Variable power supply suggestions

[atikovi]

The transformer on my typewriter died and mentioned in another post, 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,

https://www.elexp.com/products/01phsy3005variable-dc-power-supply-0-30v?

 

[eljefino]

I think they have them cheaper on ebay.

 

[GSCJR]

Maybe look for a used typewriter on Craigslist.

 

[atikovi]

Maybe look for a used typewriter on Craigslist.

A new transformer is $12. Driving around looking at typewriters is a hassle and might be worse than mine.

 

[supton]

Can you get schematics for it?

While there are variable transformers it'd be easier to find a variable DC supply, and falling out of a tree is harder than finding a fixed one. Problem is, connecting in the right place--and for all I know, some motor still would want AC to run on.

Start with a schematic, then figure out if you can redesign this to remove the transformer.

 

[Owen Lucas]

Try Amazon:

https://www.amazon.com/Adjustable-Switching-Regulated-Quick-Charge-Interface/dp/B08HYK2ZW3

 

[atikovi]

Can you get schematics for it?

Nope, cant even get parts from the manufacturer.

 

[supton]

Nope, cant even get parts from the manufacturer.

I've never been good at tracing out circuits like that, but it's probably a single layer board, real easy to follow. I'd start by looking for the diode bridge, then see what it feeds. BUT I'd also try to make sure the transformer isn't feeding AC to any motors--mind you, they might have some triacs in the way, to control them, so they would likely be on the PCB also. Triacs, relays, either one. Look to see where the AC goes. With any luck, all it feeds is a diode bridge.

Then look to see what the regulation is. Doubt it's of the 78xx type but who knows. More likely some zener diode reference. Still, figure out that out, then use DC supply with 2-3 volts overhead.

 

[berniedd]

But you don't know the output voltage that's supposed to be on the B+ line. Applying the wrong voltage on it can fry the board. Why not just buy the replacement transformer for $12 and be done with it?

 

[atikovi]

But you don't know the output voltage that's supposed to be on the B+ line. Applying the wrong voltage on it can fry the board. Why not just buy the replacement transformer for $12 and be done with it?

Because I don't know which transformer to buy. If I knew, I wouldn't have even posted this.

 

[berniedd]

Try this: Make sure no AC feeds from the transformer to other components. If none, find the filter capacitor that the diode bridge goes to. Note the voltage rating. From past experience, the working voltage of the circuit is usually the capacitor's rated voltage X 0.75

That should give you an approximate voltage to apply should you go that route. Now how much current should your power supply be capable of? Look at the rear or underside of the typewriter. The wattage consumption should be there. Take that number, divide it by the voltage you think the B+ needs, and the result is your ampere rating.

Another thought: remove the transformer, see if the secondary voltage is marked on the other side.

Finally, brute force technique: disassemble the transformer, count how many turns of wire are in the primary and secondary. By ratio and proportion, you can calculate the secondary voltage output.

 

[paulo57509]

I would look for something like this?

 

[atikovi]

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.

 

[nomas]

There's no nameplate or any kind of writing underneath that mentions power requirements ?

 

[atikovi]

nope