current limiter for dual rail power supply.

[darren adcock]

Dual rail supply is 12v 0v -12v.

Problem is i'm using it for circuit testing also and I've burned out to ac adapters. So was wanting a current limiter to add to the power supply circuit. The MFOS dual rail supply schematic is here. Just can't quite afford a proper bench supply at minute.
thank you

I'm guessing a current limiter is what i want?

Darren

 

[Audioguru]

A current regulator uses a low value resistor to sense the current then a transistor or opamp to be turned on by too much current in the sense resistor and a series Mosfet to reduce the output voltage until the current is at your current setting.

A simple series resistor at the input of each voltage regulator IC will also limit the current.

 

[darren adcock]

Thanks Audioguru.

I don't quite understand so prob just means it's a bit beyond me right now.

Is a current limiter the same as short circuit protection? Just my dc bench power supply has a current limit function which drops the voltage supply if a short happens, really useful for breadboarding and experimenting.

 

[darren adcock]

spoke to the manufacturer just. They say it has current and temperature protection but It's too difficult to add "mis wiring on the input" protection.

I'll read around short circuit protection.

But sometimes being cheap isn't the answer, I'll wait until i can afford a proper bench supply.

back to mechanics for a couple of weeks.

 

[darren adcock]

in fact I'll just make continuity beeper that sits across +/-/0 volts. Dunno why i didn't think of that earlier!

 

[Electrobrains]

Put resettable fuses (Polyswitch / Multifuse) in series with the linear regulators.
Suggestion: on the input side, for instance between C3 and C4 (positive) and between C8 and C9 (negative). If the real circuit is not made like the schematics (conductors looping from one capacitor to the other), I suppose it would work well even to put the fuses directly on the input pins of the regulators.
Resettable fuses are cheap and available in many sizes and trip currents.
Examples: https://www.digikey.com/products/en/circuit-protection/ptc-resettable-fuses/150

 

[dorke]

What kind of wall-wart are you using ?
Have a picture or specs?
Voltage/current/power .

The manufacture is correct the LMXX Ics have both thermal and short circuit protection.
Is the Wall-wart the only thing that smoked?

 

[duke37]

You say you have burned out two adaptors. This could be due to the positive and negative output being different. If they are adaptors with a proper heavy transformers then the core can saturate if they have a DC component on the output. A saturated core has very little resistance to input current. - pop.

 

[darren adcock]

pretty sure it was this adapter... but will have to check tomorrow at my workshop. Both adapters got very hot and the LED didn't light up. No voltage output afterwards.http://www.ebay.co.uk/itm/Mercury-6...249636&hash=item5d4a0992b2:g:zb4AAOSwDk5T8IR2

 

[darren adcock]

yes only the wallwart smoked.

 

[darren adcock]

Put resettable fuses (Polyswitch / Multifuse) in series with the linear regulators.
Suggestion: on the input side, for instance between C3 and C4 (positive) and between C8 and C9 (negative). If the real circuit is not made like the schematics (conductors looping from one capacitor to the other), I suppose it would work well even to put the fuses directly on the input pins of the regulators.
Resettable fuses are cheap and available in many sizes and trip currents.
Examples: https://www.digikey.com/products/en/circuit-protection/ptc-resettable-fuses/150

this looks interesting. Thank you.

 

[darren adcock]

Now I'm thinking about it, I have noticed the adapters have been getting hot even just plugged into the mains and not connected to a circuit. Maybe I just need to buy a more robust adapter.

 

[dorke]

Of course they will smoke! they are way to small for the job!!!

The load may be 12V @1.5A for each voltage ,that is twice 18W=36W.
Each regulator will dissipate about (1.4*12-12)*1.5= 7.2W, that is 14.4W
The total is about 50Watt of power.

To be on the safe side ,use a 60Watt transformer.
note: that is 10 times larger than the ones you have smoked!

 

[Audioguru]

I have one very cheap Chinese adapter that gets hot even without a load.

You are using a low current adapter with a high current voltage regulator. Why not use 78M12 and 79M12 that have outputs up to 500mA? Their peak output is about 800mA but it drops to about 240mA if the output is shorted.

 

[darren adcock]

ahhhhhh, now i see. Thanks guys.

Audioguru, what's the pros and cons of this? I don't understand if the high current is needed.

thanks all, this is a huge help and will save me money!

Need to pay more attention to wattage!

 

[Audioguru]

You never said how much current you need at plus and minus 12V. You could use a 78L12 and 79L12 that have a regulated output current up to 100mA and a peak current of 140mA. They are in a small transistor package so you need to calculate how much heating they produce so that their thermal shutdown does not become active.

 

[darren adcock]

Recommended by MFOS is 1a off their power supply. But I remember Kelly's eye saying this was a misnomer.

At the minute I have 300ma off current draw off the modules. Tech support at the manufacturers recommended me to get a bench supply and limit it at 50ma for building and experimenting.

 

[kellys_eye]

The LM317 is easily configured as a current limiter (one resistor) and can be fitted 'in-line' with your regulator output. You can even fit switched resistors to have 'variable' limiting.

In fact I'd use LM317's throughout for simplicity and ease of purchase (multiples cost less, output is set by resistors, built-in thermal protection (same as the 78/79 range) you can easily convert to a fully variable output etc) and the ability to make current limiters from them.

 

[darren adcock]

That's cool i am familiar with the LM317. I was unsure how to connect though with dual rail. But i'm sure i can figure it out.

So if I have variable current control and ressettable fuses I should be pretty much covered. And of course a suitable power adapter.

I'm looking at this as an eventual bench power supply. Any thoughts would be welcome, as you can all tell I am fairly new to synth making. https://www.rapidonline.com/catalogue/search?query=hy3002-3

 

[kellys_eye]

The data sheet for the LM317 contains all the info you need to get a simple fixed or variable suply going. The negative rail version of the device is the LM337 - equally popular and easily available.