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Linear Voltage Regulators

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darkfeffy

New Member
Hi all,
I really need some help here.
In the attached .GIF file is a schematic of my set-up to power several ICs in my system (an inverter) by using different linear voltage regulators all connected to a 24V dc source (in reality, this will come from a car battery:)). There are seven 15V regulators, one 6V and three 5V ones.
The trouble is this: The simulation works fine on ISIS, but when I cabled this stuff, it just won't work. My DC voltage supply was set at 24Vdc and always limits its output current at 3amps. When I switched on the set-up, the voltage of the DC supply went down to zero meaning the output current is higher than 3amps. How can this be when the regulators are not loaded?
I thought the input voltage was too high so I verified the data book and it's ok. Seems all L78xx regulators of the 5V-24V range can handle input voltages upto 35Vdc.
I appreciate your help in advance
 

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crutschow

Well-Known Member
Most Helpful Member
Check you wiring. Sounds like a miswire. There should be very little current if there are no output loads.
 

Wp100

Well-Known Member
Agree with Crutschow that you have got one big short.

Wiring very likely, but you do not actually state the exact regulators you are using - suggest you lookup the maunfacturers datasheet for each one to ensure that they follow your 1 2 3 layout - a few regulators do not.

I assume you are using a large heatsink for them, so any that do not use pin2, the metal tab for 0v will create a short.
 

MikeMl

Well-Known Member
Most Helpful Member
Also, you lack bypass caps on the regulator inputs and outputs. Look up the data sheets for the specific regulators you are using; the IC maker shows the minimum required caps for each regulator type. One of the possibile consequences of leaving off the caps is that the regulator oscillates, which could be why you are drawing excessive current. Look at the respective regulator outputs with an oscope.
 

darkfeffy

New Member
Thanks all;
I will "microscopically" re-verify the set-up. I think however that MikeMI's idea on by-pass capacitors is a bit amiss here because the power supply is internally stabilised and is very close to the input of the regulator. Moreso, the regulator is off-load... Any ways, getting down to it. Once more, they are L78XX regulators (5V and 15V)
 

dknguyen

Well-Known Member
Most Helpful Member
THose regulators also shut down when overheated. At 24V above 15V or 5V, they are going to overheat long before they ever output 3A (at ~250mA for 15V and ~100mA for 5V).

That current limit shutdown only comes into effect for input voltages very near the output voltage (where the regulator heats up the least).
 
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Roff

Well-Known Member
This is probably just a schematic error, but I don't see where 0VDC is connected to the regulators' common GND.
 

shimniok

Member
THose regulators also shut down when overheated. At 24V above 15V or 5V, they are going to overheat long before they ever output 3A (at ~250mA for 15V and ~100mA for 5V).
I too was thinking those regulators esp the 5V one is going to get pretty hot, assuming these are linear regulators ala 7805. In which case the voltage difference between supply and regulated out gets wasted in heat in the regulator. That may not be the current (pun) problem and if not, I would imagine it'd become one.
 

darkfeffy

New Member
Hey guys,
Cool replies. I finally got the circuit working. It was some short somewhere. Some cable was flying without my knowledge.
I also noticed the 15 V regulators get pretty hot very fast. So I am going to insert a heat sink
Thx alot.
Ed
 
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