Linear p/s question

[Bl4kkat]

Hi everyone, I had a general question about a small linear power supply I saved from the trash. It’s An Micronta brand (made by Radio Shack)
Output 13.8 VDC - 1.75 A @ 40 watts

I put my voltmeter on it and read 16.3v... will this destroy my VV-868 transceiver? I’m kind of leary of hooking up my equipment with it being over 13.8v and there is no way to adjust the voltage.
I drilled out the rivers to look inside and it’s just a transformer, soldered glass fuse and a small square shaped (assuming) bridge wave rectifier.

Any help would be great and I could not find anything on the issue. Closest I have seen are stuff like 13.8v +/-10% or 13.8v +/-15% thanks in advance.

 

[KB4MSZ]

In my opinion, that is getting into the "borderline" area for voltage. Also, the voltage should be much closer to its rated value, seems like something in the supply might be going bad.

Another odd spec is the 13.8 VDC at 1.75 amps. This would support only 24.15 watts, not 40, unless it is referring to the overall efficiency of the supply at 60%.

 

[mmckenna]

Sounds like an unregulated power supply, which means the voltage will wander as the load increases.

And while it may work for receiving only, if you try to transmit with it, it's going to bonk. 1.75 amps isn't enough to support that radio. The voltage will drop and the radio will restart each time you key up.

Id' recommend getting a switch mode 12 volt power supply that will put out 12-15 amps. That'll support most 50 watt VHF/UHF radios on the market.

 

[N4KVE]

Many of these cheap power supplies have higher voltage with no load, but then drop down to 13.8 volts when drawing the 1.75 amps. I’d toss it in the dumpster.

 

[KB4MSZ]

small linear power supply I saved from the trash.

I’d toss it in the dumpster.

Hum.

 

[Bl4kkat]

LoL but I was trying to be “Green” and fix it up. One of the older mechanic was about to throw it away cause an apprentice used it to power a car radio deck and some small speakers.

Hum.

 

[Bl4kkat]

But seriously it’s a no go? The Leixen VV-878 is the 10 watt version not 25 watt...
If the power supply is going bad could I just replace the bridge wave rectifier? I’d have to drill out the rivers again and look at what is there and compare it to the one I got. But if I remember correctly I think the one I have was something like 400VAC 50A (does that sound right?).

 

[wb6uqa]

Buy a regulated supply. BUY A VOLTAGE REGULATOR AND WIRE IT TO YOUR SUPPLY.

 

[AK9R]

A basic linear power supply should contain a transformer to drop the voltage down from line voltage to something slightly above the desired voltage, a rectifier to convert the AC to DC (actually, pulsed DC), a capacitor to level out the pulses, and a voltage regulator to maintain the voltage.

A full-wave bridge rectifier is not a voltage regulator.

If the power supply you have does not have good voltage regulation, then it will act as stated above in that the no-load voltage may be high, but the voltage will come down when you apply a load.

The specifications for the radio should indicate how much current it draws when transmitting. I'm going to jump the conclusion that it's more than 1.75 amps, but I could be wrong.

Frankly, I think you'd be better off taking this power supply to the electronics recycling center and buying a new one that is rated to supply at least the current required by your radio.

 

[MTS2000des]

The particular power supply is a model RadioShaft sold for decades. It's a basic, unregulated supply with a low quality step down transformer and basic bridge rectifier, and couple of filter caps. Nothing else. The transformer is barely capable of 2 amps. This was designed for powering LOW CURRENT devices of the day like car tape players, CB radios, etc.

All the advice is solid. If you care about your radio, a good QUALITY power supply is as essential as a good QUALITY antenna and feedline.

 

[SuperG900]

The particular power supply is a model RadioShaft sold for decades. It's a basic, unregulated supply with a low quality step down transformer and basic bridge rectifier, and couple of filter caps. Nothing else. The transformer is barely capable of 2 amps. This was designed for powering LOW CURRENT devices of the day like car tape players, CB radios, etc.

All the advice is solid. If you care about your radio, a good QUALITY power supply is as essential as a good QUALITY antenna and feedline.

Exactly. Most of us have had one of those Micronta power supplies at one time or another. They're not regulated, low amperage.

 

[needairtime]

Seems like OP has a Radio Shack PN "22-502" which is indeed unregulated - transformer to diode to capacitor. Has a cigarette lighter output it seems?

It'd be marginal to use a 7812 to get a 12V@1A regulated linear supply, but with this regulator, the unit will not have enough power for the Leixen VV-868 in high power mode - it appears to require 1A @ 4W or 1.8A @ 10W according to specifications.

If you must use this safely you'd likely need to reduce the peak voltage for when it's not transmitting. I wonder if putting in a 14.4V shunt regulator would be the easiest "hack" to get this to at least not fry the radio when it's just receiving. Else you'll need a high efficiency DC-DC converter that could generate RF noise.

 

[SuperG900]

Seems like OP has a Radio Shack PN "22-502" which is indeed unregulated - transformer to diode to capacitor. Has a cigarette lighter output it seems?

It'd be marginal to use a 7812 to get a 12V@1A regulated linear supply, but with this regulator, the unit will not have enough power for the Leixen VV-868 in high power mode - it appears to require 1A @ 4W or 1.8A @ 10W according to specifications.

If you must use this safely you'd likely need to reduce the peak voltage for when it's not transmitting. I wonder if putting in a 14.4V shunt regulator would be the easiest "hack" to get this to at least not fry the radio when it's just receiving. Else you'll need a high efficiency DC-DC converter that could generate RF noise.

Actually, it's probably a bad idea to consider it for any radio use, but as always IANAL, YMMV etc...

Hacking it, IMHO just isn't economically worth it, when decent power supplies that will do the job safely with plenty of current can be had for not a whole lot of money.

 

[WB9YBM]

I drilled out the rivers to look inside and it’s just a transformer, soldered glass fuse and a small square shaped (assuming) bridge wave rectifier.

Since you did not mention having seen a regulator (or pass transistor with voltage reference), I'd also assume it's unregulated. If you're going to ad your own regulator, make sure you heat sink it because even at 1.75A, they'll get warm. That might be worth doing if you've got any low current stuff you'd like to power on your bench.

It sounds like you'd like to stay with an analog supply, but with higher current/more stable voltage spec's. A company I've had good luck with is Astron; they come in a wide range of currents, and they do have a pot inside you can tweak voltage levels with.

 

[W5lz]

Just a suggestion. Total up all the currents to be supplied by a power supply, then double it. That's the minimum you should look for. Then look at the CCS rating of that power supply, not the ICS rating. The first is the 'continuous' current level it can supply, the other is the 'intermittent' level that it will supply in -very- short 'spurts'. That ICS rating is usually what's posted in advertisements (bigger numbers!). This is one instance where 'bigger is better' for real.

 

[WB9YBM]

the 'intermittent' level that it will supply in -very- short 'spurts'. That ICS rating is usually what's posted in advertisements (bigger numbers!). This is one instance where 'bigger is better' for real.

In many cases, very true. I've noticed not everyone does it that way: for example, with Astron it's 10 minute on / 10 minutes off as far as the maximum rating goes. That's a lot better than other manufacturers I've come across.

 

[W5lz]

Yep, a 50% duty cycle, which will pretty much cover any 'normal' amateur use. And for the not so normal use (NCS, RTTY, ETC.) is where that 'bigger is better's thingy really comes in.

 

[needairtime]

Very true, I'm constantly hearing of people frying equipment (well, amplifiers, but applies to PSUs...) running FT8 which has a higher duty cycle than using phone on sideband...

Then again, people running full legal limit with FT8 isn't really in the spirit of the mode...

 

[WB9YBM]

Very true, I'm constantly hearing of people frying equipment (well, amplifiers, but applies to PSUs...) running FT8 which has a higher duty cycle than using phone on sideband...

Then again, people running full legal limit with FT8 isn't really in the spirit of the mode...

Adding a fan's become S.O.P. ever since they started making radios smaller (more compact) because the heat sinks on the back shrank right along with the radios. (Either a thermostatic switch or manual on/off switch). (And if a regular fan won't fit, I've used fans meant for CPUs). Works great; I've had continuous key-down of a ham mobile in high power for one hour + on multiple occasions for a year or more with no signs of problems.

 

[SuperG900]

Back when I was working, a lot of the radio stuff we designed used heat pipes to wick away heat. Cool stuff.