LM7812 problem
LM7812 problem
(OP)
I've got an LM7812 conected to a 24v battery supply with a load of 63mA max. The device has failed on me twice dispite trying heatsinks (which shouldn't be necessary at such a low load).It's used on a relay circuit board for dc motor starting, so I suspect it might be a surge problem but I can't see any spikes with my osciloscope. Anybody got any bright ideas?





RE: LM7812 problem
RE: LM7812 problem
RE: LM7812 problem
http://www.wb4hfn.com/DrakeArticles/Receiver_Pwr.htm
for a little filter on the LM7812 output
RE: LM7812 problem
7812 need an output electrolytic capacitor as well that an input filtering capacitor.
The relay kick back, needs to be eliminated, place a diode ( IN4002) from the output to the input.
Anode to output, cathode ( the band ) to the input of the 7812.
The 7812 is sensitive to reverse voltages above 2 volts.
Regards
Nando
RE: LM7812 problem
A capacitor across the output of the IC will not prevent this and may make it worse as it forms a parallel resonant circuit with the relay inductance causing ringing.
Place a reverse biased diode such as a 1N4004 across the relay coil, (located at the relay coil if this is practical), to absorb the reverse voltage spike, and now a capacitor across the output of the supply (eg 1uF 35V tantalum) would be of use to ensure the LM7812 is stable all the time.
The reason for locating the diode at the coil is one of good EMI/EMC control as absorbs the high voltage transient which also has a high frequency component, before it can be carried and radiated from the connecting wires.
RE: LM7812 problem
Cheers ... Gerry
RE: LM7812 problem
It self-heals and recovers next half cycle and continues on as normal, trouble is that for 1/2 a cycle it's output was zero while the output of the regulator IC was held high by it's output capacitor and the IC was momentarily reverse biased.
Many regulator IC's fail with only a small reverse bias and so without any apparent cause, it fails. The fix is to install a reverse biased diode from output to input of the IC to drag the output down if the capacitor sputters.