Tenma 72-2630 unwanted "feature" - Page 1
Forum section: electronics / testgear

author: gtc
author_rank: Newbie
timestamp: March 15, 2017, 10:32:00 am
content: We have this Tenma power supply:http://au.element14.com/tenma/72-2630/power-supply-bench-0-30v-5v/dp/2478798#techDocsHookFor its price and functionality it's not too bad overall, but it comes with a very dangerous feature.There are, as usual, two power switches: the big main on/off button on the left, and the DC on/off switch in the column of buttons on the right.Naturally, one uses the DC switch to provide and remove DC power to the device under test.However, on this particular model, when you remove power via the main switch and then power back on, the DC switch position is remembered from last time and you can get instant powering on of the DC too -- which can be catastrophic as you don't get a chance to set the voltage before you apply the DC to the leads.In other words, if last time you had the supply set to 30 volts and power off via the main switch, then upon re-powering via the main switch some time later you get 30 volts again live to the output terminals. The supplier element14 advises that this is a "feature" of this model.With our other Tenma power supplies, the DC on/off switch is always OFF by default upon re-powering the supply, so unwanted smoke releases from equipment under test can occur with the 72-2630 if you're not careful.We'd love a hack that makes the DC switch default to 'off' when mains power is removed.Anyone familiar with these supplies and can suggest such a hack?TIA.

author: snoopy
author_rank: Frequent Contributor
timestamp: March 15, 2017, 12:25:08 pm
content: jeez that's a shocker of a design

author: carl_lab
author_rank: Frequent Contributor
timestamp: March 15, 2017, 12:39:24 pm
content: That's not a shocker, it's normal behavior of old school "analog" power supply controlled by rotary knobs.Sometimes you need same adjustments as before powered down.Generally, you should disconnect load from PS, if you don't know the adjustments.Mostly, I hate readjusting PS every time powered on. Some PS can be configured what power-up-mode is used.

author: gtc
author_rank: Newbie
timestamp: March 15, 2017, 12:52:38 pm
content: Quote from: carl_lab on March 15, 2017, 12:39:24 pm... which, of course, this power supply isn't.The described behaviour may suit some people but it doesn't suit us. As mentioned, our other Tenma models behave the way we expect and want: the default is for the DC switch to be OFF.So, back to my original question ...

author: Mark
author_rank: Frequent Contributor
timestamp: March 15, 2017, 04:36:03 pm
content: Have you checked the manual? Can you be certain that element14 have given you a the correct answer?I have two tenma dual power supplies which are essentially single units bolted together. One of them defaults to 30V output when switched on, the other three default to the last used voltage. All default to output off.What I am suggesting is that your unit may be faulty. Hopefully someone with the same model will come along soon.

author: kwass
author_rank: Frequent Contributor
timestamp: March 15, 2017, 05:18:22 pm
content: The BK 1739 linear power supply works the same way.http://www.bkprecision.com/products/power-supplies/1739-low-current-high-resolution-dc-power-supply-30-v-999-9-ma.htmlIt remembers the last output state when you turn it off. So it will power on in the output OFF state if you last had it in that state, otherwise it will power up with the output ON. I've sort of gotten used to turning off the supply in the OFF state but I'm still thinking about building in a small interface and I'm pretty sure that the programming is enabled just before the output gets turned on at startup.The Tenma is not programmable so it would take some serious hacking to do something similar.

author: gtc
author_rank: Newbie
timestamp: March 16, 2017, 02:10:05 am
content: Quote from: Mark on March 15, 2017, 04:36:03 pmThe manual is nowhere to be found, even on the Web. In any case, Tenma manuals are very poorly translated into English -- a situation that element14/Farnell as the sole suppliers ought to be ashamed of.QuoteNo we are not, but in the absence of any useful manual that's the situation.QuoteYes, that is our experience with our dual supplies. It's not the voltage setting that is the concern, it's the instant on.QuoteYes, it probably is faulty, but when we put that proposition to element14, they said that's the normal behaviour. Circular argument.

author: gtc
author_rank: Newbie
timestamp: March 16, 2017, 02:13:19 am
content: Quote from: kwass on March 15, 2017, 05:18:22 pmI guess so.Could possibly kluge it with a relay.