TekPower TP3016M power supply output funkiness - Page 1
Forum section: electronics / testgear

author: mtdoc
author_rank: Super Contributor
timestamp: February 12, 2017, 12:13:23 am
content: I've had one of these small "handheld" TekPower switching PSUs for a while now and have been using it for breadboard projects, charging USB devices, batteries and other such tasks. Voltage and current are fairly accurate and I've had no issues using it for any of those things. But recently I found that it was causing an issue when trying to power a small data acquisition board (one of trophospheres ECG boards) - apparently blowing out one of the ICs, causing a short across the power input pins.Checking the PSU I found that with power supplied but the output off, there is -640 mV across the output terminals.Opening the PSU up shows a zener and a couple of caps across the output. I suppose the diode is responsible for the -640 mV on power up. Is that is by design??On the scope it shows this when the output is switched on:So it jumps up to zero volts then up to the set output (3V in this case). No overshoot.I suspect one of the ECG board ICs is getting blown out by this small negative voltage and I believe input protection is going to be added to the ECG board.So help me understand why this PSU is designed this way. Is this common? Or maybe this is a defect or design flaw?I don't see a negative voltage like this on any of the linear PSUs I've tested.

author: mtdoc
author_rank: Super Contributor
timestamp: February 12, 2017, 10:24:31 pm
content: Bump.Anyone with any insight into this? Should I be worried using this supply on other projects or are my concerns unjustified?

author: mikeprevette
author_rank: Newbie
timestamp: April 01, 2017, 09:35:05 pm
content: I have the same unit, and also see the same negative bias to the outputs. Haven't broken into it to see if there might be a solution (not that I'd know what I was looking for).I am however surprised at it's stability and accuracy for what it is, I put the Fluke on it and was really pleased.

author: saturation
author_rank: Super Contributor
timestamp: April 02, 2017, 03:44:15 pm
content: That's a problem. V=0 [ relative to its ground] is expected at power off for any good PSU. As a rule, damage due to reverse bias has far less margin than overshoot. For example, a LP2950 3V low power regulator has max Vin of ~ -0.3V to 30V.

author: mikeprevette
author_rank: Newbie
timestamp: April 02, 2017, 10:01:11 pm
content: Did some more playing around this afternoon. I didn't have my real scope available at home, but I had a little DSONano and grabbed a plot of the off/on action. It's almost identical to the one posted above.It does seem likely it is that output diode as mentioned in the the first post. When I put that diode on the Fluke it has almost the same reading as the valu of the negative bias. Could that be a coincidence?Also I probed the USB ports, and when the output is turned off I'm seeing a -0.29v bias there.

author: saturation
author_rank: Super Contributor
timestamp: April 09, 2017, 11:27:48 pm
content: Its hard to decipher without the schematic. I think its a bug, since the output is correct at turn on, as well as it zeroes prior to rising to programmed output; so its calibrated. The negative offset can be from many things [ such as a low power switching state by the converter its using] but its clearly been compensated once the unit is turned on. The bug is, IMHO this should be set to V=0 at boot not at user turn on.Since the PSU is not really hardware 'off' once plugged and its a soft turn on, the regulator circuits are not set in the undefined state.If that diode is across the output, it and its caps maybe for reverse input protection and some output smoothing, respectively.

author: mtdoc
author_rank: Super Contributor
timestamp: April 12, 2017, 06:59:50 pm
content: Thanks mikeprevette for posting. It's reassuring to know this bug is present on other TP3016Ms.Quote from: saturation on April 09, 2017, 11:27:48 pmYes, I believe the diode is there for reverse input protection and is the cause of the negative offset at power on. I should probably desolder it to confirm this. The question is how to best fix this while maintaining reverse input protection?Yes I agree that the caps are likely there for smoothing at output turn on, as can be seen by my scope trace poster earlier.I really like this power supply but I'm a bit worried now when using it with any circuits potentially sensitive to small negative voltages. Any suggestions on a fix much appreciated.

author: saturation
author_rank: Super Contributor
timestamp: April 13, 2017, 02:13:44 pm
content: A simple solution is power the TP3016M with output ON first before connecting the leads of your project. As shown on the first post, the output is correct thereafter. Disconnect leads to project first before turning the TP3016M power off.

author: mtdoc
author_rank: Super Contributor
timestamp: April 13, 2017, 02:37:48 pm
content: Quote from: saturation on April 13, 2017, 02:13:44 pmHa ha. Yes, you could do that of course. I think I would call that a work-around and not a solution though.

author: saturation
author_rank: Super Contributor
timestamp: April 14, 2017, 10:11:03 pm
content: Yes, you need to troubleshoot the hardware and firmware for best solution, otherwise everything else is a work around. Just 2 more ideas, is it calibration? IIRC it has DIY calibration procedures, maybe the offset can be corrected.Does shorting the output remove the negative bias as well as draw negligible current? If so, the protection circuits are engaged before power ON, so a shorting switch, either mechanical or electronic, can replace dis/connecting power leads to your DUT.Quote from: mtdoc on April 13, 2017, 02:37:48 pm

author: mtdoc
author_rank: Super Contributor
timestamp: April 15, 2017, 02:09:30 am
content: Quote from: saturation on April 14, 2017, 10:11:03 pmCalibration only affects the output voltage when output is turned on. It has no effect on the -0.64 V bias present with output off.QuoteWell shorting certainly removes the negative bias (how could it not?) with about about 370uA of current.I think I'll give Tekpower a call and see if I they have a solution.

author: Platis
author_rank: Newbie
timestamp: July 10, 2017, 03:42:37 pm
content: Hi!Did you find any solution for this problem?

author: Matkovich_X
author_rank: Newbie
timestamp: August 14, 2017, 09:40:18 pm
content: Can anyone confirm if the model in amazon support 220v?https://www.amazon.com/gp/product/B015OA1J82/ref=reviewsLightboxvoterdr?ie=UTF8&asin=B015OA1J82&noCache=1502744023654&thanksvoting=cr-vote-R1KZU7MIOH2AZ4#R1KZU7MIOH2AZ4.2115.Helpful.ReviewsThanks!

author: DavidDLC
author_rank: Frequent Contributor
timestamp: September 07, 2017, 11:03:03 pm
content: Quote from: Platis on July 10, 2017, 03:42:37 pmI received a solution by email to the output off problem, I made the mod and now the output voltage when the output is off is about 0.01 V and about 0.06 V for USB output.With no load it takes some time to discharge but I don't care about this, I'm happy with the fix for the negative output problem.I have not measured the output voltage ramp with an oscilloscope yet.Now I love even more this little compact power supply.David DLC

author: guse
author_rank: Newbie
timestamp: November 29, 2017, 09:59:36 pm
content: Hello DavidDLC,Is it possible to explain what you did to fix this negative voltage ?Did you measured the output voltage ramp ?Thanks a lotGuillaume.