# Limiting variable DC power source

#### Blueboy7

##### Newbie level 5
Hello everyone,
Total noob here.
I need some help with a 24 VDC relay that I’m trying not to burn up again.
The power source is variable from 0 to 65 V depending on the time of day and how many clouds are in the sky. You guessed it, solar panels. What I’m trying to do is control a 110 VAC battery charger. The relay will be wired NC so the charger will be on when there’s no sun or too many clouds for the panels to do their job. And when the sun comes out it’ll energize the relay, opening the circuit to let the charge controller charge and maintain the batteries instead of the AC charger. I hope that makes sense. I know the AC charger contradicts the idea of having solar panels but here in Pennsylvania a clear sunny day is hard to come by.

I’m sure there’s some resistors (and other gadgets) in there somewhere, just not sure how to arrange the circuit to do what I’m trying to do.

Your help is very much appreciated!
Blue

#### KlausST

##### Super Moderator
Staff member
Hi,

there are a lot of different solutions depending on what your target is:
* from rather simple
* to most power saving
* or any other optimisation

A simple solution could be
* a series resistor, and a zener across the relay coil. (shunt regulation style)
While the zener limits the coil voltage ... the "overvoltage" has to drop at the series resistor.
It consumes a lot of power and dissipates it into heat. And there no clear threshod levels for ON and OFF.

* another solution could be by adding a BJT and a diode (for protection) --> series voltage regulation.
It wastes less power, mainly on the BJT

* one could use a "switching" solution with a comparator monitoring and PWM-ing the coil current.
This is the most power saving method. It consumes almost only the relay current.

***
But as already mentioned, I guess this applicationcalls for clean threshold levels while avoiding chattering.
This could be done with a second comparator (in same package). So the circuit consumes very low power at too low input voltage, relay is OFF. Then at a ceratin point it swtiches ON the relay. Power saving.

******
Some important informations are missing:
* relay current
* how important is power saving
* how important are clean switching levels
* your target (ideas)
* your skills / what effort you want to spend (time, money)

Klaus

#### BradtheRad

##### Super Moderator
Staff member
I measure 600 ohms coil resistance in my 24V relay. It draws 40mA at 24V.
Almost a watt. By testing it pulls in at 13V and releases at 10V.

With your 65v supply you can limit current by installing a safety resistor. 1 kohm drops 40v. The relay coil is not exposed to greater voltage than its rating.

My simulation has identical relays with differing range of source voltage. Screenshot snapped when contacts are nearly closed.

#### Blueboy7

##### Newbie level 5
Hi,

there are a lot of different solutions depending on what your target is:
* from rather simple
* to most power saving
* or any other optimisation

A simple solution could be
* a series resistor, and a zener across the relay coil. (shunt regulation style)
While the zener limits the coil voltage ... the "overvoltage" has to drop at the series resistor.
It consumes a lot of power and dissipates it into heat. And there no clear threshod levels for ON and OFF.

* another solution could be by adding a BJT and a diode (for protection) --> series voltage regulation.
It wastes less power, mainly on the BJT

* one could use a "switching" solution with a comparator monitoring and PWM-ing the coil current.
This is the most power saving method. It consumes almost only the relay current.

***
But as already mentioned, I guess this applicationcalls for clean threshold levels while avoiding chattering.
This could be done with a second comparator (in same package). So the circuit consumes very low power at too low input voltage, relay is OFF. Then at a ceratin point it swtiches ON the relay. Power saving.

******
Some important informations are missing:
* relay current
* how important is power saving
* how important are clean switching levels
* your target (ideas)
* your skills / what effort you want to spend (time, money)

Klaus
* relay current Control current is 38ma
* how important is power saving Always desirable
* how important are clean switching levels Clean enough so there is no chatter
* your target (ideas) Switch on and off like it's designed
* your skills / what effort you want to spend (time, money) I’d say my skills are decent. I don’t know the names of the components or what they do. I mean I know the common things like resistors, diodes, capacitors. I am no Electronics Technician but I can build simple things, as long as the directions are clear. Time is not an issue.
I can buy what I need off the shelf for $200 which is more than I want to spend. The BJT range would be good I just need to know where to put what. Thanks!! #### Blueboy7 ##### Newbie level 5 I measure 600 ohms coil resistance in my 24V relay. It draws 40mA at 24V. Almost a watt. By testing it pulls in at 13V and releases at 10V. With your 65v supply you can limit current by installing a safety resistor. 1 kohm drops 40v. The relay coil is not exposed to greater voltage than its rating. My simulation has identical relays with differing range of source voltage. Screenshot snapped when contacts are nearly closed. View attachment 176163 A resistor was my first thought. Needed to know what size. ( 1K thanks ) I understand the 1K resistor will protect the coil from burning up from 64 V but how does it affect the lower end? Say like when the sun starts to come out and the panels approach 24 V, with the resistor in there will it need to be a higher voltage? Thank you!! #### KlausST ##### Super Moderator Staff member Hi * how important are clean switching levels Clean enough so there is no chatter Chattering is the one thing. * above what (input) voltage level you want the relay to be safely ON * and below which (input) voltage level you want the relay to be safely OFF You need to read the relay´s datasheet. * 24V is the nominal relay coil voltage * but the ON volage maybe is at 18V (*) * and the OFF voltage maybe is at 9V (*) (*) buth values are not very clean and will depend a lot on production, temperature and for sure mechanical vibration. With additional electronics you may improve this. Like switch ON at accurately 20V (you can´t go below the relay´s specification) (you defined it as "And when the sun comes out it’ll energize the relay" .. but you need to define it a voltage level. Nobody of us can know the level.) .. and switch OFF at accurately 16V (both as examples) Another thing to consider: Your input voltage is 0V ... 65V. This is a wide range. When you connect a relay, then it draws current and the input voltage my drop. How much depends on the source impedance. Klaus #### Blueboy7 ##### Newbie level 5 Hi Chattering is the one thing. * above what (input) voltage level you want the relay to be safely ON * and below which (input) voltage level you want the relay to be safely OFF You need to read the relay´s datasheet. * 24V is the nominal relay coil voltage * but the ON volage maybe is at 18V (*) * and the OFF voltage maybe is at 9V (*) (*) buth values are not very clean and will depend a lot on production, temperature and for sure mechanical vibration. With additional electronics you may improve this. Like switch ON at accurately 20V (you can´t go below the relay´s specification) (you defined it as "And when the sun comes out it’ll energize the relay" .. but you need to define it a voltage level. Nobody of us can know the level.) .. and switch OFF at accurately 16V (both as examples) Another thing to consider: Your input voltage is 0V ... 65V. This is a wide range. When you connect a relay, then it draws current and the input voltage my drop. How much depends on the source impedance. Klaus Hi Klaus, On and off voltage really doesn’t matter. All I’m after is for the relay to turn off the AC charger when the sun is shining. I picked the 24 V relay because I thought it could handle the higher voltage, apparently not. I thought about trying a 48 V and see what that does. But I’d rather not waste any more money. And if the relays keep failing because the switching threshold is inconsistent then that’s just more wasted money. I suspect I will need to buy a PWM or something. I was considering one of these relays from McMaster but I'm not sure if they will do what I want. Need to research. https://www.mcmaster.com/catalog/128/1062 DIN-Rail Mount Multifunction Timer Relay 6 Terminals Part number 6964K4 for$72.65

Sequenced Latching Relay
DPST-1NO/1NC, 24V DC Input
Part number 6784T135 for $30.00 Blue #### crutschow ##### Advanced Member level 5 I need some help with a 24 VDC relay that I’m trying not to burn up again. So how did you burn it up the first time?. #### KlausST ##### Super Moderator Staff member Hi, I guess you need to read how solar cells voltage and current behaves. In short: It rather acts as a current source than a voltage source. This means it has relatively high impedance. * dark: zero ouput voltage, zero ouput current * full light: specified ouput voltage at specified output current * dark cloudy: without load the voltage is rather high, not as high as with full light. But as soon as you connect a load (relay, charger...) the voltage will drop to a very low value. This means: although the "open" voltage is hig you can´t dra much power from it. This may result in repeated ON/OFF: To avoid this problem the intelligent chargers/inverters don´t rely solely on the open voltage. They draw current and check how the voltage react on this. More intelligent are the MPPT chargers/inverters. They adjust voltage as well as current to get the optimal high power from the cells. To your system: Either you need to study how your system (solar cell voltage, current vs load and light quantitiy) .. or buy a good inverter/charger with MPPT function. Klaus #### Blueboy7 ##### Newbie level 5 Hi, I guess you need to read how solar cells voltage and current behaves. In short: It rather acts as a current source than a voltage source. This means it has relatively high impedance. * dark: zero ouput voltage, zero ouput current * full light: specified ouput voltage at specified output current * dark cloudy: without load the voltage is rather high, not as high as with full light. But as soon as you connect a load (relay, charger...) the voltage will drop to a very low value. This means: although the "open" voltage is hig you can´t dra much power from it. This may result in repeated ON/OFF: To avoid this problem the intelligent chargers/inverters don´t rely solely on the open voltage. They draw current and check how the voltage react on this. More intelligent are the MPPT chargers/inverters. They adjust voltage as well as current to get the optimal high power from the cells. To your system: Either you need to study how your system (solar cell voltage, current vs load and light quantitiy) .. or buy a good inverter/charger with MPPT function. Klaus I already have this. TS-MPPT-60 https://www.solar-electric.com/lib/wind-sun/TSMPPT-manual.pdf The problem is the AC charger takes priority over the solar charger 9 out of 10 That's why I want the AC charger to turn off automatically when the sun is shining. I have the AC charger on a manual switch but VERY often forget to turn it on or off. Thanks!!!!!!! Blue #### Blueboy7 ##### Newbie level 5 So how did you burn it up the first time?. Hooked it up directly to the solar panels. It was working for a little while, but it was getting hot, and then... #### KlausST ##### Super Moderator Staff member Hi, simple but maybe what you want: D1 = 24V zener Q1 = NPN, >100V, >100mA, >2W, need to be mounted on a heatsink for 2W R1 = 4k7, >1W Load = your relay coil. Add a standard diode parallel to the relay coil, anode to GND *** Please check whether "relay driver" of the charger manufacturer can be used as alternative. *** Check whether you need a more intelligent solution: I mean .. in the night there is no solar power, thus your solution switches to "mains charger". Then maybe at 6 AM the battery is full .. then the sun begins to shine, but has nothing to do... Klaus #### crutschow ##### Advanced Member level 5 picked the 24 V relay because I thought it could handle the higher voltage, apparently not. You are apparently right. Why would you think the relay could tolerate more than its rated voltage without overheating? The relay coil heating is proportional to the square of the applied voltage so if you applied 65V to the coil, that would increase the power by (65^2) / (24^2) = 7 times. Load = your relay coil. Add a standard diode parallel to the relay coil, anode to GND You don't need the diode with an emitter follower, as the negative voltage generated by the inductance will keep the transistor conducting after the power is removed, until the inductive current has dissipated. #### Blueboy7 ##### Newbie level 5 You are apparently right. Why would you think the relay could tolerate more than its rated voltage without overheating? The relay coil heating is proportional to the square of the applied voltage so if you applied 65V to the coil, that would increase the power by (65^2) / (24^2) = 7 times. Well actually I skipped one minor bit of information. When I initially checked the voltage of the panels it was early evening and was not full sun light power. The voltage was low to mid 30's. Had I checked the volts mid day I most likely would have bought a 48v relay. The batteries are set up for 24v, which is another reason why I got the 24v relay. But I did check the voltage again before I connected the relay. I rolled the dice and lost. Even though the relay was only$15 it was still a bad decision to hook it up.

Blue
--- Updated ---

Hi,

simple but maybe what you want:

D1 = 24V zener
Q1 = NPN, >100V, >100mA, >2W, need to be mounted on a heatsink for 2W
R1 = 4k7, >1W
Load = your relay coil. Add a standard diode parallel to the relay coil, anode to GND

Klaus
Components are ordered. Vielen Dank Klaus

" I'll be back "

Blue

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