Author Options:

Protecting my new power supply Answered

I've got a 12vdc 500ma power supply that I'm using to trigger a few solid state relays. I'm also using the same power supply to provide input current to a little micro PLC (Automation Direct DL05). Both the relays and the PLC have a wide range of acceptable input voltages and their current requirements are about 15ma per channel at 12 volts. The count is 6 relays and 8 PLC inputs, if all are on thats about 210ma. I once killed a similar power supply by allowing the leads to touch and short-out. I would like to protect this power supply from the same fate by protecting it from short circuit. I thought about attaching a reisistor to the end of one lead and covering all but the outlet of the resistor with heat shrink tubing... problem is, it would have to be a big, bulky power resistor to handle the wattage. Is there a simple component available that will limit the current upon short circuit that I can place in series with my loads to protect the power supply? As I said, it doesn't have to be precise and it doesn't matter if it jerks the voltage around a little.


question: in the event of a ground fault, do you want the circuit to shut down (power off) or lower power?

I thought ground fault devices were for AC circuits. A fuse is the best choice here, I believe.

a groud fault condition is when the device's positiv lead contacts either the negative lead without load, or contact another grounded objects, such as the case, a metal pipe etc. a GFCI, or ground fault circuit interupter, reacts in milliseconds to cut power in the event of a ground fault.

I'd say that the first case e.g. the positive lead connects inadvertently to the negative lead is not a ground fault. It's a simple short circuit. Nothing that a Ground Fault Interrupter can detect or do anything about. That is a job for a fuse or possibly a current limiter. The second case where there's a leak of the positive lead (to earth/ground) causing the current in the positive wire to be different than the negative wire (they are exactly the same in a normal healthy circuit) can be detected by a GFCI. This requires that the ground/earth is connected to the negative lead between the power source end the CFGI.

true. i put them together because either will damage equipment like the power supply. its also worth noting, though a tad off topic, that a gfci will not protect you from the hv output of a transformer.

wow, that was awful spelling. sorry. a fuse is probably a good choice. buy a fast blow verion though. buy somthing like a 6 pack.

I just want it to lower power. I'd just like to be able to plug-up the power supply and not have to worry about touching the leads together while I'm goofing around with my projects. I don't want to have to replace a fuse or do some kind of reset.

if you go the fuse route, go for a 12 volt 550 ma quick blow fuse type.


10 years ago

Is it regulated? Many voltage regulators will shut down automatically if the heat/current limits are exceeded. If not, how about a 12V automotive fuse?