can you open it and make a photo of what is inside ?
Do you know how old it is ? In what year was it designed ? Twenty, thirty or fourty years ago ?
I see here that is has a toroidal transformer. Those are very efficient, almost too efficient. Loading a capacitor after the rectifier can draw a large primary current peak.
The toroidal transformer is connected to the boards entirely with plug connectors. I can unplug every single connector and turn the unit on (which of course does nothing) and the breaker will still blow. There are several small blue (filter I assume) caps near the on/off switch which you can see in the lower right of the first picture. Should I suspect these? I bought the unit new maybe 5-6 years ago. I don't know the design or manufacture date. I have used it very little.
You may suspect the over-voltage protectors and spark reducers and even the circuit board on the primary side.
The blue ones are often called "MOV" : https://en.wikipedia.org/wiki/Varistor.
Can you bypass the primary circuit and connect the mains power directly to the transformer ?
The transformer is probably both for 110V and 220V, so you have to check how to connect it.
The blue discs are labeled C1,C2 and C3 and are marked 222 2KV. I looked that up and seems like a capacitor but I'm confused I also see it listed as a MOV.
All breakers in your electrical box have a limited number of trippings. I have had to replace several in my shop because they are sometimes used as switches.
Agreed but I moved it to a different circuit/breaker while troubleshooting and it exhibits the same behavior.
Two of the (as I suspect are now MOV's) are connected in parallel with either side of the on/off switch I assume for arc suppression. If either of these failed shorted or open I cannot see that they would effect the inrush current or cause a line/neutral short. The third MOV is across line/neutral and if it were shorted I could see a problem however I removed it, turned on the switch, then powered up, with transformer totally disconnected on the secondary and the breaker still blows.
Was it, perhaps, made in China?
There is much more that can go wrong. With a multi-layer circuit board, then can be carbonized traces inside the pcb board that cause trouble only with certain conditions. Sometimes a spray or glue is used that can leak current after some time. Even the jitter of a broken wire can cause peak currents. At least you know in which location the problem is.
One other thing to try is to put a 100 watt incandescent light bulb in series with the primary of the transformer and test.
Yes it was made in China.
Send it back for a refund.
Lol probably can't afford a US made one (if such a thing exists) for my hobbyist use case. BTW the light bulb appears to limit the current enough to stop the breaker from tripping. If I can figure out how to select an NTC thermistor I think that will solve the problem. It's likely there should have been one there in the first place.
That is likely the point. They look like a virtually direct connection to the capacitors.
Are you sure the 20 A breakers are not RCDs?
No they are just normal breakers.
That seems like a "serious" problem.
Koepel's link says there is 110/220V switch on the back. Is it set correctly?
I don't think you said if the breaker is tripping from the inrush current when you switch it on or randomly.
You can't really measure the temporary inrush current with a multimeter but you might want to measure the continuous current to see if it's "reasonable".
This supply has two variable outputs, so when you say 50% to 75% it trips a breaker. Does this mean....
- The supply is on and set at say 5 volts (both controllable channels)
- Nothing is connected to any of the outputs.
- You increase the supply to 15V to 18V out and the breaker trips.
What voltage is your mains? What setting is the 120/240 volt switch on?
There is no input voltage switch. It is wired for 120V only. It is only tripping on inrush current. Not randomly after it is on.
Sorry I wasn't clear. 50% to 75% of the time it is turned on it trips. No load. Trips at startup inrush. Mains are 120V and unit is hardwired to 120V there is no selector switch.