login

Author Topic: Power supply woes  (Read 337 times)

Keenite

  • Hero Member
  • *****
  • Posts: 1951
  • Country: 00
  • Thanked: 23 times
  • Change is overrated
Power supply woes
« on: 05:15:13 | 12 June, 2021 »
Question unrelated to internet radio but here goes:
Recently bought a used Radio Shack DX-394 shortwave receiver (ca. 1995). Upon receipt I noticed that the built-in power supply had been removed (presumably because it was 110V) and instead a 12V external adapter was provided. However, the jack for the external power supply on the back of the radio indicates "13.8V", so here my questions:
-is it safe to assume the nearly 2-volt difference in input power reduces overall performance?
-would prefer to find a suitable 220V power supply and install it myself, or find an external adapter putting out as near to 13.8V as possible. Googling hasn't revealed much so far. There are bench power supplies that can be adjusted to 13.8V but which are relatively expensive and might be overkill in this instance. Most of the cheapo adapters on offer are not well insulated and generally cause lots of interference that seriously disrupts radio reception.
TIA for your input.
« Last Edit: 05:34:54 | 12 June, 2021 by Keenite »

drgeoff

  • Hero Member
  • *****
  • Posts: 950
  • Thanked: 56 times
Re: Power supply woes
« Reply #1 on: 11:50:42 | 12 June, 2021 »
See page 9 of http://ve5kc.amateur-radio.ca/ftp_files/DX-394-owners-manual.pdf

It will be fine working from a 12 volt PSU of adequate current capability.  13.8 volts is what a 12 volt vehicle battery usually gives.  Hence that marking for the jack.

The schematic at http://www.monitor.co.uk/radio-mods/dx-394/dx-394-manuals/dx-394-circuit-diagram.pdf shows that the voltage sensitive parts are run from lower voltages derived by internal voltage regulators.

Yes, most PSUs now are switch mode types which do emit RF interference audible on many AM bands.  (Nothing to do with "insulation".) If the 12 volt PSU you have is the type with a heavy iron cored transformer, that is the best one to use with the radio.
« Last Edit: 12:05:13 | 12 June, 2021 by drgeoff »

Keenite

  • Hero Member
  • *****
  • Posts: 1951
  • Country: 00
  • Thanked: 23 times
  • Change is overrated
Re: Power supply woes
« Reply #2 on: 23:23:11 | 12 June, 2021 »
Thank you for the information.
I did indeed read the passage you cite, however, never having owned or even driven a motor vehicle it did not occur to me that whilst the battery yields 13.8V, the car's electrical system runs on 12V. It would have been better for me had the previous owner not removed the 110V power supply as I have several transformers that would have done the job. Can only assume that at some point it was plugged into 220 and suffered an ignominious death. He also seems to have installed a few mods such as the ability to hear stations as they appear when tuning manually. The factory default was to suppress the audio while searching.
A more pressing issue is to now replace the dead rechargable memory back-up battery - another 'detail' the seller did not share with me prior to purchase. Have since found a relatively simple solution
(http://m6rdp.blogspot.com/2019/12/realistic-dx394-ram-memory-back-up.html?m=1)
seemingly fool-proof as long as I sever the correct PCB trace.
All in all I am happy with my acquisition nevertheless - while by no means a top-of-the-line SW receiver - it does work very well for my purposes. None of these challenges are as daunting as trying to mitigate the effects of all the electronic gadgets, light bulbs etc. that emit interference. There is also an electric fence located approx. 100 yards from our house, whose incessant click can also be heard on certain bands. Have not yet attempted to properly ground the radio yet but perhaps this will solve some of these interference problems.


drgeoff

  • Hero Member
  • *****
  • Posts: 950
  • Thanked: 56 times
Re: Power supply woes
« Reply #3 on: 02:27:49 | 13 June, 2021 »
... it did not occur to me that whilst the battery yields 13.8V, the car's electrical system runs on 12V.
That is not the case.  There is nothing in a vehicle which converts the battery voltage to 12 volts for everything on the vehicle.  12 volts is just a label.  A "12 volt" car battery conventionally has 6 lead-acid cells.  That chemistry gives a variable voltage depending on the state of charge but typically between 2.2 and 2.4 volts per cell.

drgeoff

  • Hero Member
  • *****
  • Posts: 950
  • Thanked: 56 times
Re: Power supply woes
« Reply #4 on: 04:04:48 | 13 June, 2021 »
Has the internal transformer been removed or just the AC cord?  Page 23 of the Service Manual http://swl.net.ru/wp-content/uploads/2012/06/dx-394-service-manual.pdf shows the transformer location - bolted to the left side (looking at the outside front).  The bridge rectifier and reservoir capacitor are on the linear PCB.  I doubt they have been removed - if they had been damaged by powering from 220 volts a lot more of the radio would have been kaput.

There is nothing exotic about the internal power supply components.  Should be possible to mount a generic 220 volt transformer instead of the original 110 volt one. eg https://www.ebay.co.uk/itm/402643005090 single output, 220 volt, 12 volt 10 watt but check the physical size in the table against space in the radio.
« Last Edit: 05:16:32 | 13 June, 2021 by drgeoff »

Keenite

  • Hero Member
  • *****
  • Posts: 1951
  • Country: 00
  • Thanked: 23 times
  • Change is overrated
Re: Power supply woes
« Reply #5 on: 03:59:25 | 14 June, 2021 »
Got it.
Would the original transformer (indeed ripped out) have stepped down to 12v or 13.8v?

drgeoff

  • Hero Member
  • *****
  • Posts: 950
  • Thanked: 56 times
Re: Power supply woes
« Reply #6 on: 05:43:18 | 14 June, 2021 »
Transformers supply AC not DC. AC voltage is usually quoted as RMS which means root mean square. For a sine wave (which AC supplies aspire to be) their peak voltage is root 2 times the RMS value. When you rectify AC to produce DC and have a reservoir capacitor the DC voltage approaches the AC peak. The rectifier in your radio is a bridge which introduces two silicon diode voltage drops of about 0.7 volts each.

So a 12 volt transformer would give about 12 x 1.414 - 1.4 which is a shade over 15 volts. In practice slightly less. If you are thinking that is too high bear in mind these facts. When a vehicle engine is running its alternator will be trying to get the battery voltage to 14.4 volts. The only part of your radio that is running directly from the 12 to 15 volt supply is the audio amp driving the speaker. Other parts of the radio are powered  at lower voltages from voltage regulators. Both the amp and the regulators have no problem with voltages well above 15 volts. For example, the TDA1905 audio amp IC is specced at 30 volts maximum.

Keenite

  • Hero Member
  • *****
  • Posts: 1951
  • Country: 00
  • Thanked: 23 times
  • Change is overrated
Re: Power supply woes
« Reply #7 on: 04:12:10 | 21 June, 2021 »
Just saw that there are 9v and 12v power banks - would they be suitable for use with transistor radios? Several of my older radios run on D batteries, which have become rather expensive of late.

E.g. https://www.amazon.de/Varta-20000mmAh-57983-101-111/dp/B08JMD78Y3

This too might work(?) with my existing 5v power bank:
https://www.amazon.de/Qutaway-Converter-Spannung-Transformator-Stromkabel-Black/dp/B071X6VYXR/ref=sr_1_4?__mk_de_DE=%C3%85M%C3%85%C5%BD%C3%95%C3%91&dchild=1&keywords=Boost+Voltage+Converter+Adapter+Cable&qid=1624274608&sr=8-4
« Last Edit: 04:27:52 | 21 June, 2021 by Keenite »

drgeoff

  • Hero Member
  • *****
  • Posts: 950
  • Thanked: 56 times
Re: Power supply woes
« Reply #8 on: 09:48:55 | 22 June, 2021 »
Just saw that there are 9v and 12v power banks - would they be suitable for use with transistor radios? Several of my older radios run on D batteries, which have become rather expensive of late.

E.g. https://www.amazon.de/Varta-20000mmAh-57983-101-111/dp/B08JMD78Y3

This too might work(?) with my existing 5v power bank:
https://www.amazon.de/Qutaway-Converter-Spannung-Transformator-Stromkabel-Black/dp/B071X6VYXR/ref=sr_1_4?__mk_de_DE=%C3%85M%C3%85%C5%BD%C3%95%C3%91&dchild=1&keywords=Boost+Voltage+Converter+Adapter+Cable&qid=1624274608&sr=8-4
Those should work but bear in mind the following:

1.  Converters do not magically increase increase power - in fact they lose some.  Power is Volts times Amps.  So if a device requires 12 Volts and X Amps that is 12X Watts.  If the converter is being supplied at 5 Volts it would draw 12X/5 Amps if it were 100% efficient.  In practice it will be closer to 3X.  Check that the powerbank is rated to supply that.+

2.  Although a USB powerbank outputs 5 Volts there is no cell chemistry that can produce a battery with that voltage for a useful portion of its discharge cycle.  All USB powerbanks contain a DC-DC converter which takes the varying non-5 Volts from the battery and converts - again not 100% efficiently - to a constant 5 Volts.  Almost universally the internal battery is one or two lithium cells.  The voltage from these decreases from 4.2 Volts when fully charged to about 3 Volts which is generally taken as the point at which they should be recharged.  So 3.7 Volts is used as an average figure for run-time calculations.

3.  Now we come to the "sneaky" bit.  When you see the mAh capacity spec for a powerbank it is invariably the capacity of the cell(s).  The theoretical mAh figure for the 5 Volt output would be only 3.7/5 of that.  In practice less because the conversion is not 100% efficient, the capacity figure was probably on the optimistic side (for marketing reasons), taking all the available energy from a lithium cell damages it, lithium cells lose start to lose capacity as soon as they are made (so an old one won't run as long as a brand new one).   All in all a rule of thumb is to divide the advertised capacity by 2.

Which brings us to a run-time calculation for the above 12 Volt device at X Amps and running via a 5 to 12 Volt converter from a USB powerbank advertised as Y,000 mAh. (That's Y Ah but again Y,000 looks better than Y for marketing.)

Ah is Amps multiplied by hours.  To get run time divide Ah by Amps, in this case Y/2 divided by 3X which is Y/6X

The following users thanked this post: Keenite

Keenite

  • Hero Member
  • *****
  • Posts: 1951
  • Country: 00
  • Thanked: 23 times
  • Change is overrated
Re: Power supply woes
« Reply #9 on: 02:13:37 | 23 June, 2021 »
Thanks for your detailed information. Will take the time to understand the underlying theory of it all. My powerbank claims
26800 mAh/99.16 Wh
Output: 5V, 2A (max)

For me the duration of the charge is less important than providing the necessary power for radios.

Keenite

  • Hero Member
  • *****
  • Posts: 1951
  • Country: 00
  • Thanked: 23 times
  • Change is overrated
Re: Power supply woes
« Reply #10 on: 02:46:44 | 23 June, 2021 »
Here a somewhat related question:
If I understand correctly, customary batteries (D, A, AA etc) generally produce 1.5V, whereas rechargeable ones only 1.2V. So if I were to take 8 regular D-cells and use them to power a 12V device I am assuming that this would provide suffient power until they drain and need to be replaced. On the other hand, 8 x rechargables would yield only 9.6V, Does this mean I can/should use 10 rechargables to arrive at 12V?
This begs the question: why can't rechargeables be produced that yield 1.5V - is it because of the underlying chemistry? Read somewhere that a company called Kentli already has some on offer.

drgeoff

  • Hero Member
  • *****
  • Posts: 950
  • Thanked: 56 times
Re: Power supply woes
« Reply #11 on: 11:38:52 | 23 June, 2021 »
Here a somewhat related question:
If I understand correctly, customary batteries (D, A, AA etc) generally produce 1.5V, whereas rechargeable ones only 1.2V. So if I were to take 8 regular D-cells and use them to power a 12V device I am assuming that this would provide suffient power until they drain and need to be replaced. On the other hand, 8 x rechargables would yield only 9.6V, Does this mean I can/should use 10 rechargables to arrive at 12V?
This begs the question: why can't rechargeables be produced that yield 1.5V - is it because of the underlying chemistry? Read somewhere that a company called Kentli already has some on offer.
Those primary cells produce 1.5 Volts when fresh but less and less are they are run down.  Equipment should (note "should" not "must") function down to about 1.1 Volt per cell.  The NiMH rechargeables will give noticeably more than 1.2 Volts when fully charged but that will quickly drop to 1.2 Volts where it will remain for most of the discharge cycle until it falls off rapidly.  So lots of equipment will work OK fitting rechargeables on a one to one basis.

Note that there are two distinct types of NiMH rechargeables.  Those with the slightly higher capacity also have a higher self discharge rate.  Meaning they lose charge even when not being used.  This makes them less suitable for clocks, remote controls etc.  The other type is typically sold as "pre-charged" or "ready to use".  Even after sitting for a year these will still have 75% to 80% of full charge.
The following users thanked this post: Keenite

Keenite

  • Hero Member
  • *****
  • Posts: 1951
  • Country: 00
  • Thanked: 23 times
  • Change is overrated
Re: Power supply woes
« Reply #12 on: 11:42:43 | 27 June, 2021 »
Finally got the step-up converter but unfortunately it's a no-go. There is so much RF interference that it cannot be used for either shortwave or even FM. I suspect that it is the converter itself that is to blame, but can't be sure.

drgeoff

  • Hero Member
  • *****
  • Posts: 950
  • Thanked: 56 times
Re: Power supply woes
« Reply #13 on: 15:28:56 | 27 June, 2021 »
Finally got the step-up converter but unfortunately it's a no-go. There is so much RF interference that it cannot be used for either shortwave or even FM. I suspect that it is the converter itself that is to blame, but can't be sure.
Well, yes the converter has very similar circuitry to those 230 Volts AC to low voltage DC power supplies that you already know chuck out masses of RF interference.  You need a good metal enclosure to combat that.
The following users thanked this post: Keenite

jpaul

  • Jr. Member
  • **
  • Posts: 15
Re: Power supply woes
« Reply #14 on: 12:07:32 | 15 July, 2021 »
And to further reduce the RF "hash" being generated by the supply, in addition to a metal case, you might want to find a toroid and position it as close to the leads coming from the printed circuit board.  The power supply wire functions as an antenna.  Toroid cores can be found inside old PC power supplies where they were the core of a high-frequency power supply. Just remove the few windings.  The problem with a toroid is that the connector at the end often is too big to fit thru the toroid "donut". You often have to unsolder the power leads and wrap the lead around the toroid 2 or 3 times.  Radio Shack used to sell one that would open and could be closed over a power line and thus didn't need the unsoldering & resoldering step. But it has been years since I laid eyes on one of those.

 

anything