Tech Forum Questions

The short answer is no, you can’t use a variac on an induction motor.

The speed of an induction motor is controlled primarily by the line frequency, not the voltage. If you try to limit the voltage with a variac the motor will likely overheat as it draws additional current to try to compensate for the lower voltage.

What might work for you is a universal (brushed) motor. They are called universal because they can run on either DC or AC. This is the type of motor that is used in home treadmills, handheld routers, jobsite circular saws, etc. The speed can be controlled with a SCR (variable phase) speed control. These motors are usually rated for 120V single phase, up to about 15 amps or so. You could conceivably use a variac to control one of these but the SCR or Triac variable phase speed control will provide better low speed performance.

The speed of an induction motor is dependent primarily on the frequency of the alternating current driving it. Speed controls for 3-phase induction motors are often called VFDs which stands for Variable Frequency Drive. They work by converting the incoming AC power to DC and then use a 3 phase inverter to convert it back into AC with the ability to vary the frequency from as low as a few Hz to 120Hz or more for motors that can tolerate the increased speed.

If you attempt to reduce the speed of an induction motor by controlling the voltage with a Variac, you will likely overheat the motor without it slowing down much, because the current will increase as you reduce the voltage. Inexpensive variable speed power tools are powered by universal motors that have brushes and a commutator and these can be controlled with a Variac, though the common triac light dimmer type circuit is far more economical.

If you can find a grinder powered by one of these then your idea will work, but most bench mounted tools use induction motors.

Also note that the nominally fixed speed motors of any type usually  contain an integral cooling fan under a cover, mounted on the shaft. These are good enough, but if you run the motor slower for a significant time, you could fatally overheat the motor.

If it gets hot, add a separate cooling fan so the motor can take It!

Fuses are specified according to the circuits they are designed to protect. Unless you are certain that the original design specified a fuse of inadequate capacity or excessively rapid response, you should never attempt to substitute a higher current or slower acting fuse. If your power supply is blowing fuses, you need to determine why this is happening. Are you overloading the power supply? Does it have an internal fault? Do the fuses you are using match the original type specified by the power supply?

Properly designed and operated equipment that is in good working order should not ever blow fuses.

It’s not a good idea to over-fuse your power supply (PSU). The fuse was designed to protect the supply from damage (and fire) if it becomes defective internally or is operated beyond its design limits. You are probably exceeding its capabilities in some way, such as overcurrent due to too heavy load. If you’re not exceeding its specifications, there might be something wrong inside the PSU, such as bad filter capacitor(s) or a defective power transformer.

Have you checked the output under load with a scope? That will tell you if the PSU has high ripple under load, an indication of poor filtering inside the PSU. I suggest that you do a little investigation to determine whether the blown fuses are due to trying to operate it beyond its capabilities or bad component(s) in the PSU.

Your multimeter is a good tool to help do this. Watch the needle or display and see what the output voltage does right before the fuse blows. Analog meters are better in this situation. The first thing that comes to mind; does your load have a large capacitor that needs to be charged by the PSU? Large capacitors need high values of surge current from the supply until they acquire a full charge. If that’s the case, you might lower the value of the capacitance at the load. Use your multimeter as an ammeter to watch the current to the load. Is it at or beyond the specificied rating of the PSU? Again, an analog meter is best.

If you’re operating the PSU at its limits, the internal circuitry could be overheating. Mount a fan or blower so that it directs air over the heat-producing components (heat sink, power transistors, power transformer). If you absolutely need to run the PSU at its limits, you might consider getting a more robust PSU. It will be more likely to survive.

Cheap PSUs are sometimes over-specced, meaning that they meet specs only under very controlled conditions. Also, is the PSU rated for full output continuously? It might be overheating if it’s not rated for continuous operation.

The last thing I can suggest is to check your mains voltage to the PSU. Is it at or near the high limits of the PSU? If so, you might use a Variac or bucking transformet to lower the mains voltage to the supply. Hope that gives you some ideas that will help determine why the fuses are blowing so frequently.

The first thing you need to discover is why you blow fuses!

1) Are you trying to directly measure “current” by putting your meter leads directly across the supply’s output terminals? (a mistake always made by first week electronics students)

2) Does the device you want to power demand more current than the supply can deliver (i.e., 1A supply trying to power a 10A device)?

3) Is there a short circuit in the device you’re trying to power? Use your meter to measure the resistance between the device’s power terminals: a reading less than 10 ohms indicates an unwanted short circuit.

Find out what’s causing the supply fuse(s) to blow before you try substituting higher-value/slo-blo types. The power supply will be much happier overall and you’ll minimize the chance of having the supply blow up altogether!

Do not use a fuse with a higher amp rating. Try the same value fuse in a slow-blow fuse. A fast-blow fuse with a higher rating could lead to blown components, melted wires, and possibly turn your power supply into a smoke generator.

Search online for “firefly circuit 7555” and you will find what you want. (Or “unijunction transistor flasher” for the old fashioned method). The LED should be a high brightness type with diffused lens, at least 300mcd. The LED’s above 1CD typically use clear lens, so can’t be seen off axis. As an alternate, if you only need it to run a couple of days, then just solder a L-36BSRD flashing LED inside a 9V battery snap, I make these up for cavers, they leave them behind like breadcrumbs to find their way back.

It appears the output of the Sirius unit is supplying “phantom power” to the TransPod. IOW, there is a small DC voltage (1-2 VDC) present on the output signal that the TransPod is using as its’ power source. In a way, it is “magic” as phantom power schemes are regularly used in audio PA systems to supply power to electret microphones without using external batteries.

A quick Google search for “speaker reconing” will result in 100’s if not 1000’s of hits for this service or parts. Many offer a DIY kit while others are businesses offering the service or an exchange program for speakers.

We know of one business in our area that does the reconing/refoaming service:
TV Service in Crete, Nebraska. His webpage is www.reconespecialists.com
Phone: 402-826-3540
My personal opinion, I would enlist the services of someone experienced unless you really feel confident in doing the work yourself.

If the cone on your Cerwin- Vega speaker is still good, why not consider purchasing a surround kit. They are available online and maybe at your local electronics parts store. Go online and look up your particular model of speaker’s surround kit. They are usually priced around $20-$30 depending on the make and model. It’s an evenings work, but if you have some manual dexterity, with some care, you can have your speakers back up and running like new.

I have been successful doing this on several JBL, Altec, and Bose speakers. I suggest that you stick with U.S. made surrounds. Follow the instructions, and make sure to use a low frequency signal from either a signal/function generator/oscillator or the CD that comes with some of the kits to make sure that everything is centered, and that the voice coil doesn’t rub on the pole piece, etc. Good Luck!

I was recently given an old pair of Cerwin Vega speakers from a co-worker in much the same condition, it sounds, as yours are. The woofers in both cabinets were electrically good as was the physical condition of the voice coils and cones. Both woofers though, exhibited the common problem of foam rot. Not only does the foam separate but seems to dissolve leaving the cone and voice coil to rattle around without direction. 

I performed a web search on the subject and found a solution that seemed very reasonable. Simply Speakers (simplyspeakers.com) has re-foam kits for numerous speaker manufacturers including Cerwin Vega.  I’m not touting Simply Speakers as the only provider, but they were excellent to work with, sent me everything I needed to do the job and provided a youtube video on how to do it. Search youtube for “Simply Speakers” to see the video. I just finished mine last week with excellent results.

If the cone and the driver are in good condition, I would do it. It isn’t very expensive or difficult. I replaced the flexible foam on a set of old Speakerlab speakers about 5 years ago. There are a number of places on the internet and on ebay that sell the parts and they have instructions. I’d check out some of the sites and look at the instructions to see if you feel comfortable with the procedure.

For the subwoofer, I had to repair the foam on both the 10” speaker and a passive radiator. I did decide to make a ring out of 1/4” plywood to make the assembly of one of the larger rings easier but you probably wouldn’t need to do that.

That’s a very old circuit and it has at least one glaring flaw: The value of R3 is way too high. I suspect it should be 3.3K instead. Also, D3 and D4 seem unnecessary because there can never be more than about 2mA of meter current. R1 protects D1 and D2, but microamp circuit levels wouldn’t threaten them. The input bias current of a 741 op-amp through a 10K resistor produces up to 5mV offset that’s not temperature-compensated. It’s nullable, of course, but the null can drift.

Figure 1

While it’s pretty easy to make a DC voltage inverter with circuits like the one in figure 1, I believe there’s a better solution: Single-supply op-amps are available with offset null pins, completely eliminating the negative power supply! See figure 2 for my (untested) circuit suggestion. TLE2021 chips are available in plastic DIP packaging from the usual suspects, such as Digi-Key and Mouser.
 

Figure 2

The current sampling resistor should be selected for full-scale reading with 10mV drop, which is 10X lower measurement burden relative to the original circuit.

If you add the input diode protection resistor back in, then you incur only 0.7mV max uncompensated offset because of the TLE2021’s enhanced performance. And, if you do this, then there’s really no reason to keep those diodes -- you can rely on the op-amp’s input pin protection circuit.

The non-dimmable variety use a largish electrolytic capacitor in the power supply, to generate a DC voltage, and then the LEDs are driven with a constant current circuit. So because of the constant current, the LED output is unaffected by supply voltage, so the brightness won't change with voltage(or dimming). Dimmable leds only have a small film capacitor, and the control circuit sets the current according to the input voltage (so they pulsate at mains frequency) and when dimmed the light output is chopped up similar to an incandescent bulb. The above applies to screw in bulbs, larger LED supplies may still use electrolytic capacitors, but within these another circuit measures the incoming duty cycle and adjusts the LED current to match.

Chad, a small headphone amplifier could be used. Look at the ones for amps and guitar accessories.

The easiest way to listen to your TV via headphones is by using a set of computer speakers connected to your TV using a stereo 1/8” (3.5mm) jack to RCA plugs y-adapter (see below).

Plug:

• The RCA plugs to the LINE OUT jacks on your TV
• The 1/8” stereo jack to the stereo plug of the computer speakers
• The speaker AC cord into the wall
• Your headphones into the HEADPHONE jack of the computer speaker.

Turn the computer speaker on and adjust the volume to your liking.  

Here are some sources for the y-adapter:
Parts Express
www.parts-express.com/dual-rca-male-to-35mm-stereo-female-y-adapter-audio-cable--240-070
MCM Electronics
www.mcmelectronics.com/product/24-13885
RadioShack
NOTE: RadioShack doesn’t have a “direct” adapter, you need to create one from the following parts - http://comingsoon.radioshack.com/3-ft-1-8-stereo-to-dual-phono-rca-plug-y-cable/4200494.html#.VaesjHWuPIo and http://comingsoon.radioshack.com/1-8-stereo-phone-coupler/2741555.html#.VaesxHWuPIo

Finally, depending on how far away from your TV you are, you may need the appropriate length male-female stereo extension cord to patch the adapter to the computer speakers.

I would use a external computer speaker set. It will provide a cheap small amplifier, volume control and an input for your headphones to plug into. If it comes with an external power supply you won’t need to buy batteries. The computer speaker system will most likely come with a stereo mini 1/8” phono plug so you will also need to get an RCA adapter cable. The computer speakers must have a place for you to plug in your headphones.
 

Attach the adapter to your TV and then plug the speakers into the adapter. Once the speakers are connected you should be able to play audio out of them. Then plug in your headphones and the speakers will shut off and all the sound will feed into your headphones. Adjust the volume on the speakers.

Done and your total cost should be less than $25 (15-$20 speakers and $5 for the adapter).