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 on: January 14, 2019, 04:57:47 04:57 
Started by Manuel - Last post by Manuel
I had chosen 1703 since it's SOT-23.
Obviously light load will allow to work properly even with such small case, so serial resistor should not be necessary I would say.
Agree with point #4 of SIGNAL.
Fortunately there are other producer than MCHIP that realize same component with higher rated voltage up to 40V, for light loads.

Take care,

 on: January 14, 2019, 04:57:38 04:57 
Started by Manuel - Last post by Sideshow Bob
This is just a suggestion. If you have some old cell phone chargers that you do not use any more why not at least try some of them and see if they can do the job. They have an output of about 5 volt. Those chargers that are orginal with the phones tend to be better than the no branded aftermarket ones. This could solve your voltage and power power dissipation issues

Posted on: January 14, 2019, 04:44:26 04:44 - Automerged

Exactly !

The question is this: 12V wall wart is not a good thing as they have no regulation most jive out 11 to 19 volts.

Thank you pickit2!

take care,
I would say it would depend. As many switch mode power supplies are actally specified not to only have maximum load current. But also a minimum load to be in spec, regarding output voltage and ripple. In general connecting a 2A switcher to a very minimal load would be a bad desing in most cases

 on: January 14, 2019, 03:43:47 03:43 
Started by Manuel - Last post by Signal
I have some thoughts about linear regulator to 3 V powered by 12V.
1) Linear regulator (MCP1703) from 12V to 3.3V at 250mA radiates more that 2W. It is too much for sot23. (No requirements was stated for current limit so I took the maximum for MCP1703)
2) Serial resistor 30 Ohm before such regulator introduces additional 7.5V drop and radiates 1.875W at 250mA. Then linear regulator has only 0.3W to radiate that is already acceptable.
3) 1uF at MCP1703 input as noted in datasheet is placed near to regulator and needed for stability. (For some regulators the requirement of having input capacitance not less or bigger then output is explicitly noted.) It is not enough to effectively compensate big load transients though if there is no additional bulk capacitor near on PCB and device is powered by long/unspecified cable. Of course it depends on dropout reserve too.
4) I'd use at least 100uF right after serial input resistor. Then tau=3000us. So with 30 Ohm load there will be no such "first connection" drop and overshot or it will be effectively filtered out. Rapid change of load current will cause "line/input" level change slow enough to effectively regulate it.
5) If manufacturer states 18V as absolute maximum without specifying time conditions (or avalanche rating) then I consider it dies at 18V immediately without any optimistic assumptions. Although I know that the one that kills electronic devices is energy, not voltage.
6) if an input is exposed and power supply polarity is not guaranteed then reverse power protection is needed - at lease a diode.

My choice however for low power device is to use 2954 or 9036 (both with reverse polarity protection) as first step to get 5V followed by 1703. As a bonus - 3.3V will be cleaner. And of course I'd consider the use of 220VAC/5VDC power supply instead of 12V. But I do not know full requirements for given device.

 on: January 14, 2019, 01:54:36 01:54 
Started by Manuel - Last post by Manuel
b555b, GS25E12 has been obsoleted... :-)

The question was not related to price, but to a technical issue rised due to no post protection/regulation on the DC channel of power supply.

Essentially: coupling contact create ONE spike with a value that is close to the power supply double for a time of 4us.
This was not expected.

So light electronic design will find issue using such power supplies.

pickit2 underline that a component with : Input Operating Voltage Range: 2.7V to 16.0V (with Absolute Maximum Ratings Vdd +18V), will be possibly affected..

My ideas was to use such a silly TVS product like SMF11A or a VREG with higher Voltage rating.

take care,

 on: January 14, 2019, 01:42:59 01:42 
Started by Manuel - Last post by Signal
Manuel, could you post the photo of power supply PCB? I am especially interested in output capacitor value.

 on: January 13, 2019, 09:13:14 09:13 
Started by Manuel - Last post by b555b
The question is this: 12V wall wart is not a good thing as they have no regulation most jive out 11 to 19 volts.

This is especially true for noname eBay/AliExpress wall adapters. For serious things you must consider using wall adapters from renowned brands. A GST25E12 from MeanWell (12V/2A) for example is specified at 3% voltage/line/load regulation. Different specs, different price too...

 on: January 13, 2019, 06:43:21 06:43 
Started by Manuel - Last post by Manuel
Exactly !

The question is this: 12V wall wart is not a good thing as they have no regulation most jive out 11 to 19 volts.

Thank you pickit2!

take care,

 on: January 13, 2019, 06:37:55 06:37 
Started by Manuel - Last post by pickit2
Just a quick reply
you said 'Voltage regulator was: MCP1703 at 3.3V SOT23-3
This is not a good device to use in your circuit as its main use is for battery powered devices.
the max input is only 16V not 18v so your close to limit
using a 12V wall wart is not a good thing as they have no regulation most give out 11 to 19 volts.

damn spelling.

 on: January 13, 2019, 04:35:06 04:35 
Started by Manuel - Last post by Manuel
The light schematic is here attached.

Initial conditions:
-The power supply is connected to AC line (wall mount switching power supply).
-The DC will be connected to the here attached schematic.
-The DC channel is connected to the oscilloscope.

>>When DC is applied to this schematic here attached, the waveform from the DC side is the one I showed you in the very first post.

The DC side cable length is 1000mm.
The MOV was omitted in my testing, it is valid just for a 24Vdc (not this case).
The V-REG is connected to a 4.7uF capacitor an 100nF. (not present in the light schematic)

C4 tested with both 1uF and 4.7uF (unpolarized tantalium).

It is obvious the generated waveform....but personally not expected, since I would have inserted a TVS in the power supply to avoid such high vout at the first contact.

My Post is more a WARNING than an help request... but suggestions and hints are welcome..

take care,

 on: January 12, 2019, 06:43:01 18:43 
Started by Manuel - Last post by bigtoy
It's so hard to tell what's going on, from the little information you've provided here. The comments above are valid.

Most power supplies are designed to power a resistive load, at a decent percentage of their rated power. If your load is very light, and capacitive & inductive, the PSU will struggle with it. A simple test is to put a resistive load right on the output of the power supply. It needs to be a decent sized resistive load, ie, something that will draw 10% or 20% of the PSU's rated current. A 10k resistor's probably not going to do it! :-) That test will tell you a lot about the PSU's behaviour. With that kind of a load, you shouldn't see that output overshoot any longer. Then you can experiment to determine what kind of load is going to meet your requirements for your application.

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