TENMA 72-8700

[banedon]

Hi guys

Has anyone had any experience of the Tenma 72-8700?  I'm currently looking for my first bench power supply and the specs look good. However, the specs don't always tell the whole truth though . I.e. is it really good or inaccurate, does its initial voltage peak too high when turned on and is there a Alien 3 style fan in it making tons of noise? (BTW I mostly chose it as it's 5A, 2 channel, 0-32V)

Also, I've seen bench power supplies with minus voltage as well. Can anyone advise why the cheaper ones don't seem to include this as an option? All the stats I read give 0V to ?V range. Unless I'm going blind, of course!

Any help/advice would be greatly appreciated

[nanofrog]

Has anyone had any experience of the Tenma 72-8700?  I'm currently looking for my first bench power supply and the specs look good. However, the specs don't always tell the whole truth though . I.e. is it really good or inaccurate, does its initial voltage peak too high when turned on and is there a Alien 3 style fan in it making tons of noise? (BTW I mostly chose it as it's 5A, 2 channel, 0-32V)

Might want to take a look at the last post in this thread.

Also, I've seen bench power supplies with minus voltage as well. Can anyone advise why the cheaper ones don't seem to include this as an option? All the stats I read give 0V to ?V range. Unless I'm going blind, of course!

The one you're looking at actually is, as it's a dual supply. Just needs to be wired correctly. To do this, connect the Black terminal of Output 1 to the Red terminal of Output 2, and this connection becomes your center tap (+V on the Left, -V on the Right), and it's this center tap that's your 0 V reference. Think series connection of a couple of batteries, only it's the output terminals on the PSU instead. 

[banedon]

Hi nanofrog

Thanks for answering my questions.  Going by the post that you referenced it seems that the power supply is certainly not a good one - more's the pity!

Also, thanks for advising on creating a negative rail.  Still a bit puzzled that it's not an actual option to have a specific variable negative supply though. I know that they're around as I've seen an EEV blog video with one on (I think the one with the Amiga 500 repair/tear down).

[nanofrog]

Still a bit puzzled that it's not an actual option to have a specific variable negative supply though. I know that they're around as I've seen an EEV blog video with one on (I think the one with the Amiga 500 repair/tear down).

The primary reason for using multiple terminals on dual PSU's, is it allows for multiple configurations (independent, series, or parallel operation), and can be just a bit less expensive to manufacture (no switches necessary to set up series v. parallel for example). If you want even a single supply to output a negative voltage to your circuit, just invert the wires (+ to - and vice versa). No different than switching the test leads on your DMM (the minus sign will appear along with the displayed value).

IMHO, you're much better off looking at used PSU's. You get better quality, usually better specs, and most are repairable for similar, if not less money than the cheap Asian supplies (manuals tend to include full schematics, and may cover it's design as well <old HP/Agilent manuals are really good for this>). MUCH better value vs. new budget units.

It does require some patience, but it does pay off IME. Here's a popular example.

Search for "how good is XXXX Chinese PSU" types of threads, and you'll see posts that suggest used and which makes/models to go for (the linked example, HP E3610A, is one that's usually mentioned).  It will take time to wade through them, but it's worth the effort to read IMHO.