Tenma 72-10500 Dual Bench Power Supply - Teardown pictures & fan mods - Page 1
Forum section: electronics / testgear

author: spintronica
author_rank: Newbie
timestamp: February 23, 2018, 10:45:47 pm
content: Hi all. First time poster, thought I would share a bit about a bench power supply I recently purchased. Other units similar to this one have been shown on this forum as similar designs are manufactured by a few companies, but this exact brand and model haven't shown up here so I figured I'd post.Unit is a Tenma 72-10500 Dual output 30V 3A power supply with digital controls. I picked this model up from Farnell in Europe for ~138 EUR, plus shipping. For a dual output, linear power supply that goes to 30V and 3A with 7-segment LED style readouts it is pretty much the cheapest supply out there that I would touch.I've been using it as a bipolar supply for some audio projects for a few weeks without issue. The controls are good, the adjustment is a detented encoder and it easy to adjust. The adjustment 'cursor' can be set to whichever digit to adjust specifically. It also has the ability to adjust either the voltage or the current while in constant voltage or constant current mode *while the supply in enabled* which is something I needed that I am not sure all supplies do.The only issue I had right out of the box was the fan noise. Yes, there are two fans, one for each regulator board.. They are standard 12V computer fans. At idle, without drawing any current, the fans spin slow and sounded agonizing, like they had bearings going out in them. I quickly unplugged them and powered them with a separate DC supply to find that they were perfectly fine and quiet when supplied with a constant DC supply. I correctly guessed that they were being PWM speed controlled and quite badly at that. It's a 2-wire 12V fan, and the speed is being slowed by sending a ~100hz square wave with different pulse widths- looks like 25% PW for the idle speed, switches to 50% with moderate load and then just goes full on 12V when the supply is maxxed out (30V and 3A full on). The output waveform had some wild spikes too and some sort of bad chop every few cycles that would upset the square wave, like a bad interrupt. So this was causing all the horrible noise in the fan of course. Long term solution would be a separate fan controller, or even a fanless conversion with a large enough heat sink. But short term solution.... 100uF cap in parallel with the fan. Smooths out the square wave. Makes the fans at idle run slightly faster, of course, but they are far, far quieter as they lack the horrible jitter noise.Other than that, here are some pics of the teardown. You can see the regulator board with two separate linear regulators and two relays with a rectifier. The relays are there to switch between two voltage ranges done with the two regulators - the switchover happens at 7 volts if I remember correctly.Also, the supplies are identical side to side except there is a 4-wire data cable from one to the other, and the front panels of each are different, giving one of the two controllers functionality to control serial/paralleling the supplies, which is accomplished via another set of relays on a board right behind the banana jack panels.Looks like good protections throughout. Good sleeving on the mains AC cabling. Silicone (not hot melt!) to glue all the connectors and the big caps. Transformer seems quality, tight, good quality leads in and out.

author: spintronica
author_rank: Newbie
timestamp: February 23, 2018, 10:47:22 pm
content: pics...

author: spintronica
author_rank: Newbie
timestamp: February 23, 2018, 10:50:20 pm
content: I had a scope circa ~1970 on the bench at the same time I was diagnosing the fan noise.... so here are some vintage tube scope shots of that PWN signal, before and after the smoothing cap action: