Handy current monitor

Discussion in 'General Electronics Chat' started by someonesdad, Mar 2, 2011.

  1. someonesdad

    Thread Starter Senior Member

    Jul 7, 2009
    I love this HP DC power supply on my bench (I got it for $100 delivered from ebay and it was in nearly new condition). The voltage and current controls are 10 turn pots and give good resolution. However, the current display resolves to 10 mA and that's often too coarse for my tastes. For example, when powering a PIC microcontroller, you want to know what the chip is drawing; in the picture, the power supply is powering a PIC16F690 running at 1.5 volts and drawing 171 μA. A number of years ago I built the black box (shown inside the red ellipse). It lets me switch the ammeter in and out of the circuit (here, I'm using an HP 3435 as the ammeter). The box has two banana plugs at 3/4" spacing so it plugs into the power supply. I like to use BNC RF cables for connections. I wanted a smaller box, but that's all I had on hand at the time. Still, it works well. Thought some of you other folks might find the idea useful.

    The HP 3435 meter has a little story associated with it. In the 1970's, I worked in the vacuum deposition field and attended an AVS chapter meeting somewhere in Silicon Valley. At the meeting, I met a guy who worked at HP and asked him if he could get instruments at a discount. He said he could and said he'd buy me the 3435 at his employee discount. I think I paid him around $250 or so for it (that was still a pretty healthy chunk of change). I remember driving to HP up the hill after work (I worked at Varian) in Palo Alto and meeting him in the HP parking lot. He gave me the meter and I gave him a check. It still has the taint of an illegal drug transaction (we met in a remote corner so we couldn't be observed). :p