I’m trying to design a circuit that takes in a variable DC voltage between 14-32V DC and regulates it down to 12V DC @ 1.5A. I’m having trouble because I’ve tried using a L78S12CV regulator and while it almost does the job, when I get to 30-32V DC input some internal limiting kicks in and the output drops. I’ve also tried using LT3086’s but running into a similar problem. Can anyone recommend a different part or circuit design that will help me achieve what I’m trying to do?
Thanks in advance!
I’ve never enjoyed linear regulators for that exact reason, as you’re probably running against a thermal limiter. It just can’t get rid of the heat from the excess voltage.
I would look for a buck converter for something in the 1A+ range. If you want something heavier-duty , you can pick up something on amazon like a DROK which I’ve used to great success in automotive applications (protects from the 10-15V automotive normal). Otherwise there are a lot of smaller block devices you can easily incorporate into other designs.
Not a power expert, but Figure 9 is likely of interest for your application: https://www.st.com/resource/en/datasheet/l78s.pdf
For the 12V part (L78S12CV) at an input voltage of 30-32V, the ability for the device to output current is greatly reduced. In Figure 9, the drop-off appears to occur when Vin -
Vout > 16V. Note this characteristic / behavior of the device is not part of Table 8 which is likely the first place you checked to see if this would work for your application.
I’m not an electrical engineer but…
The issue you’re having is because you’re using a linear power regulator. When you drop the voltage from X to 12, the device is going to dissipate (i.e., shed as heat) power calculated like this:
PowerIn = VoltageIn x CurrentIn
PowerOut = VoltageOut x CurrentOut
PowerDissipated = PowerIn - PowerOut
For purposes of figuring out dissipated power, you can call CurrentIn and CurrentOut equal, giving you:
PowerDissipated = (VoltageIn - VoltageOut) x Current
In your case, this results in:
PowerDissipated = (32 - 12) x 1.5 = 30 Watts. That is a TON of heat for such a tiny device to dump. You are probably triggering some thermal shutdown stuff inside the part.
A linear regulator isn’t the best part for this task. You want to be looking at DC/DC Converters like those made by Traco and similar. You want a switching regulator (as opposed to linear).
I’m sure folks more knowledgeable than I will chime in and offer more suggestions. One that comes to mind is slapping a high wattage resistor between the power source and the input to the 7812. But you have a pretty high input voltage range so that might hurt you on the low end and/or not fix things on the high end.
tanners has a 3 amp variable buck for $5 and a 6 amp for $15. Can’t remember the upper input range exactly but it’s somewhere around there