Tenma 72-8700 PSU quick look - Page 1
Forum section: electronics / testgear

author: tom66
author_rank: Super Contributor
timestamp: April 29, 2014, 10:34:15 pm
content: I recently bought a Tenma 72-8700 off TopLoser on this forum.This PSU has two independent 0-32V 0-5A channels. The channels can be put in parallel for a total of 0-10A at 0-32V (although this does require some external wiring, and matching of both channels) or in series (using a supplied shorting block) for 0-64V 0-5A. With 24V @ 8A I can power an LCD backlight inverter for a 42" LCD TV! I'm not certain it will run such a high load continuously (320W at full load, probably 400VA+ from the wall), but it will be OK for testing and quick jobs. From the size and weight of it I guess it probably has a 200VA transformer in it.The PSU has a rather loud fan, which is quite annoying, and it runs at full speed continuously. I'm not quite sure what its purpose is, given it doesn't cool the main heatsink (which is passively cooled.) I'll probably end up turning its speed down. It's considerably louder than my Rigol DS1074Z fan, and my laptop/desktop PC fans.This thing eats fuses for breakfast!The original 3.15A(T) fuse popped after 3 power cycles. A second fuse popped after about 10 power cycles. I've since put in a 5A(T) fuse, which seems to be OK for now. I am guessing the original rating is insufficient for the torodial transformer in-rush current.It has a dual channel tracking mode. However! It will not work with the channels floating, which is a pain. If the grounding block is not connected the other channel will go to zero. So to get parallel tracking, you have to short both channels together, balancing the voltages. I have tried setting parallel sources for a grounded common terminal; the result is the output of CH2 is half CH1... so I'm guessing the tracking mode is intended only for series channels/split supplies.Transient analysis: very good. I tried both CH1 and CH2, they are identical. There is almost no overshoot except when starting from 0V in which case it overshoots to about 1V with no load. The other spikes on the image are due to the switch arc being picked up by my probe. Turn off is also very subdued.Attachments:-Start up/turn off 0V, 0A current limit.Start up/turn off 0V, full current limit.Start up/turn off 3.3V no load full current limit.Start up/turn off into 5W LED @ 350mA current limit, 14.4V.Transient connection of 5W LED while running, from 15.5V to 14.4V, 350mA.

author: Pinkus
author_rank: Frequent Contributor
timestamp: April 30, 2014, 07:25:39 am
content: EDIT: I just recognized, the Tenma 72-8700 from tom66 is not the one I have (I have TENMA - 72-10500 which is totally different). Sorry for the confusion.Because of this I am deleting my post.

author: tom66
author_rank: Super Contributor
timestamp: April 30, 2014, 09:07:50 am
content: This is a slightly different PSU: it's 100% analog unlike the others.I like the 10-turn pots for setting the voltage a lot.Not sure who made it, some OEM probably in China; overall build quality is OK, the capacitors are cheap, and they use 4 x 3A diodes in FWB for a 5A output.There is a small heatsink inside, but the majority of the cooling is passive and external. Hence why I might end up turning down the fan speed.

author: mariush
author_rank: Super Contributor
timestamp: April 30, 2014, 03:43:19 pm
content: Can you make some pictures of the insides? I'd like to know how it differs from 8695 (the 3A version) that I have and picturedhere- mine has no fan.Also, if you fix that one, tell us/me what was the problem (the 5v-37v on second channel).. my first guesses were some issue with the opamp or a zener diode on that channel, or possibly something related to the fixed 5v output (which afaik is tied to second channel)

author: grumpydoc
author_rank: Super Contributor
timestamp: April 30, 2014, 07:13:43 pm
content: Circuit diagram probably not a million miles from this one for the basic dual supply bit.

author: Kevin.D
author_rank: Frequent Contributor
timestamp: April 30, 2014, 08:09:56 pm
content: They are rebaged uni -t utp3705 I have repaired a couple of these .I remember the very load annoying fan ,which looks like it was placed in there as an afterthought on the bottom of the case where it hardly does anything .It wants fixing on the back heatsink where it will at least be effective and a normally open 45 deg C thermal switch fixed to the heatsink to keep the fan off when its not needed .

author: grumpydoc
author_rank: Super Contributor
timestamp: April 30, 2014, 08:14:51 pm
content: QuoteThere are so many clones of this PSU design I'm not sure who has the "original". Probably not UNI-T though.