[stinkpot]

hi all,

so i'm assembling a usb dac. i've rigged a power supply w/ a radioshack transformer. the dac works but ....

my 5V regulator gets awfully hot. also, i measure 5A passing back to ground! that seems really high.

or is it normal? i've really psyched that this thing works, but is it currently drawing far too much power? should i be looking for a short?

thanks!
-ld

 

[BrokenEnglish]

Quote:

Originally Posted by stinkpot my 5V regulator gets awfully hot. also, i measure 5A passing back to ground! that seems really high.

while you're measuring that value, you're just about shorting the circuit with your dmm: never measure amperes from + to -! read it from two measure points along the positive rail.

 

[stinkpot]

Quote:

Originally Posted by BrokenEnglish while you're measuring that value, you're just about shorting the circuit with your dmm: never measure amperes from + to -! read it from two measure points along the positive rail.

heh, yea, wasn't being that dumb

. (would have measured much more than 5A then!) got 5A by measuring along negative rail.

 

[stinkpot]

ok, so i was being silly earlier - remeasured just now and got about 20 mA - much more reasonable!

nevertheless, my 7805 voltage regulator gets awfully hot - so hot that i can't really touch it. its stepping about 16V down to 5V. is that normal (that it gets so hot)? should i get a heat sink or just not worry about it?

on the bright side, the DAC sounds great

 

[BrokenEnglish]

Quote:

Originally Posted by stinkpot ok, so i was being silly earlier - remeasured just now and got about 20 mA - much more reasonable! nevertheless, my 7805 voltage regulator gets awfully hot - so hot that i can't really touch it. its stepping about 16V down to 5V. is that normal (that it gets so hot)? should i get a heat sink or just not worry about it? on the bright side, the DAC sounds great

20mA or 200mA? to calculate the power dissipation and find appropriate heat sinks, read this

 

[stinkpot]

much thanks broken english!

 

[rickcr42]

your problem is not the current but the input/output voltage differential on the regulator chip.
All that excess voltage (9.4 volts considering the 1.6 used by the chip to regulate) is being cast off as heat and this will shorten the life of the vregs.You need to bang the voltage down to around 7- 8 VDC before it enters the regulator.

As for current,you can have 100 amps powering a circuit that only needs 20ma with no harm unless or until you have a short somewhere.That is when a circuit will draw current until something gives or burns up and is the reason for fuses in electronic devices.

It is the safety valve for the short circuit situation to prevent exess current draw from ruining everything downstream

 

[stinkpot]

Quote:

Originally Posted by rickcr42 your problem is not the current but the input/output voltage differential on the regulator chip. All that excess voltage (9.4 volts considering the 1.6 used by the chip to regulate) is being cast off as heat and this will shorten the life of the vregs.You need to bang the voltage down to around 7- 8 VDC before it enters the regulator. As for current,you can have 100 amps powering a circuit that only needs 20ma with no harm unless or until you have a short somewhere.That is when a circuit will draw current until something gives or burns up and is the reason for fuses in electronic devices. It is the safety valve for the short circuit situation to prevent exess current draw from ruining everything downstream

So just measure current in front of the regulator and stick in front an appropriate resistor?

 

[rickcr42]

no

it is not the current causing your problems but the extreme voltage differential between the input and output of the regulator.
you need only 1.6 volts over the regulation voltage for the regulator to operate correctly and other than dissapating the actual current put out by the Vreg the rest is cast off as heat and if the differential between input voltage and regulated output voltage is wide you wil have one hot mama on your hands-assuming the rest of the circuit is correct-so you need to "lose" about 8 volts before the chip or heat sink the crap out of the IC

 

[stinkpot]

Quote:

Originally Posted by rickcr42 no it is not the current causing your problems but the extreme voltage differential between the input and output of the regulator. you need only 1.6 volts over the regulation voltage for the regulator to operate correctly and other than dissapating the actual current put out by the Vreg the rest is cast off as heat and if the differential between input voltage and regulated output voltage is wide you wil have one hot mama on your hands-assuming the rest of the circuit is correct-so you need to "lose" about 8 volts before the chip or heat sink the crap out of the IC

right - i meant measure current in front so that i can calculate appropriate resistance to stick in and achieve desired voltage drop.