Changed AC power supply to DC. What about the fuse? - Page 1
Forum section: electronics / rf-microwave

author: vinlove
author_rank: Frequent Contributor
timestamp: April 22, 2021, 08:21:36 pm
content: Hi AllMy old active antenna / preamp used to have a built in AC power supply. It was working OK, but failed.I ripped the AC Power supply out from the set, and decided to run a lead for DC power externally.The AC Power Supply was 230V input and was outputting 15V.Now it is fed by 12V DC power from separate power pack.At the back of the there is a fuse rated for 0.2A. That was for the AC power supply 230V AC input and 15V DC output for the set.Now with 12V DC, it seems working OK, but what rate of fuse should it have?Sometimes the new DC rechargeable battery power pack could drop to 8V DC before put for recharging.The original 0.2A fuse has blown. Checked all the connections and cabling, and they are all fine.Thanks

author: TheMG
author_rank: Frequent Contributor
timestamp: April 22, 2021, 11:42:01 pm
content: Get your multimeter out and measure how much DC current the device uses, and choose a fuse rated a bit higher than that.No other way to know, unless the manufacturer manual specifies the DC current rating, you'll have to measure it yourself.

author: ejeffrey
author_rank: Super Contributor
timestamp: April 23, 2021, 01:55:43 am
content: If your external dc supply can't deliver dangerous fault currents you don't even need a fuse. If the external source is a car battery then you should use a fuse.

author: vinlove
author_rank: Frequent Contributor
timestamp: April 23, 2021, 05:30:08 pm
content: OK, thanks for the info and advice. Much appreciated.

author: coppercone2
author_rank: Super Contributor
timestamp: April 23, 2021, 05:33:23 pm
content: A fuse is always recommended to make repair easier, its worth the investment IMOits nice to work on a PCB without sand paper because someone put 50 cents into itif you want to do it the best you need to look at I^2*T value (look up littlefuse documents), preferably measured with a current clamp and oscilloscope.plus then its ready to go and connect to whatever you want if you decide to make the amp battery powered (highly useful). I try to make all my little boxes run on batteries if possible, making the battery holder is usually a fun craft project IMO.A cheap wall adapter can break, you might not have the right low current one on hand if you want to continue work.

author: vinlove
author_rank: Frequent Contributor
timestamp: April 23, 2021, 05:46:32 pm
content: Originally the set ran at 15V DC converted from the dead 230V AC main power supply.But now, it is fed with 12V DC rechargeable battery or 12V Linear Power Supply.I have noticed the rechargeable battery drops voltage when the charge runs out.It even goes down to 7V DC.I was wondering if the current consumption in the set would go up as the DC voltage drops, henceneeding higher rated fuse 0.2V originally to 0.3 or even 0.5A? I was not sure on this.Should be able to figure out this from the Ohms Law, but I am rusty in Electronics and Mathrecently, and was not sure what is right.The original fuse in the set (0.2A) blew after about 10 minutes running, and then the replaced0.25A fuses were keep blowing too. Kept checking any shorts on the new leads but all seem ok.

author: A.Z.
author_rank: Super Contributor
timestamp: April 23, 2021, 07:22:47 pm
content: Quote from: vinlove on April 22, 2021, 08:21:36 pmI wonder which kind of problem you have with the fundamental ohm law; given that V*I=W ...

author: coppercone2
author_rank: Super Contributor
timestamp: April 23, 2021, 08:56:04 pm
content: depends on the power supply typeare you powering a boost converter?

author: vinlove
author_rank: Frequent Contributor
timestamp: April 23, 2021, 09:56:28 pm
content: Quote from: coppercone2 on April 23, 2021, 08:56:04 pmIt is just a rechargeable battery with 12V rating. Originally it was a car engine starter.But it has been reconditioned to be used as a battery. Outputs solid 12V in full charge.But drops to 7V in about a few hours.The other PSU is a linear PSU for Ham Radio with 3A rating. Puts out 12V DC.A good PSU.

author: vinlove
author_rank: Frequent Contributor
timestamp: April 23, 2021, 10:00:29 pm
content: Quote from: A.Z. on April 23, 2021, 07:22:47 pmNo problem as such. Just have not been dealing with these problems for long time doing other stuff.Have been trying to save the old active preamp. Seems really well made stuff with heavy casing and parts.Its AC PSU has died, and thought would be better off with DC psu.I was under impression the answer to the OP could come from Ohms Law.

author: coppercone2
author_rank: Super Contributor
timestamp: April 23, 2021, 10:18:55 pm
content: I am asking what the battery load is, the thing you are powering, does it have a boost converter

author: vinlove
author_rank: Frequent Contributor
timestamp: April 24, 2021, 07:21:31 am
content: Quote from: coppercone2 on April 23, 2021, 10:18:55 pmNo it doesn't have a boost converter. Battery load of the thing I am powering? I am not sure.The set was originally running with 15V DC from the AC PSU (230V AC Input => 15V DC out)Nominal fuse rating was 0.2AThats the only info I have on the set.

author: TheMG
author_rank: Frequent Contributor
timestamp: April 24, 2021, 01:43:51 pm
content: I guess you don't have a multimeter to measure the actual current with?Mathematically determining the DC current consumption from the AC power supply's input fuse rating is not possible as the fuses are often oversized by a certain amount to account for inrush current and other factors. It's just a safety device to protect the wiring, and sometimes the transformer, from going up in smoke, nothing more. There is no direct correlation between the fuse rating and the real power consumption, other than it has to be less than the fuse rating (240V*0.2A=48W).

author: sleemanj
author_rank: Super Contributor
timestamp: April 25, 2021, 02:05:43 am
content: The device works normally?200mA fuse blows itself after 10 minutes?Put in a 315mA, 500mA or 630mA fuse. Sure you could measure and calculate and estimate, but popping in the next one up and observing is probably just as good.

author: coppercone2
author_rank: Super Contributor
timestamp: April 25, 2021, 06:54:03 am
content: being able to measure AC current is useful for advanced electronics, its not a bad tangent to go off on from a repair to get that kind of capability.Oscilloscope digital signal processing math function differential connection might be good enough to measure what you are looking at, but I never played with it much since I own a differential probe. The key here is to make an adapter that splices in current measurement.its really not that much effort given how long this thread is going lol

author: vinlove
author_rank: Frequent Contributor
timestamp: April 25, 2021, 10:56:12 am
content: I put a new fuse in (250mA), and powered it on last night.The set came back on, and was passing the radio signal from the RX wire antenna to the Radio.It didn't sound as if it was doing any amplifying or pre amping.I opened the set, and had close look on all the parts with a LED torch.There I noticed a small resistor burnt like barbecued sausage.And one of the mica capacitor has been charred on the top.So, 2x damaged parts are spottedThe set was not blowing the fuse anymore, and powering on and switching on and off and attenuation seems work,but with those 2x damaged parts it couldn't be working properly.Needing more work on this set.

author: coppercone2
author_rank: Super Contributor
timestamp: April 25, 2021, 06:50:40 pm
content: yeah, you upped the current too much where the parts overloaded.Something tells me the boilerplate voltage was wrong, or something is wrong in the circuit.Alot of equipment I buy on ebay is functional with a oversized fuse, probobly because its ancient and they did not want to buy the recommended one. I see them doubled alot.