Intro

There is often a need to have nice low-noise power supply on the bench to power various equipment and experiments. Sure, one can use laboratory adjustable and programmable power supply with all bells and whistles, such as Keysight E36312A but it’s not a good choice if you need cheaper source that get stuff done without taking valuable bench space.

Also many volt-nut experience involve running small power modules, such as LTZ1000A-based voltage reference for months and years of the time. To help with this task I’ve bought good industrial +24V power supply from US manufacturer Acopian, which we will look at in this article.

Acopian Gold Series A power supply datasheet

Power supply was acquired from secondary market in used, but working condition. It is built in metal enclosure with vented shields. All inputs and outputs are available on a single screw terminal block. Output voltage adjustment trimmer also mounted on same side.

Features and specifications of our particular model from Acopian site :

A24MT350 Specification	Value	Unit	Notes
Nominal output voltage	24	VDC	Adjustable within ±500 mV
Maximum load current	3.5	ADC	at +40°C, derate to 2.5A at +71°C
Regulation line	±50	ppm	or 2mV, whichever is greater
Regulation load	±50	ppm	or 2mV, whichever is greater
Ripple noise, RMS	250	µVRMS	Unknown band
Case size	M9
Nominal input voltage	100-120	VAC	Option for 230V also available
Undervoltage/overvoltage Alarm	None
Overvoltage protection	None
Terminal cover	None
MSRP	$360	USD

Sides of the power supply are made of anodized aluminum heatsink bars. One side has three power transistors in TO-3 package mounted to it.

Based on labels, this power supply was manufactured in year 2001.

Output voltage terminals have Nichicon 330 µF 50V capacitor, mounted directly to the screw block pins. Round component by the left side is single-turn wiper for output voltage adjustment, sitting in series with negative sense line.

High-voltage mains input contacts are carefully heatshrinked with protection tubing, to reduce risk of electrical shock.

Power pass transistors are good old 2N3055 in TO-3. There are three of them here. Other power supply models have different amount of pass transistors to share the load current. Each transistor have 0.5 Ω cement resistor in emitter node.

PCB is cheap single-layer FR4 with thru-hole only components. There are no opamps or other integrated circuits, just discrete design with BJTs, resistors and capacitors. Large capacitors are nice quality, made by Nichicon.

There is no silkscreen or soldermask on the top layer, but circuit is simple enough for possible repairs and servicing.

Main output capacitor with screw terminals rated for +85 °C working temperature and has 5000 µF at 50 VDC rating.

Few transistors have little heatsink embedded into TO package. These are 2N6715 NPN BJTs .

Schematics of A-series Gold PSUs

Schematics of these series available right in the datasheet. This is appreciated move, Acopian!

Benchmarks with no to light load.

Test setup was easy and simple:

• Power supply under test, Acopian A24MT350 model
• Mains cable with a switch
• Keithley 2400 SMU as 105mA DC load
• Fluke 87V to monitor voltage
• Tektronix DPO7104C oscilloscope
• Tektronix ADA400A differential amplifier to measure noise, A=1, AC coupled
• Tektronix TCPA300A current probe to measure mains current
• Tektronix TMDP200 high voltage probe to measure mains voltage

Test results, noise measurements

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