ok, well i decided to make a simple power supply for signal processors in my car, I just hooked up the 4 components, which are 1 1000uF 16v filter cap, 1 5v 3A regulator, 1 470uF 10v filter cap, and a 5v to +-15v DC-DC converter module. I however get +-20v. I also don't have any form of overvoltage protection for the 16v cap, and i know it is possible to get more than 16v in a surge from a car battery. the voltage after the regulator is about 7v if that helps any. I have tl082cp op amps which specify a +-15vDC power supply. What can i do to fix, or diagnose this problem better, or should i do anything?
C
CryingDragon
well i would start by replacing the caps with 25 or 35V ones cause giving yourself a 1 volt leeway (or with most caps being -+ 20% 
) a lower rating than your output could cause some cap top poppin. I can't see why it would be acting as you say without looking at it for real but i would reccomend hooking a 15V regulator on the output of your DC-DC if infact it runs at 20 volts with no problems. So it would be:
12V car battery--->25 or 35V filter cap(say 1000Uf)---> 5V Regulator----> dc-dc converter----> 15V regulator---->output smoothing cap (agian 25 or 35V,1000Uf)----> smooth +- 15V out.
) a lower rating than your output could cause some cap top poppin. I can't see why it would be acting as you say without looking at it for real but i would reccomend hooking a 15V regulator on the output of your DC-DC if infact it runs at 20 volts with no problems. So it would be:12V car battery--->25 or 35V filter cap(say 1000Uf)---> 5V Regulator----> dc-dc converter----> 15V regulator---->output smoothing cap (agian 25 or 35V,1000Uf)----> smooth +- 15V out.
Caps don't like overvoltage or overcurrent:
especially when it's in the thousands of volts:
http://www.tech-diy.com/RD100R.htm
especially when it's in the thousands of volts:
http://www.tech-diy.com/RD100R.htm
Hi theChris,
First thing - a 5V 3A regulator should give 5V near enough - say +/-5% at the outside. 7V is too much and indicates something is wrong in this area. Your 5V input DC-DC may not like getting 7V ...
If correcting the 7V input to 5V doesn't fix the 20V output of your DC-DC then you may need to load the DC-DC to get it to stabilise - some DC-DC expect to see a minimum load and don't react well if it's not present. Add a couple of load resistors - enough to draw a few tens of milliamps may be sufficient - try some experiments to find out the value required.
James
First thing - a 5V 3A regulator should give 5V near enough - say +/-5% at the outside. 7V is too much and indicates something is wrong in this area. Your 5V input DC-DC may not like getting 7V ...
If correcting the 7V input to 5V doesn't fix the 20V output of your DC-DC then you may need to load the DC-DC to get it to stabilise - some DC-DC expect to see a minimum load and don't react well if it's not present. Add a couple of load resistors - enough to draw a few tens of milliamps may be sufficient - try some experiments to find out the value required.
James
the adjustable regulators require loading
about 3ma minimum for an LM317, but I don't think that an LM320 or 78L05 requires loading -- at least it isn't specified in the datasheets (as it is for the adjustable regulators.)
I find the acuracy of most fixed regulators to be between 1% and 2%.
speaking of loading -- smps power supplies switch out the loading when the circuit is up and running -- saves milliwatts. (Do you know that in India, at least in Tamil Nadu, they drive (during the night) with their headlights off to save gas? -- almost got killed one night down there! what they save by keeping the lamps off is offset by what they lose by constantly blowing the horn!).
about 3ma minimum for an LM317, but I don't think that an LM320 or 78L05 requires loading -- at least it isn't specified in the datasheets (as it is for the adjustable regulators.)
I find the acuracy of most fixed regulators to be between 1% and 2%.
speaking of loading -- smps power supplies switch out the loading when the circuit is up and running -- saves milliwatts. (Do you know that in India, at least in Tamil Nadu, they drive (during the night) with their headlights off to save gas? -- almost got killed one night down there! what they save by keeping the lamps off is offset by what they lose by constantly blowing the horn!).
I will try the loading. i had kinda wondered about that. would it be better to load the DC-DC converter at all times (as current is limited) or load the voltage regualtor at all times. how low of a resistor would i need. i ran a test on another "5v"regulator with a test load pot and the voltage seemed to stay at 7v and the pot just heated up (which i found odd, but it wasn't smelling too good at the end)
ok, i loaded both dc-dc and the regualtor.
i put a 50ohm (49 measured) load across the regualtor's output. i calculated that if the voltage was 5 volts, then the power dissipated should be about a half a watt, and that was the rating of the resistor. next i looked at what the the resistance required for 150mA output would be. i got 200 ohms across the 30v differance. i have some 240ohm resistors, close enough for testing. so i hook up 2 of the 240s, one from +15 to ground and the other from -15 to ground. (which was a mistake). got 44v...
then i looked and said, hmm, 2 240ohm resistors in series, that'd be almost 500 ohms. 2.5 times more than i wanted. i know i'll hook up a 240 ohm resistor across the 30v differance (which actually gave me about 170 ohms...) i finally got the 30v differance and was happy. then i decided to remove the resistors. i hadn't accounted for the extra wattage dissipated by the resistors, and it was hot. i had 3.75 watts on a 1/4 watt resistor. i probably should be more careful in the future...
i put a 50ohm (49 measured) load across the regualtor's output. i calculated that if the voltage was 5 volts, then the power dissipated should be about a half a watt, and that was the rating of the resistor. next i looked at what the the resistance required for 150mA output would be. i got 200 ohms across the 30v differance. i have some 240ohm resistors, close enough for testing. so i hook up 2 of the 240s, one from +15 to ground and the other from -15 to ground. (which was a mistake). got 44v...
then i looked and said, hmm, 2 240ohm resistors in series, that'd be almost 500 ohms. 2.5 times more than i wanted. i know i'll hook up a 240 ohm resistor across the 30v differance (which actually gave me about 170 ohms...) i finally got the 30v differance and was happy. then i decided to remove the resistors. i hadn't accounted for the extra wattage dissipated by the resistors, and it was hot. i had 3.75 watts on a 1/4 watt resistor. i probably should be more careful in the future...
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