For those who don’t know, a dim bulb tester is a device that uses a light bulb as a resistive current limiter, so short circuits don’t damage things or start fires. This has a few cool features:
I had a 5A circuit breaker lying around (actually, literally, it’s a spare from another project), so even a short is somewhat protected.
The power cord has strain relief. It’s the little things…
There’s a bypass switch so you can cut the dim bulb out and then you’re not really protected anymore… but this way you don’t have to unplug stuff when you’re satisfied whatever you’re testing is working.
I have a tendency to overengineer things, and this is no exception. So there’s a nice power light, a light when the dim bulb is bypassed, and one when “test” mode is on.
Speaking of test mode…
That’s supposed to put a short across the load so the tester can be, well, tested. But I haven’t figured out how to do that without causing some interesting failure modes. So all that does right now is turn on a light. Buuuut I guess that gives me something to think about. I might need a bit of relay logic or something so that test mode can only be activated when it won’t actually cause an active short. I guess that’s fodder for another video.
If you’re interested in the schematics or more indepth build instructions, let me know. It’s pretty well built and I think will serve me well, though I might have to recrimp some spade connectors…
So, I spent tonight in kicad, and I made a schematic. I also think I solved the test switch problem with a couple of relays. Arguably not the most elegant of solutions, but whatever.
So it’s obviously pretty simple. Turn the power on, the power light comes on (I used some cheap lights I found on Amazon but anything will do). Flip the bypass switch one way, the bulb is in circuit. Flip it the other way, it’s not.
If the bulb is in circuit, RL1 is activated. If it’s bypassed, it is not.
The “test” switch activates RL2. If both are activated, then the circuit is completed and the load is shorted. Benefit is, the circuit can’t test if it’s being bypassed, so… no popping breakers. That’s the problem I was trying to solve. I didn’t want to be popping breakers or fuses if the switches were set wrong. One could also use a DPDT for the test switch and cut the load out while it’s activated, but that’s optional. OR I could just use the other throw for RL2 to cut the load out. Decisions, decisions…
There’s probably an easier way to do that, but that’s what I came up with, and I’ll be modifying the circuit that’s on my desk as soon as I get the relays.
Only really important note: these aren’t low voltage relays. I’m using a 5A 110V coil relay.
If you want the kicad file, let me know. But hopefully this will be enough to go on.
This is revision 2 of the schematic. I added functionality to cut off the output when the test switch is enabled, the test switch is now an SPST (I use an SPDT but that’s just for convenience), and I also added a ground isolation switch, which I could see being useful in some situations.
I completed the prototype last night, and I’m going to be working on a more polished version over the next couple of weeks. The prototype has exposed 120V relay terminals, and I really, really don’t like that.
I may be misreading things but it looks like those 2 DPDT relays could just be one.
Having a switch to remove ground may not be accomplishing much. This is a job for an isolation transformer (BTW there’s one in the lab).
Also, a variac is a similar tool for uncommon cases where you might reduce the voltage in to test something. But limiting the input voltage/current on 120VAC is often impractical because the switching power supplies will go haywire under these conditions.
Best thing is just a circuit breaker without a lot of extra capacity. An isolation transformer is still important in some cases too. But I’d move away from series resistance to the load.
Yes, the relays could be just one. There’s even a note to that effect on the schematic. I have this propensity for overengineering, and since I’m not designing to a price point, I indulge it sometimes. Are the lights and stuff strictly necessary? Not at all. But they’re fun.
That, btw, would be advice I’d give to anyone starting out in electronics as a hobby - if adding a light or a switch or something makes it a bit more fun or cool, and you don’t mind spending money for it and have the space, just add it! It’s fun!
As you mention, having a resistive load in-circuit is not appropriate under all circumstances. Neither, for that matter, is a variac. There are, however, situations where they’re really useful. For example, reforming capacitors on an old television or radio that hasn’t been used in years. Or testing something that you don’t trust for short circuits. For what they are, they have their uses. A variac can double as an isolation transformer, and that’s mostly what I’m using it for in this case. You raise an interesting point, though - I wonder if I can put an isolation transformer in-circuit and leave the variac out unless it’s necessary. I’ll have to think about that.
The bypass switch cuts the light bulb (the resistive load) out of the circuit and then it just becomes a glorified power transfer switch with a 5A circuit breaker. Even that has its uses.
A Variac by definition is always an autotransformer, it is never isolating. The isolation transformer adds that.
Yes you can operate an isolation transformer without a Variac, but you don’t need a bypass you’d just dial the Variac to 1:1 and go
There are isolated variacs. That said, i think you may be right that mine isnt one. That is an oversight on my part, and I will need to rectify that. Isolation is important to me. Suitable transformers are expensive, but worth the cost.
All that said, I stand by the utility of the rest of my circuit, and will continue to build it out.
