Power supply

Hi I’m trying to build a power supply on a breadboard, and it works, but the voktage regulator gets very hot, sometimes smokes, before i turn it off. I have a thermistor between the volage and the voltage reg, which has to work everytime its turned on. I have been follwing the tut at sparkfun, and started by copying the photos, but i had to change and just copy the schematic because the problem was occuring everytime. There are photos on the link below, can anyone see where i’m going wrong?


Getting warm or hot is sometimes alright for electronic components, but they should never ever smoke. A general rule of thumb is if smoke came out of it, it’s ruined (unless of course it was meant to make smoke, like a model steam train or those weird USB-powered aromatherapy sticks).

What’s the make/model number of your voltage regulator?

Sadly, even though components like voltage regulators come in a few standard chip packages, the pin order can be completely different, so even if you followed the SparkFun tutorial exactly you might have mis-wired this particular regulator. It sounds like either your regulator was wired up incorrectly, or a component on the board was short-circuiting regulated voltage and ground (it’s hard to tell from the pictures).

I would take a look at your voltage regulator’s datasheet, and make sure the pins are what you think they are (top/bottom views in diagrams are sometimes confusing!) then try reconstructing your board with a new regulator, and very quickly measure the regulated voltage with a meter and see if it’s what you expect.

Don’t worry about it though, we’ve all done it. I hooked up a voltage regulator backwards at a robot demonstration once and had it physically explode into multiple pieces! Fortunately it was just an internal university science-fair type competition, and it was really the poster that was being judged anyway!


Ok, Although it might sound supid, if it were ruined, then 9v would be travelling to my led, and burn it out, but the led is absolutely fine, it only goes out, because the thermistor is monitoring current? I think i have clearer shots on that link now at the bottom… i just dont see what can cause it, unless the power coming in is too great - or can the voltage reg, handle any voltage?

the datasheet says i have the order right: pin1 - power source, pin 2, ground and pin3, outputs 5v

Aah, much better. Your board layout looks fine, although I would still like to know the part numbers of your voltage regulator and thermistor. There is no way that a single LED by itself should be drawing enough current to trip the thermistor. A thermistor is a little bit of protection, but it takes a (small) amount of time to ‘trip,’ and even a tripped thermistor will still let a good deal of current flow. It’s possible that the regulator was damaged by a short circuit before the thermistor kicked in.

Voltage regulators do have a maximum input voltage, but for big regulators like that it’s usually like 30V-50V. They also have a maximum current they can safely deliver, but one LED with a proper resistor should only draw a few milliamps. I would expect an LED with a resistor sized for 5V operation to work fine at 9V, it just wouldn’t last as long (like maybe only 100 hours of operation as opposed to like 100,000 hours). Do you have a volt-meter you can measure the actual output voltage with? That could clear all this up.


Yeah need to buy a voltmeter…

OK the thermistor is part
sparkfun.com/datasheets/Comp … _rev_H.pdf

and the voltage regulator is part
sparkfun.com/datasheets/Comp … LM7805.pdf

the power supply i’m using is the same as the one i am using to power the orangutan, so i dont think it could be that…

is my circuit correct?

Your circuit looks totally fine, and the power supply should be too (9V?). Is it possible that some of the long bend component leads may have brushed against each other? Specifically I’m thinking that if the leads of the output capacitor brushed against each other it would have shorted the regulator’s output to ground, which could have damaged it before the thermistor tripped. Once it’s damaged internally it could keep burning up inside whenever you power it. Do you have another one you could try?

You might want to try just a bare bones circuit, without the bus lines or the capacitors or the thermistor, just the power supply, regulator, resistor, and LED.

And yes, a multi-meter is a most useful tool.


how would that circuit look?

Like this (photo-edit of the SparkFun tutorial):

Capacitors are great at soaking up fluctuations in power that can interfere with some electronics, but for just lighting an LED you probably wouldn’t notice the difference.

By the way, what is your 9V power source, a battery or a wall adapter?


the power is mains power, but it runs the orangutan fine…

which one should i get, i am trying to find the parts in the uk…
rapidonline.com/Electronic-C … 0regulator

Mains power, as in straight out of your wall outlet???

made your circuit above, the thermistor still has to cut the power off, now it must be the regulator or the power supplY?

Hold on,

The power going into your board should be coming from a DC power supply, so you need to use either batteries, or an AC to DC adapter of some sort, a “wall wart” for example:

Are you wiring power straight from your wall outlet into your breadboard?

And if so, STOP!


the thing i am using looks just like your picture.

it says on it, ac input: 230V

dc output: 9V 2.0 Amps typical
that sounds ok to me?

above is the link for the place i buy components in the UK, please can you help me choose a new reg?

PS. I took the thermistor out, of the circuit, and turned it on (figuring if the reg was already broken why not experiment with it). It continued to light the led for over two mins before the led went out. Now I have to find out whether the led is broken or the reg is broken (one definately is now). The eg got too hot to touch and every so often a small whisp of smoke came from it. Capacitors didnt pop so they are the right way around, I need a new reg…

Aah, okay. So you’re not using the mains power directly, you’re converting it to lower voltage DC power. Good good, you had me worried there for a second!

Those are all negative output regulators, which is not exactly what you want. You could make it work, but it’s more expensive and you really don’t need it.

I would suggest one of these: 82-0678

It’s very similar to the regulator you’re using now, and even has the same pinout. Get a couple though, just in case!


Cool, will do.

My multi meter arrived today, and had a quick poke around with the voltmeter sertting. 9volts is coming in from the wall adaptor, and 2.2 is coming out of the reg and across the LED. A that seems low, because its supposed to hold the voltage at 5 isnt it? and B, my bread board has been melted by the reg!!! theres about 5 holes around it that cant be used anymore because the holes have shrunk with the heat, With my basic electronincs knowledge, if Voltage decreases current must increase, so if V is supposed to be 5 and is 2.2 then maybe the current is too high which is causing the reg to get hot??? Does that make sense or did i make all of that up?

the thermistor though is BEFORE the switch, and for that to cut the flow, must mean something is drawing too higher current???

Faulty reg?

Sorry to hear about your breadboard melting, but congratulations on your new multimeter!

One quick question, although this won’t necessarily answer much since your regulator is undoubtedly already messed up inside. Is the LED still lighting up? What is the output voltage of the regulator if you remove the led from the board?


Thats weird!

The capacitor that goes to ground, from the output of the reg, If i measure across its two legs, then the voltage is 7.96, but if i mearsure the voltage by touching on the output pin of the reg, and the leg of the capacitor

[reg output leg]----|----[cap leg]-|-[cap]–[cap leg]—[ground]

so if i measure where the two | are (thats where i’m touching) then there is no voltage…

I’m not sure I understand what you’re describing, but I wouldn’t trust the regulator to act in ways that make sense, now that it’s given up it’s magic smoke.


Thats annoying lost my whole message.

So my new regs arrived this morning, and everything is working perfectly! I must have made an error like you said, ruined the old regulator, and then it was never the same again…

Quick question about the other post, that is relevent to this one. If i manage to get the other one to work with interupts, rather than delays will i need an external crystal to keep ultra accruate time? I have one, just wondered whterh it would be used?

I can never upload photos here???