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Cen-Tech/Harbor Freight #92020 Digital Multimeter Constant Backlight Modification

Cen-Tech 92020 Digital Multimeter with Backlight Amongst Harbor Freight's many digital multimeter options is the lesser known #92020 variant. This meter differs from the #98025 and #69096 by way of an orange backlight for the display. In addition to my being a sucker for almost anything that lights up, the backlight does lend considerable utility to these inexpensive meters. There's a dollar worth of difference between the two, and I'm certainly willing to spend the dollar. (Update as of December 2017: HFT has now placed these on clearance, with a price of less than $4! If you want one, get it while you can.)

You'll find these meters sold under many different names, colors and model numbers, but they're all the same thing: variants of the Chinese DT-830 manual ranging multimeter. I'd never recommend them for use on the AC powerline (mainly because so many of them really aren't very safe) but for low voltage DC and many resistance measurements they're as good as you'll ever need. And if you're new to your electronics hobby, the manual range selection forces you to think about what you're measuring and why. That might slow you down enough to avoid making a mistake, or it might not. Even if it doesn't, or if you decide that working with electronics isn't your hobby, you aren't out much. Or if you do get into electronics repair, you can easily move up to a better meter in time.

I like the backlit variant (and wish Harbor Freight would, if just the one time, offer them for free with a coupon) because almost any higher quality multimeter has one and you'll be surprised how often you find yourself working with a multimeter where there isn't sufficient light.

The problem is that the backlight only stays on for about five seconds. Here again, you'll be surprised where you find your hands busy enough, or that you need to concentrate without the hassle of having to turn the backlight on. Wouldn't it be nice if the backlight on the #92020 meter could just stay on? Why, yes it would!

I'd been thinking for a long time that this couldn't be that hard to do, but I never sat down and tried to figure it out. Here's what you do. But first...

Let's talk about SAFETY. You're more likely to hurt the multimeter than you are anything else here, and it's not like replacing it is expensive. However, the number of stupid people in the world is absolutely mind boggling. Oh sure, we've all had our moments where we did dumb things. Yet we can all think of people who seem to exist only as a warning to others, or those that make you honestly wonder how they're even still alive. Make sure you aren't measuring anything with the multimeter when you go to modify it. Remove the battery before you work on the circuit board. Be extremely careful with a soldering iron and only work with solder in a well ventilated area. If you've never soldered anything before, don't make this your first project. Practice on some scrap circuit boards or components, build an electronics kit or two and then come back here if you're still interested in doing this modification.

While I have found that Harbor Freight is the only source for these backlit meters in my part of the world, your situation may be different and this procedure should be easily applied to other backlit versions of the DT-830 meter, since all the designs are almost identical.

You will VOID ANY WARRANTY that Harbor Freight makes on this meter if you modify it.
Don't come crying to them or to me if you break your multimeter.

You'll need a roll of solder and a low wattage soldering pencil. I personally use 60/40 leaded solder on everything because it's cheap, easy to work with, available everywhere and even a small roll will last you a very long time. You can go lead free if you want. I have no idea what type of solder is used in these meters. As for the soldering iron, if you have a high end temperature controlled Weller pencil-type iron, that's great. You can certainly use it. Don't go out and buy one if you don't: Harbor Freight also sells a soldering iron that will work perfectly well for this. You don't need much more than a 15-20 watt model. It will cost you $10 or less.

Other things you'll need include a small Philips screwdriver (number one ought to do nicely), a needle-nose pliers, some patience, and a steady hand. If your vision isn't that great, get a magnifying glass or perhaps some reading glasses. Got all that? Here we go. Get your soldering iron ready and make sure it can't roll off somewhere to start a fire.

1. Start by taking the meter apart, as though you were going to change the battery. All that needs to come off is the rear cover. I'd strongly advise against getting too rambunctious with the screws, as they only thread into cheap plastic. Once you're there, you'll see the printed circuit board within. The backlight assembly itself is a white plastic rectangle running the length of the display. It's held in place by a black plastic strap that is screwed down at each end. You can separate the white plastic rectangle from the display. The LEDs within operate from five volts and draw around 35mA. (If ever the meter broke down or was damaged, this might be a useful part for some other project. That's something to think about before you just throw the whole thing into the trash electronics recycling.)

It should go without saying that the meter's test leads need to be disconnected. The safest thing to do would be to remove them entirely from the meter and store them in a safe place.

Cen-Tech 92020 Printed Circuit Board

I strongly advise against removing the circuit board. If you do, the ball bearings that serve to give the range/mode selector wheel its "clicks" will get loose and they're a lot of fun to chase down on any kind of flooring surface. A "zebra stripe" piece of plastic links the display to the circuit board and this has to be secured pretty tightly in order to work properly. What I've also found in several of these is that the screw holes used to hold the circuit board in place are oftentimes either stripped or right on the edge. Hence my suggestion not to complicate your life. (You will also have to contend with the wires for the backlight, which are none too sturdy looking and not soldered down very well.)

2. The area of interest is in the upper left corner of the circuit board, near the backlight wires and the small adjustment potentiometer (if your meter has one -- not all do). You'll see two small black cased transistors (marked as "6Q2" and "6Q3" in the Harbor Freight version of this meter). It is the former (6Q2) that is of the most interest to us, as it is what ultimately turns on the backlight. I believe, but haven't closely verified, that the circuit theory goes something like this: upon pressing the backlight button, transistor 6Q3 is momentarily turned on. This charges up a small capacitor and turns transistor 6Q2 on. The transistor remains on as the capacitor (C8) slowly discharges through resistor R11 (91K ohm) into transistor 6Q2's base. Resistor R12 (47K ohm) is the current limiter for the LEDs.

With the cost to produce these meters being 3/10ths of approximately nothing, and successive revisions having more and more "cost optimization" done to them, I have to wonder why the designers implemented what seems like an "extra" transistor in the form of 6Q3. It's not likely that these transistors have a meaningful cost in the vast quantities ordered by whoever makes these meters. Perhaps the makers put it there to reduce accidental or atmospheric triggers to the backlight. At this price and quality point, though, I'm sure one doesn't get the best electrical engineering talent and that might really be why the "extra" transistor is there: because someone thought they needed it and nobody's come along to review the design and say it isn't necessary.

Let's take a closer look at the actual board. (Somehow, I feel I ought to be saying "computer, enhance!" This is about as good as it gets with any digital camera that I have.)

Cen-Tech #92020 Multimeter Printed Circuit Board Closeup

6Q2 is the small black component in the top row, third from the upper left edge. (Be careful not to mistakenly adjust the potentiometer (white/blue rotary control in this picture). Doing so will affect the accuracy of the meter.) It's different from the larger transistors you may be used to seeing. (This is an SOT-23 packaged NPN transistor, type S9014, if you really want to know.) There is one electrical contact at the top and two at the bottom. It's the top contact that represents the collector (C), while the bottom left contact is the base and the bottom right the emitter. We're going to short the collector (which gets its power directly off of the power switch) to the emitter, which will force the backlight to turn on continously.
Transistor 6Q2 Before Modification
I hope your soldering iron is warmed up and ready! Tack a wire between terminals 3 and 2. I found it helpful to add a bit of solder to the terminals, and to use a needle-nose pliers so as to avoid burning the daylights out of my fingers. For the wire I used a clipped off component lead and carefully bent it into the needed shape before tacking it down.
Transistor 6Q2 After Modification
Note: It's possible (though rather unlikely) that the transistor might be damaged by this. I certainly don't (yet) know the long term effects this modification may have, if any. Reports of the backlights failing in these meters are not uncommon. Of the eight or so that I now have, only one suffered a backlight failure and did so right out of the box. Here is what the completed modification looks like on the circuit board.

HFT/Cen-Tech 92020 Digital Multimeter Modification

At this point you should have a multimeter whose backlight stays on continually. (Wesley made the suggestion that I could install a switch, if ever I wanted to turn the backlight off. Which either you or I might well want to do. See the notes below.)

Cen-Tech 92020 Digital Multimeter with Constant Backlight Modification

Power Consumption

That suggestion does bring to mind something worth considering. Does modifying this meter mean that it's going to start wolfing down three rectangular batteries a day, for breakfast lunch and dinner? I set up a contraption (warning: not dial-up or low/limited bandwidth friendly) to find out. (The irony of using a well known name brand, much higher quality meter has not gone unnoticed.) The bad news is that doing this will reduce your battery life considerably. That's doubly true if you're running from the carbon-zinc "heavy duty" battery these meters come with. With the backlight off, current drawn is around 0.27 milliamps or 270 microamps. That's assuming my measurement equipment is actually accurate enough to go down that far. Turning on the backlight pushes the current draw up to around 38-42 milliamps.

Some time ago, I tested one of these meters to see how long it'd run from a decent quality alkaline battery. It lasted about six weeks before the low battery symbol came on. That happens around six volts, so the meter gets basically everything a nine volt battery can possibly give it. In a more realistic usage scenario, that may be years of normal use! It also means that if you're forgetful and leave the meter on by mistake (it has no auto power off circuit), you won't have to worry too much.

A figure I found online suggests that a "good quality" nine volt alkaline battery (PP3) is rated to deliver 500 milliamp-hours worth of current. A number of different sources seem to back this up. Only nine volt batteries don't take well to heavy loads, thus you would not get anything like an hour's worth of operation with a 500 milliamp current draw. In the best case, with the backlight on, you might get 10-11 hours worth of operation from the meter. Realistically, I think you're going to get less.

Does it actually matter? I don't really think so. The convenience to me far outweighs the cost. Most people only use a multimeter for a few minutes worth of troubleshooting before setting it aside. The only "gotcha" I can really think of is that of leaving the meter on by mistake, since these don't have any means by which to shut themselves off after being ignored for a while. I've never found myself having that problem with these.

One could insert more resistance to further reduce current flow and thus consumption of the battery. I may experiment with this in time.

Other Caveats

Multimeters are continually exposed to potentially dangerous conditions. It can be argued that a meter like this should never see high energy circuits or line voltage of any kind, but the fact of the matter is that they do. It's also a very valid point that a multimeter should never be modified in any way to ensure that its safety features remain intact. Even the most well thought out of modifications might well invalidate one or more safety measures built into the original design. Truth be told, the DT-830 family of multimeters have essentially no safety measures (some -- including other variants sold by Harbor Freight -- don't even have a fuse!). The backlight circuitry is well away from anything else within the meter.

You'll have to decide if this modification is for you. It should go without saying that if you perform this modification, you and you alone are responsible for the results. Don't overestimate your capabilities. Killing the meter means you're only out about $6. Killing or injuring yourself is a bit more serious.

Exercises For The Reader

I'd love to know if you attempt any of these. I may try some of them myself in time.

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Copyright 2017 by William R. Walsh. Some rights reserved. Your rights to this material are governed by the terms and conditions linked from the top level page of this server. Please take a moment to read them. Any project involving the use of electricity can be hazardous. Your safety and wellbeing are your own responsibility -- not mine. This article is presented in good faith, but no guarantee is made as to its suitability or safety for any purpose.