Electronics

Seeqer · Guest

I'd like to be able to determine exactly what the start-up
surge of power to an appliance (air conditioner)mite be. Not just
this appliance but any. The problem is this particular socket
works with most things in our house i.e.. vacuum cleaner, lites, radios etc.
But one a/c I have will always blow the fuse (on start up) on only this
socket.
Any other socket is fine. The problem is easily avoided, yes; all I need do
is switch a/c with a newer one I have that is energy efficient and probably
has some electronics designed to avoid problems like this.
However I got to thinking it would be cool to be able to measure
the start up current as well as consumption of electricity of any particular
device in my house. What would be the easiest, cheapest method to
accomplish this.
Any help would be appreciated.
thx

awright · Joined: 20 May 2005 Posts: 10 Location: Oakland, CA USA

There are several methods of documenting the peak current drawn by an appliance during startup, including:

1) Use of a DVM (Digital Volt Meter) having a recording function with a fast sampling rate or peak hold;

2) Use of a DSO (Digital Storage Oscilloscope) with a current transformer;

3) Use of a specialized power quality monitor;

4) Construction of a circuit with a bipolar peak track-and-hold function.

I'm guessing that you are not presently equipped with any of the instrumentation mentioned. Are you interested in and able to construct a circuit with a few op-amps, diodes, capacitors, meters, etc., and are you comfortable messing around in power line circuitry?

The simplest method that comes to mind to document power transients (referring herein to peak startup currents, not power line disturbances) is to buy a DVM with peak recording capability, being sure that it has either an AC peak capture and hold function or has a fast enough sampling rate on record function that it is likely to capture something near the peak of an AC line frequency sine wave. Many DVMs these days provide a recording function that allows you to review a sampled period for peak values. I wouldn't be surprised to find that you could buy at an appliance service parts supplier a DVM specialized for appliance servicing with a surge current recording function. If you are interested in this approach, be sure the instrument can capture the peak of a half-sine wave.

A simple method of capturing peak surge current is to set up a DSO on continuous store with a fairly fast, repetitive sweep and capture the turn-on surge. You end up with the mid-range of the screen completely filled with stored traces and with one or two distinct peaks rising above the mass of stored data. Works great, if you have access to a DSO. Use a current transformer to isolate the DSO from the power line and scale the current. Use an appropriate loading resistor on the transformer and measure the voltage across the loading resistor . (NEVER put current through a wire in a current transformer core unless the transformer has a low-value loading resistor across its terminals. This protects the transformer from insulation breakdown.)

Rent, borrow or buy a used power quality monitor. I've picked up three or four over the years for less than $100 each. Buying a used one is obviously a gamble, especially if you are not into equipment repair. You will probably at least have to replace internal NICAD batteries that have usually died in a machine old enough to be affordable. But once you get it working, it will tell you more than you ever wanted to know about your power quality and surge currents.

If you are into circuit construction, it is fairly simple to construct a +/- peak detector or precision rectifier/peak detector circuit that you can then read out with a VOM or DVM. There have been very many perfectly suitable peak capture and hold circuits presented in op-amp application notes and data sheets since the beginning of the op-amp era. You would want to use a (loaded) current transformer with this approach, also, for isolation (safety) and current scaling. You can buy current transformers surplus for $5 to $15, and building the circuit would be very low in cost.

Let us know what approach interests you.

awright

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