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Anyone know where yo get tiny switch mode supplies?

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danrogers

Member
Hi all, I'm working on making a thermostat for my heating. I need to find a way to drop 240v to something like 9v. It needs to be really small to fit in the wall box, I was going to use a standard inductive transformer http://uk.rs-online.com/web/p/pcb-mounting-transformers/7320531/ but its really a bit to tall. Something low profile would be far better.

I need it to be able to deliver max 200mA, anyone know of anything available?
 

kubeek

Well-Known Member
Most Helpful Member
Maybe you could tear down some usb charger adaptor or similar.
 
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danrogers

Member
hi there. I was thinking about that possibility, I opened up a nokia charger from 2 or 3 years ago (a nightmare to open!) but it outputs 5.7v, so I'd need to add another regulator and I'm trying to keep the overall component footprint really small

maybe I'll get hold of a newer one and see what it puts out if there are no other off the shelf solutions?
 

ChrisP58

Well-Known Member
Most Helpful Member
The output voltage is usually set by a pair of resistors in a divider. You can often change the output voltage by changing one of those values. But you'll need to reverse engineer the supply to get a schematic to know what resistor to change, and how far you can go before other components exceed their limits.

But be very careful about the high voltage that is in the supply.
 

dougy83

Well-Known Member
If it's just a USB charger/adapter it's likely that the output is regulated using either TL431 and resistor divider (as mentioned above) or just a zener; this provides feedback to the converter IC via an opto coupler. Usually there are very few components, so it's very easy to work out what to change.

Increasing the output voltage may cause problems for the converter chip however - doubling the output voltage will double the spike across the primary coil as well (which is connected directly to the transistor switch). Reducing the output voltage won't have this issue however (e.g. from a 12VDC o/p to 9VDC).
 

audioguru

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My local surplus electronics store had a few thousand small switched-mode power supplies for modems. They are rated at 5V-2A but have a trimpot to adjust the output voltage up to 35VDC. They were Name-Brand and costed less than $2.00 each.
 

dr pepper

Well-Known Member
Most Helpful Member
Compact fluorescent lights have inverters so arent much good, but the later generation led lights have a mini smpsu inside and are fairly cheap, and leds run at a few v, the only thing is they tend to be constant current so you'dd need to mod the feedback circuit.
 

WTP Pepper

Active Member
Something we are always looking for at work. Have a look at TRACO or V-Infinity. They do some very small units that conform to EU regulations for "touch current" etc.
It's easy to botch something together using capacitor dividers, but if it burns your house down......
RS Comps do some very small ST devices on a PCB (I can get the details if required). They seem to work OK but are not certified for general use, not EU approved despite working very well in my test circuits.
 

dr pepper

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If you can live with 5v you can get 8 pin chips you put 240vac in and get 5vdc out, they are common in tellys to power the standby circuit.
Dunno if they are isolated from the mains, probably not.
 

tcmtech

Banned
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Around here our heating systems use a 24 VAC power source for the thermostats and controls which make for a easy to tap into power source for small low power applications relating extra functions on specialty thermostats and the like.

Any chance your systems over there use the same or a similar low voltage power source that could be tapped into?
 

tvtech

Well-Known Member
Most Helpful Member
Mike says, Be careful with "cheapo" switching power supplies

Now that is a Video ALL should watch. Absolute crap coming out of China now....

This rubbish has the ability to destroy your expensive PC or Lappie. Trash.

Regards,
tvtech
 
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danrogers

Member
Thanks that actually looks perfect for my application? I had no idea you could get AC/DC converters I thought there were only DC/DC! Anyone think of any downsides to using it or something similar?

Something we are always looking for at work. Have a look at TRACO or V-Infinity. They do some very small units that conform to EU regulations for "touch current" etc.
It's easy to botch something together using capacitor dividers, but if it burns your house down......
RS Comps do some very small ST devices on a PCB (I can get the details if required). They seem to work OK but are not certified for general use, not EU approved despite working very well in my test circuits.
That's one of the things, my device will be powered 24/7 so I don't really want to hack anything unless I'm 100% confident lol.

If you can live with 5v you can get 8 pin chips you put 240vac in and get 5vdc out, they are common in tellys to power the standby circuit.
Dunno if they are isolated from the mains, probably not.
#

Thanks for the tip, any idea what they are called? can't find much using a parametric search

Around here our heating systems use a 24 VAC power source for the thermostats and controls which make for a easy to tap into power source for small low power applications relating extra functions on specialty thermostats and the like.

Any chance your systems over there use the same or a similar low voltage power source that could be tapped into?
This is a good point, I think there is 24VDC on the control board for the boiler, I guess tapping that might be an option.

Now that is a Video ALL should watch. Absolute crap coming out of China now....

This rubbish has the ability to destroy your expensive PC or Lappie. Trash.

Regards,
tvtech
Pretty incredible, I knew they would be junk but didn't think they could potentially be that dangerous!
 

dougy83

Well-Known Member
Thanks for the tip, any idea what they are called? can't find much using a parametric search
It's possible he's referring to an AC-DC linear regulator, such as http://www.electro-tech-online.com/custompdfs/2012/11/FSAR001B.pdf ; it has a very limited output current, e.g. 35mA.

Or he may be referring to an off-line smps controller IC; but it requires support components including a transformer. You can see some here: http://au.element14.com/jsp/search/...rtial&suppressRedirect=true&originalQueryURL=
 

dr pepper

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I cant find one now either, next time I have a tv apart I'll make a note, seeing as they have no external components I have to assume the output is not isolated from the mains, so safety issues from the capacitor dropper will aplly here too.
 

simonbramble

Active Member
If you know what you are doing, you can drop the voltage using a capacitor. Theoretically a capacitor has zero resistance, so dissipates zero heat. I have a wireless thermostat in my house that communicates to the valve in the garage. They use this technique in there. Series capacitor 'dropper' down to a resistor with a Zener across it to regulate the voltage. However, beware that you dont get any isolation. Also use a good quality cap. The cheap one in mine dried out, so the Zener voltage was too low and the circuit stopped working. replacing the cap with a better quality one fixed it. Have a look at this:
http://www.bitsbox.co.uk/infocontent/capdropper.html
 

tvtech

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Most Helpful Member
I hear you Simon

There is NOTHING more reliable than an X2 mains cap doing the current limiting/voltage dropping. I have a circuit that I promised to publish here a while back...I bailed out because the circuit is way too dangerous for amateurs to try and build.

You can get shocked very easily if you are not aware of the risks when holding the PCB when it is powered up.

However, in a suitable insulated casing, the thing is unburstable. And will not shock the user.

SMPS: Can handle up to a Max of around 280VAC before the Main Smoothing cap lets go....blows up...or something else goes in the PSU. Result = dead unit. All this happens very quickly. Less than a few Seconds.

X2 Supply: Tested @ 395VAC for a full 60 Seconds. No drama. No nasty surprises. Still works and is ready for more if it is necessary.

Albeit, my X2 supply does not provide Amps as a SMPS can....but I don't need Amps. I run @ 140mA for a 1W LED head and control circuit @ a system Voltage of around 10 Volts :D:D

Regards,
tvtech
 
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