Voltage Regulator Question

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Voltage Regulator Question

Postposted on Wed Jan 15, 2014 10:55 am

Hi Folks,

A quick question about a voltage regulator chip -- If the regulator is designed to produce a consistent 9 volts and it is supplied about 6 volts, does any voltage get past the chip? I would like to activate a relay only if the supply voltage goes above 9 volts, but not have it activate if it is getting only about 5 or 6 volts. So I was figuring on just using a simple circuit with the chip series with the relay's coil. Do I understand this correctly? The drop out voltage is listed on the data sheets of these chips, but I don't know how to apply that figure, if I need to consider it at all.
Thanks!!!!!

Take care, Joe.
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Re: Voltage Regulator Question

Postposted on Wed Jan 15, 2014 11:19 am

iirc from EE classes and doing some basic labs on the 7805, it did output a low output voltage at low input voltages. ie: give it 2V and it would be outputting about 3V at low current instead of 5V. It's a very quick dropoff after a certain point.

I think you'll need some kind of switch like a BJT or FET that activates at a certain voltage to build exactly what you want. Do you have the datasheet handy?
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Re: Voltage Regulator Question

Postposted on Wed Jan 15, 2014 11:36 am

Hi Duct Tape Dude,

Thanks for the reply! I was looking at this one: http://www.jameco.com/Jameco/Products/ProdDS/1958641.pdf to see if I understood the drop out voltage issue, but if lower voltages get past the chip, then it won't work for me. I will look into a BJT or FET. I have used basic transistors, but a BJT or FET would be new for me. Just kicking around an idea for a personal toy train electronics project; no immediate need yet.

Take care, Joe.
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Re: Voltage Regulator Question

Postposted on Wed Jan 15, 2014 12:10 pm

In general, linear regulators are really not intended to be used that way. The "drop out" voltage refers to the voltage below which the regulator ceases to operate properly; it is not intended to indicate the voltage at which it shuts down completely.

What are the "on" and "off" voltages for the coil of the relay you're using, and how high can the input voltage go? If the input voltage doesn't go too far above 9V you may be able to do this with just a zener diode in series with the relay coil. Otherwise you're going to need a more complex circuit to drive the relay.

Keep in mind that if you are using a solid state device that is not designed to drive relays directly, you will also need a flyback diode connected across the relay coil. Otherwise the voltage transients generated when the relay switches will fry the device that is driving it. Even low-voltage relay coils can generate brief spikes in the hundreds of volts when current to the coil is shut off suddenly, due to the fact that the relay coil is a highly inductive load.

@DTD - I think you have the voltages reversed in your post. Since this is a linear regulator it cannot produce an output voltage greater than its input.
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Re: Voltage Regulator Question

Postposted on Wed Jan 15, 2014 12:10 pm

A zener diode would be a simpler device to use in this situation. Get a 9V calibrated one and stick it reversed in series with your relay. The zener is calibrated in such a way that once the voltage across it passes above 9V, it undergoes zener breakdown and becomes conducting - and turns on the relay.
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Re: Voltage Regulator Question

Postposted on Wed Jan 15, 2014 12:17 pm

LASR wrote:A zener diode would be a simpler device to use in this situation. Get a 9V calibrated one and stick it reversed in series with your relay. The zener is calibrated in such a way that once the voltage across it passes above 9V, it undergoes zener breakdown and becomes conducting - and turns on the relay.

Actually, the zener voltage needs to be 9V minus the "on" voltage of the relay coil. When a zener breaks down the voltage across it does not go to zero; it stays at the zener breakdown voltage as the input voltage increases further.

Another consideration with this approach is that relays typically have a lot of hysteresis. Voltage will probably have to drop a fair bit below 9V before the relay disconnects. E.g.: If the "on" voltage of the relay coil is 5V and the "off" voltage of the relay is 2V, you would need a 4V zener to get the relay to switch on at the correct voltage; but once activated the voltage will need to drop to 6V before the relay shuts off.

Unit-to-unit variation between the relays will also result in slightly different trip points, if multiple units are being built. Using a more sophisticated circuit to drive the relay (e.g. voltage comparator) would eliminate this.
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Re: Voltage Regulator Question

Postposted on Wed Jan 15, 2014 12:31 pm

Hi Just Brew It and LASR,

Thanks for the replies!

I experimented with a zener diode and a relay a while back, but I gather I didn't have the correct combination or didn't have it oriented correctly. (I do understand regular diodes and use them for polarity techniques and the "snubber" effect.) The relay was 12 volts but would activate at as low as 6-7 volts. I wouldn't be able to deliver much more than the 12 volts anyway. Unless I made an error, I still seemed to get some lower voltage past the zener. Is there a particular type of zener to look for? If I remember correctly, the zener seemed to conduct in both directions.

Thanks again, take care, Joe.
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Re: Voltage Regulator Question

Postposted on Wed Jan 15, 2014 12:38 pm

OP: if you can describe the actual application, there may be another way to get you there.

Usually, the most reliable way to achieve a simple threshold switch is by connecting an opamp as a comparator. Resistor dividers or Zeners can be used to set the reference voltage, and a FET can be wired to the output as as simple switch. The IRF510 is one of my favorite FETs for this use because it is widely avaialable, cheap, rugged, and has a gate turn-on voltage somewhere in the range of 3-5V, hence is ideal as a DC "relay." It can readily switch the coil of a power relay.
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Re: Voltage Regulator Question

Postposted on Wed Jan 15, 2014 12:39 pm

just brew it! wrote:
LASR wrote:A zener diode would be a simpler device to use in this situation. Get a 9V calibrated one and stick it reversed in series with your relay. The zener is calibrated in such a way that once the voltage across it passes above 9V, it undergoes zener breakdown and becomes conducting - and turns on the relay.

Actually, the zener voltage needs to be 9V minus the "on" voltage of the relay coil. When a zener breaks down the voltage across it does not go to zero; it stays at the zener breakdown voltage as the input voltage increases further.

I was going to add this, but you beat me to it. You'll still draw current through the diode (and through the relay), but you have to remember that you "lose" the voltage across the zener. Otherwise you won't have enough voltage on the other side for your intended application.

Another consideration with this approach is that relays typically have a lot of hysteresis. Voltage will probably have to drop a fair bit below 9V before the relay disconnects. E.g.: If the "on" voltage of the relay coil is 5V and the "off" voltage of the relay is 2V, you would need a 4V zener to get the relay to switch on at the correct voltage; but once activated the voltage will need to drop to 6V before the relay shuts off.

Unit-to-unit variation between the relays will also result in slightly different trip points, if multiple units are being built. Using a more sophisticated circuit to drive the relay (e.g. voltage comparator) would eliminate this.

And another good point. However, I think that you can get around the shut off voltage with a little help from a pull-down resistor. If you put one in parallel with the relay coil, then it should ground the relay when the zener diode shuts off. You would need something with a lot of resistance compared to the relay so that you don't draw current when the relay is turned on.
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Re: Voltage Regulator Question

Postposted on Wed Jan 15, 2014 12:43 pm

Zeners are normally used "backwards" (i.e. cathode to the more positive side of the circuit). They start to conduct when the voltage reaches the zener voltage, and conduct as much current as necessary to keep the voltage across themselves at the zener voltage. If you want to activate at 9V, but the relay activates at 6V, you need a 3V zener. Four regular rectifier diodes wired in series (and installed "forwards" instead of backwards like a zener) will also give you close to the desired voltage since silicon diodes have approximately 0.7V drop when forward biased.

Resistor across the coil will reduce the switching spikes, but not eliminate them. You really want a reverse-connected diode there to take the brunt of the surge.
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Re: Voltage Regulator Question

Postposted on Wed Jan 15, 2014 1:02 pm

Zeners have a much more reliable threshold voltage vs. current than forward-conducting diodes. Depending on current through a standard switching diode, forward voltage can change from as little as 0.5V to 1.0V. Several of these in series will produce an even larger swing (9x0.5 = 4.5V, 9x1.0 = 9V, not very accurate!). A zener will vary maybe be +/-0.1V over the rated current range. (8.9V to 9.1V)

The best way to realize a threshold activated circuit is to use a comparitor with a built in open-drain driver that can handle the relay current, or to drive an external transistor. One leg of the comparitor is tied to a reference voltage like a zener with a low current to turn it "on", the other input to your sense node. The problem with using OpAmps is their switching speed is slow and they don't handle large differential voltages on the inputs well like comparitors do. (they "Stick" in states until an unpredictable differential is large enough)

If you need a little help selecting parts let us know.

To answer your original question, it depends on the linear regulator choice. Absolute but-loads of variety out there.

The 780x series will essentially act like a series resistor until your input voltage surpasses the output set voltage by at least the input voltage + dropout voltage. Otherwise it does nothing for you except burn power.
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Re: Voltage Regulator Question

Postposted on Wed Jan 15, 2014 1:17 pm

Hi Everyone,

Again, very much appreciate all the help and the ability to kick these ideas around!!!!!!!!!!!
Here is what I tested back in April 2013:Image

I wanted a compact, cheap, simple, homemade alternative to a manufactured reversing board. I couldn't get it to work and gave up, perhaps too soon. In this situation, track is supplied with AC, otherwise just a matter of reversing polarity to the rails if DC is used. For backward compatibility with older toy trains, AC would be on the rails. Cannot remember what components I actually used. I drew the diagram before I experimented.

I must have done something wrong, but as I said, gave up too soon.
Thanks!!!!!

P.S. Just noticed an error in my drawing with the connections to the motor -- will post corrections shortly!
Corrected diagram above. And when I tested the idea, I didn't have a motor connected, just listened for the relay to "click." Perhaps some issues with the feedback from the motor I hadn't thought about.
Take care, Joe.
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Re: Voltage Regulator Question

Postposted on Wed Jan 15, 2014 1:49 pm

You wont get the current you need through the coil of the relay to close it until the voltage of the track gets pretty high. Then you will likely burn up the zener if it is not rated to deal with the power it will dissipate.

Example: 10V zener drop + 100mA coil load to activate = 1W of dissipation (I * E). If the relay requires 12V to activate the coil, your circuit will need 22V (maybe 20V) to get the relay to close (22V(supply) - 10V(zener) = 12V(coil))
So over 10V current will begin to flow through the zener, but not until you get to 22V will there be enough current to activate it.
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Re: Voltage Regulator Question

Postposted on Wed Jan 15, 2014 2:14 pm

One other thing: I am not sure what you are trying to achieve:

1. Ramp up voltage to ~10V, train increases in forward speed.
2. Surpass 10V and moderately fast train reverses direction at the same 10V speed?
3. Increase past 10V and train goes even faster in reverse?

Or is the AC on top of the DC on the train tracks? If so, you will need a capacitor in-line with one of the lines connected to the track to block the DC component. The relay switch nodes then need to be tied directly to the track, not the bridge output. In your case to drive a relay, a pretty large non-polarized capacitor unless the AC frequency is high. In other words, no, your circuit will not work without a little more circuitry.
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Re: Voltage Regulator Question

Postposted on Wed Jan 15, 2014 2:22 pm

As noted previously, the zener voltage needs to be (desired trip voltage) - (voltage needed to activate the relay). A zener voltage of 10V will not activate the relay until the voltage hits 16V or so.
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Re: Voltage Regulator Question

Postposted on Wed Jan 15, 2014 2:24 pm

Hi Liquidsquid,

Thanks for the info!!!!

In theory, what I would be doing is running the loco at about 6-7 volts and it would be going forward. To reverse it, I would cut power to the rails, turn the supply to 10 volts Ac or whatever, then apply power to the rails. Assuming the relay would kick in no later than the motor would turn, then the train would run in reverse. To handle extra speed, a string of diodes would be used to reduce voltage to the desired speed in reverse. So on a motor lead, one diode forward and about 7 in reverse. I figured that if the idea worked, I could fine-tune the values of the components and adjust speed with diodes. But once you have to make room for the relay, several diodes, even a cap to cut relay chatter, already it is too big. Guess better minds than mine have tried the idea.

Take care, joe.
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Re: Voltage Regulator Question

Postposted on Wed Jan 15, 2014 2:27 pm

just brew it! wrote:As noted previously, the zener voltage needs to be (desired trip voltage) - (voltage needed to activate the relay). A zener voltage of 10V will not activate the relay until the voltage hits 16V or so.


Hi Just Brew It,

So is it just a matter of using a zener at a lower rating?

Take care, Joe.
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Re: Voltage Regulator Question

Postposted on Wed Jan 15, 2014 2:30 pm

If you are able to easily cut the DC and apply AC to the rails to try and trigger this thing, seems to me it would be pretty easy to just reverse the polarity of the DC to the rails instead? Problem solved!
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Re: Voltage Regulator Question

Postposted on Wed Jan 15, 2014 2:33 pm

jprampolla wrote:
just brew it! wrote:As noted previously, the zener voltage needs to be (desired trip voltage) - (voltage needed to activate the relay). A zener voltage of 10V will not activate the relay until the voltage hits 16V or so.


Hi Just Brew It,

So is it just a matter of using a zener at a lower rating?

I think so. But it depends on the characteristics of the relay. The point at which it drops into forward again is going to be somewhat unpredictable since it will depend on the
"off" voltage of the relay (which will be lower than the "on" voltage).

Make sure the zener can handle the power dissipation of the current flowing through it as well. (Zener voltage) x (relay coil current) must be less than or equal to the wattage of the zener.
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Re: Voltage Regulator Question

Postposted on Wed Jan 15, 2014 2:34 pm

just brew it! wrote:If you are able to easily cut the DC and apply AC to the rails to try and trigger this thing, seems to me it would be pretty easy to just reverse the polarity of the DC to the rails instead? Problem solved!


But for backward compatibility, AC on the rails is the standard. Presently that is how I reverse the things that don't have a reversing board inside, I just run them on DC and reverse polarity to change direction.

Take care, Joe.
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Re: Voltage Regulator Question

Postposted on Wed Jan 15, 2014 2:35 pm

jprampolla wrote:But for backward compatibility, AC on the rails is the standard. Presently that is how I reverse the things that don't have a reversing board inside, I just run them on DC and reverse polarity to change direction.

Ahh, OK. Understood.
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Re: Voltage Regulator Question

Postposted on Wed Jan 15, 2014 2:46 pm

Hi Folks,

Thanks, everyone, for the help!!!!!!!!!!!

Next time I order stuff, I will buy some zeners of different ratings and fool around a little with them. I have a selection somewhere, but don't recall the values. The relays are miniature or even sub miniature PC mount types. I buy most of my components from All Electronics. I fly by the seat of my pants with these things. If something burns up, then I just up-size it. I was thinking that if I could find a relay that wouldn't activate until it was supplied with about 10-12 volts, I wouldn't need anything other than a small cap on the relay if it bounced. Or use something like a 3 volt relay with some resistors in series with the coil.

Take care, Joe.
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