electrical gurus: fuel sender-gauge voltages?

[General23cmp]

Ok. I understand the gauge works on 5v. Is that what goes to the sender as well (or is it 12v to the sender)? Does anyone have a schematic to show how all that ties together with the 5v limiter, gauge, sender, battery? At the end, I am trying to determine how many amps flow through that 5v voltage limiter, max (I know the coolant sensor runs on the same limiter, with the same resistance as the fuel sender). I understand circuits pretty well. I just don't know how this one is laid out. I am designing my own limiter as my mechanical one is fried. I understand they make drop in replacements for it, and that there are schematics for making DIY ones for cheap. I like DIY. But when doing the calculations without knowing the whole circuit, I cannot see how those DIY units do not burn up with a 7805 if you have a full tank of fuel and hot coolant (lowest resistance, more amps). My car is a 1969 Plymouth Satellite, but it should be similar for many models. Thanks.

 

[dadsbee]

All senders are regulated NEGATIVE feeds.

 

[General23cmp]

I know that.

Is it a 5v or 12v circuit to the sender? I assume 5v though the gauge, but there could be something different than a simple 5v loop through it all. I cannot see how a single 7805 will survive regulating both fuel and temp circuits if they are simple, single loop circuits.

 

[dadsbee]

I know that.

Sorry I tried to help! Sender is negative/ground/frame period with a variable resistance output to the negative side of the gauge. The other side of the gauge (basically a volt meter) gets the 5V.

 

[General23cmp]

Sorry, it was not meant that way. I'm just trying to calculate the electrical demands of the 5v limiter. There has to be some sort of voltage through the sender in order for the variable resistance to have any use, right? It's likely minimal amperage and understanding the circuitry of the fuel gauge likely has the answers, but I am just trying to understand.

 

[dadsbee]

Sorry but as an Electo-Mechanical Engineer I have trouble explaining electricity to people. Look at the wire size of a gauge and ask yourself how high the current could possibly be without frying them. Basically zilch !

 

[FrnkNsteen]

Dadsbee is right. Amps through that circuit are nearly zero.

Remember that Voltage = Ohms x Amps or in this case Amps = Volts/Ohms. Our fuel, oil pressure, and temperature senders work on a 10 - 80 ohm resistance range (80 ohms equals empty or low pressure/temp - 10 ohms for Full or High pressure/temp).

So 5 volts divided by 80 ohm max resistance of our senders equals .0625 amps (per gauge).

 

[General23cmp]

I also am an engineer, so I'm glad I can talk more detail. I'm still curious how much is expected from the limiter. When looking at the specs for a 7805 IC, zilch is about all you can get from it when regulating from 14.7v (lets say 15v) down to 5v. For example, given a junction to case thermal resistance of 3 C/Watt, a free-air thermal resistance of 62.5 C/Watt, and the maximum allowable junction temperature of 150C of a typical 7805 (assuming a high ambient temperature of 60C, inside a hot car), then I can only absorb about 1.4 Watts of heat from the 7805 without it overheating. I can add a heat sink, but it doesn't buy me a ton. So, for a 7805, to get that amount of heat, assuming ~15 volts applied, the math says (15v-5v) * current = 1.4 W. Thus my maximum current the IC can provide is 140 mA and stay "ok". Also keep in mind that this is for the two gauges that use the one limiter. I'm not sure the thickness of those wires, but it doesn't take much current to overheat the 7805 IC either.

 

[General23cmp]

Dadsbee is right. Amps through that circuit are nearly zero.

Remember that Voltage = Ohms x Amps or in this case Amps = Volts/Ohms. Our fuel, oil pressure, and temperature senders work on a 10 - 80 ohm resistance range (80 ohms equals empty or low pressure/temp - 10 ohms for Full or High pressure/temp).

So 5 volts divided by 80 ohm max resistance of our senders equals .0625 amps (per gauge).

.
But 5v divided by 10 ohms is 0.500 amp....enough to easily overheat a 7805 IC being stepped down from 12-15v.

I think the circuitry of the gauge makes the simple voltage/ohm of the fuel sender not exactly what the gauge's wires are seeing.

 

[tmaleck]

I also am an engineer, so I'm glad I can talk more detail. I'm still curious how much is expected from the limiter. When looking at the specs for a 7805 IC, zilch is about all you can get from it when regulating from 14.7v (lets say 15v) down to 5v. For example, given a junction to case thermal resistance of 3 C/Watt, a free-air thermal resistance of 62.5 C/Watt, and the maximum allowable junction temperature of 150C of a typical 7805 (assuming a high ambient temperature of 60C, inside a hot car), then I can only absorb about 1.4 Watts of heat from the 7805 without it overheating. I can add a heat sink, but it doesn't buy me a ton. So, for a 7805, to get that amount of heat, assuming ~15 volts applied, the math says (15v-5v) * current = 1.4 W. Thus my maximum current the IC can provide is 140 mA and stay "ok". Also keep in mind that this is for the two gauges that use the one limiter. I'm not sure the thickness of those wires, but it doesn't take much current to overheat the 7805 IC either.

I would just make up a small switcher and lose the 7805 linear reg. I just did one for work based on the LMR14030SDDAR with 6 passive components and a 3.5 Amp output. At 2A load the die temp is about 100 deg. F. Total BOM cost is about 6 bucks. Osh Park does small PCB's cheap and fast. I was too lazy to do one for my dash, so I bought one instead.
I bet a lot of eyes glazed over when you started talking about Theta JC.

 

[General23cmp]

I would just make up a small switcher and lose the 7805 linear reg. I just did one for work based on the LMR14030SDDAR with 6 passive components and a 3.5 Amp output. At 2A load the die temp is about 100 deg. F. Total BOM cost is about 6 bucks. Osh Park does small PCB's cheap and fast. I was too lazy to do one for my dash, so I bought one instead.
I bet a lot of eyes glazed over when you started talking about Theta JC.

Did you test to see how steady the output was given the possible variation in voltage input (12-14.7v)? That was my concern with a simple converter off of Amazon for $7. It was rated for 3A output and accepted a wide range of inputs; however, the output likely has difficulty staying steady at 5v when the input varied.

 

[66chargerpat]

I have used the 7805 for 16 years on a 66 charger running the fuel, oil and temp all at the same time. You are forgetting to add the resistance of the heater coil in the gauge. The gauge and sending unit are wired in series, so the load is very low.

 

[JimCoronette]

Ok. I understand the gauge works on 5v. Is that what goes to the sender as well (or is it 12v to the sender)? Does anyone have a schematic to show how all that ties together with the 5v limiter, gauge, sender, battery? At the end, I am trying to determine how many amps flow through that 5v voltage limiter, max (I know the coolant sensor runs on the same limiter, with the same resistance as the fuel sender). I understand circuits pretty well. I just don't know how this one is laid out. I am designing my own limiter as my mechanical one is fried. I understand they make drop in replacements for it, and that there are schematics for making DIY ones for cheap. I like DIY. But when doing the calculations without knowing the whole circuit, I cannot see how those DIY units do not burn up with a 7805 if you have a full tank of fuel and hot coolant (lowest resistance, more amps). My car is a 1969 Plymouth Satellite, but it should be similar for many models. Thanks.

7805 is good for 1.5A.

Therefore 5v/10ohms=.5A max draw for the fuel sending unit.

What is the current draw for hot resistance of the coolant sensor? My guess would be another .5A
for a total draw of 1.0A.

That's 66% of max, seems fine to me.

 

[General23cmp]

7805 is good for 1.5A.

Therefore 5v/10ohms=.5A max draw for the fuel sending unit.

What is the current draw for hot resistance of the coolant sensor? My guess would be another .5A
for a total draw of 1.0A.

That's 66% of max, seems fine to me.

A 7805 is not good for 1.5A if regulating 12-15v to 5v (at least not for long), especially in warm areas. My equations above detail that. It’s good for 1.5A under specific conditions, none of which apply to our vehicles.

 

[General23cmp]

I have used the 7805 for 16 years on a 66 charger running the fuel, oil and temp all at the same time. You are forgetting to add the resistance of the heater coil in the gauge. The gauge and sending unit are wired in series, so the load is very low.

I believe you are likely correct. Does anyone know the resistance of the gauges?

 

[tmaleck]

Did you test to see how steady the output was given the possible variation in voltage input (12-14.7v)? That was my concern with a simple converter off of Amazon for $7. It was rated for 3A output and accepted a wide range of inputs; however, the output likely has difficulty staying steady at 5v when the input varied.

Percent regulation is quite good, Ti has a website you can plug in desired parameters and it will give you tons of analysis to look at.

 

[451Mopar]

A good 1.5A rated 7805 regulator can power two gauges (around 0.3 Amp) from 15v to 5v without going into thermal shutdown (tested at about 80-85 ambient temp, and just using the old regulator case as a heat sink.)
On a three gauge cluster (about 0.460 to 0.500 Amp max), it needs a larger heat sink or it can go into thermal shutdown after about 15-30 minutes.
Your gauges could appear to be working correctly, but actually reading low if the regulator starts shutting down due to overheating.
Only way to really know is monitor the regulator output voltage.
From the really old data I have, Oil pressure gauge resistance on '69 rally cluster seems to be around 16.3 Ohms for most of the sweep (calculated from 5-volt/current - sender unit resistance (simulated with resistors). It looks to change at the low and high ends, around 12.5 ohms around zero PSI, and 19.3 Ohms at 80 PSI. Max current at 80 PSI was 172 mA.
Fuel gauge resistance changes alot. Max current at Full was 150 mA. Calculated gauge resistance seemed to swing from 23 Ohms at Full to 67 ohms at 1/4 tank?
Temp gauge resistance was fairly consistent around 18 to 19 Ohms. Max Current at full scale was 163 mA.

 

[General23cmp]

Thanks. Good info. My concern is really when the inside of my car is more like 120F. That’s not unusual in the south (maybe more) with everything closed up. It’s probably fine anyway. Thanks again.