I’ve got several power supplies that I use on my bench or wherever, and I really don’t know why I bought this new one. But It’s getting very close to Xmas and I wanted a present from Santa. I guess I’ll have to explain further to justify my frivolity.
Hewlett Packard, AKA Agilent I have several HP power supplies, and also several non HP power supplies, which all seem to suffer from one limitation or another. One of the first things I did back in 2004 when I started to buy on eBay was to buy some power supplies. In college classes and in jobs I had, I used HP power supplies, so I knew that they made some well-made, good performing power supplies. I found that they were readily available on eBay, but at a price. HP PSes are like a Mercedes Benz: if you buy one you can get a decent one for a premium price, or you can get a piece of junk for cheap. Anything in between is out there being used by its owner and he doesn’t want to part with it. Since I am an electronic tech and I can fix electronic equipment, I decided to bid on some cheap, well-worn HP power supplies.
I started out with just about the oldest of the HP power supplies, the model 723. These were built starting in 1959, and are 0 to 30 volts at 10 to 225 milliamps – unimpressive performance, but okay for some low power transistor circuits and they are built like a brick outhouse: even though they used old germanium parts, they still were well engineered. I bid on one and won, and received it soon after. Well, the seller had claimed it was working, but it would only put out enough current to move the wiggle stick meter needle; if I put just a small load on it the voltage would drop to nearly zero. I opened it up and took a few voltage measurements and found that as soon as the small load was put on it, the voltage across the main filter capacitor (right after the rectifier) was dropping dramatically. I knew then that the electrolytic filter cap was so old it had dried out and was not doing its job. I put a 470 microfarad electrolytic across it, and Bingo! The power supply started working normally. Fortunately this power supply is built old fashioned just like tube equipment, on a chassis, with plenty of room for the added ‘lytic capacitor. I emailed the seller about this, and we eventually came to the agreement that he would refund some of the money I paid for it. So I didn’t do too bad.
The Worst HP Power Supplies The disadvantage of the 723 was that it could only put out 225 mA, and even then the current limiting was in just a few steps with a rotary switch. I wanted something that could put out an amp with fully variable current limiting from 0 to 1 amp. I found that the HP 6214 was good for up to 1 amp at 0 to 10 volts, and other models less current but higher voltage. I bid on one and won, and when I received it, I was highly disappointed. The bozo seller had packed it in a small package with no space or padding between the case and the package. I opened the package and found that the plastic case had shattered into splinters and there was a gaping hole. I took pictures and attached them to an email to the seller, and finally after much disagreement, I managed to get a refund. This was in the days before Paypal and eBay came up with the dispute resolution system, so the buyer was on his own to pursue the seller about getting his money partially or fully refunded.
This episode taught me one very important lesson: the HP 6214 series of power supplies were encased in a cheap plastic case that snapped together, and after a few years the plastic became very brittle and would easily break. If you tried to open the case to repair it, the plastic ring around the back would most likely shatter, and you would then have to tape up the case with some duct tape (if there was enough left to tape up!). Or the case itself would break, especially when shipped. I also learned that many of them had a bad wiggle stick meter that would stick part way up the scale. Or, as in the earlier PS, the electrolytic capacitors would dry out and have to be replaced – not a major hassle. But the electronics was well engineered, and the performance was good. I bought some service manuals for these power supplies, which were then going for as much as the power supplies on eBay. I found a guy who was selling the original manuals for a lot less. I’m guessing that he lived near a military base (see note below) and got some of the manuals from the base when they surplussed them. They have been a great help in getting the old HP power supplies up and running.
Other PSes I found on eBay some Power Design precision power supplies, and I bid on them and won. These were at least a few decades, old, but were very well made. They were accurate to less than a millivolt when calibrated, but the switch contacts (there are lots of them!) got corrosion from the air, so they were intermittent. I got a can of Electrosol contact cleaner and sprayed and cleaned the contacts thoroughly and that helped a lot. I use one of these a lot, but the problem with this is it only puts out 1/2 amp. Also, it has an edge mounted analog ‘wiggle stick’ meter that is very hard to read and gives poor accuracy; if it’s left on the current range, the reading is not very good. But the switches set the voltage to the exact amount on the dials, so no problem with voltage. But it’s a real pain to crank a bunch of dials around just to change a few volts.
Power Supply From Hell I have several other power supplies that I bought on eBay, most of them not worth mentioning. I also have several that I built over the years, one in particular that was very troublesome. Decades ago, Radio Shack was trying to make some money by getting into the kit selling business, so they came out with plans for a power supply kit. The “kit” was an assembly manual and front panel decal, and a list of parts that had to be bought off their shelf. After sending a lot of money on the “kit”, the case, and all the parts, and then spending a lot of time assembling it, you had a dual power supply that put out 0 to 30 volts at about an amp.
My neighbor bought one, assembled it, and came over to see if I could help him. What would happen is that when the power supply was first turned on, the output voltage would peak at near maximum for a fraction of a second no matter where the voltage knob was set. He put a small 6 volt light bulb on the output and turned it on, and the light lit up so bright that it looked like it had burned out – like a camera flash! After a few times of trying this, it really did burn out. Both sides of the dual supply did the same thing. I looked at the power supply schematic and the circuit board and found nothing wrong, and since it was occurring on both supplies, we came to the conclusion that the problem was caused by poor circuit design. He was so disgusted by the situation that he didn’t want the supply, and sold it to me for a small fraction of the total amount he had invested in it. Needless to say, Radio Shack undoubtedly had many such complaints, and it is probably one of the main reasons why RS is no longer in the kit selling business.
Another problem PS Last year I bought a PS kit consisting of a project box, a metal plate for the PS’s front panel, power transformer, rectifiers and filter caps, and a LM317 regulator chip which used the front panel as the heatsink. Instead of assembling it as it was intended, I decided to add a negative supply and regulator to give a few volts negative, enough to offset the 1.25 volts of the LM317, and allow the output to be reduced to zero. The negative supply did what it is supposed to do, the output can be turned down to zero. But whenever the PS was turned on or off, the output spiked at a few volts for a fraction of a second, no matter what the pot was set to. Again I had a PS that could do damage to a low voltage circuit if it was powered on or off, similar to the Radio Shack PS. I tried several things to get it to stop the spiking, but it still had the problem. My final solution was to connect the output to a relay that delayed connecting the output at power on so that the circuit had a chance to stabilize. At power off it immediately disconnected the output. But it really bugged me. What started out as a relatively simple PS ended up being much more complicated just to prevent its nasty behavior of putting out voltage spikes during power on and power off.
New Mastech HY1503D I guess I should get back to the power supply I bought. The Mastech HY1503D seems to be an older design that may no longer be produced. They have marked down the price to $50.00 USD, and with tax and shipping it may cost around $70 delivered to your door. It is an all linear power supply, it is not a switching supply. I have noticed that the newer bench power supplies being sold often say they are switching power supplies. The switching power supply is lighter, usually cheaper, and more efficient than the older linear power supplies. But they have one drawback: they can radiate EMI/RFI (electromagnetic interference / radio frequency interference). This is extremely difficult to filter out completely, and when it leaks out of the power supply, it can cause noise in the circuits powered by the supply. It can be especially noticeable in low level audio circuits such as preamps or in receivers. This was the reason I chose to buy this all linear power supply, along with the ability to put out 3 amps, which most of my other PSes can’t do. Linear power supplies are free of this switching noise, so there is less concern about it affecting measurements or reception in a receiver. The low price was attractive, too.
I opened it up (the screws had some Loc-tite on them) to see what was inside. I saw a massive transformer and circuit board but I didn’t see any regulator chips. I’m guessing that they have their own circuit design. There is a fan on the back, which seems reasonably quiet when it’s running, but it’s not as quiet as a power supply with a big heatsink on the back that depends on convection cooling.
I R Losses One thing that this Mastech PS does not have that some of my other PSes have: remote sensing. As the current (I) travels from the banana jacks on the front to the alligator clips of the test leads, the I flows through the resistance of the copper in the wires, and we have an I R or voltage drop (remember, one of the fundamental electrical formulas is: E (voltage) is equal to I (current times R (resistance)). On the back of the Power Design PSes, there are terminals where I can connect up a cable to the device to be powered. This cable has the Sense terminals connected through separate conductors to the far end of the cable where the alligator clips normally are. When the equipment is drawing high current, the voltage drop along the cable is sensed and corrected to the set value at the alligator clips, instead of at the banana plugs on the front of the PS. If there is any voltage drop along the cable, it is automatically removed. I have a short 2 foot cable with alligator clips and banana plug, and I plugged it into the Mastech’s banana jacks. I shorted the clips together and turned up the PS to 3 amps. I measured over 0.3 volts drop across this short 2 foot cable at the banana jack, even though I have connected the cable’s multiple conductors together to get minimum voltage drop.
To solve this Vdrop problem, I built up a heavy duty cable. I used four stranded conductors, two each for plus and minus, with each conductor being equivalent to 17 AWG. The length of the cable is about 8 feet, and the voltage drop at 3 amps is 0.148 volts, with about 0.042 volts of that in the short smaller 18 gauge cables to the alligator clips. When it’s supplying 1 amp, the drop should be about 0.05V or 50 millivolts. That’s pretty low, and should not be a problem when I’m experimenting with high power JTs. But I’m gonna stop by the hardware store and see if I can buy 10 feet of four conductor 14 gauge flexible power cable, which will have even lower V drop.
Update Dec 5 – I stopped by the Big Box home improvement store and bought 16 feet of 2 conductor, 12 AWG “low voltage garden light cable”. It looks like really heavy lamp cord or speaker cord, it’s black, and it’s made of fine wires and is very flexible. My wire table says that 12 AWG has 1.59 milliohms per foot. I haven’t assembled it yet but I plan on cutting the cord into two 8 foot lengths and putting the alligator clips directly on the ends. This should reduce the IR drop to 75 millivolts or about half of the amount I measured in the cable I built above. It won’t look so tacky and home made, either.
A concern I am doing all this so that I can get exactly 1.5 volts DC at the alligator clips at the end of the lead, so it will be just the same as if I had a battery at the point where I clip the leads on. But is this realistic? Is it the same as a battery? Truthfully it is not. The actual AA cell has internal resistance that will reduce the voltage across its contacts as the current increases. If I used thinner wire in the test leads, it would be more like the internal resistance of the battery. All I will be doing with this resistance is making the test as real as if I were using a real battery. By using the heavy cable, I’m actually deviating from reality into an artificially ideal voltage source with (almost) zero resistance. But one can argue that when I test a device such as a Joule Thief, I should be doing the test at a voltage which has been established as the supply voltage for that device, which is 1.5V for a Joule Thief. If I lose a fraction of a volt in the clip leads, that looks to me like I’m denying a fair test to the device because it is not getting its full share of the 1.5 volts that is supposed to be getting. Which is the right way? What is real, or what is artificial?
This also doesn’t take into account if the device was designed to be used with different batteries such as Ni-MH rechargeable batteries which are 1.25 volts per cell. Testing this device at 1.5V would not give the results the consumer would expect with the correct cells installed.
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Note: Any institution has times when the shelves are full, and storage areas are crammed with equipment, and there comes a time when there is a massive clean up effort. I know all about this because I work at a college where after every semester we receive a bunch of new computers and the old ones stack up like cordwood. Then when things get too crowded, the boss orders a flat bed truck with a bobtail on the back and we load all the stuff onto dollies and pile it up in the flat bed. Literally hundreds of perfectly good PCs and monitors get scratched up in the toss-around, and it happens again when the stuff is unloaded at the warehouse. This is exactly what happens to all those pieces of test equipment that are sold on eBay. The stuff looks pretty roughly handled (missing/broken knobs?), and if it was built well, the dents and scrapes on the outside don’t affect the operation much. But when you buy one of these pieces of surplus equipment you take a risk that something may be loose and come back to haunt you.
Back to experimenting…