a dumb beginner question

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snowday
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a dumb beginner question

Post by snowday » Thu Sep 05, 2019 5:54 pm

so today I started my diy journey towards designing my own stuff. I tried my hand at breadboarding a simple circuit, powered from my doepfer PSU2, and...
I successfully made some magic smoke. I tried twice more, this time simply connecting a resistor once from 12V to ground, and once from 12V to -12V, both times magic smoke. Clearly there is a huge gap in my understanding.

The PSU2 is rated for 1.2A, surely it isn't dumping that all? I read[/i] that 12V was constant, and current was drawn according to the needs of the circuit. What's the problem?

Thanks for the help!

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guest
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Post by guest » Thu Sep 05, 2019 5:57 pm

where was the smoke coming from? what value resistor did you use?
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snowday
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Post by snowday » Thu Sep 05, 2019 6:02 pm

guest wrote:where was the smoke coming from? what value resistor did you use?
the first circuit was some power filtering and a linear voltage regulator.

the resistors were sorta random, 100ohm and 10ohm or something

update: some simple math V=IR, 12=I*100, I = .12.
P = IV, 12*.12 = 1.44W, way too much.
still begs the question, what happened to the voltage regulator? My suspicion is short circuit.

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Post by infinitemachinery » Thu Sep 05, 2019 6:51 pm

snowday wrote: still begs the question, what happened to the voltage regulator?
Voltage regulators are typically polarized. If you hook then up wrong there will be smoke.

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Post by guest » Thu Sep 05, 2019 7:14 pm

yes, the resistors definitely couldnt handle that sort of voltage across them, they are too small. if it was the 10ohm, that would be too much current for the powersupply. the other thing to consider is how much heat you are burning in the linear regulator. a linear regulator will take some input voltage and reduce it and keep it constant. but, all the current that goes out the output, also must flow into the input of the regulator. so you get (Vin-Vout)*Iout for power dissipated in the regulator. if that heat doesnt get removed, the device fails. although magic smoke situations tend to be when a device is over-voltaged, and it very quickly and catastrophically fails. most linear regulators have thermal shutdown inside of them, and will fail more gracefully.
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m.o
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Post by m.o » Fri Sep 06, 2019 8:53 am

I would recommend getting a "Lab" power supply, with these you can usually set max allowed current (as well as desired voltage), and if your circuit it drawing too much it will simply turn off the output with an audible "click".

You'll want a double supply (so it can give both the + and the -).

Costs a bit of money of course ($200 maybe?) but well worth it IMO.


I don't know if maybe there's some simpler (cheaper) hack, like a resettable fuse...

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Post by devinw1 » Fri Sep 06, 2019 10:39 am

Also be careful with negative voltage regulators as the body (mounting tab) is usually hot to the negative supply.

Also agree with getting a a lab power supply. MUCH easier to use when prototyping and they will have lovely protection circuits to click off when you inevitably do something silly as we all have :)

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snowday
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Post by snowday » Fri Sep 06, 2019 12:27 pm

thanks for the advice, hopefully my next post will be something that doesn't set itself on fire!

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Post by B0bcat » Fri Sep 06, 2019 2:52 pm

while we're at it, any recommendations for an affordable lab power supply that would meet the demands of Euro prototyping?
WTB: Intellijel Dixie v1, Intellijel Spock, Plan B / SubCon Model 15 VCO

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Post by devinw1 » Fri Sep 06, 2019 3:19 pm

I've got this one and it works quite well for me:


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Post by cornutt » Fri Sep 06, 2019 3:57 pm

snowday wrote:thanks for the advice, hopefully my next post will be something that doesn't set itself on fire!
We've all done it. I'm embarrassed to say that I did it a few months ago, when I connected a power distro board backwards, and transformed one of the filter caps into a little torch. :doh:
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Post by Synth Con Meo » Fri Sep 06, 2019 4:23 pm

devinw1 wrote:I've got this one and it works quite well for me:

That one shows "currently unavailable" which usually if I see that then it never comes up for sale again. I keep thinking about getting a desktop power supply too. And of course you see people using power supplies anywhere from $50 to several hundred$ so it's a little hard to figure out how much to really spend. I don't do a ton of builds so I can't justify getting some NASA approved unit but a half way decent power supply would be nice.

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snowday
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Post by snowday » Sun Sep 08, 2019 1:12 pm

Synth Con Meo wrote:I keep thinking about getting a desktop power supply too. And of course you see people using power supplies anywhere from $50 to several hundred$ so it's a little hard to figure out how much to really spend. I don't do a ton of builds so I can't justify getting some NASA approved unit but a half way decent power supply would be nice.
I've been researching old supplies on ebay. HP and a brand called Power Designs. Seems like a good go between. Strong feature set, at a low price, as well as easy to service and generally reliable. Only problem is the size!

Update: I've bought an Instek GPC 3020 off ebay, seems like a solid unit.

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