Apologies if this is answered in another thread.
I am cascading a number of LDO voltage regulators, 7815, 7812, 7809 & 7805, to create a general power supply for my workbench. However, the output from the regulators seems to be all over the place. If I install the 15-volt reg, the output is correct; however, as soon as I add another regulator downstream, the output from the first reg reduces, and the output from the second reg is low. I have changed the voltage regulators for different units just in case I bought fake units from ebay. The unregulated supply seems to be good at 36v. I have reviewed the datasheets and the internet and can find details of any issues with cascading. Possibly I am caught in a "Trap for young players" or not so young in my case. Can anyone help and possibly provide a sketch if I am missing something?
You need to remember that, with cascaded regulators, the current drawn from the lower voltage regulators will add to the current, and power dissipation, through all the higher voltage regulators up stream. I suspect you may be causing the 7815 regulator to go into thermal shutdown due to excessive power dissipation.
For example, if you draw 1A from the 7805 and 100mA each from the 7809, 7812 and 7815 the currents and power dissipations (ignoring each regulators own small bias current) will be:
7805 Iout = 1A Pd = (Vin-Vout)*Iout = (9V-5V)*1A = 4W
7809 Iout = 1.1A Pd = (12V-9V)*1.1A = 3.3W
7812 Iout = 1.2A Pd = (15V-12V)*1.2A = 3.6W
7815 Iout = 1.3A Pd = (36V-15V)*1.3A = 27.3W
All of these power dissipations are above the free air power dissipations limits of the 78xx devices so heat-sinks will be required to avoid the regulators shutting down due to thermal overload. In the case of the 7815 you will need an absolutely enormous heat-sink to handle this level of power dissipation. The problem is worsened by the 78xx regulators having a from of thermal overload protection which further limits the output current when operating with high input voltage to constrain the power dissipation to acceptable limits. In the case of the 7815 the input voltage you are applying exceeds the manufacturer's absolute maximum input voltage of 35V and so the the protection circuits will come into action for even lower output currents, causing the regulator to shut down.
The solution to the problem is to start with a much lower input voltage to the regulator chain. An input voltage of 20V would be more suitable for the regulators you are using and will significantly reduce the power dissipation although you will still need to pay close attention to heatsinking.
If you are stuck with the 36V input then you could use a 7824 regulator prior to the 7815 regulator. The 7824 is allowed to go up to 40V input and it will drop the excess input voltage to the 7815 to a more acceptable level and more than halve the 7815's power dissipation.