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Homebrew Equipment A place to show, design and discuss the weird and wonderful electronic creations from the hands of individual members. |
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20th Jan 2018, 11:57 pm | #21 |
Nonode
Join Date: May 2007
Location: Redruth, Cornwall, UK.
Posts: 2,573
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Re: Wanted: Circuit for HV PSU
Hi Al.
That's an impressive looking and very useful power supply and thanks for posting the details, I will have a read through the circuit description later. When I can find the time, I'll probably have a go at building your circuit. Regards Symon. |
21st Jan 2018, 12:53 am | #22 |
Nonode
Join Date: Oct 2010
Location: Tintinara, South Australia, Australia
Posts: 2,339
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Re: Wanted: Circuit for HV PSU
Can I mention one common mistake in Al's design (and perpetuated in many other designs).
The volt meter should go after the mA meter to read the true output voltage. In a HV psu I doubt it would make a great deal of difference, but in low voltage designs it can lead to reading differences of up to a couple of volts on either side of the Amp meter. A couple of volts may not sound like much, but if you're running a 5v circuit and only getting 3v after the amp meter, it can lead to a lot of head scratching. |
21st Jan 2018, 1:18 am | #23 |
Dekatron
Join Date: May 2008
Location: Derby, UK.
Posts: 7,735
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Re: Wanted: Circuit for HV PSU
But if you put the ammeter upstream of the voltmeter, then the ammeter is going to be measuring the current drawn by the voltmeter on top of the current flowing through the load .....
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21st Jan 2018, 10:56 am | #24 |
Nonode
Join Date: Aug 2010
Location: Resolven, Wales; and Bristol, England
Posts: 2,608
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Re: Wanted: Circuit for HV PSU
Isn’t that why the Farnell power supplies have a dual-scaled meter with a selector switch to read either current or voltage?
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21st Jan 2018, 11:41 am | #25 |
Dekatron
Join Date: Sep 2007
Location: Oxfordshire, UK.
Posts: 4,311
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Re: Wanted: Circuit for HV PSU
I don't think a switched meter helps the fundamental problem does it ? If you have the voltmeter switched in and set the voltage to 5V, then you switch the voltmeter out and the current meter in, the actual voltage on your load will no longer be 5V because now you have the current meter in, which you didn't when you set the voltage. This is one of the real beauties of modern digital voltmeters - they typically draw hardly any current at all, so if you put them after a current meter they won't change its reading. One solution using old technology was to set up a potentiometer circuit. When a voltage potentiometer is balanced it draws zero current https://en.wikipedia.org/wiki/Potent...ing_instrument.
Cheers, GJ
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21st Jan 2018, 12:58 pm | #26 |
Rest in Peace
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Location: Chard, South Somerset, UK.
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Re: Wanted: Circuit for HV PSU
Thanks for all your responses.
V before I or I before V? (Nothing to do with spelling!) I suppose it's just another example of trying to have one's cake and eating it: where do all those currents go? Al. |
21st Jan 2018, 1:03 pm | #27 |
Hexode
Join Date: Aug 2010
Location: nr. Hannover, Germany
Posts: 372
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Re: Wanted: Circuit for HV PSU
Guys,
many thanks for all your constructive comments. I have put together the attached circuit and I would be extremely grateful for your comments and suggestions. I have merged one or two ideas from others into the circuit so can claim no originality for it. I am not sure how much voltage to expect at TP1 which makes further calculations a bit dodgy. I would like to achieve an output range of about 150 to 265V and imagine I will have to experiment with Rtop and Rbottom. Components have been chosen because I have them in my cupboard. Does this circuit seem realistic and viable? Many thanks, as always
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Eddie BVWS Member. Friend of the BVWTM Last edited by eddie_ce; 21st Jan 2018 at 1:09 pm. Reason: spelling |
21st Jan 2018, 1:45 pm | #28 | |
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Re: Wanted: Circuit for HV PSU
Quote:
Inspired by your post, I did a few sums for the two cases of a H.V. and a L.V. PSU. In both cases, I assumed an ammeter with a native FSD of 1 mA and an internal resistance of 1000 Ω Case 1: L.V. Assume load current= 1-amp. For FSD of 1-amp., the mA meter needs a shunt of 1 Ω. So volt-drop across mA meter = 1v. Hence, for a voltmeter reading of 5v., 4v. across the load. Case 2: H.V. Assume load current= 100 mA. For FSD of 100 mA, the mA meter needs a shunt of 10 Ω. So volt-drop across mA meter = 1v. Hence, for a voltmeter reading of 100v., 99v. across the load. Case 1 and Case 2. With the voltmeter across the load instead and assuming it also has a native FSD current of 1 mA and an internal resistance of 1000 Ω, a resistor is obviously required to be in series with that meter to produce the required FSD. However, the max. current drawn by that meter for both cases will still be 1 mA: negligible compared to 1-amp. and 100 mA drawn by the load. When I produced my design, did I take Terry's considerations into account? Can't recall now - all too long ago. But Terry's point is well-worth bearing in mind for future reference. Al. |
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21st Jan 2018, 1:56 pm | #29 |
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Re: Wanted: Circuit for HV PSU
Eddie: the EL86 has a max. anode voltage of 200-v. Your design will substantially exceed that value. I would also be inclined to choose a choke input filter, anyway. And a potential divider for the screen-grid voltage of the EF80 might be a better choice, too.
Al. |
21st Jan 2018, 3:29 pm | #30 |
Hexode
Join Date: Aug 2010
Location: nr. Hannover, Germany
Posts: 372
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Re: Wanted: Circuit for HV PSU
AL,
I was under the impression, that having a floating heater supply and an assumed max anode voltage of 350V and a minimum cathode voltage of 150V I would just be in the safe region of 200V. So is the 200V rating an absolute value irrespective of the potential of the valve's other electrodes? If so, it looks like EL34s.
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21st Jan 2018, 4:07 pm | #31 |
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Re: Wanted: Circuit for HV PSU
Eddie - my remark was based on data from the National Valve Museum:
http://www.r-type.org/exhib/abv0002.htm plus the Mullard datasheet. (As below as an attachment). If you can obtain them, my favourite for series-pass regulator valve duty has always been the 12E1, but they tend to be a bit pricey. OTOH, the venerable 807 is a bit lower on Va, Ia and Pa max., but usually substantially cheaper. On account of their popularity in the valve Hi-Fi world, EL34 valves are becoming seriously expensive. Al. Last edited by Skywave; 21st Jan 2018 at 4:19 pm. |
21st Jan 2018, 4:36 pm | #32 | |
Dekatron
Join Date: Sep 2007
Location: Oxfordshire, UK.
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Re: Wanted: Circuit for HV PSU
Quote:
Cheers, GJ
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21st Jan 2018, 5:07 pm | #33 | |
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Re: Wanted: Circuit for HV PSU
Quote:
As for those voltages, yes, it can safely be assumed that they are relative to the cathode - although I didn't see that explicitly stated. Al. |
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21st Jan 2018, 5:11 pm | #34 |
Dekatron
Join Date: Sep 2007
Location: Oxfordshire, UK.
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Re: Wanted: Circuit for HV PSU
Mostly it looks good. But there is one serious flaw. You cannot put a 10uF capacitor across the 85A2 gas-discharge valve. Valves of this type have a 'negative resistance' region in their characteristic curves and too much parallel capacitance turns them into relaxation oscillators. It is often claimed that they will destroy themselves like this, even explosively !
This is perhaps the main drawback with gas-discharge voltage references. They are inherently noisy and it seems like a good idea to smooth this noise away. But you can't do it with just a capacitor (if you use a solid-state Zener diode you can, but I have seen too many blown Zener diodes to trust them in valve circuits). Steve Bench, in my earlier reference, suggests (his page 3) that you use no more than 10nF across the gas-discharge tube. Other people are braver and will risk up to 100nF. If you want to quieten the reference further then one way to do it is to use a triode-pentode instead of the pentode error amplifier. the pentode section still acts as the amplifier, but the triode is used as a cathode follower with the gas-discharge tube setting its grid voltage and its cathode wired to the pentode's cathode. Now you can put a large capacitor across the common cathode resistor to eliminate the reference noise without blowing the reference valve up. Cheers, GJ
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22nd Jan 2018, 1:08 am | #35 |
Nonode
Join Date: Oct 2010
Location: Tintinara, South Australia, Australia
Posts: 2,339
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Re: Wanted: Circuit for HV PSU
Cheers Al
Same thing happens using DMM's/multimeters etc when measuring current. Caught me out once |
22nd Jan 2018, 8:44 am | #36 |
Dekatron
Join Date: Jul 2009
Location: Sleaford, Lincs. UK.
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Re: Wanted: Circuit for HV PSU
There's this circuit that has been posted before - http://www.parcgwyn.talktalk.net/valvetester.htm
uses an EL81, but any circuit that uses line OP valves has got to be better than using EL84's. For not so simple variable valve PSU's see here - http://diyaudioprojects.com/mirror/m...ench/reg1.html Andy.
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22nd Jan 2018, 10:08 am | #37 | |
Dekatron
Join Date: Sep 2007
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Re: Wanted: Circuit for HV PSU
Quote:
I should say that with an EL86 and an EF86 delivering 250V/50mA I was able to achieve an output impedance of well below an ohm. At this point the resistance of the umbilical cable to the device I was powering started to matter ! Cheers, GJ
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22nd Jan 2018, 5:20 pm | #38 |
Pentode
Join Date: Mar 2013
Location: Leeds, West Yorkshire, UK.
Posts: 246
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Re: Wanted: Circuit for HV PSU
How about using "part" of a Farnell circuit for a "E350" also uses EL34s (as requested). Here is the manual..
regards S-W |
22nd Jan 2018, 6:54 pm | #39 |
Hexode
Join Date: Aug 2010
Location: nr. Hannover, Germany
Posts: 372
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Re: Wanted: Circuit for HV PSU
Short Wave
Building an E350 is still on the to do list, I have most of the parts but lack time. So I want to knock up a relatively simple supply over the next couple of weeks. GrimJosef Many thanks for your helpful comments, I did not know about the perils of putting a capacitor across a stabiliser. I have now redesigned and am using an ECF80, the triode as a cathode follower as you suggested. I'm away for a few days, but when back would like to re-post the modified circuit for members' scrutiny. Is there a formula to calculate the expected off-load voltage at TP1 on my circuit? This would make further calculations a lot easier. I really appreciate the time you all take to reply to this thread.
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23rd Jan 2018, 12:41 am | #40 | |
Rest in Peace
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Re: Wanted: Circuit for HV PSU
Quote:
At low load currents, the voltage to the anode of the pass valve will approach that figure: so there could be problem then (Va max. rating). That's one of the reasons why I usually advocate using a choke input filter: the off-load / low-load voltage (at your TP1) is lower; the inherent regulation of the cct. is better. And at low O/P voltages & with substantial load currents, the pass valve has an easier life. Al. |
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