LT30 PSU - Crowbar Circuit? - UK Vintage Radio Repair and Restoration Discussion Forum

jamesinnewcastl · 22nd Jun 2017, 11:14 am

Hi All

I have one of the ubiquitous Farnell LT30s (Mine goes up to 12V at 10A, can't remember the full part).

I was driving my motor generator with it - very inductive I imagine - and when I switch off at the motor the PSU voltage drops to about 1V and it appears to still be outputting 10A, at first I suspected my equipment but that was OK as I disconnected completely from the PSU but the 'short circuit' stayed.

A bit of head scratching and the answer must be that a 'crowbar' circuit in the PSU is reacting to spikes as I switch off the motor. However, when I look at the multitude of manuals available there is no sign of a crowbar circuit even though the text talks about SCR101.

Further reading appears to reveal that the 'over-voltage' protection might be an add-on goodie and so might have a seperate circuit in the manual.

So, after some essentially pointless rambling, my questions are: Is there a seperate crowbar and does anyone have the circuit diagram?

Cheers
James

karesz* · 22nd Jun 2017, 12:49 pm

Hi James,
maybe your Over Voltage Protection cct is build onto the SCR?
Regards, Karl

jamesinnewcastl · 22nd Jun 2017, 2:23 pm

Hi Karl

Yes that's the text and it refers to SCR101 - which I take to be a silicon Controlled Rectifier performing the 'crowbar' - the point is that I can't find that component on the circuit diagrams I have seen, and I've looked at some 3 or 4.

It's possible that I am going senile or that they have drawn the symbol differently from the usual and I've missed it! (Have to say that the circuits are very badly drawn too).

I'll have another look!

James

karesz* · 22nd Jun 2017, 2:43 pm

Hello James,

No-no, you aren`t senil, but our circuits (I have such public versions) are all only for L(T)30/xx types, nothing i.e. for your exact L12/10C (or 10/3 too), even if in the documentations title it was wroten :-( ...
Hopfely has WME_Bill some circuit?

Regards, Karl

jamesinnewcastl · 22nd Jun 2017, 3:04 pm

Hi Karl

Yes, it looks as if the crowbar is not in the more usual version of the PSU and I need the 'C' version circuit diagram.

(Thinks 'I got an even better buy off that auction site than I had imagined')

(Thinks 'Better do something about that back EMF, shouldn't really be battering my bargain PSU more than I need....)

Cheers
James

short wave · 22nd Jun 2017, 4:23 pm

Hi James, i posted a hand drawn one i did of mine will try to find forum page..
S-W

short wave · 22nd Jun 2017, 4:27 pm

Found it at
https://www.vintage-radio.net/forum/...ad.php?t=97029
hope it helps
regards S-W

jamesinnewcastl · 22nd Jun 2017, 5:18 pm

Hi Short-Wave

Just got home and checked, mine is the same as yours an L12-10C that nails the explanation and I get a full circuit diagram - thanks!

Now I just need to play with some supression components - the crowbar is all very well but it is a pain to see the ammeter slam up and having to reset it everytime I switch off!

Cheers All
James

kalee20 · 22nd Jun 2017, 9:41 pm

That crowbar circuit which Shortwave kindly put up seems quite complicated!

It's quite possible that the act of switching 'off' at the motor us causing some switch chatter, the power supply is bouncing from heavy load to no load and back - and simply not reacting fast enough, so that when the load drops to zero the output rises, and while the stabilisation circuitry is reacting to bring it back to what it should be, the crowbar fires. A small steady load will prove this - or even a chunky big electrolytic across the output.

If you think about it, an inductive load would actually try to take things negative, not more highly positive, on switch-off. Unless, that is, there's some stray capacitance giving at least half a cycle of oscillation! But an anti parallel diode straight across the motor would fix this, if so.

jamesinnewcastl · 22nd Jun 2017, 11:51 pm

Hi Kaylee

I don't think it was due to a chattering load as the supply was coping happily at 12V and the switch off is very positive via a pair of large relay contacts.

It turns out that the monster motor is actually quite well suppressed and the culprit was the GPO relay that switches it on/off. If I manually operated the armature the motor and PSU played nicely together.

In fact an ammeter in series with the system also stops the crowbar issue - and that confused me even further at the time....

The kit is WW2 aircraft vintage and the relay has no diode across the coil or any other form of supression, as you suspected a reverse diode (across the equipment terminals to avoid altering the kit) has cured the problem!

Cheers
James

QQVO6/40 · 23rd Jun 2017, 3:13 am

Greetings to the forum.
Just a little idea I have used for a long time on the many power supplies I have here. Some of which go up to 200Amps at 24 Volts.
I put a large reverse biased diode directly across the output terminals of the power supply. Saves all sorts of back EMF problems.
I have never had any problems with this practice changing any parameters of the respective power supply.
This has saved me a lot of grief over many years as I am old school and tend to use relays for controling my "stuff" rather than electronics.
As a side note I have one power supply here that has a hockey puck SCR for crowbar over voltage protection. Short term this SCR will handle around 1200Amps.
Just my 2d worth.

22nd Jun 2017, 11:14 am	#1
jamesinnewcastl Heptode Join Date: Jul 2009 Location: Newcastle upon Tyne, Tyne & Wear, UK. Posts: 722	LT30 PSU - Crowbar Circuit? Hi All I have one of the ubiquitous Farnell LT30s (Mine goes up to 12V at 10A, can't remember the full part). I was driving my motor generator with it - very inductive I imagine - and when I switch off at the motor the PSU voltage drops to about 1V and it appears to still be outputting 10A, at first I suspected my equipment but that was OK as I disconnected completely from the PSU but the 'short circuit' stayed. A bit of head scratching and the answer must be that a 'crowbar' circuit in the PSU is reacting to spikes as I switch off the motor. However, when I look at the multitude of manuals available there is no sign of a crowbar circuit even though the text talks about SCR101. Further reading appears to reveal that the 'over-voltage' protection might be an add-on goodie and so might have a seperate circuit in the manual. So, after some essentially pointless rambling, my questions are: Is there a seperate crowbar and does anyone have the circuit diagram? Cheers James

22nd Jun 2017, 2:23 pm	#3
jamesinnewcastl Heptode Join Date: Jul 2009 Location: Newcastle upon Tyne, Tyne & Wear, UK. Posts: 722	Re: LT30 PSU - Crowbar Circuit? Hi Karl Yes that's the text and it refers to SCR101 - which I take to be a silicon Controlled Rectifier performing the 'crowbar' - the point is that I can't find that component on the circuit diagrams I have seen, and I've looked at some 3 or 4. It's possible that I am going senile or that they have drawn the symbol differently from the usual and I've missed it! (Have to say that the circuits are very badly drawn too). I'll have another look! James

22nd Jun 2017, 2:43 pm	#4
karesz* Heptode Join Date: Jun 2010 Location: Zurich, Switzerland Posts: 538	Re: LT30 PSU - Crowbar Circuit? Hello James, No-no, you aren`t senil, but our circuits (I have such public versions) are all only for L(T)30/xx types, nothing i.e. for your exact L12/10C (or 10/3 too), even if in the documentations title it was wroten :-( ... Hopfely has WME_Bill some circuit? Regards, Karl

22nd Jun 2017, 3:04 pm	#5
jamesinnewcastl Heptode Join Date: Jul 2009 Location: Newcastle upon Tyne, Tyne & Wear, UK. Posts: 722	Re: LT30 PSU - Crowbar Circuit? Hi Karl Yes, it looks as if the crowbar is not in the more usual version of the PSU and I need the 'C' version circuit diagram. (Thinks 'I got an even better buy off that auction site than I had imagined') (Thinks 'Better do something about that back EMF, shouldn't really be battering my bargain PSU more than I need....) Cheers James

22nd Jun 2017, 4:23 pm	#6
short wave Pentode Join Date: Mar 2013 Location: Leeds, West Yorkshire, UK. Posts: 246	Re: LT30 PSU - Crowbar Circuit? Hi James, i posted a hand drawn one i did of mine will try to find forum page.. S-W

22nd Jun 2017, 4:27 pm	#7
short wave Pentode Join Date: Mar 2013 Location: Leeds, West Yorkshire, UK. Posts: 246	Re: LT30 PSU - Crowbar Circuit? Found it at https://www.vintage-radio.net/forum/...ad.php?t=97029 hope it helps regards S-W

22nd Jun 2017, 5:18 pm	#8
jamesinnewcastl Heptode Join Date: Jul 2009 Location: Newcastle upon Tyne, Tyne & Wear, UK. Posts: 722	Re: LT30 PSU - Crowbar Circuit? Hi Short-Wave Just got home and checked, mine is the same as yours an L12-10C that nails the explanation and I get a full circuit diagram - thanks! Now I just need to play with some supression components - the crowbar is all very well but it is a pain to see the ammeter slam up and having to reset it everytime I switch off! Cheers All James

22nd Jun 2017, 9:41 pm	#9
kalee20 Dekatron Join Date: Feb 2007 Location: Lynton, N. Devon, UK. Posts: 7,081	Re: LT30 PSU - Crowbar Circuit? That crowbar circuit which Shortwave kindly put up seems quite complicated! It's quite possible that the act of switching 'off' at the motor us causing some switch chatter, the power supply is bouncing from heavy load to no load and back - and simply not reacting fast enough, so that when the load drops to zero the output rises, and while the stabilisation circuitry is reacting to bring it back to what it should be, the crowbar fires. A small steady load will prove this - or even a chunky big electrolytic across the output. If you think about it, an inductive load would actually try to take things negative, not more highly positive, on switch-off. Unless, that is, there's some stray capacitance giving at least half a cycle of oscillation! But an anti parallel diode straight across the motor would fix this, if so.

22nd Jun 2017, 11:51 pm	#10
jamesinnewcastl Heptode Join Date: Jul 2009 Location: Newcastle upon Tyne, Tyne & Wear, UK. Posts: 722	Re: LT30 PSU - Crowbar Circuit? Hi Kaylee I don't think it was due to a chattering load as the supply was coping happily at 12V and the switch off is very positive via a pair of large relay contacts. It turns out that the monster motor is actually quite well suppressed and the culprit was the GPO relay that switches it on/off. If I manually operated the armature the motor and PSU played nicely together. In fact an ammeter in series with the system also stops the crowbar issue - and that confused me even further at the time.... The kit is WW2 aircraft vintage and the relay has no diode across the coil or any other form of supression, as you suspected a reverse diode (across the equipment terminals to avoid altering the kit) has cured the problem! Cheers James

23rd Jun 2017, 3:13 am	#11
QQVO6/40 Hexode Join Date: May 2009 Location: Newcastle, Australia Posts: 319	Re: LT30 PSU - Crowbar Circuit? Greetings to the forum. Just a little idea I have used for a long time on the many power supplies I have here. Some of which go up to 200Amps at 24 Volts. I put a large reverse biased diode directly across the output terminals of the power supply. Saves all sorts of back EMF problems. I have never had any problems with this practice changing any parameters of the respective power supply. This has saved me a lot of grief over many years as I am old school and tend to use relays for controling my "stuff" rather than electronics. As a side note I have one power supply here that has a hockey puck SCR for crowbar over voltage protection. Short term this SCR will handle around 1200Amps. Just my 2d worth.