# Mini power supply - legit or death trap?

#### R!f@@

Joined Apr 2, 2009
9,784
Unless we know or see the PCB, I cannot say if it is isolated PSU or capacitor coupled one.
The later is dangerous and should not be used if you have even the slightest chance of touching either of the output pins..
But from the looks of it I believe it is an SMPS. So should be safe to handle and is designed for PCB mounting

#### Lucky-Luka

Joined Mar 28, 2019
173
Unless we know or see the PCB, I cannot say if it is isolated PSU or capacitor coupled one.
The later is dangerous and should not be used if you have even the slightest chance of touching either of the output pins..
But from the looks of it I believe it is an SMPS. So should be safe to handle and is designed for PCB mounting
I just have an Arduino on it and a couple of relays driving a lamp and an alarm system

#### AlbertHall

Joined Jun 4, 2014
12,019
From the circuit diagram in your link it is an isolated SMPS supply and I expect that it is encapsulated so no PCB view will be available.

#### Hymie

Joined Mar 30, 2018
1,081
As AlbertHall says, it is an isolated supply – besides the transformer, it has two capacitors (in series) and an opto isolator between the mains and low voltage out (based on the circuit diagram).

For increased assurance of a safe product, buy one having third party safety certification/approval (by an organisation such as UL, TUV, VDE etc), which will be indicated by their approval/certification logo on the product, and third party certification details in the specification.

#### R!f@@

Joined Apr 2, 2009
9,784
I just have an Arduino on it and a couple of relays driving a lamp and an alarm system
From the PDF, it is an isolated supply so you should be good.
If your total load is around 80% of the rated PSU power, than you can run without overloading or heating it up too much.

Yes, you can run an arduino and couple of relays of it 24/7

#### R!f@@

Joined Apr 2, 2009
9,784
As AlbertHall says, it is an isolated supply – besides the transformer, it has two capacitors (in series) and an opto isolator between the mains and low voltage out (based on the circuit diagram).
Two caps in series ??
where did you get that from ?

It's TX isolated Supply with an opto feedback loop.

#### DickCappels

Joined Aug 21, 2008
8,704
It is probably isolated. To get 3W at 240 VAC you would need an about 100 uF low leakage capcitor. 120 VAC, a 200 uf capacitor. This power supply is a bit expensive buy you can buy one and check the AC leakage current to earth by measuring the AC voltage across a 1 k resistor to earth. I do this with the little US$1 and$2 cellphone charger cubes. If I see more than a couple of voltage (couple of ma) then I know I would get a tingle from it, so I discard it.

It would be irresponsible to advertise a power supply for Arduino and Raspberry Pi that was not sufficiently isolated for normal use without giving a warning.

Just noticed that safety marks are not visible. There is a slight risk but not much in my opinion as long as you check it for AC leakage.

#### Lucky-Luka

Joined Mar 28, 2019
173
check the AC leakage current to earth by measuring the AC voltage across a 1 k resistor to earth. I do this with the little US$1 and$2 cellphone charger cubes. If I see more than a couple of voltage (couple of ma) then I know I would get a tingle from it, so I discard it.
I haven't understood how you do that check.

There is the class II insulation symbol on this module.
I should open it to see if it's true though...

#### DickCappels

Joined Aug 21, 2008
8,704

#### MisterBill2

Joined Jan 23, 2018
11,876
C31 and C32.
One cap is common in this position but two in series is equivalent.
https://www.power.com/community/for...capacitor-between-primary-and-secondary-sides

View attachment 266611
Looking at this circuit, it is NOT ISOLATED!! The capacitors couple the mains voltage to the output negative side and if it happens that the input side is at the mains potential a shock of some magnitude will happen (Can Happen).
Consider how effective coupling capacitors are in an audio amplifier and you will understand what I am talking about. These supplies are OK for charging a cell phone because the user never contracts the internal circuits, and the micro USB connector does not provide an easy way to touch the contacts. But they are not safe for general use powering non-insulated equipment.

#### AlbertHall

Joined Jun 4, 2014
12,019
Looking at this circuit, it is NOT ISOLATED!! The capacitors couple the mains voltage to the output negative side and if it happens that the input side is at the mains potential a shock of some magnitude will happen (Can Happen).
The capacitors are very small value and do not pass enough current to be harmful.
With, for instance, most laptop supplies you can just about feel the current.

#### MisterBill2

Joined Jan 23, 2018
11,876
The capacitors are very small value and do not pass enough current to be harmful.
With, for instance, most laptop supplies you can just about feel the current.
Given that the current flowing in those series capacitors must be at least the output current, it is not "small" .
This supply may be safe enough for battery charging but not for other purposes. It is a definite-purpose device and only safe for that purpose. AND it demands a great deal of trust in the stability and voltage resistance of those capacitors.
ALSO, all line noise present on the input will be transferred to the output.

#### Hymie

Joined Mar 30, 2018
1,081
Besides limiting any ‘leakage’ current from the mains supply to the output, those two capacitors (being a few nF in value) should be Y2 safety capacitors meeting IEC 60384-14.

Such a construction is common in switch mode power supplies, with safety capacitors bridging the insulation barrier (primary to secondary); it is permissible to use a single Y1 safety capacitor rather than two Y2 capacitors in series in most applications.

#### DickCappels

Joined Aug 21, 2008
8,704
The capacitors are very small value and do not pass enough current to be harmful.
With, for instance, most laptop supplies you can just about feel the current.
The specification says 3 watt at 5 volts out. A significant amount of current, but how could they get such large capacitors needed to have a low enough impedance to get the 3/5 amps of current? I think it is much more likely that the supply is transformer isolated for reasons given in post #9.

Looking at this circuit, it is NOT ISOLATED!! The capacitors couple the mains voltage to the output negative side and if it happens that the input side is at the mains potential a shock of some magnitude will happen (Can Happen).
Consider how effective coupling capacitors are in an audio amplifier and you will understand what I am talking about. These supplies are OK for charging a cell phone because the user never contracts the internal circuits, and the micro USB connector does not provide an easy way to touch the contacts. But they are not safe for general use powering non-insulated equipment.
Where I live many cell phone/game chargers are sold that do not comply with any particular safety standard. A number of time I have read in the news about people using their phones and hand-held games as they were charging and were electrocuted (not just a shock or tingle).

#### Hymie

Joined Mar 30, 2018
1,081
Indeed, I have seen many of these mains switch mode PSUs that do not meet recognised safety standards – that is why it is important to buy from a reputable source.

Having a mains supply system fitted with earth leakage protection (ground fault protection) will protect users from such devices that go faulty – but this should not be considered ‘an excuse’ to allow connection of dangerous products to the mains supply.

#### Boggart

Joined Jan 31, 2022
13
If you are not sure, buy a Meanwell IRM-03-5 instead, it's around the same price (probably less actually) and meets all required safety specs etc. Alternatively, a Recom RAC03-05SK. When it comes to power supplies, buy well known name brands.

#### MisterBill2

Joined Jan 23, 2018
11,876
Referencing post #16: Like I said, that circuit, as shown, does present a hazard.
Evidently the folks who were electrocuted somehow were able to connect with parts that should have not been touched.
The fact remains that just because a circuit functions does not mean that it is safe to use in all conditions.

The unintended secondary effects of actions can be far more unfortunate than anticipated.

#### Hymie

Joined Mar 30, 2018
1,081
Early in my working life, I was examining a faulty light touch switch with a colleague. From the switch construction, we could see that the conductive touch plate of the light switch was directly connected to the mains circuit via a single capacitor.

We both concluded that the construction of the touch switch was dangerous, since the failure of the capacitor would result in the touch plate becoming live.

But we were both wrong – Y1 (IEC 60384-14) safety approved capacitors are considered fail-safe, such that it is permissible to have un-earthed (floating) accessible parts connected to the mains via an approved Y1 capacitor – on the proviso that the current through the capacitor is within prescribed limits. Thus, these Y1 safety capacitors are often in switch mode PSUs, bridging the insulation (primary to secondary).