Newbie bench supply question - UK Vintage Radio Repair and Restoration Discussion Forum

Brettski · 5th Feb 2018, 10:23 pm

Hi, after getting fed up with swapping PP9s in and out of my small collection of Roberts radios, I decided to buy a cheap 30v 5 amp bench supply. Now this might sound like a stupid question : I can set the voltage but what do I do when setting the current? Would I be right in saying just select the max current and let the radio draw what it wants?

Help please .

Many thanks.

Herald1360 · 5th Feb 2018, 10:35 pm

Yeeeessss......

But better to set the current limit at slightly above max likely current draw.

For a big tranny (Hacker, Roberts etc capable of a watt or so) say 200mA.

If the voltage drops on loud bits, up the limit a bit more!

30V, 5A and cheap suggests switchmode innards. I hope it's nice and quiet!

Brettski · 5th Feb 2018, 11:18 pm

Thanks Chris. It’s not Landed yet, but looking forward to having a mess around.

Thanks for the reply.

Brett

Diabolical Artificer · 6th Feb 2018, 8:02 am

If you have a bit of gear or circuit that hasn't been powered up in ages or you are powering up a circuit you have made changes to then it's a good idea to set the current to a low value, 1/4 to 1/2 to make sure nothing goes bang. The idea being to give the DUT (device under test) just enough power to turn it on, but not at full power.

If you have some big resistors it might be an idea to have a play first to see how your PSU works. A big 470 ohm say - 5v current set to 10mA.

Andy.

Radio Wrangler · 6th Feb 2018, 8:50 am

If all is good, you can set the current limit to max. The power supply then provides the chosen voltage and the radio takes as much current as it needs.

But if something goes wrong and the radio takes too much current, the current setting on the power supply controls how much damage is done.

If you don't know whether everything is OK or not, then set the current limit down very low, set the voltage to the correct level, switch the power supply output on and start slowly increasing the current limit setting. Watch the voltage and current come up. If the current goes too high it's time to stop and go looking for the reason.At this point it's probably not caused any collateral damage. Large capacitors will take significant current for a moment as their voltage increases.

If all is good and the current has levelled off at a reasonable value, and the voltage has come up to the intended value, then setting the current limit a little higher and leaving it there means that the set is left with some protection should something go wrong.

Some transistor amplifiers, particularly the larger hifi types can behave oddly with reduced voltage or limited current supplies even though they can be fine when on full voltage. The ones with dual supplies are even more critical. But these concerns don't apply to powering a radio in lieu of PP9s.

Be cautious about cheap power supplies. A fault in a power supply can snd its output voltage higher than whet the device being powered can usually survive. In theamateur radio world, people used to spend £1,000s on a new transceiver, spending ages poring over the feature lists of the various models, spanding ages saving up. Then they'd try to save every penny possible when buying a 13.8v power supply for it. Yeah! one shorted regulator transistor in a cheapo PSU and every semiconductor in the expensive radio would fry. The properly good power supplies were designed by people who recognised that the radio was worth a lot more than the power supply, and included a crowbar circuit to monitor the output voltage and to short it if it went over a set limit. The moral is that although power supplies aren't things people get enthusiastic over, they can easily kill your favourite equipment.

David

Brettski · 6th Feb 2018, 1:27 pm

Brill, thanks ever so much for the replies. I'm still in learning mode.

I am using said device for my own ebay sourced Roberts ( and Hackers maybe at some point) transistors , with a price limit of £20.00 so If I accidently fry a set, I would be disappointed, but it wouldn't be the end of the world. In fact it would give me another fault finding mission to go on.

Kindest regards.

Brett.

kalee20 · 6th Feb 2018, 1:53 pm

A cheap 30V 5A definitely sounds like switch mode!

You may be lucky, but I'd expect there to be a fair bit of interference and 'mush' when in use. It may clean up if you operate the radio 20 feet from the power supply, with long connecting cables between, and it will probably behave differently between LW/MW and FM.

One thing well worth doing, is to connect a chunky diode (1N5402 sort of thing) permanently across the battery connector. If you connect the supply backwards, the diode will conduct all the current and prevent damage.

Brettski · 6th Feb 2018, 3:09 pm

https://www.ebay.co.uk/p/Adjustable-...d=282761066316

This is the one. I had planned on using the unit with a good distance from the set as the workbench dictates this, but alas not 20ft.

I could always use it to power my Scalextric set and revert back to PP9s . I can understand the noise/interference concerns, at least was not too expensive if I have wasted my time on it.

I will report my finding .

Cheers .

Refugee · 6th Feb 2018, 3:41 pm

One of the reviews says it is heavy enough that there is a traditional transformer in it.

dave cox · 6th Feb 2018, 4:32 pm

It would be worth testing it out with a dummy load (resistor) or a bulb, this will give you some confidence using it and with its capabilities. Maybe a stop/tail 12v bulb would be a sensible test.

dc

barrymagrec · 6th Feb 2018, 4:40 pm

Be aware that with some bench supplies if you set the current very low, 10mA say for testing LEDs and use the output switch for switching on and off there may be a large cap after the current limit circuit which can supply enough current to blow your device before the current limit kicks in - I speak from experience.

Brettski · 6th Feb 2018, 5:57 pm

Quote:

Originally Posted by dave cox

It would be worth testing it out with a dummy load (resistor) or a bulb, this will give you some confidence using it and with its capabilities. Maybe a stop/tail 12v bulb would be a sensible test.

I've got loads of these. I will try this first.

Noted about low voltage testing. Cheers.

Diabolical Artificer · 7th Feb 2018, 8:10 am

I've got a Roberts power supply somewhere, someone gave me to fix, am sure it'd be ok to pass it on to you if you want it.

Andy.

Brettski · 7th Feb 2018, 9:29 am

PMd

mhennessy · 7th Feb 2018, 1:05 pm

The series lamp limiter concept works at DC as well

A 12V lamp, rated at a couple of watts or thereabouts, can work well. Given the increasing scarcity of germanium output transistors - the most vulnerable part of a transistor radio - I'd regard that as cheap insurance until you've got more confidence in the power supply.

OldTechFan96 · 7th Feb 2018, 1:33 pm

I've been after one of these bench top power supplies as I think that it will come in handy. It would be nice not to be tethered to batteries when working on the bench. Some of them look like they don't come with probes.

Keep us updated.

kalee20 · 7th Feb 2018, 1:48 pm

Quote:

Originally Posted by Brettski

https://www.ebay.co.uk/p/Adjustable-...d=282761066316

This is the one. I had planned on using the unit with a good distance from the set as the workbench dictates this, but alas not 20ft.

I could always use it to power my Scalextric set and revert back to PP9s . I can understand the noise/interference concerns, at least was not too expensive if I have wasted my time on it.

It looks good value and well specified for the money! If it is a large, mains-frequency transformer jobbie (heavy, inefficient, but 'clean' output) then it's even better value.

The caveats that I and others have said, still apply, but once you have it you can let us know how you get on. Really, that's the worst things can get, and reality could be a lot better. And yes, eminently suitable for Scalextric!

Brettski · 7th Feb 2018, 2:08 pm

Thanks all for the great array of replies. I expect to see it this weekend so If mods can leave this thread open until next week, I will keep you posted .

Thanks to Diabolical Artificer for hopefully supplying me with some clean power too.

Cheers. Brett.

MrBungle · 7th Feb 2018, 3:33 pm

Just a point on that power supply. If you're lucky enough to get a linear one, they have a massive cost cut in them to be aware of. They use a switched transformer tap which kicks in and out as voltage increases to keep the voltage drop over the pass transistors as low as possible. If the relay that switches this dies, which they do regularly, it'll fail into 30v switched mode. This will cause the pass transistors to overheat if you pull more than about 1A through it and then it's a 50/50 if they go short or open when they give up. If it goes short you'll get about 50v unregulated DC out suddenly.

Ed_Dinning · 8th Feb 2018, 7:08 am

Hi Brettski, if it has a transformer in it it will provide the basis of a bench power supply.
As you gain confidence you should be able to find a circuit that can form a variable "front end" for the transformer to give variable volts and current limiting.

There may be some power supplies at the Golbourne meeting in early April.

Ed

5th Feb 2018, 10:23 pm	#1
Brettski Tetrode Join Date: Dec 2017 Location: York, North Yorkshire, UK. Posts: 65	Newbie bench supply question Hi, after getting fed up with swapping PP9s in and out of my small collection of Roberts radios, I decided to buy a cheap 30v 5 amp bench supply. Now this might sound like a stupid question : I can set the voltage but what do I do when setting the current? Would I be right in saying just select the max current and let the radio draw what it wants? Help please . Many thanks.

5th Feb 2018, 10:35 pm	#2
Herald1360 Dekatron Join Date: Feb 2007 Location: Leominster, Herefordshire, UK. Posts: 16,535	Re: Newbie bench supply question Yeeeessss...... But better to set the current limit at slightly above max likely current draw. For a big tranny (Hacker, Roberts etc capable of a watt or so) say 200mA. If the voltage drops on loud bits, up the limit a bit more! 30V, 5A and cheap suggests switchmode innards. I hope it's nice and quiet! __________________ ....__________ ....\|____\|\|__\|__\_____ .=.\| _---\__\|__\|_---_\|. .........O..Chris....O

5th Feb 2018, 11:18 pm	#3
Brettski Tetrode Join Date: Dec 2017 Location: York, North Yorkshire, UK. Posts: 65	Re: Newbie bench supply question Thanks Chris. It’s not Landed yet, but looking forward to having a mess around. Thanks for the reply. Brett

6th Feb 2018, 8:02 am	#4
Diabolical Artificer Dekatron Join Date: Jul 2009 Location: Sleaford, Lincs. UK. Posts: 7,658	Re: Newbie bench supply question If you have a bit of gear or circuit that hasn't been powered up in ages or you are powering up a circuit you have made changes to then it's a good idea to set the current to a low value, 1/4 to 1/2 to make sure nothing goes bang. The idea being to give the DUT (device under test) just enough power to turn it on, but not at full power. If you have some big resistors it might be an idea to have a play first to see how your PSU works. A big 470 ohm say - 5v current set to 10mA. Andy. __________________ Curiosity hasn't killed this cat...so far.

6th Feb 2018, 8:50 am	#5
Radio Wrangler Moderator Join Date: Mar 2012 Location: Fife, Scotland, UK. Posts: 22,869	Re: Newbie bench supply question If all is good, you can set the current limit to max. The power supply then provides the chosen voltage and the radio takes as much current as it needs. But if something goes wrong and the radio takes too much current, the current setting on the power supply controls how much damage is done. If you don't know whether everything is OK or not, then set the current limit down very low, set the voltage to the correct level, switch the power supply output on and start slowly increasing the current limit setting. Watch the voltage and current come up. If the current goes too high it's time to stop and go looking for the reason.At this point it's probably not caused any collateral damage. Large capacitors will take significant current for a moment as their voltage increases. If all is good and the current has levelled off at a reasonable value, and the voltage has come up to the intended value, then setting the current limit a little higher and leaving it there means that the set is left with some protection should something go wrong. Some transistor amplifiers, particularly the larger hifi types can behave oddly with reduced voltage or limited current supplies even though they can be fine when on full voltage. The ones with dual supplies are even more critical. But these concerns don't apply to powering a radio in lieu of PP9s. Be cautious about cheap power supplies. A fault in a power supply can snd its output voltage higher than whet the device being powered can usually survive. In theamateur radio world, people used to spend £1,000s on a new transceiver, spending ages poring over the feature lists of the various models, spanding ages saving up. Then they'd try to save every penny possible when buying a 13.8v power supply for it. Yeah! one shorted regulator transistor in a cheapo PSU and every semiconductor in the expensive radio would fry. The properly good power supplies were designed by people who recognised that the radio was worth a lot more than the power supply, and included a crowbar circuit to monitor the output voltage and to short it if it went over a set limit. The moral is that although power supplies aren't things people get enthusiastic over, they can easily kill your favourite equipment. David __________________ Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done

6th Feb 2018, 1:27 pm	#6
Brettski Tetrode Join Date: Dec 2017 Location: York, North Yorkshire, UK. Posts: 65	Re: Newbie bench supply question Brill, thanks ever so much for the replies. I'm still in learning mode. I am using said device for my own ebay sourced Roberts ( and Hackers maybe at some point) transistors , with a price limit of £20.00 so If I accidently fry a set, I would be disappointed, but it wouldn't be the end of the world. In fact it would give me another fault finding mission to go on. Kindest regards. Brett.

6th Feb 2018, 1:53 pm	#7
kalee20 Dekatron Join Date: Feb 2007 Location: Lynton, N. Devon, UK. Posts: 7,081	Re: Newbie bench supply question A cheap 30V 5A definitely sounds like switch mode! You may be lucky, but I'd expect there to be a fair bit of interference and 'mush' when in use. It may clean up if you operate the radio 20 feet from the power supply, with long connecting cables between, and it will probably behave differently between LW/MW and FM. One thing well worth doing, is to connect a chunky diode (1N5402 sort of thing) permanently across the battery connector. If you connect the supply backwards, the diode will conduct all the current and prevent damage.

6th Feb 2018, 3:09 pm	#8
Brettski Tetrode Join Date: Dec 2017 Location: York, North Yorkshire, UK. Posts: 65	Re: Newbie bench supply question https://www.ebay.co.uk/p/Adjustable-...d=282761066316 This is the one. I had planned on using the unit with a good distance from the set as the workbench dictates this, but alas not 20ft. I could always use it to power my Scalextric set and revert back to PP9s . I can understand the noise/interference concerns, at least was not too expensive if I have wasted my time on it. I will report my finding . Cheers .

6th Feb 2018, 3:41 pm	#9
Refugee Dekatron Join Date: Apr 2012 Location: Worksop, Nottinghamshire, UK. Posts: 5,553	Re: Newbie bench supply question One of the reviews says it is heavy enough that there is a traditional transformer in it.

6th Feb 2018, 4:32 pm	#10
dave cox Nonode Join Date: Jan 2009 Location: Bristol, UK. Posts: 2,062	Re: Newbie bench supply question It would be worth testing it out with a dummy load (resistor) or a bulb, this will give you some confidence using it and with its capabilities. Maybe a stop/tail 12v bulb would be a sensible test. dc

6th Feb 2018, 4:40 pm	#11
barrymagrec Octode Join Date: Dec 2017 Location: Morden, Surrey, UK. Posts: 1,557	Re: Newbie bench supply question Be aware that with some bench supplies if you set the current very low, 10mA say for testing LEDs and use the output switch for switching on and off there may be a large cap after the current limit circuit which can supply enough current to blow your device before the current limit kicks in - I speak from experience.

7th Feb 2018, 8:10 am	#13
Diabolical Artificer Dekatron Join Date: Jul 2009 Location: Sleaford, Lincs. UK. Posts: 7,658	Re: Newbie bench supply question I've got a Roberts power supply somewhere, someone gave me to fix, am sure it'd be ok to pass it on to you if you want it. Andy. __________________ Curiosity hasn't killed this cat...so far.

7th Feb 2018, 9:29 am	#14
Brettski Tetrode Join Date: Dec 2017 Location: York, North Yorkshire, UK. Posts: 65	Re: Newbie bench supply question PMd

7th Feb 2018, 1:05 pm	#15
mhennessy Dekatron Join Date: Jul 2007 Location: Evesham, Worcestershire, UK. Posts: 4,244	Re: Newbie bench supply question The series lamp limiter concept works at DC as well A 12V lamp, rated at a couple of watts or thereabouts, can work well. Given the increasing scarcity of germanium output transistors - the most vulnerable part of a transistor radio - I'd regard that as cheap insurance until you've got more confidence in the power supply.

7th Feb 2018, 1:33 pm	#16
OldTechFan96 Octode Join Date: Jul 2017 Location: Newcastle Upon Tyne, Tyne & Wear, UK. Posts: 1,178	Re: Newbie bench supply question I've been after one of these bench top power supplies as I think that it will come in handy. It would be nice not to be tethered to batteries when working on the bench. Some of them look like they don't come with probes. Keep us updated.

7th Feb 2018, 2:08 pm	#18
Brettski Tetrode Join Date: Dec 2017 Location: York, North Yorkshire, UK. Posts: 65	Re: Newbie bench supply question Thanks all for the great array of replies. I expect to see it this weekend so If mods can leave this thread open until next week, I will keep you posted . Thanks to Diabolical Artificer for hopefully supplying me with some clean power too. Cheers. Brett.

7th Feb 2018, 3:33 pm	#19
MrBungle Dekatron Join Date: Jun 2016 Location: London, UK. Posts: 3,687	Re: Newbie bench supply question Just a point on that power supply. If you're lucky enough to get a linear one, they have a massive cost cut in them to be aware of. They use a switched transformer tap which kicks in and out as voltage increases to keep the voltage drop over the pass transistors as low as possible. If the relay that switches this dies, which they do regularly, it'll fail into 30v switched mode. This will cause the pass transistors to overheat if you pull more than about 1A through it and then it's a 50/50 if they go short or open when they give up. If it goes short you'll get about 50v unregulated DC out suddenly.

8th Feb 2018, 7:08 am	#20
Ed_Dinning Dekatron Join Date: Jan 2004 Location: Newcastle upon Tyne, Tyne & Wear, UK. Posts: 8,194	Re: Newbie bench supply question Hi Brettski, if it has a transformer in it it will provide the basis of a bench power supply. As you gain confidence you should be able to find a circuit that can form a variable "front end" for the transformer to give variable volts and current limiting. There may be some power supplies at the Golbourne meeting in early April. Ed