Power supply problem - UK Vintage Radio Repair and Restoration Discussion Forum

MotorBikeLes · 10th Feb 2018, 12:28 am

I have a Farnell LS30-10 PSU which I have used for powering stuff and charging batteries for the last 30 or so years. But I need a PSU for my shed to fill in where conventional chargers can not be used. I have a 24v Hitron SM unit acquired here so I thought "use that". A couple of volts adjustment, so I set it to 26v and checked it with a BIG 20 ohm ww resistor. Perfect. In the shed, I connected it to a 24v battery (2 xs 12v 8Ah in series), but there was a big spark and a hot smell from the Hitron. Disconnect quickly.
Back to electronics room and check it visually but no evidence of damage, so connect to the 20 ohm again. Relief, it still works OK.
So, the question is, can I use this for battery charging, and if so how?
I suppose (hope) I can just add a big diode out of a computer PSU in series, but if so, do I keep the sense joined to the output, or connect it to the diode's output. Or is it simply a "no-no"?
Les.

bikerhifinut · 10th Feb 2018, 12:41 am

I wonder Les, could you have accidentally got the polarity wrong?

I did something similar a couple of months ago with a car battery charger that didn't have polarity protection, pure carelessness I freely admit and I was lucky in that it only blew an internal fuse on the charger. In my defence the + and - markings on the battery in question are not too easy to spot, no excuse really though

A

Skywave · 10th Feb 2018, 1:03 am

Reverse polarity protection when charging a battery.
Inside the PSU, fit a reverse-biassed diode - something chunky. Then make up a battery charging cable that contains a suitably rated fuse. In the event of a reverse polarity connection of battery to PSU, the diode protects the PSU and puts an effective short-circuit across the battery . . . fuse blows. Just choose the diode carrying current to be substantially bigger than that of the fuse - the idea is to replace the fuse, not the diode!

Al.

Radio Wrangler · 10th Feb 2018, 7:00 am

Some lab PSUs include 'Down-programming' transistors which dump charge on the output when the voltage setting is at all lower than the voltage on the output. They are smaller than the main series pass devices and burn out when people try to use them to charge batteries.

It's worth checking if a supply has these, or else just adding a series diode to battery charging leads and jacking the voltage up to compensate.

David

MotorBikeLes · 10th Feb 2018, 10:09 am

No, I definitely did not connect the wrong way. In fact I initially connected to a lowish Ah battery about 6v below the set voltage, and there was a spark. It was when I connected to the "full fat" 24v lead acid cells that the spark was bigger, plus that familiar smell. You can get that with a resistor getting hot as well as when semis commit the final surrender.
I will try with a series diode, but where to connect the sense lead?
Thanks, Les.

Herald1360 · 10th Feb 2018, 10:18 am

I have found that connecting batteries to PSUs which are OFF can be bad news for the PSU. I now set the voltage and current limits with the leads connected to the battery but with one disconnected at the PSU, turn on the PSU, then connect the other lead to the PSU. (Keeps any sparks away from the battery- a good idea if it's a wet Pb SO4 type.) Obviously it's better to have plug connections at the PSU rather than binding posts when doing this!

10th Feb 2018, 12:28 am	#1
MotorBikeLes Nonode Join Date: Dec 2012 Location: Kirk Michael, Isle of Man Posts: 2,350	Power supply problem I have a Farnell LS30-10 PSU which I have used for powering stuff and charging batteries for the last 30 or so years. But I need a PSU for my shed to fill in where conventional chargers can not be used. I have a 24v Hitron SM unit acquired here so I thought "use that". A couple of volts adjustment, so I set it to 26v and checked it with a BIG 20 ohm ww resistor. Perfect. In the shed, I connected it to a 24v battery (2 xs 12v 8Ah in series), but there was a big spark and a hot smell from the Hitron. Disconnect quickly. Back to electronics room and check it visually but no evidence of damage, so connect to the 20 ohm again. Relief, it still works OK. So, the question is, can I use this for battery charging, and if so how? I suppose (hope) I can just add a big diode out of a computer PSU in series, but if so, do I keep the sense joined to the output, or connect it to the diode's output. Or is it simply a "no-no"? Les.

10th Feb 2018, 12:41 am	#2
bikerhifinut Octode Join Date: Nov 2013 Location: Penrith, Cumbria, UK Posts: 1,993	Re: Power supply problem I wonder Les, could you have accidentally got the polarity wrong? I did something similar a couple of months ago with a car battery charger that didn't have polarity protection, pure carelessness I freely admit and I was lucky in that it only blew an internal fuse on the charger. In my defence the + and - markings on the battery in question are not too easy to spot, no excuse really though A

10th Feb 2018, 1:03 am	#3
Skywave Rest in Peace Join Date: Jun 2006 Location: Chard, South Somerset, UK. Posts: 7,457	Re: Power supply problem Reverse polarity protection when charging a battery. Inside the PSU, fit a reverse-biassed diode - something chunky. Then make up a battery charging cable that contains a suitably rated fuse. In the event of a reverse polarity connection of battery to PSU, the diode protects the PSU and puts an effective short-circuit across the battery . . . fuse blows. Just choose the diode carrying current to be substantially bigger than that of the fuse - the idea is to replace the fuse, not the diode! Al.

10th Feb 2018, 7:00 am	#4
Radio Wrangler Moderator Join Date: Mar 2012 Location: Fife, Scotland, UK. Posts: 22,868	Re: Power supply problem Some lab PSUs include 'Down-programming' transistors which dump charge on the output when the voltage setting is at all lower than the voltage on the output. They are smaller than the main series pass devices and burn out when people try to use them to charge batteries. It's worth checking if a supply has these, or else just adding a series diode to battery charging leads and jacking the voltage up to compensate. David __________________ Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done

10th Feb 2018, 10:09 am	#5
MotorBikeLes Nonode Join Date: Dec 2012 Location: Kirk Michael, Isle of Man Posts: 2,350	Re: Power supply problem No, I definitely did not connect the wrong way. In fact I initially connected to a lowish Ah battery about 6v below the set voltage, and there was a spark. It was when I connected to the "full fat" 24v lead acid cells that the spark was bigger, plus that familiar smell. You can get that with a resistor getting hot as well as when semis commit the final surrender. I will try with a series diode, but where to connect the sense lead? Thanks, Les.

10th Feb 2018, 10:18 am	#6
Herald1360 Dekatron Join Date: Feb 2007 Location: Leominster, Herefordshire, UK. Posts: 16,535	Re: Power supply problem I have found that connecting batteries to PSUs which are OFF can be bad news for the PSU. I now set the voltage and current limits with the leads connected to the battery but with one disconnected at the PSU, turn on the PSU, then connect the other lead to the PSU. (Keeps any sparks away from the battery- a good idea if it's a wet Pb SO4 type.) Obviously it's better to have plug connections at the PSU rather than binding posts when doing this! __________________ ....__________ ....\|____\|\|__\|__\_____ .=.\| _---\__\|__\|_---_\|. .........O..Chris....O