Mini review: Tenma 72-8350A small footprint bench power supply

[zapta]

I got recently this little bench power supply and l like it a lot so I thought I should share.

This is the Tenma 72-8350A which the 20V/5A version of the power supply Mike reviewed here

https://www.eevblog.com/forum/reviews/tenma-72-8345-bench-psu-review/

This is an upgraded model (hence the 'A') suffix and the original version seems to be discontinued.  The display is now white on black with excellent contrast, it shows the current limit setting without having to short the output and a click or two on the voltage and current button gets you into one of two course adjustment modes. Internally it looks just the same as in Mike's review.

Fro reference, I tried the Korad KA3005D and didn't like it. Bulky, complex panel, and once every few times I turned it on the top cover made a loud sudden noise, probably from  magnetic field change in the transformer below it.

Like:
* Compact, light, small footprint
* Large clear display
* 100W output (way more than I need, Mine is 20V 5A, they also have 60V 1.6A and 36V 3A versions.).
* Accurate voltage/current settings and readings.
* Simple and intuitive to operate.
* Output on/off switch  (my HP E3610A doesn't have it).
* Good construction overall.
* Reasonable price (~$160).

Doesn't like:
* Can't set current limit below 150ma (I often goes down to 40ma limit when bringing up boards).
* No post binds.  (I am using these http://radioshacklb.com/index.php?route=product/product&product_id=2656 )
* Handle (removed).
* No UL certification.

Overall this power supply is a very good fit for my hobby needs. I think I will put my HP E3610A and Protek 303 for sell, and use only this one.

[echomancer]

I purchased one of these today but have noticed an odd behavior.  If I set the voltage to say 5.00, the unit will show the output voltage as 5.32.  it's as though the set point isn't calibrated the same as the actual VO.  This happens whether there is a load or not.  It seems no matter what, it is always displaying .32V above the set point.  Have you noticed this?  Have you discovered any type of calibration point?  I will get my Fluke 287 out tomorrow to compare values.  A third of a volt over is no fun.

[zapta]

I purchased one of these today but have noticed an odd behavior.  If I set the voltage to say 5.00, the unit will show the output voltage as 5.32.  it's as though the set point isn't calibrated the same as the actual VO.  This happens whether there is a load or not.  It seems no matter what, it is always displaying .32V above the set point.  Have you noticed this?  Have you discovered any type of calibration point?  I will get my Fluke 287 out tomorrow to compare values.  A third of a volt over is no fun.

I just tested mine at 5V and 20V, no load. I get at most one or two digits discrepancy between the set voltage and actual voltage (the voltage display and my Fluke are in close agreement). 0.32V seems way too high.

It reminds me one of Dave's episode about the disconnected sense line. This power supply also has internal sense lines, visible in one of the pictures I posted. I wonder if one of them disconnected. That's a long shot.

BTW, I just found that if I just click once on the voltage or current knobs it display the set value for a second or two. Very nice. Before, I used to turn them a click to see the set value.

[GlowingGhoul]

The "A" in the model name stands for "approximate".

[zapta]

The "A" in the model name stands for "approximate".

I assumed it stands for "awesome".

[rolycat]

The "A" in the model name stands for "approximate".

I assumed it stands for "awesome".

I have an older version of this PSU. It is also a 20V 5A unit but only has three-digit meters. On the other hand, the current limit works down to 3mA.

It's definitely not "awesome", though. It's not a bad unit while it's working but it has packed up twice, on both occasions due to failed capacitors on the low voltage side. From your pictures it appears that you have the same C(r)apXon brand, so be prepared to replace them.

[GreyWoolfe]

I am amazed that CapXon is still around.  They have to be the world's worst capacitors.  I don't think I would get rid of my E3610 for the Tenma.  By the way, Zapta, if you really want an output on/off switch, just add one.

[echomancer]

I'll be heading back to MicroCenter today to exchange this for the other one they have in stock - hopefully this is just a bad sample and not indicative of the whole line.  I've also made a video to demonstrate the issue (linked below).  I also learned that even though the unit appears to come up with the output turned off, voltage is infact present during startup.

[Fraser]

I bought one of these as a customer return a couple of years ago. The fault was pretty obvious...... the switching controller has self destructed in a big way  I bought a replacement chip  (thankfully its a COTS device) and new capacitors, but I have yet to bother fitting them. Other priorities came higher up on the 'to-do' list.

I do like the small footprint and it will be a useful little power supply to have around for those occasions when you just want to quickly apply power to a circuit without having to go to the garden office to do so. Lazy I know, but convenience is king 

Aurora

[zapta]

From your pictures it appears that you have the same C(r)apXon brand, so be prepared to replace them.

I don't mind doing it now as a preventive measure. Which capacitors should I replace?

[GreyWoolfe]

All that say CapXon.

[artag]

Fro reference, I tried the Korad KA3005D and didn't like it. Bulky, complex panel, and once every few times I turned it on the top cover made a loud sudden noise, probably from  magnetic field change in the transformer below it.

My Farnell B30/10 makes a loud crash when it's switched on, as the top steel plate rattles in the extrusions. It's one of the things I like best about it.
 

[zapta]

I also learned that even though the unit appears to come up with the output turned off, voltage is infact present during startup.

I tested it on mine, and this is the output voltage when powering on with 20V setting and no load.  I think I can live with that.

Hmm, that's quiet a few. Two big ones on the high side and two big ones on the low side and then ~7 small ones. It's easy to access though both sides of the main board.
All that say CapXon.

[nctnico]

Such a spike can be big enough to kill components. What happens with 3.3V or 5V at the output?

[rolycat]

All that say CapXon.

Hmm, that's quiet a few. Two big ones on the high side and two big ones on the low side and then ~7 small ones. It's easy to access though both sides of the main board.

It was the large caps on the low side that failed on my unit, but I have a little pot of failed/bulged capacitors of various sizes and voltages from several boards and 90% of them are CapXon.

They must be unbelievably cheap to be so infamously useless and yet so common.

[zapta]

Such a spike can be big enough to kill components. What happens with 3.3V or 5V at the output?

It's going down to -0.4V on the negative side and 2.6 on the positive.

It was the large caps on the low side that failed on my unit, but I have a little pot of failed/bulged capacitors of various sizes and voltages from several boards and 90% of them are CapXon.

I am lucky in general so chances are I got good caps that will not fail.  I can also tie to it a rabbit's foot just to be on the safe side.

BTW, can I predict the failure using a IR camera? Do caps get hot before they fail?  If not, can I test the caps in circuit? For example with DE-5000?

[echomancer]

I returned my unit today for a new one and all is right with the world again. 

[kopashoff]

Hi everyone!

I have recently purchased 72-8350A. It shows voltage, but it does not show current consumption. What it can be?
I have opened it, but have not found any visual defects.
I can not return it, as i purchased it on ebay without warranty.

Thanks

[Fraser]

The output to the terminals is switched on and off by a MOSFET. Possibly the MOSFET has died.

I have some schematics for these power supplies. If you PM me an email address I will send them to you.

Fraser