I’ve been a fan of the relativly cheap power supplies sold at MPJA. In the past I’ve been used to the larger supplies that will trip relays when over-currenting the supply and that is usually enough to get my attention. The newer solid state supplies are much lighter and don’t have any relays in them. I have not yet blown up any circuits, but prememptivly I decided to add a constant-current buzzer.
Overall, these are nice supplies. We got a few at work and other then the fan being loud on one of them we have no complaints.
I knew that the signal to show whether these supplies were in contant-voltage or contstant-current mode was being sent to the low-voltage LCD display so I started probing around in the connector between the power board and the LCD board.
The white connector on the bottom of the board connects to the actual high-power regulator board. I discovered that pin 4 (or two if you count from the other side), carries a signal that is 12V when in constant-voltage mode, and 0V when in constant-current mode.
The plan: use buzzer, turn it on when constant current mode is enabled. First I tried just connecting the buzzer between the signal line and 12V. This didn’t work and the LCD got very cranky – must be this line is not as low-impedance as I hoped, time for plan B:
I needed a way to drive an active-low signal without having to drive 74xx logic or anything. The solution: P-type Mosfet. When the gate is at 12V (Vgs = 0V), the FET is off, and therefore the buzzer is off. When the gate is low, Vgs = 12V, the P-FET turns on and the buzzer is on. The great thing about using MOSFETs instead of BJTs is that there is no gate current when the circuit is not switching (I don’t anticipate this thing jumping in and out of constant-current mode, that means I need to turn the currrent limit up).
I wired in a TO-92 leaded PMOS, connecting R2 to the gate, C3 (12V) to the source, and connected ground on the buzzer to the ground on the backlight for the LCD. This seems to be working great and doesn’t seem to be too loud once it’s in the enclosure.