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[nextpage title=”Introduction”]

The Coolmax V-500 costs only USD 22 at Newegg.com and, therefore, we were very skeptical that it could deliver its labeled wattage, 500 W. Wattage, as we always explain, isn’t everything. Let’s see the other downsides of using a cheap power supply.

The reviewed power supply doesn’t have an active PFC circuit, has a manual 115 V/230 V switch, and is based on the obsolete half-bridge design.

Figure 1: Coolmax V-500 power supply

Figure 2: Coolmax V-500 power supply

The Coolmax V-500 is 5.5” (140 mm) deep, using a 120 mm sleeve bearing fan on its bottom (LY 1225L12S ND1).

This unit doesn’t have a modular cabling system, and only the main motherboard cable has a nylon sleeve. This power supply comes with the following cables:

• Main motherboard cable with a 20/24-pin connector, 14.6” (37 cm) long
• One cable with an ATX12V connector, 15.7” (40 cm) long
• One cable with one SATA power connector, 15.4” (39 cm) long
• One cable with two peripheral power connectors, 15.4” (39 cm) to the first connector, 5.9” (15 cm) between connectors
• One cable with two peripheral power connectors and one floppy disk drive power connector, 15.4” (39 cm) to the first connector, 5.9” (15 cm) between connectors
• One ATX12V/EPS12V extension, 6.7” (17 cm) long

There are several major flaws here.

First, all wires are 20 AWG, thinner than the minimum recommended (18 AWG).

Second, the number of connectors is simply ridiculous for a power supply labeled as a 500 W product, with no video card power connector and only one SATA power connector. Even very basic computers nowadays require at least two SATA power connectors, one for the hard drive and another for the optical drive.

Third, there is a -5 V (white) wire on the main motherboard cable, wire that was removed from the ATX12V specification in January 2002, almost 10 years ago. Therefore, this power supply uses an obsolete design. Inside the power supply, we could clearly read a marking saying that it is an ATX12V 1.3 model (the ATX12V specification is 2.3), but the box says this unit is an “ATX12V 2.01” unit. This is wrong for two reasons. First, there was never a 2.01 ATX12V specification. Second, the correct name is ATX 2.01 specification (and not ATX12V), which only specifies the physical dimensions of the power supply housing.

Figure 3: Cables

Let’s now take an in-depth look inside this power supply.

[nextpage title=”A Look Inside the Coolmax V-500″]

We decided to disassemble this power supply to see what it looks like inside, how it is designed, and what components are used. Please read our Anatomy of Switching Power Supplies tutorial to understand how a power supply works and to compare this power supply to others.

On this page we will have an overall look, and then in the following pages we will discuss in detail the quality and ratings of the components used.

Figure 4: Top view

Figure 5: Front quarter view

Figure 6: Rear quarter view

Figure 7: The printed circuit board

[nextpage title=”Transient Filtering Stage”]

As we have mentioned in other articles and reviews, the first place we look when opening a power supply for a hint about its quality, is its filtering stage. The recommended components for this stage are two ferrite coils, two ceramic capacitors (Y capacitors, usually blue), one metalized polyester capacitor (X capacitor), and one MOV (Metal-Oxide Varistor). Very low-end power supplies use fewer components, usually removing the MOV and the first coil. 

This power supply doesn’t come with the two MOVs that are required on the half-bridge design, which would be installed between the two electrolytic capacitors of the voltage doubler circuit.

Figure 8: Transient filtering stage

In the next page we will have a more detailed discussion about the components used in the Coolmax V-500.

[nextpage title=”Primary Analysis”]

On this page we will take an in-depth look at the primary stage of the Coolmax V-500. For a better understanding, please read our Anatomy of Switching Power Supplies tutorial.

Instead of using a ready-made rectifying bridge, this power supply uses four discrete diodes. Four RL207 diodes are used, each one supporting up to 2 A at 75° C. So, in theory, you would be able to pull up to 230 W from a 115 V power grid. Assuming 80% efficiency, these diodes would allow this unit to deliver up to 184 W without burning themselves out. Of course, we are only talking about these particular components. The real limit will depend on all the components combined in this power supply. Here we can clearly see that it is impossible for this power supply to be a 500 W product.

Figure 9: Rectifying bridge

The electrolytic capacitors of the voltage doubler circuit are Chinese, from Tongjia, and labeled at 105° C.

In the switching section, two AT350 NPN transistors are used in the obsolete half-bridge configuration. Unfortunately, we couldn’t find the datasheet for these components.

Figure 10: Switching transistors

The primary is controlled by an ATX2005 PWM controller, which is a very popular controller among “generic” power supplies (i.e., power supplies with fake wattages). This integrated circuit is physically located in the secondary of the power supply.

Figure 11: PWM controller

Let’s now take a look at the secondary of this power supply.

[nextpage title=”Secondary Analysis”]

The Coolmax V-500 has three rectifiers attached to the secondary heatsink.

The maximum theoretical current each line can deliver is given by the formula I / (1 – D) where D is the duty cycle used and I is the maximum current supported by the rectifying diode. Since this unit uses the half-bridge design, the duty cycle is 50 percent.

The +12 V output uses one MUR2020CT rectifier (20 A, 10 A per internal diode at 145° C, 1.15 V maximum voltage drop), giving us a maximum theoretical current of 20 A or 240 W for this output. It is important to understand that this device is not a “Schottky” model, meaning it presents a higher voltage drop and, therefore, lower efficiency.

The +5 V output uses one SB3040ST Schottky rectifier (30 A, 15 A per internal diode), giving us a maximum theoretical current of 30 A or 150 W for this output.

The +3.3 V output uses one SB2045CT Schottky rectifier (20 A, 10 A per internal diode at 135° C, 0.57 V maximum voltage drop), giving us a maximum theoretical current of 20 A or 66 W for this output.

Figure 12: The +12 V, +5 V, and +3.3 V rectifiers

Notice how the +5 V rectifier is “stronger” than the +12 V one. This was a typical scenario 15 years ago. Nowadays, the +12 V output needs to be “stronger” than the other outputs, since the components that pull most of the current/power (the CPU and video cards) are connected to this output.

The PWM controller shown in Figure 11 is also in charge of monitoring the unit’s outputs. It provides over voltage (OVP) and under voltage (UVP) protections.

The electrolytic capacitors available in the secondary are also Chinese, from JEE and Sapcon, and are labeled at 105° C.

[nextpage title=”Power Distribution”]

In Figure 13, you can see the power supply label containing all the power specs.

Figure 13: Power supply label

According to the label, this power supply has two +12 V rails, but this information is false, since this power supply doesn’t implement over current protection (OCP). Since there is no OCP, this unit uses a single-rail design. Click here to understand more about this subject.

How much power can this unit really deliver? Let’s find out.

[nextpage title=”Load Tests”]

We conducted several tests with this power supply, as described in the article Hardware Secrets Power Supply Test Methodology.  

Because we had no clue as to the real wattage of this power supply, we tested it differently. Starting from 85 W, we increased the load little by little until we could see the maximum amount of power we could extract from the reviewed unit.

 If you add all the powers listed for each test, you may find a different value than what is posted under “Total” below. Since each output can have a slight variation (e.g., the +5 V output working at +5.10 V), the actual total amount of power being delivered is slightly different than the calculated value. In the “Total” row, we are using the real amount of power being delivered, as measured by our load tester.

The +12VA and +12VB inputs listed below are the two +12 V independent inputs from our load tester. During this test, both inputs were connected to the power supply’s single +12 V rail.

Input	Test 1	Test 2	Test 3	Test 4	Test 5	Test 6
+12VA	3 A (36 W)	3.5 A (42 W)	4.5 A (54 W)	5.5 A (66 W)	6.25 A (75 W)	7.5 A (90 W)
+12VB	2.5 A (30 W)	3.25 A (39 W)	4 A (48 W)	5 A (60 W)	6 A (72 W)	7 A (84 W)
+5 V	1 A (5 W)	1 A (5 W)	1.5 A (7.5 A)	1.5 A (7.5 A)	2 A (10 W)	2 A (10 W)
+3.3 V	1 A (5 W)	1 A (5 W)	1.5 A (4.95 W)	1.5 A (4.95 W)	2 A (6.6 W)	2 A (6.6 W)
+5VSB	1 A (5 W)	1 A (5 W)	1 A (5 W)	1 A (5 W)	1 A (5 W)	1 A (5 W)
-12 V	0.5 A (6 W)	0.5 A (6 W)	0.5 A (6 W)	0.5 A (6 W)	0.5 A (6 W)	0.5 A (6 W)
Total	90.6 W	99.5 W	124.4 W	147.1 W	171.9 W	196.4 W
% Max Load	18.1%	19.9%	24.9%	29.4%	34.4%	39.3%
Room Temp.	42.2° C	41.6° C	41.3° C	41.2° C	41.2° C	40.2° C
PSU Temp.	47.4° C	46.2° C	45.6° C	45.3° C	45.3° C	44.7° C
Voltage Regulation	Pass	Pass	Pass	Failed on +5 V	Failed on +5 V	Failed on +5 V
Ripple and Noise	Pass	Pass	Pass	Pass	Pass	Pass
AC Power	119.4 W	130.4 W	161.4 W	191.5 W	224.3 W	260.4 W
Efficiency	75.9%	76.3%	77.1%	76.8%	76.6%	75.4%
AC Voltage	115.6 V	115.9 V	115.0 V	114.0 V	114.6 V	113.5 V
Power Factor	0.649	0.652	0.658	0.662	0.665	0.668
Final Result	Pass	Pass	Pass	Fail	Fail	Fail

Input	Test 7	Test 8	Test 9	Test 10	Test 11	Test 12
+12VA	8.25 A (99 W)	9.25 A (111 W)	10 A (120 W)	11 A (132 W)	12 A (144 W)	13 A (156 W)
+12VB	8 A (96 W)	9 A (108 W)	10 A (120 W)	11 A (132 W)	11.75 A (141 W)	12.75 A (153 W)
+5 V	2.5 A (12.5 W)	2.5 A (12.5 W)	3 A (15 W)	3 A (15 W)	3.5 A (17.5 W)	3.5 A (17.5 W)
+3.3 V	2.5 A (8.25 W)	2.5 A (8.25 W)	3 A (9.9 W)	3 A (9.9 W)	3.5 A (11.55 W)	3.5 A (11.55 W)
+5VSB	1 A (5 W)	1 A (5 W)	1 A (5 W)	1 A (5 W)	1 A (5 W)	1 A (5 W)
-12 V	0.5 A (6 W)	0.5 A (6 W)	0.5 A (6 W)	0.5 A (6 W)	0.5 A (6 W)	0.5 A (6 W)
Total	220.2 W	240.8 W	263.4 W	282.9 W	304.4 W	329.4 W
% Max Load	44.0%	48.2%	52.7%	56.6%	60.9%	65.9%
Room Temp.	40.6° C	41.4° C	42.3° C	43.4° C	44.3° C	44.4° C
PSU Temp.	45.3° C	46.3° C	47.5° C	48.9° C	50.7° C	49.3° C
Voltage Regulation	Failed on +5 V	Failed on +5 V and +12 V	Failed on +5 V and +12 V	Failed on +5 V and +12 V	Failed on +5 V and +12 V	Failed on +5 V and +12 V
Ripple and Noise	Pass	Pass	Failed on +12 V	Failed on +12 V	Failed on +12 V and -12 V	Failed on +12 V, +5 V, and -12 V
AC Power	296.0 W	330.5 W	370.6 W	410.5 W	456.0 W	487.0 W
Efficiency	74.4%	72.9%	71.1%	68.9%	66.8%	67.6%
AC Voltage	114.0 V	114.2 V	113.2 V	112.8 V	112.2 V	112.2 V
Power Factor	0.672	0.676	0.68	0.685	0.69	0.672
Final Result	Fail	Fail	Fail	Fail	Fail	Fail

The Coolmax V-500 is a piece of junk. We could only pull around 330 W from it. Above that the power supply burned.

Efficiency was always very low, between 66.8% and 77.1%. In fact, when we consider that power supplies achieve their peak efficiency when delivering around 50% of their maximum load, the Coolmax V-500 would be a 250 W unit, since its peak efficiency was achieved during test three, with the power supply delivering around 125 W.

Voltage regulation is one of the main issues with this unit. From test four (150 W) on, voltages were outside their proper range. The +5 V output failed on test four at +5.26 V (the maximum allowed is +5.25 V), increasing all the way to +5.39 V on test 12. The +12 V output failed from test eight (240 W) on, dropping to +11.38 V during this test (the minimum allowed is +11.40 V), decreasing to +10.99 V during test 12. The ATX12V specification says positive voltages must be within 5% of their nominal values, and negative voltages must be within 10% of their nominal values.

Noise and ripple levels got above the proper level from test nine (260 W) on. During test 12, noise levels at the +12 V outputs were at 207 mV, at the +5 V outputs were at 60 mV, and at the -12 V outputs were 186 mV. The maximum allowed is 120 mV for +12 V and -12 V outputs, and 50 mV for +5 V, +3.3 V and +5VSB outputs. All values are peak-to-peak figures.

[nextpage title=”Main Specifications”]

The main specifications for the Coolmax V-500 power supply include:

• Standards: ATX12V 2.0 (fake, it is actually 1.3)
• Nominal labeled power: 500 W
• Measured maximum power: 329.4 W at 44.4° C ambient
• Labeled efficiency: NA
• Measured efficiency: Between 66.8% and 77.1%, at 115 V (nominal, see complete results for actual voltage)
• Active PFC: No
• Modular Cabling System: No
• Motherboard Power Connectors: One 24-pin connector and one ATX12V connector
• Video Card Power Connectors: None
• SATA Power Connectors: One
• Peripheral Power Connectors: Four on two cables
• Floppy Disk Drive Power Connectors: One
• Protections (as listed by the manufacturer): Over voltage (OVP) and short-circuit (SCP) protections
• Are the above protections really available? Yes. The unit also has under voltage protection (UVP).
• Warranty: One year
• More Information: https://www.coolmaxusa.com
• Average Price in the US*: USD 22.00

* Researched at Newegg.com on the day we published this review.

[nextpage title=”Conclusions”]

The Coolmax V-500 is a textbook example of why you should not buy a USD 20 power supply. Its inability of delivering its labeled wattage is not its worst problem. It presents lousy efficiency between 66.8% and 77.1%, its voltages are out of range, and noise and ripple levels are above the maximum allowed. A perfect weapon of mass destruction. Stay away.