transformless power supply or explosives? :D

[ikalogic]

transformerless power supply or explosives?

hello.. i am trying to build a transformerless 5v power supply that can deliver about 100 to 500 mA (100 mA is a minimum..)

- i don't want to use HI WATT resistor to dispate the excessive voltage...
- I want to use 7805 regulator..

So i thought of a way to try to reduce the voltage from 220 to something like 20 or anything under 35V (which is the max input voltage of a 7805)

here is my base idea..
**broken link removed**

The resistor R2 is to protect the transistor from the over voltage.. (TIP122 datasheet says MAX VCE = 100 V)

i think that if i adjust (by any mean) the base voltage to smth like 20V, the emitter voltage should be 20 - 2.5 = 17.5 V , which then can easily filtered and regulated though capacitors and a 7805...

i am not quite sure if i shoukd go on and test this on my bread baord... any advices?

EDIT: i also wonder if i do need a power transistor like the TIP, or a 2N2222 can do?

Don't make me wait for too long! or i'm gonna try this right now.. if you don't see me on the forum again.. you'll know why..

 

[ericgibbs]

Hi akalogic,
As you are using 220v ac, [ at 60hz ] you can use a non polarised paper
capacitor. Rz= 1/ (2 * 60 * cap), rated at a minimum of 250v ac.

Say you want 220mA then Rz = 220/.22 = 1000R

Cap = 1/(1000 * 2 * pi * 60) = 2.6mF

This is the same the method used in low cost consumer equipment.

REMEMBER: there is no isolation, so be careful!

Regards
EricG

 

[audioguru]

Your rectified 220VAC makes a peak voltage of 311VDC. So say goodbye to the TIP122.
The peak power in the TIP122 will be (311V-17.5V) x 500mA= 145W! It will explode.

 

[Sceadwian]

This isn't the first thread that wants to use a transformerless power supply... Aside from the chance for vaporizing plastic, semi conductors, or human tissue, what is the reason for not using a transformer? Especially for 500ma output they're hardly ungainly, and switchers can be built hardly larger than the circuit described.

 

[ikalogic]

This isn't the first thread that wants to use a transformerless power supply... Aside from the chance for vaporizing plastic, semi conductors, or human tissue, what is the reason for not using a transformer? Especially for 500ma output they're hardly ungainly, and switchers can be built hardly larger than the circuit described.

the problem with thansformers is size and weight.

i'll search the forum for other threads.. but usualy the search feature is not working very well..

 

[Olihou]

How about pulse width modulation of the rectified 311VDC, and then filtered by an LC filter for the required DC level ?

TIP122 can only stand 100 V? You will need some other transistors !

 

[ikalogic]

TIP122 can only stand 100 V? You will need some other transistors !

i know but.. wont R2 protect it from over voltage?

 

[ikalogic]

This isn't the first thread that wants to use a transformerless power supply... Aside from the chance for vaporizing plastic, semi conductors, or human tissue, what is the reason for not using a transformer? Especially for 500ma output they're hardly ungainly, and switchers can be built hardly larger than the circuit described.

forget about the 500 mA. 100mA will do.

i searched the rest of the forum.. but didn't rally find what i need..

Even if my circuit is wrong... i only need a way to reduce voltage.. am sure a voltage divider is not an option..

any alternatives..?

 

[ikalogic]

after lot of searching in old threads, i found a link for an app. note from uChip,

and i found this ckt:
**broken link removed**

it should work, and deliver about 50mA with the resistor they speciefied, and given an input voltage of 220V..

-now, according to my calculations, the resistor needs to be 15 W at least, not 5W as specified right?

-can i exchange the zener diode with a friendly 7805 ?

 

[Nigel Goodwin]

after lot of searching in old threads, i found a link for an app. note from uChip,

and i found this ckt:
**broken link removed**

it should work, and deliver about 50mA with the resistor they speciefied, and given an input voltage of 220V..

-now, according to my calculations, the resistor needs to be 15 W at least, not 5W as specified right?

It's probably for 110V? - you need to increase the resistor value (and wattage) for 230V. Really all these type of things are pretty useless, it's FAR easier and safer to use a transformer - there are odd occasions when it's OK, and everything being live isn't a problem (such as a light dimmer) - but they are few and far between.

-can i exchange the zener diode with a friendly 7805 ?

No, the zener stops the voltage going above 5.1V, a 7805 doesn't limit the input. You could use a 12V zener, and feed from that to a 7805.

 

[SPDCHK]

Microchip had **broken link removed** article on one of their CD's before. I have not used it myself, BUT you will not get the high current supply you require. Maybe you can use it to further develop your own power supply.

Good luck

 

[Sceadwian]

Circuits like that are very dangerous, failure of any of several components or contact with the wrong portion of the circuit will give you full mains voltage without current limiting.
You list size and weight as a problem with your circuit, but if it's attached to mains power how significant is that size and weight problem really?
Why not just canibalize some moderatly recent cell phone power pack? The switcher on my fiances cell phone is 5 volt 700ma's and it's about the same size as a deck of cards cut in half, and much lighter. Remove the internals and use that, it's UL listed so it's likley to be more reliable and safer than anything you can home brew.

 

[ikalogic]

Circuits like that are very dangerous, failure of any of several components or contact with the wrong portion of the circuit will give you full mains voltage without current limiting.
You list size and weight as a problem with your circuit, but if it's attached to mains power how significant is that size and weight problem really?
Why not just canibalize some moderatly recent cell phone power pack? The switcher on my fiances cell phone is 5 volt 700ma's and it's about the same size as a deck of cards cut in half, and much lighter. Remove the internals and use that, it's UL listed so it's likley to be more reliable and safer than anything you can home brew.

great idea.. i found in my junk box an old PALM charger.. it outputs 2A 6Vdc..

Can i feed the 6V into a 7805 to get clean 5Vdc, i mean, doesn't the 7805 require at least 7.5 Vdc input?

Can i attache 6V directly to a TTL uController? (it says 5.5 Vdc max in the AT89S52 datasheet... )

any simple solution to get rid of this extra 0.5 volt..?

A 1N4004 Diode in series will do the tric?

 

[technogeek]

Hi akalogic,
As you are using 220v ac, [ at 60hz ] you can use a non polarised paper
capacitor. Rz= 1/ (2 * 60 * cap), rated at a minimum of 250v ac.

Say you want 220mA then Rz = 220/.22 = 1000R

Cap = 1/(1000 * 2 * pi * 60) = 2.6mF

This is the same the method used in low cost consumer equipment.

REMEMBER: there is no isolation, so be careful!

Regards
EricG

question about this method - how do you know the current capacity of the capacitor? (i.e. how difficult to create >0.7A for instance?) And why not something like a polyester film cap?

 

[Sceadwian]

A capacitor as a current limiting device on mains power is stupidity in action excepting in ma power levels (well under 100ma's) They generally can't handle the ripple current, and are a prime failure point, possibly dead short to AC mains.

 

[ericgibbs]

Hi,
With ref to previous posts regarding the doubts about capacitors
for limiting the current.

For many years mains radio's were using capacitors as current
limiting devices. [ both home made and commercial ]

eg: a non polarised 4mF cap rated at 400v ac limits the current
flowing in a radios series connected valve heater chain to 300mA.

Zr= 1/(2 * pi * 50 * 4E-6)= 795R . I= 240/795 = 0.3A

The individual valves can have different operating voltages, typically
6.3v or 12.6v but must have the same heater current requirement, thats
why most GP valves have a heater current rating of 300mA or 150mA.

As there where about 5 valves in the radios, the total 'voltage'
across the valves was about 30 to 40 volts.

I think audioguru has missed the point, no one suggested that
the cap be connected in series with the bridge/regs.

Consider a 240v 50hz supply and we require a pre-regulated voltage of
say, 30v dc, at a max current of 0.3A

The way that its connected is the full wave bridge comprising
400v 2amp rectifiers, connected as a normal bridge.
Across the +/- outputs of the bridge is a 100R 10W resistor
in series with a darlington power transistor wired as a shunt regulator.

The supply to the external regulator and load is also
connected across the +/- terminals of the bridge.
OFF load the 100R and series shunt transistor hold the bridge output
at approx 30v dc. ON Full load the series shunt is virtually off.

With regard to current carrying capacity and 'blowing' up.
Almost every single phase motor and ac generator has a high voltage,
high current cap installed. As anyone seen, one blow up?, I havn't.

Although I have used and seen other engineers use these
techniques for mains driven transformer less psu's in the past.

I would NOT recommend anyone who is not a fully qualified
electrical/electronics engineer to use this technique, from the
point of view of personal safety and statutory regulations.

Regards
EricG

 

[Nigel Goodwin]

With regard to current carrying capacity and 'blowing' up.
Almost every single phase motor and ac generator has a high voltage,
high current cap installed. As anyone seen, one blow up?, I havn't.

Yes I have, but it's certainly not common - but they are seriously big and expensive capacitors!.

 

[Sceadwian]

The original posters intent was to come up with a a transformerless power supply because they were too big and heavy. What is the size of the capacitors used for motor starts on those motors? Probably larger than the transformer required to get 700ma at 5 volts... This post has because of various respondants preconceptions gone WAY out of line with the original intent of the post.

 

[Hero999]

Your rectified 220VAC makes a peak voltage of 311VDC. So say goodbye to the TIP122.
The peak power in the TIP122 will be (311V-17.5V) x 500mA= 145W! It will explode.

Come on, he did say he wanted to make an explosive.

 

[ikalogic]

The original posters intent was to come up with a a transformerless power supply because they were too big and heavy. What is the size of the capacitors used for motor starts on those motors? Probably larger than the transformer required to get 700ma at 5 volts... This post has because of various respondants preconceptions gone WAY out of line with the original intent of the post.

thx i was going to complain that most replies didn't help me with my problem.. but in a second thought i said, as long as someone is learning something.. it's worth it anyway..