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Old 24th May 2023, 12:30 pm   #1
bill knox
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Default PSU for R1155

Hi all

I am building a PSU for an R1155 from bits that I have laying around.

The mains transformer is a little too energetic, it is giving out 260 0 260 Volts, I have put resistors in the anodes to assist in lowering the output but to get the output down to 220 Volts I am thinking of using a regulator valve, something like a VR150/30.

I don't want to use Zener's or complicated circuitry as space is limited.

Does anyone have suggestions for a regulator valve and/circuits that can be used?

Regards

Bill
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Old 24th May 2023, 3:39 pm   #2
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Default Re: PSU for R1155

260-0-260 volts seems about right for a transformer driving valve equipment, it would be unlikely to cause any damage to valve devices. The critical supply is the heater voltage, this needs to be within 10% to avoid premature ageing of the filaments and thoriated cathodes.
Neil
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Old 24th May 2023, 3:57 pm   #3
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Default Re: PSU for R1155

Bill is right to be concerned, the R1155 was designed to operate with an HT of 217 volts from the rotary convertor. Why overstress everything, capacitors in particular?

How are you planning to smooth the HT? A choke input filter will give a lower DC output than a capacitor input.

Andy
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Old 24th May 2023, 4:28 pm   #4
M0FYA Andy
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Default Re: PSU for R1155

Don't forget that HT-ve must not be connected to the chassis.

Andy
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Old 24th May 2023, 8:17 pm   #5
bill knox
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Default Re: PSU for R1155

Hi All

Unfortunately, I have now allocated most of the space on the chassis.

The input is a capacitor input and I do not have a choke with enough impedance to be of any use, unless someone can sell me one.

I am looking at using two valve regulators, in series, I believe that they are 105 volt
ones, never used them before so watch this space.

Also I am very aware that -HT and the radio chassis must not touch, I had a problem last night with one of the R1155s in that the potential difference between the two chassis was 75 V approx. soon found out that the 2K resistor that is across the bias chain was O/C, using a 1.5K temp. and ordered a supply of replacement resistors. I am in the process of changing all the .1 uf capacitors etc. I have a supply of ones that I have already restuffed with modern caps, when I am done I will restuff the ones removed.

No sign of anyone with knowledge of regulator circuits yet, but am going tomorrow to collect a couple, the guy there says that he has a couple of books showing some circuits so maybe have an answer tomorrow.

Regards

Bill

Last edited by bill knox; 24th May 2023 at 8:24 pm.
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Old 24th May 2023, 8:44 pm   #6
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Default Re: PSU for R1155

Bill,

My suggestion is not a series regulator solution, but a choke input filter after the rectifiers. DC output from a series L - parallel C filter, is approximately 0.9 times the AC voltage from the transformer. So 260V x 0.9 = 234 volts. Still a bit high, but not outrageously so.

A big advantage of an L-C filter is its tight regulation with varying load currents, which is useful for a receiver with no regulation system for its local oscillator.

You say that you do not have a choke with enough impedance to be any use. My recommendation is to go through the design steps for a L-C filter using whatever choke you do have. These days it is quite practical to use relatively low values of L, because much higher values of C are readily available in sensible sizes. The best design tool I have found (other than the traditional build and test it) is simulation with PSU Designer II on Duncan's Amp Pages at https://www.duncanamps.com/psud2/

Richard
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Old 24th May 2023, 9:11 pm   #7
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Default Re: PSU for R1155

A choke input filter needs a minimum load current to have it average-rectifying. Drop the load and the voltage goes up to the peak rectifying value - about 1.414 times that 260v =367v but less some rectifier drop.

You need to work out what the minimum likely HT current could be and calculate the choke inductance to suit. They can be a bit large, a significant fraction of the size of the power transformer and as you seem to be tight on space....

David
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Old 24th May 2023, 9:57 pm   #8
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Default Re: PSU for R1155

Shoot me down in flames but if want to keep it really simple and can accommodate some heat you could consider simple shunt zeners (prob have to use two or more in series to get 217V) (one set each side) with appropriate series resistors to drop the excess volts. Viability will depend on HT current and current range and I am afraid I have no idea whether you are talking 1, 10, 100, or more mA and what the operating range is. For larger currents especially variable currents the zeners and series resistors may need to dissipate impractical powers.

This has worked for me in the past at lower voltages but make sure you do the calcs and dont forget the winding resistance.
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Old 25th May 2023, 9:48 am   #9
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Default Re: PSU for R1155

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Originally Posted by Radio Wrangler View Post
A choke input filter needs a minimum load current to have it average-rectifying. Drop the load and the voltage goes up to the peak rectifying value - about 1.414 times that 260v =367v but less some rectifier drop.

You need to work out what the minimum likely HT current could be and calculate the choke inductance to suit. They can be a bit large, a significant fraction of the size of the power transformer and as you seem to be tight on space....

David
All very good points, David, and my comments were simplistic to the point of being misleading. Having noted that the choke inductance must not be less than a critical value, the question then is how to calculate that value.

I turned up this fairly useful summary of the design of a choke input filter, which covers the calculations that you outline: https://www.dhtrob.com/overige/pdf/ps_07.pdf

Here's my brief attempt at putting numbers into that equation. The output voltage is 220V or thereabouts. The current is the minimum that is likely to be drawn by the receiver. And that will never be less than that through the various bias resistor chains between HT+ and HT-. In other words the current through the receiver with all valves removed. I make that minimum current 21mA.

Those figures put the critical inductance at 10.5H.

In practice the minimum current will be far higher than 21mA, because the receiver will not be operated with all the valves removed. I don't have a figure for that minimum current, which would have to be obtained on a test bench with a regulated power supply and a suitable test receiver. Typical R1155 operating current (with the DF section valves out) is around 70mA.

Its obvious that even if the minimum current is only 40mA (for the sake of argument), the critical inductance is then only 5.5H. A 10H choke would seem to fit the bill here - with a current rating of say, 70mA.


Richard
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Old 25th May 2023, 10:04 am   #10
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Default Re: PSU for R1155

Quote:
Originally Posted by Radio Wrangler View Post
A choke input filter needs a minimum load current to have it average-rectifying. Drop the load and the voltage goes up to the peak rectifying value - about 1.414 times that 260v =367v but less some rectifier drop.

You need to work out what the minimum likely HT current could be and calculate the choke inductance to suit. They can be a bit large, a significant fraction of the size of the power transformer and as you seem to be tight on space....

David
Yup. I restored the Lorenz SZ42 cipher machine for Bletchley. That has a choke input filter on one of the power supply lines, and a power resistor across the capacitor for exactly that reason. There were operating modes when the current draw was too low for the LC, so a bleeder resistor was necessary.

Craig

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Old 25th May 2023, 11:23 am   #11
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Default Re: PSU for R1155

Are you using valve or solid state rectifiers and have you measured the voltage with the R1155 connected? The open circuit voltage will drop under load, more so with a valve rectifier and any sort of choke.
The 1155 uses about 70 -80 mA.
If you find that the voltage is a bit high all you have to do is calculate the value of a resistor to correct the voltage.
In my opinion a few extra volts is not going to be a problem as long as it's not something silly like 300V!
Don m5aky

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Old 25th May 2023, 12:13 pm   #12
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Default Re: PSU for R1155

Bearing in mind Craig Sawyers note on the Lorenz machine he restored, its worth pointing out that the "official" mains power supply for a R1155 also had choke input filters. I say "filters" because it has two L-C sections in series, presumably because hum was an issue, with the modest (4uF) capacitors they had available.

The power supply in question is a RAF Type 114, described in AP1186E. Its a brute of thing, floor standing, with plenty of chunky iron-cored components in there. I attach the schematic of the HT and LT supplies, which I have marked up with values from the text of AP1186E. In some previous life I actually had a Type 114 power unit to play with, and I measured the choke L3 and L4 resistances at 105 ohms each.

Note the two bleeder resistors - 50k each - R3 and R4. The bridge rectifier shown is actually a metal rectifier, and in the power unit I had this was defunct, so I couldn't test this aspect of the circuit. It's worth noting though that T3 secondary voltage I have as 270V, so there must have been a substantial voltage drop in the metal rectifier, to end up with 210V (under load) as shown on the schematic.

I have some simulation results (Duncan Amps PSU II) which show that the output voltage of this HT supply was 234V, with a load current of 22mA (not including the current in bleeder resistor R2).


Richard
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Old 25th May 2023, 12:56 pm   #13
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Default Re: PSU for R1155

Hi Bill - I wonder if you are in danger of letting the tail wag the dog here by insisting on using an inappropriate transformer that you have hanging around, and then asking if anyone has a suitable inductor for sale - in spite of having limited space available?

Maybe take a step back and just look for a suitable transformer instead? There are plenty of radio amateurs who have built R1155 PSUs over the decades from magazine articles etc so it can't be an insurmountable problem.

Good luck with your project.

Cheers
Chris
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Old 25th May 2023, 1:15 pm   #14
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Default Re: PSU for R1155

Looking at that circuit, it's L4 that does the real work.

When the rectifier turns on with the transformer secondary giving a useful voltage, the first capacitor, C4 starts to charge up. Less obviously, the current going into C4 in passing through L4 revs up a magnetic field in the core of L4. So as C4 is charged up, some of the energy from the transformer is robbed to make the field in L4. There is energy stored in this field (half * L * Isquared) When the transformer voltage starts to fall, the current tries to diminish, but the choke L4 doesn't let it. Energy comes OUT of the choke now and this contributes to keeping C4 charged. When the transformer has got to zero volts, the slowly collapsing field sucks current out of the rectifier, keeping some diodes still conducting, as C4 gets charge from the energy stored in L4

So both C4 and L4 act as energy reservoirs. The art in designing one of these circuits is in having enough inductance in L4 that L4 is still powering things when the transformer voltage has risen enough to take over recharging C4.

Get enough L in L4 and the rectifiers never get to turn off. One diode swaps with the other, but there isn't any significant off period. L4 can make transient voltages to not allow that.

So L4 has to be a big enough inductance to hold enough energy to not run out between half-cycles. It also has to be a physically big enough inductor to not saturate at its max current.

All this massaging of current gets rid od the short-spike high current transients that capacitor input filters take, and the transformer, rectifier, capacitor seem to be in a nice smooth low-stress constant-current situation.

Just so long as there's enough L to not drop out of continuous conduction.

Choke input supplies can be larger, heavier and costlier than capacitor input, but in avoiding high peak current surges on the peaks of the voltage waveform, they are kind to rectifiers and avoid the multiplication of losses due to those high currents.

You just have to watch out for minimum inductance and saturation, and you can have good regulation.

David
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Old 25th May 2023, 2:06 pm   #15
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Default Re: PSU for R1155

A choke input filter, provided you don't go below the minimum current has quite reasonable voltage regulation. If you go below it, though, the voltage shoots up, to eventually 40-ish percent high at no load.
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Old 25th May 2023, 3:44 pm   #16
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Default Re: PSU for R1155

2x 105 volt stabilisers in series has been mentioned, it strikes (!) me that choke-input smoothing and shunt stabilisers ought to be a happy marriage, or at least an armed truce. Any high-voltage initial state resulting from directly-heated or solid-state rectifiers will assure rapid stabiliser striking and the drawing of sufficient current to clamp voltage rise. E.g. the VR105 has a maximum continuous rating of 40mA but a short-term rating of 100mA to cater for this kind of situation. A combination of initial theory and suck-it-and-see practice would be needed to get to the optimum value of feed resistor but it ought to provide a workable solution that avoids over-volting the R1155 and provides for efficient PSU operation- as mentioned, choke-input is kinder to rectifier and transformer, and most ex-radio transformers tend to have an HT secondary of 250V or more.
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Old 26th May 2023, 8:56 pm   #17
bill knox
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Default Re: PSU for R1155

Hi All

I have been reading all your replies and they make interesting reading, I followed a couple of the replies and managed to get hold of a choke that measured about 20H on my little Chinese tester and it has reduced the voltage very well, it came down to around 190 volts which I thought is slightly to low (being picky). I measured the dc resistance of the choke and it measures 400 ohms

What I have done is to put a 1K resistor across the choke and this has raised the o/p voltage to 210 volts. I have absolutely no hum, or should I be looking for a choke about the same inductance but lower resistance.

Is this defeating the use of a choke input, also, is this system also known as a swinging choke input??

Regards

Bill

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Old 26th May 2023, 9:06 pm   #18
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Default Re: PSU for R1155

Hi Richard trh01UK, I was following your page on filters but when I came to the end I found that the end is missing, I think another page, but I may be wrong on that, but very interesting.

Regards

Bill
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Old 26th May 2023, 9:17 pm   #19
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Default Re: PSU for R1155

Quote:
Originally Posted by bill knox View Post
Hi All

I have been reading all your replies and they make interesting reading, I followed a couple of the replies and managed to get hold of a choke that measured about 20H on my little ying tong tester and it has reduced the voltage very well, it came down to around 190 volts which I thought is slightly to low (being picky). I measured the dc resistance of the choke and it measures 400 ohms

What I have done is to put a 1K resistor across the choke and this has raised the o/p voltage to 210 volts.

Is this defeating the use of a choke input, also, is this system also known as a swinging choke input??

Regards

Bill
Bill,

it isn't really possible to make a sensible comment in response to your post above unless you describe the circuit as you have it now? Do you have a choke input filter? If so, what's the value of the capacitor?

And you need to describe the conditions of your test. Is the 190V with the R1155 connected and working? What HT current is it drawing at 190V? What is the current when the resistor is added, and the output voltage increases to 210V?

Putting a 1K resistor across the choke will lower the effective DC resistance of this series element from 400 ohms to 286 ohms. If we assume the R1155 is taking 70mA from the HT supply, then the average voltage drop in the series element will reduce from 28 volts to 20V due to the connection of the 1K resistor. But you are reporting a 20V change, so something else must be going on here. Maybe you altered the load current in the R1155 inadvertently?


Richard
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Old 26th May 2023, 10:28 pm   #20
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Default Re: PSU for R1155

Quote:
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Hi Richard trh01UK, I was following your page on filters but when I came to the end I found that the end is missing, I think another page, but I may be wrong on that, but very interesting.

Regards

Bill
Bill,

the article goes on to discuss swinging chokes, which is really a bit off-topic here. A swinging choke's inductance falls as the current through it increases - but it never falls below the critical value of inductance to maintain regulation.

I believe the main reason for using a swinging choke is to keep the size and cost of the choke to a minimum. Since you are using components from your junk box cost/size is not really relevant here.

Richard
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