Tenma 72-8700 Power Supply Blows Fuses

[JimmyK]

I recently purchased a new but not working Tenma 72-8700 power supply off EBay.  The power supply arrived yesterday and I found the fuse blown and a power wire to the right LED display broken.  I put in a fuse and attached the display wire and plugged in the power supply with the 2 prong plug that it came with.  The power supply came on and appeared to work fine.  I then wondered why it had a 2 prong plug when it has a 3 wire grounded connector on the chassis.  I hooked up a 3 wire cord and plugged it in.  The power supply blew the fuse immediately as soon as I punched the power button.

I've looked through the unit for any obvious wiring shorts and tested for continuity between ground and most circuit board traces and pins but found nothing.

This power supply appears to be the same as the Uni-T UTP3705 and possibly the Mastech HY3005-2

I could use some suggestions on where to look next.  Thanks for your help.

Jim

[SeanB]

What is the rating of the fuse? A regular fuse will blow if it feeds a toroidal transformer very often, as the inrush current is quite large. Replace it with the same value ( or the right value fuse) in a slow blow variety, and check again. This often cures the problem.

[JimmyK]

Thanks for the reply.

But it didn't blow when the power supply was hooked up with a 2 wire power cord.  Wouldn't it have the same inrush of current with a 2 wire cord?  I thought there is probably a short to ground and with the 3 wire cord it blows because the short has a return path to ground generating the high current to blow the fuse.

It was also a rather quick and violent fuse blowing, I heard it and saw a flash of light.  The fuse is 4a

Jim K.

[metalphreak]

Use a multimeter and test the resistance between the Live/Neutral pins of the power inlet and the ground pin. Also test for continuity/resistance between all of those pins and any exposed parts of the chassis. Obviously, do this when it's not plugged in. You will need to replace the fuse before you do this (or probe on the other side of the fuse holder instead of the live pin).

Any shorts between the Active/Neutral AC lines and the ground should be obvious then. It sounds like there is a short between one of them and the ground connection. This is quite dangerous if its the Active (Hot Live whatever you want to call it) side of the AC power. The whole chassis is grounded and thus now live. If it's a short to the neutral side of the AC supply, it probably won't kill you if you touch it assuming the neutral side is close to ground potential (it isn't always the case). I would not plug it in and use it (fused, 2 or 3 pin cable) until you have rectified the issue.

[JimmyK]

I will run those checks when I get back to the garage.

I just plugged the unit it again with the 2 wire power cord a new fuse and the cover off.  The unit started fine.  I measured voltage between the power supply chassis and the house outlet ground.  There was 46V AC and 1.7V DC between the chassis and  house mains ground.  It looks like the chassis is live with AC power which is why the fuse blows when the 3 wire cord is used?  Does the fact that it's 46V AC mean that the short is before the rectification stage?

I'll run the other checks you suggested and post back.  Thanks for the help.

[JimmyK]

With the power unplugged, the fuse in:

Front panel switch off: (measurements at power outlet on back of power supply)
hot-ground= no continuity (OL)
hot-neutral= no continuity (OL)
neutral-ground= no continuity (OL)

Front panel switch ON: (measurements at power outlet on back of power supply)
hot-ground= no continuity (OL)
hot-neutral= 1.3 ohms
neutral-ground= no continuity (OL)

There was no continuity between the rear power outlet pins and the chassis except between ground pin and chassis (obviously).

Jim

Use a multimeter and test the resistance between the Live/Neutral pins of the power inlet and the ground pin. Also test for continuity/resistance between all of those pins and any exposed parts of the chassis. Obviously, do this when it's not plugged in. You will need to replace the fuse before you do this (or probe on the other side of the fuse holder instead of the live pin).

Any shorts between the Active/Neutral AC lines and the ground should be obvious then. It sounds like there is a short between one of them and the ground connection. This is quite dangerous if its the Active (Hot Live whatever you want to call it) side of the AC power. The whole chassis is grounded and thus now live. If it's a short to the neutral side of the AC supply, it probably won't kill you if you touch it assuming the neutral side is close to ground potential (it isn't always the case). I would not plug it in and use it (fused, 2 or 3 pin cable) until you have rectified the issue.

[Matje]

But it didn't blow when the power supply was hooked up with a 2 wire power cord.  Wouldn't it have the same inrush of current with a 2 wire cord?

...

Yes, it would.

But the nasty thing about big toroidal transformers is the following: the inrush current depends on the exact moment these things were switched off before, to be more specific whether switch-off occurred near zero crossing or near the peak of the input voltage, so inrush current can vary wildly. There are articles online describing this.

Properly designed devices have an inrush current limiter for this reason.

These can also be bought as external devices - which I use to tame my big power supply, which used to blow the circuit breaker for the whole room uncomfortably often.

[JimmyK]

I have 2 young boys who go in the shop with me.  I can't have a power supply with 46 volts AC on the chassis.  This is a shock hazard that could hurt someone and is not due to torroidal transformer start-up surge current.

If anyone has any suggestions based on my measurements above on where to look to home in on the problem, I'd appreciate it.

Thanks much

JimmyK

[don.r]

Check the insulation on the toroid. Usually there is a big rubber gasket between the retaining plate and the windings. If not, any tear in the winding cover (I assume its clear kapton tape in your case) could short to the chassis. You may also have a short between one of the secondary outlets and the chassis but before the rectifier, as you have identified. You really need to unbolt that toroid and have a good look. Lots of red threadlock on that nut. You may need to heat it first a little. Also check where the secondaries plug into the board with the rectifier diodes on it.

[bilko]

I have 2 young boys who go in the shop with me.  I can't have a power supply with 46 volts AC on the chassis.  This is a shock hazard that could hurt someone and is not due to torroidal transformer start-up surge current.

If anyone has any suggestions based on my measurements above on where to look to home in on the problem, I'd appreciate it.

Thanks much

JimmyK

Firstly I would suggest that you have a RCD installed in your workshop. If you have and it is working correctly (when did you last test it ?) you should be protected from electric shock hazards.

When you disconnect the ground from the power supply, the output voltage on the secondary will float. Measuring between utility ground and the PSU ground will show a reading, but is usually high impedance unless there is a ground fault, and that is what the RCD is there to protect you against.

To test, you could connect a 1K resistor between the utility ground and PSU chassis ground. If you still measure 46V you have a fault. I would guess the voltage will drop significantly close to zero.

Once you have established that there is no ground fault, you can proceed to check the secondary of the transformer.

Does the PSU have any secondary fuses that you can remove ?
If not, unsolder the secondary windings, tape the ends, insert a grounded utility lead and see if the fuse still blows. If it does you have identified a transformer fault or an incorrectly sized fuse.
Is your fuse antisurge or fast blow ?
Most torroids have huge input surge and require antisurge fusing.

If the fuse doesn't blow, it is highly likely that the fault lies with the electronics.

[JimmyK]

Thanks for all the suggestions. 

I took the unit apart and checked all traces on circuit board for ground shorts, checked capacitors and looked for anything that was obvious.  With my analog meter I discovered the voltage on the chassis was ghost voltage.  I used a dim bulb tester and plugged in the unit with the 3 wire cord and the light glowed brightly very briefly and then went out.  I knew then that it was surge current popping the fuse as SeanB and Matje suggested.  I installed a 3.1a slow blow and the unit works great.  I now have a new power supply for $40.  The unit was sold new by Newark Elec. and I assume a customer returned it and Newark sold as unworking to the EBay vendor.

Thanks again.

JimmyK