Regulating a switching power supply?

Huff360 · Jul 07, 2003, 10:55 PM

I work in the controls and sensors field and the majority of my work is done on 24V systems. I have access to 24V power supplies, but no 12V without buying a new one. I have a couple questions about converting one of these power supplies to use for my charger.

(Like I say, I work with this stuff, not on it. I know about how to use the things, not how they work!!)

1. How do I know if a supply is switching or regulated? (I have access to an o-scope if this would help.) Which is better for this application?

2. If the supply is switching would it cause problems to use a regulator on the output to drop it to 12V?

These power supplies are VERY clean. Data indicates <.005mV ripple on the outputs.

Carey

jmbig · Jul 07, 2003, 11:00 PM

Try hooking one of nationals LDO's to dropp it to??
CU
http://www.national.com/catalog/AnalogRegulators.html

flatfour · Jul 08, 2003, 07:50 PM

I think it would be much easier to do a PC power supply conversion. You can't tell a switcher or linear by the output with a scope.

What kind of amps do your chargers require?

Huff360 · Jul 08, 2003, 09:11 PM

I'm using an Orbit, but not to fill capacity. Probably never more than 1.5 or 2 amps. It's just that I have a couple of these 24V units and I don't have any spare PC supplies!

I am looking for one. I just thought I would bounce this question off you guys to see what you thought.

flatfour · Jul 08, 2003, 11:06 PM

I was unable to find any pictures of switcher guts on the net.

Switchers have a relatively tiny transformer. Toward the output side there will be chokes (coils) and lots of capacitors.

A linear supply will have a big heavy transformer and very large capacitors (beer can size) to filter the output.

mtucker · Jul 09, 2003, 01:20 AM

Hello,
Less than 5mV probably shows that it is a linear. 5mV is hard to do with a switcher, but not impossible. Either way, it doesn't matter if you want to drop the voltage down to 12V. The most efficient way would to use a switching power supply to do so, but since using your power supplies is to save money....... you might just want to slap a linear regulator on the 24VDC output. Most simple IC regulators have three pins (input, output, and ground). You will find 7812 regulators just about anywhere (including the shack). The standard size is only good for up to 1.5A with a very large heatsink. If you drop 12V (24 V to 12V) and have 1.5A flowing, you will be dumping 18W of power and need a good sized heatsink. There are larger size voltage regulators available. Also, depending on your electronic knowledge and handiness with a soldering iron, you could make one using a large transistor, zener diode and a few resistors. If interested, let me know.
Matt

Huff360 · Jul 09, 2003, 12:21 PM

Based on what flatflour says it is most likely a switcher. I was curious as to the chokes, I haven't seen them in any of the supplies I have opened. This must be the first switcher I've looked in.

Matt- I've built a few circuits before and I feel comfortable working with that type stuff. I am familiar with what you are talking about doing. My main concern was about trying to regulate a switching supply. I'm not too familiar with switchers and wasn't sure if this would cause trouble or not.

I'll most likely pick up a transistor and start assembling my own regulator.

mtucker · Jul 09, 2003, 08:27 PM

Hello,
Adding a linear regulator to a switcher will not cause you any problems. One more thing to keep in mind. If it is a switcher, you can adjust the output voltage by changing one resistor (in the feedback network). There are a couple things that could stop you from doing that, but most likely it would work. The big advantage would be the efficiency. You wouldn't need a big heatsink or transistors added to the output (wasting power). Switchers adjust their pulswidth to give the correct output voltage so you wouldn't be losing any more power in the supply. If you don't have a schematic to follow, then your best bet is as you have planned.
Good luck,
Matt

Jul 07, 2003, 10:55 PM	#1
Huff360 Registered User Join Date: Jun 2003 Location: Auburn, Alabama & Augusta, Georgia Posts: 52	Regulating a switching power supply? I work in the controls and sensors field and the majority of my work is done on 24V systems. I have access to 24V power supplies, but no 12V without buying a new one. I have a couple questions about converting one of these power supplies to use for my charger. (Like I say, I work with this stuff, not on it. I know about how to use the things, not how they work!!) 1. How do I know if a supply is switching or regulated? (I have access to an o-scope if this would help.) Which is better for this application? 2. If the supply is switching would it cause problems to use a regulator on the output to drop it to 12V? These power supplies are VERY clean. Data indicates <.005mV ripple on the outputs. Carey

Jul 08, 2003, 07:50 PM	#3
flatfour Right Honorable Member Join Date: Jul 2003 Location: Chicago Midway, Illinois, United States Posts: 516	I think it would be much easier to do a PC power supply conversion. You can't tell a switcher or linear by the output with a scope. What kind of amps do your chargers require? Last edited by flatfour; Jul 08, 2003 at 08:09 PM.

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Jul 07, 2003, 11:00 PM	#2
jmbig Registered User Join Date: Dec 2002 Location: North Highland,ca Posts: 551	Try hooking one of nationals LDO's to dropp it to?? CU http://www.national.com/catalog/AnalogRegulators.html

Jul 08, 2003, 09:11 PM	#4
Huff360 Registered User Join Date: Jun 2003 Location: Auburn, Alabama & Augusta, Georgia Posts: 52	I'm using an Orbit, but not to fill capacity. Probably never more than 1.5 or 2 amps. It's just that I have a couple of these 24V units and I don't have any spare PC supplies! I am looking for one. I just thought I would bounce this question off you guys to see what you thought.

Jul 08, 2003, 11:06 PM	#5
flatfour Right Honorable Member Join Date: Jul 2003 Location: Chicago Midway, Illinois, United States Posts: 516	I was unable to find any pictures of switcher guts on the net. Switchers have a relatively tiny transformer. Toward the output side there will be chokes (coils) and lots of capacitors. A linear supply will have a big heavy transformer and very large capacitors (beer can size) to filter the output.

Jul 09, 2003, 01:20 AM	#6
mtucker Registered User Join Date: Jul 2003 Location: Escondido CA. Posts: 192	Hello, Less than 5mV probably shows that it is a linear. 5mV is hard to do with a switcher, but not impossible. Either way, it doesn't matter if you want to drop the voltage down to 12V. The most efficient way would to use a switching power supply to do so, but since using your power supplies is to save money....... you might just want to slap a linear regulator on the 24VDC output. Most simple IC regulators have three pins (input, output, and ground). You will find 7812 regulators just about anywhere (including the shack). The standard size is only good for up to 1.5A with a very large heatsink. If you drop 12V (24 V to 12V) and have 1.5A flowing, you will be dumping 18W of power and need a good sized heatsink. There are larger size voltage regulators available. Also, depending on your electronic knowledge and handiness with a soldering iron, you could make one using a large transistor, zener diode and a few resistors. If interested, let me know. Matt

Jul 09, 2003, 12:21 PM	#7
Huff360 Registered User Join Date: Jun 2003 Location: Auburn, Alabama & Augusta, Georgia Posts: 52	Based on what flatflour says it is most likely a switcher. I was curious as to the chokes, I haven't seen them in any of the supplies I have opened. This must be the first switcher I've looked in. Matt- I've built a few circuits before and I feel comfortable working with that type stuff. I am familiar with what you are talking about doing. My main concern was about trying to regulate a switching supply. I'm not too familiar with switchers and wasn't sure if this would cause trouble or not. I'll most likely pick up a transistor and start assembling my own regulator.

Jul 09, 2003, 08:27 PM	#8
mtucker Registered User Join Date: Jul 2003 Location: Escondido CA. Posts: 192	Hello, Adding a linear regulator to a switcher will not cause you any problems. One more thing to keep in mind. If it is a switcher, you can adjust the output voltage by changing one resistor (in the feedback network). There are a couple things that could stop you from doing that, but most likely it would work. The big advantage would be the efficiency. You wouldn't need a big heatsink or transistors added to the output (wasting power). Switchers adjust their pulswidth to give the correct output voltage so you wouldn't be losing any more power in the supply. If you don't have a schematic to follow, then your best bet is as you have planned. Good luck, Matt