Tech Forum

You didn’t give specs, but, if you size a power supply, leave plenty of head room, tube amps are power hungry, transformer supplies have a soft drop off, and switching supplies simply drop out when pressed beyond their limit. Also I’ll bet that the transformer has two or three voltage outputs, that is a lot to spec and integrate, and expensive.

The thin aluminum oxide dielectric (energy-storing) layer in electrolytic capacitors is formed on the specially-treated anode metal, and the electrolyte contacts the outer can (on early parts) or the cathode foil (on later “dry” parts). When radios have been unpowered for a very long time, electrolytic capacitors tend to lose their dielectric layer and their voltage rating, but not usually much capacitance. They may destroy themselves and other parts when re-powered, unless slow-start techniques are used to renew their dielectric.

When restarting long-idle sets, it’s best to use a variable-voltage (Variac) transformer to increase voltage slowly over several days. Note that Variacs are usually not isolated. An alternative method is to power up with an incandescent lamp socket in series with the AC line, starting with a 60-watt bulb and increasing the wattage gradually. Turn the set off occasionally and check for excessive heating.

I have repaired many radios using higher voltage TV capacitors, and never known one to take more than a week, unless it was defective. If the TV parts aren’t ancient, they may still be sufficiently formed for your new (lower) voltage, but for safety, follow the above procedures.

There is a strong shock hazard posed by AC/DC sets. Many early versions of these sets had all B(-) connections grounded to the chassis, including one side of the switched line cord, no matter which way it’s plugged in. Missing or wrong size screws or knobs or rotting rubber mounting grommets could make outer metal cabinets lethal. Some otherwise-well-built later radios, such as the Hallicrafters SX-41, had a live chassis. Most later sets used an isolated internal ground system to minimize the hazard. It’s safest to work on AC-DC sets using an isolation transformer, especially if you’ll be connecting any AC-powered test gear. You can make your own, using back-to-back filament or power transformers of appropriate power. Good luck with your repair!

Let me start with a rule of thumb, electrolytic caps drop some value if formed to higher voltage (within working range). This is because of thickening of barrier layer, BUT this can vary, that is why most are rated plus or minus 20%. They generally only need to be big enough (power filter, or bypass). Don’t expect much value drift moving to lower voltage. Other concerns are, internal resistance (can screw with bypass performance in audio stages, and heats cap at high duty cycle).

Depending on your design layout and other components, I would suggest using a TVS (Transient Voltage Suppression) diode. While a conventional freewheel diode may be sufficient, not many clipping networks work in the picosecond range of a TVS. Placement of the TVS should be close to the source of the transient with as short a connection as possible.

Common TVS operating voltages range from 5V to 440V. They may be unidirectional (fine for relay coil flyback clipping) or bidirectional (AC transient suppression). If you can’t find one with the exact clipping level needed there are methods for adding to the clipping voltage by placing fast silicon diodes in opposing series (cathode to cathode) with the TVS. Adding a fast silicon diode in this way will increase the transient suppression clipping point by the silicon diode junction voltage drop (.7V).

Another option would be to use a Snubber Network. Again, use of these would depend upon your circuit layout and conductor lengths. These are simple RC networks. You can build your own by placing the correct capacitor and resistor in series across the relay coil or purchase a snubber as a unit. A TVS is usually less expensive and easier to add to the design.

The inductive reaction from opening a circuit which contains an inductor (relay coil) is due to the reverse voltage generated by the collapsing magnetic field around the coil when the current is interrupted by opening the contact. This high voltage can cause arcing across the contacts or spike on electronic components which leads to their demise.

For a DC powered circuit, you could add a snubber circuit (series resistor and capacitor) in parallel with both the diode and relay coil as shown in the Figure. Just make sure the resistor and capacitor can handle the power discharged from the diode coil when the contact opens. For an AC circuit you can use a properly sized Metal Oxide Varistor (MOV) to snub the spike.

When controlling inductive loads, I like to use an N-channel MOSFET such as a 2N7000, BS107 or equivalent to control the device in addition to the flyback diode. This provides further isolation and reduces the current requirement of the Arduino just in case you want to control more that a couple of relays.

Get an inexpensive 24 hour timer and adjust it so it is on for as much of the 24 hours as you can. If that provides a short enough off time, you are done. If not, you can do the following. Make a 555 monostable timer circuit to energize a relay for about 30 sec. Circuits to trigger a relay using a 555 can be found in many places. Power the 555 circuit using a (surplus) wall cube plugged into your 24 hr timer.

Power your equipment through the relay contacts (but not through the timer) so that when the relay is in the de-energized state, your equipment is on. When the 24 hour timer turns on, the 555 timer will go high for about 30s, energizing the relay which turns off your equipment for a short time.

You may not have resolution down to 30 seconds, but a simple light switch timer would do the job. Set it to switch power off in the middle of the night or at a time of minimal use.

You could use this timer designed for resetting a router on a timed schedule! It is a plug in timer! www.amazon.com/NetReset-NR-1000US-Automated-Cycler-Routers/dp/B00HUEU9H8

You can try this particular timer from Amazon www.amazon.com/Digital-Programmable-Socket-switch-Energy-Saving/dp/B00WHPNON6/ref=zg_bs_495340_1 Or, Google for household lamp timer, there are many available.

There are a lot of solar to li-ion battery charger ICs. You might get one on a breakout board, like the following: www.ladyada.net/make/solarlipo

I made a small solar powered solar tracker that has a similar IC, and charges 2 li-ion batteries to about 8.0 volts. Works well, in operation over a year now.

I think an couple of quad op amps would enable you to scale the temperature range you want to 0 to 5 volts. That way, the resolution would be around 0.5degrees on each a/d input.

I suggest you consider the Maxim MAX31820 temperature sensor. Its accuracy is +- 3,6 Degrees F over the range of -67 F to 257 F. It has a digital output (rather than analog) with a "1-wire" interface using only 1 controller port pin. A large number of these sensors can be paralleled on the one pin so you could even put several together and average the readings. For a datasheet, see: [url=https://datasheets.maximintegrated.com/en/ds/MAX31820.pdf]https://datasheets.maximintegrated.com/en/ds/MAX31820.pdf[/url]

Have you considered a digital sensor, such as the DS18B20? They are inexpensive (about $4), readily available, is accurate to +/- 0.5 C (so, about 1 degree F), wide temperature range, and then you only need 1 digital pin. I used one in a little oven controller that used a PIC, a while back. Programming is a little more work, and it can be a little tricky to use with longer cables.

Your proposal will not work. It reminds me of students who try to connect two 8-bit DACs to make a 16-bit DAC. It just doesn't work. But I can't offer a solution without more information. What's the resolution of the analog-to-digital converter(s)? Do you mean you can set any of the analog inputs for a 0-to-0.5V range anywhere within the 10-volt span, say, from 1.2 to 1.7 volts? What is the voltage output from the thermistor circuit? More information, please.

You don't say what kind of controller you are using but if it has SPI capability, why not use a DS18B20 serial sensor. It is accurate to .5°C in the temperature range you specified and software routines for most MCUs are readily available.

The linear type wallwarts operated on line frequency of 60 cycles, and even though they only have 3-6 parts, these had to be large to handle the current and squelch ripple.  What you are looking at are the newer units which are switch mode type supplies, operating in the 100-200 kilocycle range.  Even though they may have 40 or so parts, these parts due to surface mount technology, and the high frequency, can be much smaller and lighter.  These supplies also provide a stable voltage of more current and less load droop than their older linear cousins.

Personally I always use the "machined" gold plated contacts (Machined contacts are round, and have 3 to 4 gold fingers internally) the cheaper tin sockets require more force, and this force can (and does) bend the IC pins. Once you get above 18pins it is really difficult to insert IC's into tinned leaf sockets.

Gold plated contacts are highly resistant to corrosion. If you are designing equipment that is going to potentially operate in humid environments or where reliability is paramount, then using sockets with gold plated contacts can be a good investment. For most applications though, conventional machined-pin sockets provide excellent performance and the added cost of gold plating will not buy you much.

Most consumer equipment uses inexpensive stamped pin sockets and even those are normally adequate.

Depends on the environment that the project will be operating in. Gold plated sockets are usually specified for high-reliability, that is, in mission-critical or life-critical applications. Tin plated contacts don’t like lots of high frequency vibration, such as near heavy industrial equipment. It also doesn’t tolerate humid environments. Gold plating is great for use in humid environments, and is better than tin in high vibration environments.

There’s not a lot of difference in contact resistance, and you won’t gain anything with gold. If your project will be used in a humid or corrosive environment, then I suggest a conformal coating be applied over the entire circuit board. Gold plating comes with its own unique problems, the main one being separation of the gold plating from the plated surface, causing a failed connection, or worse, an intermittent connection.

I’ve used tin plated sockets for many years, and never a problem that could be traced to the sockets. My money is on the tin plated sockets.

Always go for the gold with chip sockets and “wire wrap” socket pins! The main reasons are:
 

1) Excellent corrosion resistance, which translates to more reliable connections. Tinned contacts will electrically degrade due to oxidation, even after the chips are inserted.
 

2) Better spring (mechanical) contact between the socket and chip pin. Tinned sockets are simple “leaf compression” types that are prone to “chip creep” (i.e., the chips push out of the socket over time) caused by thermal effects (i.e., heating when powered on and cooling when powered off), which means you have to occasionally re-seat the chips (i.e., pushing them back into their sockets with a “crunch-like” sound when they re-seat). Gold sockets are machined and use “grip fingers” that, literally, clamp down on the chip pins, virtually eliminating chip creep. Also, after crunching a loose chip in a tinned socket, it’ll creep out again at an increasingly faster pace as the tension in the leaf springs increasingly degrades with each re-seat operation (also resulting in degraded electrical connections - see 1 above).
 

3) Mechanical security of installed chips. Referring to 2) above, a creeped-out chip will eventually fall out of its’ socket if the board is jarred. Chips installed in gold sockets, again because of the clamping effect on the pins, will never fall out if the board gets jarred. FYI: I type this from 35 years of industry experience having to find and re-seat loose chips in tinned sockets that caused intermittent (and frustrating!) device malfunctions. I rarely (almost never) had creep, etc. problems with gold-plated (machined) chip/wire-wrap sockets.

I would get the less expensive tin sockets, unless you will have the project in a corrosive environment (and you would also have to deal with possible wire corrosion). In using many sockets over the years, I have not had any problems with tin. Get the “springy” side-contacing socket like (www.digikey.com/product-detail/en/A08-LC-TT/AE9986-ND/821740) rather than the machined pin type of socket. I have had some machined pin sockets making bad contact with the IC.

Your calculations are probably correct, however you may have used the worst case current from the datasheet, if you use the nominal current it should be good. A couple of months is the expected retention at room temperature. Just be careful if you use schottky diodes in your circuit (to power the RTC from normal 5V) the leakage current in a poorly selected diode can exceed the standby current. (Schottky diodes are made with 3 doping levels, producing forward drops of 200, 300, 400mV, the 400mV has lowest leakage current, it will usually have a H in the part number). A supercapacitor is a electrolytic double layer capacitor (EDLC), and each electrode is coated in very fine carbon granules, the total surface area is about 1000 times higher than just aluminium foil, with a much thinner dielectric, hence the increase in capacitance (and drop in maximum operating voltage). There is no actual oxide layer for the dielectric, an EDLC has charged layers of ions a few molecules thick instead, and charge and discharge just move the ions back and forth across the layer.

“Super-caps” are, indeed, electrolytic capacitors. They’re construction typically uses tantalum (not aluminum) plates to obtain a large capacitance value in a relatively small package compared to standard aluminum electrolytics. Plus, their electrolyte formulation differs enough from aluminum capacitors to allow a larger charge vs. size capability.

Because of their large charge capability, they are ideal for use in short-term backup applications where low currents (typically, less than 100 microamperes) are required (i.e., real-time clock chips). However, they are not the same as a battery (i.e., lithium coin cell) as, like regular electrolytics, super-caps do self-discharge over time and they can not deliver a large supply current (i.e. >1 mA) for longer than a couple of seconds.

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.