Battery charger experiment - UK Vintage Radio Repair and Restoration Discussion Forum

avocollector · 21st Oct 2016, 2:17 pm

Not sure if this is the place to post this item but a few years back I wanted to generate a high voltage using a car coil. So I took an old 12 volt post office relay and wired the push to break contacts to the coil, then used a 7 amp 12 volt battery charger to drive it - I had a separate (battery) 12 volts supply wired to another set of contacts on the same relay and to the car coil. So the battery charger simply drove the relay to make and break all the time which gave me the square wave from the second set of independent contacts and battery to drive the car coil.

All worked quite well but there was a heck of lot of sparking on the relay contacts. Remembering some advice from Practical electronics, I wired a 10 uF 25 volt electrolytic capacitor across both sets of contacts. This stopped the sparking but 5 mins later the relay stopped working. When I pulled the battery charger to bits, the diodes (4 separate ones) inside had all literally melted.

I was surprised as the charger was supposed to be 12 volts up to 7 amps which I would have thought was well above any current drain even with the capacitor across the relay contact.

Any thoughts on why this happened??

Nicklyons2 · 21st Oct 2016, 3:58 pm

I would imagine the back e.m.f. from the coils had exceeded the P.I.V. of the battery charger's diodes and, in a bridge, once one diode fails the others are soon 'goners'. If you'd replaced the diodes with say 200 V PIV versions I'd have thought they would have survived OR you could have shunted each diode with say 0.1uF. This sort of spikiness would not be expected in a simple battery charger but was why many mains equipments of the live chassis era had diode shunt capacitors to prevent nastiness on the mains + the mains voltage exceeding the diode's (often BY100's) P.I.V.

GMB · 21st Oct 2016, 6:03 pm

By putting a capacitor across the contacts you just fed the high voltage rebound from the relay into the supply. It saves the contacts but not the supply.

A way to snub a relay without capacitors is to put a resistor in parallel with the coil. It only has to be somewhat larger than the coil resistance to mean that the consumption is not much affected but the high voltage pulse will see it differently. A diode and small resistor is another way to do it as the pulse is of opposite polarity to the supply.
A capacitor across the supply would also help to protect the supply but take care how you wire it or a very short pulse might bypass the capacitor.

When using capacitors around switches, never forget that they are a double-edged solution. If the capacitor ends up charged and the switch in parallel with it closes - now the capacitor damages the contacts.
So such a capacitor should have a small series resistor to avoid that problem too.

These things are never so simple!

kalee20 · 21st Oct 2016, 7:29 pm

A circuit would help a lot - I'm a bit puzzled right now!

Ed_Dinning · 21st Oct 2016, 9:40 pm

Hi Gents, usually a diode is wired in inverse parallel to the relay coil. This will give a longish drop out time but a back emf of about supply voltage +1 diode drop.
The addition of a resistor in series with the diode will quicken up the drop out time but increase the back EMF.
Usual trick was to make the resistor the same value as the coil resistance and back EMF is then 2* supply voltage.

In your scenario it should be possible to add a blocking diode in line with the supply to stop anything getting back to the supply, but use a freewheel diode across the relay coil as well.

Ed

TonyDuell · 22nd Oct 2016, 6:31 am

Another method is to put a zener in series with the protection diode (with the appropriate polarity). The back EMF is then clamped to diode-drop + zener voltage.

If you have multiple coils running from one supply line you can return all the protection diodes to a single zener.

This was often done on things like computer paper tape punches and dot matrix printheads where you had 8 or 9 coils. Keeping the release time short was important, equally you had to limit the back emf to protect the drivers.

trobbins · 23rd Oct 2016, 11:59 pm

Did you measure the relay coil resistance, or measure the continuous current through the relay? Did you check if the battery charger had just a bridge diode rectifier, or was there more circuitry such as a freewheeling diode or a filter capacitor or ? It does sound strange that the diodes would be physically degraded from your application (perhaps it had had a tough previous life, and it was the relay contacts that stopped your experiment).

emeritus · 24th Oct 2016, 1:54 pm

I guess floating a 12V battery across the battery charger ought to take care of transient voltage spikes. I am presently using an old computer uninterruptable power supply to generate a variable frequency 240V AC supply for adjusting the speed of the synchronous motor of a cine projector for copying cine film onto DVD. The power source is a 25A 12V battery charger, but I connect an old small (nominal 12Ah) 12V sealed lead-acid battery across the terminals to stabilise the voltage. The battery has less than a quarter of its original capacity, but as it simply floats across the supply, it is never called on to deliver current.

avocollector · 25th Oct 2016, 5:25 am

Quote:

Originally Posted by trobbins

Did you measure the relay coil resistance, or measure the continuous current through the relay? Did you check if the battery charger had just a bridge diode rectifier, or was there more circuitry such as a freewheeling diode or a filter capacitor or ? It does sound strange that the diodes would be physically degraded from your application (perhaps it had had a tough previous life, and it was the relay contacts that stopped your experiment).

The relay was an old UK Post Office 3000(?? not sure) 12 volt one with two sets of single make to break contacts and the diodes in the charger were in bridge form but separate. No caps or other circuit in the charger apart form a transformer. As I said worked happily without the caps but their well intentioned addition was not so great.

Many thanks to everybody for their explanations and help - I had thought the point of the caps was also to absorb any back emf etc but I do now dimly remember a diode being wired across the contacts and cap. Must give it another go sometime - I've put hefty new diodes (think they were rated at 10amps) with 1kv PIV in the charger so they at least should not melt again.

avocollector · 25th Oct 2016, 5:31 am

Quote:

Originally Posted by kalee20

A circuit would help a lot - I'm a bit puzzled right now!

Very simple - one battery charger terminal connected to one side of relay coil, other side of relay coil connected to one make to break contact, other make to break contact connected to other battery charger terminal. Idea is that voltage energises relay which pulls contacts apart which breaks circuit to relay coil. Relay contacts then connect again supply voltage to relay coil and whole thing starts again. Basically a low frequency interrupting mechanism for a second set of contacts with it's own supply.

trobbins · 25th Oct 2016, 7:47 am

If there was only a standard bridge rectifier in the charger, then the relay coil inductance would have used the bridge diodes and the transformer secondary winding as a free-wheeling current path, up to the point of the relay contact opening enough for the contact arc to extinguish.

I doubt the relay contacts are working at twice mains frequency - it would be interesting to measure that somehow.

A reverse diode directly across the coil should keep freewheeling current away from the bridge and relay contact.

21st Oct 2016, 2:17 pm	#1
avocollector Heptode Join Date: Jul 2010 Location: Wellington, New Zealand. Posts: 653	Battery charger experiment Not sure if this is the place to post this item but a few years back I wanted to generate a high voltage using a car coil. So I took an old 12 volt post office relay and wired the push to break contacts to the coil, then used a 7 amp 12 volt battery charger to drive it - I had a separate (battery) 12 volts supply wired to another set of contacts on the same relay and to the car coil. So the battery charger simply drove the relay to make and break all the time which gave me the square wave from the second set of independent contacts and battery to drive the car coil. All worked quite well but there was a heck of lot of sparking on the relay contacts. Remembering some advice from Practical electronics, I wired a 10 uF 25 volt electrolytic capacitor across both sets of contacts. This stopped the sparking but 5 mins later the relay stopped working. When I pulled the battery charger to bits, the diodes (4 separate ones) inside had all literally melted. I was surprised as the charger was supposed to be 12 volts up to 7 amps which I would have thought was well above any current drain even with the capacitor across the relay contact. Any thoughts on why this happened??

21st Oct 2016, 3:58 pm	#2
Nicklyons2 Octode Join Date: Jan 2013 Location: Barnsley, South Yorkshire, UK. Posts: 1,517	Re: Battery charger experiment I would imagine the back e.m.f. from the coils had exceeded the P.I.V. of the battery charger's diodes and, in a bridge, once one diode fails the others are soon 'goners'. If you'd replaced the diodes with say 200 V PIV versions I'd have thought they would have survived OR you could have shunted each diode with say 0.1uF. This sort of spikiness would not be expected in a simple battery charger but was why many mains equipments of the live chassis era had diode shunt capacitors to prevent nastiness on the mains + the mains voltage exceeding the diode's (often BY100's) P.I.V.

21st Oct 2016, 6:03 pm	#3
GMB Dekatron Join Date: Aug 2003 Location: near Reading (and sometimes Torquay) Posts: 3,099	Re: Battery charger experiment By putting a capacitor across the contacts you just fed the high voltage rebound from the relay into the supply. It saves the contacts but not the supply. A way to snub a relay without capacitors is to put a resistor in parallel with the coil. It only has to be somewhat larger than the coil resistance to mean that the consumption is not much affected but the high voltage pulse will see it differently. A diode and small resistor is another way to do it as the pulse is of opposite polarity to the supply. A capacitor across the supply would also help to protect the supply but take care how you wire it or a very short pulse might bypass the capacitor. When using capacitors around switches, never forget that they are a double-edged solution. If the capacitor ends up charged and the switch in parallel with it closes - now the capacitor damages the contacts. So such a capacitor should have a small series resistor to avoid that problem too. These things are never so simple!

21st Oct 2016, 7:29 pm	#4
kalee20 Dekatron Join Date: Feb 2007 Location: Lynton, N. Devon, UK. Posts: 7,087	Re: Battery charger experiment A circuit would help a lot - I'm a bit puzzled right now!

21st Oct 2016, 9:40 pm	#5
Ed_Dinning Dekatron Join Date: Jan 2004 Location: Newcastle upon Tyne, Tyne & Wear, UK. Posts: 8,195	Re: Battery charger experiment Hi Gents, usually a diode is wired in inverse parallel to the relay coil. This will give a longish drop out time but a back emf of about supply voltage +1 diode drop. The addition of a resistor in series with the diode will quicken up the drop out time but increase the back EMF. Usual trick was to make the resistor the same value as the coil resistance and back EMF is then 2* supply voltage. In your scenario it should be possible to add a blocking diode in line with the supply to stop anything getting back to the supply, but use a freewheel diode across the relay coil as well. Ed

22nd Oct 2016, 6:31 am	#6
TonyDuell Dekatron Join Date: Jun 2015 Location: Biggin Hill, London, UK. Posts: 5,215	Re: Battery charger experiment Another method is to put a zener in series with the protection diode (with the appropriate polarity). The back EMF is then clamped to diode-drop + zener voltage. If you have multiple coils running from one supply line you can return all the protection diodes to a single zener. This was often done on things like computer paper tape punches and dot matrix printheads where you had 8 or 9 coils. Keeping the release time short was important, equally you had to limit the back emf to protect the drivers.

23rd Oct 2016, 11:59 pm	#7
trobbins Heptode Join Date: Oct 2009 Location: Melbourne Australia Posts: 901	Re: Battery charger experiment Did you measure the relay coil resistance, or measure the continuous current through the relay? Did you check if the battery charger had just a bridge diode rectifier, or was there more circuitry such as a freewheeling diode or a filter capacitor or ? It does sound strange that the diodes would be physically degraded from your application (perhaps it had had a tough previous life, and it was the relay contacts that stopped your experiment).

24th Oct 2016, 1:54 pm	#8
emeritus Dekatron Join Date: Jan 2012 Location: Brentwood, Essex, UK. Posts: 5,346	Re: Battery charger experiment I guess floating a 12V battery across the battery charger ought to take care of transient voltage spikes. I am presently using an old computer uninterruptable power supply to generate a variable frequency 240V AC supply for adjusting the speed of the synchronous motor of a cine projector for copying cine film onto DVD. The power source is a 25A 12V battery charger, but I connect an old small (nominal 12Ah) 12V sealed lead-acid battery across the terminals to stabilise the voltage. The battery has less than a quarter of its original capacity, but as it simply floats across the supply, it is never called on to deliver current.

25th Oct 2016, 7:47 am	#11
trobbins Heptode Join Date: Oct 2009 Location: Melbourne Australia Posts: 901	Re: Battery charger experiment If there was only a standard bridge rectifier in the charger, then the relay coil inductance would have used the bridge diodes and the transformer secondary winding as a free-wheeling current path, up to the point of the relay contact opening enough for the contact arc to extinguish. I doubt the relay contacts are working at twice mains frequency - it would be interesting to measure that somehow. A reverse diode directly across the coil should keep freewheeling current away from the bridge and relay contact.