power supply voltage - UK Vintage Radio Repair and Restoration Discussion Forum

Mike Brett · 4th May 2008, 6:52 pm

Hello all
I have constructed a constant voltage supply for my radio using a MIC29302BT voltage regulator to supply 12 volts exactly. There is only 5 parts in the circuit so I thought it would be simple. I adjusted the 10k preset and it gave out 12volts, brilliant I thought. But when I connected it to the radio circuits the voltage fell to about 8 volts.So I adjusted it again to 12 volts. Is this normal as I thought the idea of a constant voltage chip was to keep the voltage at what it is set at regardless of load, or have I got the wrong end of the stick.

Mike

RobinBirch · 4th May 2008, 7:22 pm

Hi Mike,
Looking at the data sheet I would have thought that it should be ok. The only thing that I can think of is if the quiescent current through your divider network is too small and maybe smaller resistors are what is required.

Hope this helps and if you find out what the problem is then let me know just for my own education.

Cheers

Robin

ppppenguin · 4th May 2008, 7:30 pm

Not sure about this particular regulator but many regualtors require a minimum load current to ensure proper operation. This is usually just a few mA. Operation without a load may well give excess output.

If the regulator is oscillating due to inadequate input or output capacitors then the output voltage can be very strange indeed. 50MHz is quite feasible for many regulators so you may not see it on your scope.

evingar · 4th May 2008, 7:46 pm

Quote:

50MHz is quite feasible for many regulators so you may not see it on your scope.

....Although the mere application of a 'scope probe may stop it oscillating, especially if it's a low impedance X1 sort

ronbryan · 4th May 2008, 8:54 pm

Mike

Have you checked the on-load input voltage to the regulator? Most regulators require an input to output differential voltage of at least a couple of volts, excluding ripple.

Ron

XTC · 4th May 2008, 9:21 pm

I've no experience of this voltage regulator, but what others have said regarding possible spurious oscillations, drop out voltage and a mA or two minimum O/P current is worth thinking about.

If you want a fixed 12V output, I'd have thought you've be better off using a 7812 in one of its variants, rather than using a variable regulator and adjusting it to 12V. The fixed regulators are dirt cheap and very accurate.

Pete.

KeithsTV · 4th May 2008, 10:15 pm

Checking the datasheet this regulator requires a minimum load of 7mA. With a reference voltage, the voltage from the output pin to the ADJ pin, of 1.24v and setting a minimum load current of around 10mA a suitable value of resistor from the output pin to ADJ pin would be 120 ohms. To make a 12v regulator connect a 1.3k resistor from ADJ pin to ground. To make it adjustable use a 1k in series with a 500ohm preset.

These regulators require a capacitor on the output to maintain stability, a 10uF electrolytic should be OK. If this is too small the regulator will oscillate.

The datasheet for the MIC29302BT shows it is a 3A low dropout device with typically a dropout voltage of 250mv at 3A. The input voltage should not go below12.25v which includes the ripple voltage on the input. I assume the radio does not consume more than 3A, If it does this would cause the regulator to current limit and drop the output voltage.

Using a low dropout regulator can reduce the disipation in the regulator. With a 1A load this regulator would dissipate 250mW at the minimum dropout voltage compared to approx 3W with a 78 series regulator which have a dropout voltage of around 3v.

Hope that helps

Keith

Ed_Dinning · 5th May 2008, 5:03 pm

Hi Mike, measure the input voltage ans see what it drops to on load. There are low dropout regs but you typically need 15V on load if you want 12v out on load. These things are also notorious for oscillating. Add a 0.1uF cap from both input and output pins to the ground pin as close as possible to the device and with short leads.

Ed

Mike Brett · 5th May 2008, 9:09 pm

Hello all
Thanks for all your great help. Good news is it now works ok. What was wrong with it, thats a tricky one. I took the unit out of the radio and recleaned all of the tracks. I then increased the voltage to 20 volts for a short while to see if that made a difference, this seemed to make the output more stable. Although I did not actually change any of the components on replacing the unit back in the radio and supplying 13.8 volts it worked perfectly. Now adding circuits the output stays rock steady on 12 volts. Its got me beat, but while its ok I will leave it alone and see how it goes. Thanks a lot anyway.
Regards Mike

ppppenguin · 5th May 2008, 9:29 pm

Glad to hear it's working now. Sounds like a dry joint might have been the original problem.

4th May 2008, 6:52 pm	#1
Mike Brett Retired Dormant Member Join Date: Dec 2005 Location: Wisbech, Cambridgeshire, UK. Posts: 452	power supply voltage Hello all I have constructed a constant voltage supply for my radio using a MIC29302BT voltage regulator to supply 12 volts exactly. There is only 5 parts in the circuit so I thought it would be simple. I adjusted the 10k preset and it gave out 12volts, brilliant I thought. But when I connected it to the radio circuits the voltage fell to about 8 volts.So I adjusted it again to 12 volts. Is this normal as I thought the idea of a constant voltage chip was to keep the voltage at what it is set at regardless of load, or have I got the wrong end of the stick. Mike

4th May 2008, 7:22 pm	#2
RobinBirch Hexode Join Date: Apr 2003 Location: Cotswolds, UK. Posts: 465	Re: power supply voltage Hi Mike, Looking at the data sheet I would have thought that it should be ok. The only thing that I can think of is if the quiescent current through your divider network is too small and maybe smaller resistors are what is required. Hope this helps and if you find out what the problem is then let me know just for my own education. Cheers Robin

4th May 2008, 7:30 pm	#3
ppppenguin Retired Dormant Member Join Date: Dec 2003 Location: North London, UK. Posts: 6,168	Re: power supply voltage Not sure about this particular regulator but many regualtors require a minimum load current to ensure proper operation. This is usually just a few mA. Operation without a load may well give excess output. If the regulator is oscillating due to inadequate input or output capacitors then the output voltage can be very strange indeed. 50MHz is quite feasible for many regulators so you may not see it on your scope.

4th May 2008, 8:54 pm	#5
ronbryan Octode Join Date: Jan 2003 Location: Guildford, Surrey, UK. Posts: 1,960	Re: power supply voltage Mike Have you checked the on-load input voltage to the regulator? Most regulators require an input to output differential voltage of at least a couple of volts, excluding ripple. Ron

4th May 2008, 9:21 pm	#6
XTC Retired Dormant Member Join Date: Oct 2004 Location: Bristol, UK. Posts: 837	Re: power supply voltage I've no experience of this voltage regulator, but what others have said regarding possible spurious oscillations, drop out voltage and a mA or two minimum O/P current is worth thinking about. If you want a fixed 12V output, I'd have thought you've be better off using a 7812 in one of its variants, rather than using a variable regulator and adjusting it to 12V. The fixed regulators are dirt cheap and very accurate. Pete.

4th May 2008, 10:15 pm	#7
KeithsTV Nonode Join Date: Aug 2007 Location: South Bradford, West Yorkshire, UK. Posts: 2,574	Re: power supply voltage Checking the datasheet this regulator requires a minimum load of 7mA. With a reference voltage, the voltage from the output pin to the ADJ pin, of 1.24v and setting a minimum load current of around 10mA a suitable value of resistor from the output pin to ADJ pin would be 120 ohms. To make a 12v regulator connect a 1.3k resistor from ADJ pin to ground. To make it adjustable use a 1k in series with a 500ohm preset. These regulators require a capacitor on the output to maintain stability, a 10uF electrolytic should be OK. If this is too small the regulator will oscillate. The datasheet for the MIC29302BT shows it is a 3A low dropout device with typically a dropout voltage of 250mv at 3A. The input voltage should not go below12.25v which includes the ripple voltage on the input. I assume the radio does not consume more than 3A, If it does this would cause the regulator to current limit and drop the output voltage. Using a low dropout regulator can reduce the disipation in the regulator. With a 1A load this regulator would dissipate 250mW at the minimum dropout voltage compared to approx 3W with a 78 series regulator which have a dropout voltage of around 3v. Hope that helps Keith

5th May 2008, 5:03 pm	#8
Ed_Dinning Dekatron Join Date: Jan 2004 Location: Newcastle upon Tyne, Tyne & Wear, UK. Posts: 8,195	Re: power supply voltage Hi Mike, measure the input voltage ans see what it drops to on load. There are low dropout regs but you typically need 15V on load if you want 12v out on load. These things are also notorious for oscillating. Add a 0.1uF cap from both input and output pins to the ground pin as close as possible to the device and with short leads. Ed

5th May 2008, 9:09 pm	#9
Mike Brett Retired Dormant Member Join Date: Dec 2005 Location: Wisbech, Cambridgeshire, UK. Posts: 452	Re: power supply voltage Hello all Thanks for all your great help. Good news is it now works ok. What was wrong with it, thats a tricky one. I took the unit out of the radio and recleaned all of the tracks. I then increased the voltage to 20 volts for a short while to see if that made a difference, this seemed to make the output more stable. Although I did not actually change any of the components on replacing the unit back in the radio and supplying 13.8 volts it worked perfectly. Now adding circuits the output stays rock steady on 12 volts. Its got me beat, but while its ok I will leave it alone and see how it goes. Thanks a lot anyway. Regards Mike

5th May 2008, 9:29 pm	#10
ppppenguin Retired Dormant Member Join Date: Dec 2003 Location: North London, UK. Posts: 6,168	Re: power supply voltage Glad to hear it's working now. Sounds like a dry joint might have been the original problem.