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Variable power supply suggestions

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atikovi

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LSS the transformer in a 40 year old electronic typewriter died, and searching the numbers on the transformer got me nowhere. I'm thinking if I can test different voltages where the transformer output connects to, like 5, 12, 24, etc. volts, and see which voltage makes it work again, I can buy a matching transformer with that voltage rating. Looking for suggestions on a power supply under about $100 like one of these for example,
www.elexp.com

Variable DC Power Supply with LED Display (Voltage adjustable 0-30V ;

•Voltage Adjustable 0-30V (Model HY3005)•Current adjustable 0-5 Amp (Model HY3005)•LED displays for both current and voltage.•Constant voltage and current operation.•Load Effect: .01% +/- 5mv•Dimension: 11" x 6.3" x 5.3"
www.elexp.com www.elexp.com
 
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5ubtle

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A 'transformer' converts AC voltage to another AC voltage. Do you really have a transformer, or is it a 'DC power supply'?
 
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5ubtle

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American Locomotive

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It's a two component system. That's the transformer, it just outputs AC. It then feeds power into the power supply board (in the middle), which will convert it to DC at the appropriate voltages.

I see a bridge rectifier (the metal thing with fins that says D5FB10) and a filter capacitor next to it. This is what converts the AC to DC.

I looked up the part number of the rectifier (D5FB10), and it's rated for 100v maximum. So that at least creates an upper limit for the voltage of the transformer.

What is the voltage rating of the large black capacitor next to the rectifier? If you look at the power supply board, it may also list the input voltage where the transformer plugs in.
 
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atikovi

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American Locomotive said:
It's a two component system. That's the transformer, it just outputs AC. It then feeds power into the power supply board (in the middle), which will convert it to DC at the appropriate voltages.

I see a bridge rectifier (the metal thing with fins that says D5FB10) and a filter capacitor next to it. This is what converts the AC to DC.

I looked up the part number of the rectifier (D5FB10), and it's rated for 100v maximum. So that at least creates an upper limit for the voltage of the transformer.

What is the voltage rating of the large black capacitor next to the rectifier? If you look at the power supply board, it may also list the input voltage where the transformer plugs in.
Click to expand...
IMG_3846.JPG
 
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American Locomotive

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The capacitor is rated for 50 volts. Generally good practice is to double the voltage of your capacitors. I suspect the system is probably running on 24vDC, which means the transformer is probably rated for about ~17 VAC.

Do any of the motors have a tag or model number?
 
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American Locomotive

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So the motor is rated for 12vdc. That's a good hint.

In that case, it's probably a 120v > 9.5 VAC transformer. When you rectify AC into DC, the voltage gets bumped up by 1.4x or so. So 9.5 VAC will become 13.3 VDC in theory.

However there are some losses. The full-bridge rectifier I mentioned earlier will eat about 1.3v, giving you 12vDC with 9.5vAC input.

I would feel comfortable feeding 9-10 vAC into that input on the board.
 
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atikovi

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So this transformer would be the right specs? The dimensions are right.

www.hammfg.com

166Q12

Hammond Manufacturing is a leading manufacturer of industrial enclosures, electronic enclosures, racks & rack cabinets, transformers, outlet strips and climate control products.
www.hammfg.com www.hammfg.com
 
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tool_scrounge

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That is a stepper motor. To achieve good higher speed performance, you would design the drive circuit with a constant current drive chip being fed from a DC power supply at a higher voltage than the motor. If you drive a 12v stepper motor at 12v, you cannot run very fast due to the back EMF of the coil when switching windings to take a step.
 
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Steve_P

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Is the capacitor shown in the OPs photo leaking? If so, that can destroy the PCB and should be changed if it's leaking.
 
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Gozo

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Transformers are pretty rugged components. I’ve got equipment that’s 60-70 years old and the transformers are the last thing to go. Are you sure it’s the transformer? Have you checked the input and out voltages (as in if there’s volts in are there volts out or not)? Sometimes it’s simply some oxidation at the connectors. I’d change the electrolytic cap just as a matter of cause, as they do have a finite lifetime and dry out. It’s not modern auto repair; where you just keep growing parts at it. 😁
 
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American Locomotive

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Is there any way you can get more pictures of the power supply board? It may have give us a better clue as to what voltage you need. My hunch is it's going to run on either 12v or 24v, but it's difficult to tell. After thinking about it, tool_scrounge may be right, and it might actually operate on a slightly higher voltage than the motor is rated for.

At least at a minimum, we're looking at 12v.

Since it appears to rectify all of the incoming AC into DC anyways, you can actually hook up a DC power supply into the input, and it will probably work okay.
 
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Steve_P

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So, I'm no electronics expert but will confirm that transformers are extremely robust as compared to the rest of what's on a PCB. Transformers typically only die when they get hit by a massive surge from lightning or similar. You can easily test them.
 
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atikovi

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Got a $50 power supply off Ebay delivered today. Hooked it up starting with 5 volts, then 10, 12, 15.... and around 20v it started to work but not very well. Wasn't until I hit 24 volts that it seems fully functional. Will put all the rollers back in for another test to confirm everything works and it takes a 24v transformer before searching for one.

typewriter.JPG
 
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oldschoolcraft

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Great work Atikovi! I have one of these Variable Power Supplies on my wish list to eventually buy. It seems handy though I dont have any immediate need for one. I would like to source a variety of the different "tips" that various wall DC adapters use, that work with one of these, and then it becomes a universal power supply for those kinds of devices as needed.

I could have used one to jump my M12 batteries that refused to charge on the charger. I wound up "jumping" it by taking it on and off the charger in 10 seconds increments until the error lights flashed. I'd get 10 seconds of charge each time and after 20 minutes, it had enough charge that the charger recognized it at enough of a threshold to charge it normally.

If I had one of these I think I could set it to 12V and connect the wires to the leads and either set my living room on fire, or very efficiently jump start the battery. 50% of the time I avoid setting my apartment on fire, every time.
 
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jayemm

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I bought one of these a couple of years ago. Only used it a few times so no extended experience. But I will say that to give it a workout I hooked up to a cheap 12VDC tire inflator (hose not attached, open to the air). It smoothly ran with no jumps or dead spots up to 30 volts and 8.7 amps with the pump just screaming. Lucky the damn thing didn't throw a rod (or shrapnel) LOL. I believe the power supply could output it's rated 30V at 10 amps if given a sufficient load. The ~$50 price has been the same for at least the last 3 years on Amazon.


1715314771296.png
 
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American Locomotive

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If it seems to work properly at 24vDC, you will need a ~17vAC transformer. I wouldn't go much higher than 18vAC. It will likely still function at higher voltages, but the voltage regulation circuits will start burning off more heat.
 
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N_Jay

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The issue is how AC voltage is measured.
RMS or peak. 17VAC RMS is about 24V peak.
 
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American Locomotive

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atikovi said:
So you're saying 24 volts DC equals 17 volts AC?
Click to expand...
It gets a little complicated. AC voltage swings up and down in a sine wave, so how do you meausure the voltage? Do you measure the peak voltage, the middle, the average? This was a problem in the early days of electricity, when we had competing AC and DC electrical systems. They wanted to make sure a lightbulb would work identically on AC and DC power.

Well some smart people figured out that if you multiply the peak voltage the AC hits, by 0.707 (squareroot of 2), it would get you the equivalent voltage that would "effectively" be the same to the lightbulb as DC voltage. So 120 VAC has a peak voltage of 170v, but to a lightbulb, it's effectively the same as 120VDC. The line labeled "VRMS" (root-mean-square) is the "DC Resistive Equivalent Voltage".
Sine wave voltages.svg.png

The problem is, your typewriter is not a simple dumb resistive light bulb. The diodes in the rectifier act as a check valve. They only allow the electricity to flow in one direction, and then that electricity "fills" up the capacitor. That means the capacitor sees and stores the full "peak" voltage of the AC, rather than just the RMS.

So if your system converts AC into DC, you need to mulitply your desired DC voltage by 0.707 to get the AC voltage you need.
 
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atikovi

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I connected the power supply back and cranked it up to 29 volts. Everything works the same and the heat sinks don't get hot. At idle it shows 0.50 amps and it maxes out at 1.25 amps when the carriage is moving.
 
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RPH

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