Tenma Model 72-2685 Digital Controlled DC Power Supply - First Impressions

RE: Tenma Model 72-2685 Digital Controlled DC Power Supply - First Impressions

jw0752 — Sat, 26 Oct 2019 22:13:04 GMT

Hi Frank,

Thanks for taking the time to give us a report on this power supply. It is always helpful to have some personal insights.

John

RE: Tenma Model 72-2685 Digital Controlled DC Power Supply - First Impressions

three-phase — Sat, 26 Oct 2019 20:28:24 GMT

Great blog on your new power supply, hope it gives you many years of service. I have seen encoders for control on oscilloscopes before and do like them, never seen them on a power supply like this though.

Be interesting to see what quality they are and how long they last.

Kind regards.

RE: Tenma Model 72-2685 Digital Controlled DC Power Supply - First Impressions

koudelad — Sat, 26 Oct 2019 20:00:23 GMT

Hello Frank,

Thanks for this quick review of an entry level power supply. I think many hobbyists might look for such power supply as their first.

May I ask about the noise (sound) of it? There are reviews on Farnell eshop that it produces annoying hum and buzz.

Thank you,

David

RE: Tenma Model 72-2685 Digital Controlled DC Power Supply - First Impressions

baldengineer — Sat, 26 Oct 2019 17:59:53 GMT

I also have one of the 72-2685s. I agree with your points (and experience with the constant current control.) I wish it had output on/off control, but it behaves well turning AC turn-on. Overall, a good single-output supply for use on any bench.

Your fantastic write-up hit all of the relevant details.

RE: Tenma Model 72-2685 Digital Controlled DC Power Supply - First Impressions

shabaz — Sat, 26 Oct 2019 17:59:43 GMT

Hi Frank,

Nice review : )

I've got the almost-identical model 72-270572-2705, internally it looks the same, I've had mine unscrewed on my desk for the past week because I wanted to make a mod to it.

The boards look identical to the ones in your photo, the only difference being that mine is the European variant so the input mains voltage is different, and also there's a small additional board for the USB/serial.

I wanted to make a mod to have a simpler front panel on/off toggle switch (it is still possible to do that using some mode, but its not as easy as a toggle switch!).

The user interface could have done with an extra button or two : ) And a better display, because the red LEDs everywhere get confusing. Apart from that, the supply is great.

Anyway, I've aborted the idea to add a switch because I couldn't find a suitable way; there is no space to be able to attach a switch, because the front panel PCB takes up the entire space, and also because it didn't seem easy. I probed the pins that go off to a connector between the front panel and the rear PCB, it looked (to me) that pin 3 is 0V, pin 7 *might* be the DAC output (probably PWM), I didn't check with a 'scope, and pins 5 and 6 are the relay controls (they are at either 12V or 0V depending on the voltage level. Both are high (12V) at the lowest output voltages.

The small offset in the current sounds reasonable, looking at your CC mode measurements and readback. Mine only fares slightly better, in that if I just connect an ammeter directly across the supply terminals, then the output current was to within 1mA of the set current, but the readback was off for me by about 2mA (I only tested from 0 to 40mA, in 1mA increments), so it is in the same ballpark as yours. Since the display resolution on the unit is down to the 1mA value, then I expected a small error to be honest, so it felt better than I expected.

Anyway, apart from the UI, it seems to be a very nice, low-noise supply : ) It's got less ripple and harmonics compared to an old Thurlby supply, so I'm thinking of selling that as a result.

[View:https://players.brightcove.net/1362235890001/NkxiVJdjx_default/index.html?videoId=6098091422001:740:466]

Old Thurlby supply as a comparison (for context, this is a 20+ year old PSU though):

[View:/resized-image/__size/910x337/__key/commentfiles/f7d226abd59f475c9d224a79e3f0ec07-fa630962-f930-4eac-961c-07c30320b5b6/6663.contentimage_5F00_204954.png:910:337]

RE: Tenma Model 72-2685 Digital Controlled DC Power Supply - First Impressions

genebren — Sat, 26 Oct 2019 17:52:31 GMT

Frank,

Great write-up on your new power supply. Well done!

Gene