Watch your voltages
Kaos Kidd
Posts: 614
Ok, Just thought I'd share what happened within the last few weeks.
I have been quietly working on a few projects with my propeller Pdips, and noticed hot voltage regulators and power supplies.
Hence my question about "ganging up" voltage regulators some time ago.
The suggestion I followed was to implement two separate 3.3V and two separate 5.0v power rails.
Well, the long and short of it is I ended up placing a poorly labeled 5.0v regulator in the 3.3v side of my power supply.
No smoke, just some very hot chips. Now this wouldnt have been a problem excluding the fact that my entire system is
solder less breadboard, and my vga video adapter has some poorly soldered joints, leading me to believe my issue was there.
Well, here I am, with two new professionally designed and assembled (Thank you Parallax!) development boards, and a working system.
each with it's own properly powered supply!
My question is this: I'm supplying power to both units with one 9.0V 300 mili wall wort, but the regulators are getting hot.
Yes, now I'm watching and checking. I take it if I used a 7.1V 1 amp supply from Parallax, the regulators wont run as hot.
By hot I mean, I can touch the regulators, but they are very warm.
Suggestions anyone? I really dont want to fry another prop. Two is more then enough for me!
KK
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Propeller + Hardware - extra bits for the bit bucket =· 1 Coffeeless KaosKidd
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I have been quietly working on a few projects with my propeller Pdips, and noticed hot voltage regulators and power supplies.
Hence my question about "ganging up" voltage regulators some time ago.
The suggestion I followed was to implement two separate 3.3V and two separate 5.0v power rails.
Well, the long and short of it is I ended up placing a poorly labeled 5.0v regulator in the 3.3v side of my power supply.
No smoke, just some very hot chips. Now this wouldnt have been a problem excluding the fact that my entire system is
solder less breadboard, and my vga video adapter has some poorly soldered joints, leading me to believe my issue was there.
Well, here I am, with two new professionally designed and assembled (Thank you Parallax!) development boards, and a working system.
each with it's own properly powered supply!
My question is this: I'm supplying power to both units with one 9.0V 300 mili wall wort, but the regulators are getting hot.
Yes, now I'm watching and checking. I take it if I used a 7.1V 1 amp supply from Parallax, the regulators wont run as hot.
By hot I mean, I can touch the regulators, but they are very warm.
Suggestions anyone? I really dont want to fry another prop. Two is more then enough for me!
KK
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Propeller + Hardware - extra bits for the bit bucket =· 1 Coffeeless KaosKidd
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Comments
Again, thanks!
KK
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Propeller + Hardware - extra bits for the bit bucket =· 1 Coffeeless KaosKidd
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(12 - 3.3)V * 0.5A = 4.35W minimum power rating.
(12 - 3.3)V / 0.5A = 17.4 ohms max resistance value.
So in this case a 15 ohm, 5 watt resistor would work.
Like I said, "in a pinch". Adding a heat sink to the regulator is a better solution if you can do it.
It's unlikely you'll fry a Prop by a regulator getting hot.· Most regulators have thermal shutdown built-in.
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Post Edited (Ken Peterson) : 5/29/2008 8:16:21 PM GMT
1... You say that you have multiple power rails... what are you doing about the grounds? Does each rail have its own ground?
2... I'm running two Parallax stepper motors, using a really stupid implementation of a stepper motor driver and some Reed switches... which I really love because it looks so retro and is so dog simple, but I find it all incredibly embarrassing to talk about...
The motors runs off a 9V 1.5 watt RadioShack supply... and my 5V regulator gets really hot and I don't care, because I don't know why I should care... what difference does it make if the regulator gets hot? If it burns up will it hurt anything but itself? (My Prop uses the same power supply set up as in the demo.)
I have run the set up for several days at a time and it didn't fail.
(If I burn up my 5V regulator... I go around the corner to RadioShack and get another one. If I burn up my 3V regulator, I have a FedEx issue.)
Thanks,
Rich
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If you drink the coffee because you like the taste... you might have a problem. But if you just want the caffeine, then the best buy in caffeine is JetAlert from Walmart... each pill is equivalent to 2 cups of coffee and you get 90 pills for about $3...which is about the cost of a 5V regulator at RadioShack once you factor in the surcharge for gas.
I think the only way a failed regulator would hurt the Prop is if it passes the supply voltage through to the output upon failure. I would guess that regulators are carefully designed NOT to fail in that manner, but that may not always be the case.
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Always hard to say without a full fault mode analysis on a particular circuit. As Ken says worse case would be a failure when the regulator passed its input voltage, but most IME will usually go into thermal shutdown and kill the output.
You could have an interesting situation if the 3V3 went into shutdown but the 5V didn't. I don't know about the Propeller but some chips might not like having voltages on pins > Vdd+N.
I know from experience some PICmicros do some pretty odd things when running from parasitic power sucked through I/O lines and that could have catastrophic effects; closing two relays to form a dead short across power rails etc.
Keep everything within the operational spec envelope and you usually won't have to worry about it