Mastech 3003D Lab Power Supply Overview

Foreword

Greetings to the community. This short article will give an overview of the Mastech Laboratory Power Supply. I will not discuss the topic of model selection, but I hope that this article will help other electronics lovers make their choice.

Contents of delivery

Everything is as usual: cardboard box, polyethylene, polystyrene.
The package includes the unit itself, a power cord and a four-page user manual in English.
There are 3 large series of power supplies for Mastech: with directional indicators, with seven-segment LED indicators and LCD displays. The differences between similar products of different series end there.

What's inside?

The appearance of the device inspires confidence: it does not creak, does not play, potentiometers do not turn. Alright, now take a look inside

The heart of the power supply is a weighty transformer, which occupies most of the volume and mass of the PSU. Manufacturers did not stint on copper.



On top of the stabilizer board itself.





Build quality is good, the only thing is that powerful wire resistors are not all neatly mounted.



As regulating transistors, the widely used 2N3055 are used. As many as 2 pieces, although our power supply has a maximum current of 3A, and each of the transistors can provide current up to 15A. Good stock however.
Voltage regulation occurs with the switching of the secondary windings. Those. at low installed voltages, low-voltage windings are switched, with increasing - higher-voltage windings. Switching occurs at the following voltages: 7.5V, 15V and 22V. Switching occurs without surges. This structure helps to reduce the amount of heat generated by the control transistors. The filtering capacitor costs 4700 uF * 63V, which may seem insufficient for some, but as we will see later, its capacity is quite sufficient. The outputs are equipped with screw terminals, allowing you to still connect connectors such as "banana".



Transistors are isolated from the body and covered with a metal plate:

Measurements

For a more or less adequate assessment of the characteristics of the product, it was decided to conduct 3 small tests: idling, working under load, working in current limit mode. All measurements were carried out using an oscilloscope in the closed input mode.



This mode of operation is needed to cut off the input of the constant component so that you can clearly see the ripple.

The first test should show the amount of noise at the PSU output at different voltages; the second - if there are any artifacts when working on a load; third, how well the current limitation works (again, does the voltage form turn into a saw, etc.)

Idling.

Shows waveforms idling at different output voltages.

2B: ripple 1 mV.

10V: ripple 0.7 mV

15V (made with different horizontal values). Ripples 0.8-1 mV.
50 μS / div:

Here you can see periodic emissions with a small amplitude. Using cursors, reducing the scale to 20 μS / div, you can measure the frequency: approximately 14 kHz is obtained:



And the final picture with a scale of 500 nS / div:



20V: 0.4 mV ripple



and 30V: 0.45 mV ripple

Under load

The load is a wire resistor of 20 Ohms with a power of 5W of domestic production C5-5V. However, during the experiments, the power dissipated on it four times exceeded the nominal, and nothing happened to it.
Voltage 20V, current 1A:





As you can see, no artifacts appeared, the ripple level only slightly increased.

Current 3A, as a load - 3 parallel connected resistors of 20Ω * 5W.



Ripple <1mV.

Current limit mode

Now let's try to limit the current by half - to 500 mA:



And again, everything is quite predictable.

Of course, in the power supply there is protection against a short circuit at the output, which was “fortunate” to test several times (by connecting the device in the wrong polarity). The protection worked very quickly.

The manufacturer declared the following characteristics:

Parameter	Value
output voltage, V	0-30
output current, A	0-3
ripple current, mA	<= 3
ripple voltage level, mV	<= 0.5
load influence coefficient,% current	<0.2 + 5mA
load influence coefficient,% voltage	<0.01 + 5mV
the coefficient of influence of the supply voltage,% current	<0.2 + 1mV
the coefficient of influence of the supply voltage,% voltage	<0.01 + 1mV
Indication of output current and voltage	3 1/2-digit LCD indicators
Overall dimensions, mm	206 x 153 x 110
food, In	~ 220V ± 10%

I will also give an oscillogram of the transient response during switching on.



At the request of the community, he made oscillograms for connecting and disconnecting the load.
Connection:





Disconnect:

Verdict

Although the ripples turned out to be slightly higher than stated, they still have a fairly low level. These characteristics are enough to configure and debug most amateur radio designs. Of course, the raspberries were somewhat spoiled by transient characteristics when connecting / disconnecting a load.