Old and basic linear power supply

[bdunham7]

In this case, there is no PCB - so as long as there is air flow around the resistor, and it is rated for the heat produced, everything will be fine for another several decades!

It's still in a closed box, along with a transformer that is constantly at or near full power.  Also, we have no idea if it has lasted 'several decades', as the total runtime might actually be quite low.  It may have failed years ago, then sat in a closet for 20 years at which time it was sold on eBay as 'pulled from a working environment'. 

I think it would be a perfectly interesting and rewarding exercise to see how cheaply and easily it could be modified using 'modern' components. I think the design objectives are a well-regulated +/- 17VDC supply up to 20mA with short circuit tolerance.  I think it can be done with a BOM under $2, but perhaps we should make it a contest!

I would:

a) retain point-to-point wiring, no need for PCBs
b) parallel the secondaries with a 75R current 3W resistor in series with each winding to limit short circuit current to 200mA (0.2A x 15V = 3VA, the rating of the xformer) and to balance the secondary currents to some extent, as the primaries are in series.
c) retain the half-wave voltage doubler configuration.
d) replace R2 and the zeners, which appear to be externally mounted, with an LM317/LM337 combo + 4 resistors to set the voltage.  I don't think there would be any need for heat sinks in any case, but if you really wanted to make it bulletproof (impervious to sustained overload or reverse transient currents) you could add small heatsinks and regulator bypass diodes.

[tony359]

Bdunham,

I am not ruling out to improve the PSU - My approach here is to understand what's happening, why it happened, fix the PSU and then maybe improve it. I am in stage 2 right now, let me have it fully up and running and then I may move to phase 3 The LM317 is an interesting item. I was wondering how I would be able to get 17V when regulators are usually 5-12V.

SilverSolder

Many thanks for taking the time to draw the schematic for me. Let me study it and experiment with the PSU and I'll get back to you

[bdunham7]

Bdunham,

I am not ruling out to improve the PSU - My approach here is to understand what's happening, why it happened, fix the PSU and then maybe improve it. I am in stage 2 right now, let me have it fully up and running and then I may move to phase 3 The LM317 is an interesting item. I was wondering how I would be able to get 17V when regulators are usually 5-12V.

SilverSolder

Many thanks for taking the time to draw the schematic for me. Let me study it and experiment with the PSU and I'll get back to you

OK!  It's just regulation by dissipation, relatively common in old low-power designs.  It's not just cheap stuff, even fairly expensive audio equipment used this method for secondary power supplies.  It's quite common for these things to burn up, especially as they get older and line voltages go up, encroaching on their thermal margins.  I'm fixing some Fluke calibrators right now (very expensive units) that use zeners as dropping resistors before LM-series regulators to get a +/- 15V supply from +/-40V rails.  It isn't pretty--the power supplies look like they've been on fire.  Your current design will work with the proper size components, and as you said, it will run as hot as the sun inside that box.  Perhaps it  really wasn't intended for continuous use if it was a battery replacement device.

And, just in case you missed it, the LM317 is not (specifically) a 17 volt regulator! 

[Wallace Gasiewicz]

What happens if you ground the link  between the primary windings of the two transformers (to the 240V third wire)
Essentially making them two 120 V transformers.

Or linking the secondaries together in parallel??

[MarkF]

What happens if you ground the link  between the primary windings of the two transformers (to the 240V third wire)
Essentially making them two 120 V transformers.

Or linking the secondaries together in parallel??

You NEVER ground the primaries.
You either put them in parallel or series (noting the winding direction).

[bdunham7]

You NEVER ground the primaries.
You either put them in parallel or series (noting the winding direction).

I had to go back and look because your drawing of the transformer(s) shows a single core where the OP shows two transformers.  In fact, the OP's photo shows a single xformer, as you have drawn.  That's an important difference!   

No worrying about balancing primary winding voltages and no choice in how you connect the windings.

[tony359]

apologies for that, I could not find a way to draw a single transformer with two windings and I forgot to mention that detail.

[Wallace Gasiewicz]

Yea I think I meant wiring the transformer primaries for 120, Since there is no center tap so cannot be done.
Just wondering why so much electricity is wasted and the resistor has such a load.
Does not seem right.

[bdunham7]

apologies for that, I could not find a way to draw a single transformer with two windings and I forgot to mention that detail.

No worries.  But if you want to configure the new supply with dual full-wave rectifiers, that will work nicely as well.  The primaries can't become unbalance if they share a core.

[MarkF]

Yea I think I meant wiring the transformer primaries for 120, Since there is no center tap so cannot be done.
Just wondering why so much electricity is wasted and the resistor has such a load.
Does not seem right.

Can be done.
You wire the two primary coils in parallel to double the current for 115V.
For 240V the coils are in series which requires half the current for the same power rating.

I don't understand your question.
The photo of his transformer is exactly the same as the wiring diagram I posted.
You can wire it for either voltage.

If he builds a +/- supply with a full wave bridge, he will also tie the secondary windings in series and create the center tap.

[Kleinstein]

In the old days designers were not that afraid of more heat as they were used to power hungry tubes.
Electrolytic capacitors were more expensive so they used lower capacitance. With more heat wasted at the resistors the regulation from the zeners gets a little better. It is still a slightly odd circuit.
Another point is that in the old times the mains voltage was more variable - so they may have designed for a 20% drop in mains voltage.

2 x 15 V AC is normally just enough to get some +-16 V DC.
Something like LM317 would however not include a current limit to some 60 mA like the original circuit. For most circuit it would still be OK.

The 33 Ohms resistor may also be a fusible one and serve as a fuse.

[SilverSolder]

This circuit would never be designed this way today, but it is kind of cute and probably gets the best out of the handful of parts that are used.

In the 1970's, the decisions might have looked like this:

1) Using just 2 diodes for rectification is a cost saving over a bridge rectifier.

2) Using a voltage doubler allows use of smaller (cheaper) capacitors as @Kleinstein said - makes more ripple but is dealt with by the drop resistor and the zeners which we have to have anyway, so this advantage comes essentially for free.

3) Voltage doubler may allow getting away with less copper windings on the transformer, so we can use a cheaper model

4) The circuit gets away with just one expensive power resistor to drop the voltage for both positive and negative rails.


The guy that designed this circuit could only dream of what we have today... where it would be cheaply possible to use a switch mode power supply having dozens of advanced components, including a microprocessor if we wanted, with a total BOM cost possibly lower than he had!

[bdunham7]

5) The power factor is probably pretty good compared to most 'better' designs! 

[tony359]

@silversolder

I studied the diagram and did some tests on the actual unit. I do not measure 100V but 85V - I believe @Kleinstein mentioned that because of R1 and the capacitors the value was not going to be exactly VAC x 2.8 but more in the 80 range - which is exactly what I am measuring.

I now understand why R2 is reading 50V across it - the way you draw the circuit is very clear, thank you a lot!

Indeed I see the voltage slightly dropping when I connect a static load. Coincidentally, I played with Zeners just a few days ago on a breadboard and I know what you mean that a tiny change in voltage causes a large change in current - which is how Zeners work indeed. I have also tested with my thermal camera and indeed I can see the Zeners getting much hotter when I switch the unit off from the selector (which only disconnects the main PCB but not the PSU).

After replacing the resistors with a 5W and 2W versions and verified that the whole thing still burns like a star, I think I am happy to move to phase 3 and improve on the PSU as many here recommended

I understand the LM317 (and I guess LM337 for the negative rail) are the variable regulators of choice - Wouldn't a voltage regulator get very hot when dropping lots of volts though? . I do not have the skills to design something like that by myself so I would appreciate your help.

Based on the typical drawings found on the datasheets, it could be something like that - but this is the first time I am doing something like this so I am 100% sure that it'll make many of you cringe! Please be kind! (Yes, I know 3.024Kohm resistors do not exist, but I am struggling a bit finding the correct values so for now I put the ones given by the calculator!)

https://crcit.net/c/d71559e0902e4b2d84d78f4fad22bbb7

Thank you!

[bdunham7]

Connect the point where the two secondaries are common to the point where the two input capacitor are common.  Add 75R (or something) resistors to the non-common sides of the secondary if you want short circuit current limiting. 

Also, I would put the larger capacitors on the input side, you shouldn't need so much on the output side.  You need reservoir caps to supply power when the transformer can't, but the regulators only need small output caps to be stable. And the term 'mF' is confusing, nowadays it refers to a really, really big capacitor.  Years ago, when capacitors that size were all but impossible, this term was used to mean 'microfarad', but nowadays 'uF' is needed because 100 millifarad capacitors are perfectly common--and there are even 'kF
 kilofarad units!

The regulators will not get hot because the current through them is very small--just the load and the small current through the divider resistors.  They will be dissipating less than 1 watt and likely won't need heat sinks at all.

[MarkF]

A summary of changes.
Use the LM337 on the negative side instead of two LM317's

The power(P) dissipated by the regulators is the (voltage across regulator) * (current)
It's the combination of both voltage and current, not just the voltage.

I would use 5K multi-turn pots instead of the 3K resistors.
   https://www.mouser.com/ProductDetail/Vishay-Sfernice/T93YA502KT20?qs=%2Fha2pyFaduiha0Qd7DX8mLYS%252BFwYHADuGWgWC1%2F3SdC2ktWgCdFBcA%3D%3D


Edit-  Your LM317 on the negative side will NOT work as you have it.

[SilverSolder]

@silversolder

I studied the diagram and did some tests on the actual unit. I do not measure 100V but 85V - I believe @Kleinstein mentioned that because of R1 and the capacitors the value was not going to be exactly VAC x 2.8 but more in the 80 range - which is exactly what I am measuring.

I now understand why R2 is reading 50V across it - the way you draw the circuit is very clear, thank you a lot!

Indeed I see the voltage slightly dropping when I connect a static load. Coincidentally, I played with Zeners just a few days ago on a breadboard and I know what you mean that a tiny change in voltage causes a large change in current - which is how Zeners work indeed. I have also tested with my thermal camera and indeed I can see the Zeners getting much hotter when I switch the unit off from the selector (which only disconnects the main PCB but not the PSU).

After replacing the resistors with a 5W and 2W versions and verified that the whole thing still burns like a star, I think I am happy to move to phase 3 and improve on the PSU as many here recommended

I understand the LM317 (and I guess LM337 for the negative rail) are the variable regulators of choice - Wouldn't a voltage regulator get very hot when dropping lots of volts though? . I do not have the skills to design something like that by myself so I would appreciate your help.

Based on the typical drawings found on the datasheets, it could be something like that - but this is the first time I am doing something like this so I am 100% sure that it'll make many of you cringe! Please be kind! (Yes, I know 3.024Kohm resistors do not exist, but I am struggling a bit finding the correct values so for now I put the ones given by the calculator!)

https://crcit.net/c/d71559e0902e4b2d84d78f4fad22bbb7

Thank you!

Learning by doing is the best way, well done! 

For your improved circuit, the suggestions from @bdunham7 and @MarkF appear to be sound, although I have never actually used the LM337 or LM317 in a project - but these things tend to be quite docile.

The way these regulators work isn't like a zener diode.  These things actually pass the entire current through a large pass transistor, which is controlled by a built-in op amp.  This way, pretty much all the current goes to the load.  Only a very small current flows in the "voltage setting" part of the circuit.   So the power dissipated by them is equal to V*I,  where V is the voltage drop across the regulator, and I is the current going to the load (I guess, 22mA max).   

My penny's worth on the design discussion so far is that I would stick with fixed resistors instead of putting a pot or a trimmer in the circuit.  The old design that is being replaced was not ultra precise, so using fixed 1% resistors will be more than good enough to set the voltage reliably. 

The less parts used, the more reliable the result will be (all else being equal).   The reason for disliking trimmers and pots is that over time, they have a tendency to go bad (usually by the wiper arm going open circuit or intermittent against the track).  Depending on the circuit configuration, this can cause the voltage to either max out or drop to zero...   there are commercial power supplies where it maxes out!  - needless to say, that usually isn't good for the load connected to the supply... 

The other thing to consider is that the improved circuit as it stands will be able to supply much more current than the 40 odd mA that the old one was capable of.  It is probably wise to add resistors on the transformer outputs to the bridge as @bdunham7 suggested, in order to put a limit to this - making sure they are high enough wattage to handle a short circuit of infinite duration if necessary.  Also, it would be wise to calculate how much power will be dissipated by the regulator(s) during a short circuit condition - this might influence your decision as to whether the regulators should be bolted onto the case for cooling, or if it is OK with a small heat sink internally, or even no heat sink at all!

[MarkF]

I'm just going to toss this out there.

If you do want the pots to be able to adjust the voltage (your preference), I saw a configuration where by the output
output voltage drops to 0V if the wiper goes open.  I never wired this up and would be interested in how well the voltage adjust works.

I wouldn't be surprised though if you couldn't just get away with +/-15V fixed regulators.

[SilverSolder]

You can get 18V fixed regulators as well -  a couple of those might be a good solution?

[ArthurDent]

I think the easiest way to fix what you have is to and not have 100 VDC across a string of 4 zeners is to rewire the circuit as shown in the redrawn schematic below. The resistors in series with the rectifiers can limit inrush and can also be selected so too much current doesn't flow through the zeners. If the secondaries are 15 VAC each, the rectified peak would be about 23 VDC. By adjusting the values of the resistors before and after the rectifiers you can adjust the current and spread the dissipation between the resistors.

[SilverSolder]

I think the easiest way to fix what you have is to and not have 100 VDC across a string of 4 zeners is to rewire the circuit as shown in the redrawn schematic below. The resistors in series with the rectifiers can limit inrush and can also be selected so too much current doesn't flow through the zeners. If the secondaries are 15 VAC each, the rectified peak would be about 23 VDC. By adjusting the values of the resistors before and after the rectifiers you can adjust the current and spread the dissipation between the resistors.

This would only be a half wave rectifier for each rail, which may lead to too much ripple?

If the two diodes were replaced with two bridge rectifiers...

[ArthurDent]

I think the easiest way to fix what you have is to and not have 100 VDC across a string of 4 zeners is to rewire the circuit as shown in the redrawn schematic below. The resistors in series with the rectifiers can limit inrush and can also be selected so too much current doesn't flow through the zeners. If the secondaries are 15 VAC each, the rectified peak would be about 23 VDC. By adjusting the values of the resistors before and after the rectifiers you can adjust the current and spread the dissipation between the resistors.

This would only be a half wave rectifier for each rail, which may lead to too much ripple?

If the two diodes were replaced with two bridge rectifiers...

The best repair would be to throw it all away and start over! With the low current drawn I think the half wave rectifiers should work. Without knowing more about the load (audio preamp or battery charger) we can't be sure. Maybe increasing the value of the caps may be needed. This is pretty much guess work without the power supply and known load to test. The original wiring didn't indicate that much filtering was needed so I went with the simplest modifications. The caps are probably dried out by now as well.   

[tony359]

it's a vintage Signal generator. Generates up to 1Khz sines and square. The system is already a half wave setup and I have 1V ripple on the rails already.

As mentioned before, it seems the PSU is replacing 4 batteries which I am not familiar with - PP9? which seem to be 9V each so 18V both. So maybe an 18V regulator could work. I see rechargeable PP9s are 8.4Vx2 = 16.8. To be honest I feel that 18V could be ok.

yesterday after I posted my sketch I realised that I don't think it's going to work. The way it's configured won't give me a positive and negative output from the bridge, am I right? If I am not mistaken I would need to ground the "central" ends of each 15V winding?

Or, as suggested by someone, I could just keep the half wave configuration - which is what I have now.

@MarkF
Thanks. Are the TIP3055 and 2N2905 in your drawing for dropping the voltage to zero in case the regulation opens?

[vk6zgo]

You can get 18V fixed regulators as well -  a couple of those might be a good solution?

Especially as, if you fit new batteries, 18v is what you get!

The OP's Levell Signal Generator sounds like the one I have.

Mine didn't come with a power supply, but with four horribly corroded Brit 276 batteries, (the whole thing being in a very nice leather carry case, which was the original reason for my purchase).

The Brit 276 batteries are different from the 276P batt which were sold in Oz, in that the former uses "press stud" type connectors, but both types are quite large.
The battery compartment is, hence, also large, & my reading of the available information was that the PSU fitted in there, in lieu of the batteries.

Even so, the PSU described by the OP seems like it would be even larger, so if it is separate, I would suggest it may be a home brew effort, hence the "sneaky" way of getting a split supply.

In my case, I stuck with batteries, using four 6 cell AA battery holders.

[tony359]

Here is the signal generator!